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Characteristics of the South China Sea Monsoon from the Onset to Withdrawal before and after 1993/94

Characteristics of the South China Sea Monsoon from the Onset to Withdrawal before and after 1993/94 Hindawi Advances in Meteorology Volume 2020, Article ID 8820460, 13 pages https://doi.org/10.1155/2020/8820460 Research Article Characteristics of the South China Sea Monsoon from the Onset to Withdrawal before and after 1993/94 1,2 1 Zhao Xiaofang and Wang Lijuan Key Laboratory of Meteorological Disaster, Ministry Education, Joint International Research Laboratory of Climate and Environment Change, Collaborative Innovation Center on Forecast and Evaluation of Meteorological Disasters, Nanjing University of Information Science and Technology, Nanjing 210044, China Wuhan Regional Climate Center, Wuhan 430074, China Correspondence should be addressed to Wang Lijuan; wljfw@163.com Received 8 April 2020; Revised 21 August 2020; Accepted 3 September 2020; Published 16 September 2020 Academic Editor: Enrico Ferrero Copyright © 2020 Zhao Xiaofang and Wang Lijuan. ,is is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. ,e characteristics and possible impact factors of the South China Sea summer monsoon (SCSSM) evolution from onset to withdrawal before and after 1993/94 are investigated using ERA-Interim, CPC rainfall, and OLR data. During the late-onset period of 1979–1993, the SCSSM was characterized by stronger onset intensity and a gradual withdrawal, resulting in a con- tinuous, strong preflood season in Southern China and a slower rain-belt retreat from north to south China in September. In addition, the rain-belt in the Yangtze River basin persisted much longer during summer. However, during the early-onset period in 1994–2016, the SCSSM is associated with a weaker onset intensity and comparatively faster retreat. ,e advanced preflood season lasted intermittently throughout May and the whole eastern China precipitation lasted until October when it retreated rapidly, making the rain-belt in Southern China persist for an extended duration. Further analysis indicates that a strong modulation of SCS intraseasonal oscillation (ISO) on the SCSSM evolution is observed. ,ere are two active low-frequency oscillations over the SCS in summer during the late-onset period but three during the early-onset period. ,e wet ISO in the Northwest Pacific propagating northwestward into the SCS and enhanced SCSSM ISO activity may contribute to the early onset and faster withdrawal after 1993/94. ,e effect of warm western Pacific sea surface temperatures (SST) on the SCSSM evolution is also discussed. interannual variability of the SCSSM onset. A warm (cold) El 1. Introduction Niño-Southern Oscillation (ENSO) event in the previous Asian monsoons are one of the main energy and moisture winter and the warm (cold) tropical Indian Ocean (TIO) in sources of the global atmosphere [1]. ,e SCSSM has a the spring can delay (advance) the SCSSM onset [16–19]. significant position in the Asian monsoon, forming a vital ,ere are fewer studies that focus on SCSSM withdrawal link between the East Asian and South Asian summer when compared to the SCSSM onset [20]. Hu et al. [21] monsoons [2–4]. Climatologically, the SCSSM onset is found that SCSSM withdrawal mainly occurred due to the generally accompanied by a continuous eastward retreat of westward intrusion of the WNP subtropical high, which is the western North Pacific (WNP) subtropical high, con- accompanied by the retreat of the weakening low-level in- vection enhancement, and the reversal of low-level zonal tertropical convergence zone (ITCZ) and rain-belt in the wind in the SCS [5, 6]. ,e synoptic-scale circulation sys- SCS-WNP. ,e factors contributing to the interannual tems, the disturbance of mid-latitude, and the ISO activities variability of the SCSSM withdrawal may include tropical can trigger SCSSM onset [7–15]. In addition, the sea surface cyclones (TCs) and ISO [22]. Luo and Lin [23] suggested that temperature anomaly (SSTA) is closely related to the El Niño can delay the monsoon onset and advance the 2 Advances in Meteorology SCSSM withdrawal, thus shortening the length of the wind and convection are the two common and significant summer monsoon season. indices for identifying the SCSSM onset [21, 35]. In order to Recent research has shown that the SCSSM onset date take low-level features for a more comprehensive consid- has advanced by about two weeks since 1994 [24–29]. eration into account the, we corrected the onset date by Kajikawa and Wang [25] suggested that the enhanced combining circulation and the onset date defined by the northwestward intraseasonal variability (ISV) and TC ac- National Climate Centre. ,e National Climate Centre tivity from WNP could be responsible for the interdecadal defines the SCSSM onset by the first pentad when 850 hPa change in the SCSSM onset. Xiang and Wang [30] indicated zonal wind in the SCS changes to westerly wind for two that the interdecadal advance of the Asian summer monsoon constant pentads stably and the average potential pseu- onset may be attributed to a grand La Niña-like pattern doequivalent temperature is greater than 340 K. As a result, change in the Pacific, while that in the South China Sea (SCS) we corrected the onset date of 1982, 1997, 2000, 2008, and is primarily determined by the abrupt SST warming near the 2009. ,e SCSSM withdrawal pentad for 1979–2016 is also Philippine Sea. Liu et al. [31] highlighted the impact of defined by MTG. ,e MTG-defined SCSSM onset and southern Indian Ocean (SIO) on the SCSSM onset ad- withdrawal originate from thermal differences between vancement after 1993. Many efforts have been made to north and south, which is the essence of monsoon forma- understand the cause driving interdecadal change of the tion. In order to extract the more obvious signal caused by SCSSM onset, and interdecadal change in the SCSSM the SCSSM interdecadal transition, the years with later than withdrawal after the mid-2000 has been found recently [32]. average onset are regarded as the typical late-onset years Considering the integrity of SCSSM evolution, particularly during 1979–1993, which are 1981, 1982, 1985, 1987, 1991, from onset to withdrawal, the following questions can be and 1993. ,e years with an earlier than average onset are raised: (1) What changes have happened in the SCSSM regarded as the typical early-onset years during 1994–2016, evolution from onset to withdrawal since 1993? (2) Previous which are 1994, 1996, 2000, 2001, 2002, 2005, 2006, 2008, studies have highlighted the role of SSTA and ISO during 2012, 2013, and 2014. this advanced SCSSM onset. What are the effects of ISO and SSTAs on the SCSSM evolution from onset to withdrawal in 3. The Decadal Shift of the SCSSM before different periods? ,ese questions will be answered in this and after 1993/94 paper. 3.1. Difference of the SCSSM Onset Characteristics during the Two Epochs. To show the reliability of the MTG-defined 2. Data and Methods SCSSM, the time series of the SCSSM onset date is plotted in ,e datasets used in this paper include (1) the daily mean Figure 1, which shows a significant advancement of the interpolated outgoing longwave radiation (OLR) data from SCSSM onset date after 1993/1994 and the SCSSM onset the National Oceanic and Atmospheric Administration time we defined is basically consistent with that defined by ° ° (NOAA) satellite with resolution of 2.5 latitude by 2.5 the National Climate Centre and its correlation is 0.58. ,e longitude from 1979 to 2016; (2) the daily mean ECMWF mean SCSSM onset occurs in about pentad 29 during Interim Re-Analysis dataset (ERA-Interim) with resolution 1979–1993, while during 1994–2016 it occurs in about ° ° of 2.5 latitude by 2.5 longitude from 1979 to 2016 (the pentad 27. ,e change is about two pentads and the standard variables used in this paper include temperature and three- deviation (STD) of the SCSSM onset time is 1.67. ,is dimensional wind fields); (3) the monthly mean Hadley interdecadal change of the onset date is consistent with previous studies [24–26] and passes the sliding t-test (not Centre sea surface temperature (SST) dataset for the period of 1979–2016 [33]; and (4) daily precipitation set from the shown). Based on the interdecadal change of SCSSM onset, CPC Unified Precipitation Project which is underway at the circulation and convection differences during the NOAA Climate Prediction Centre (CPC). SCSSM onset should be researched further. In the present study, the SCSSM onset is identified by the Figure 2 shows the composite OLR and 850 hPa winds of seasonal transition of the mid- to upper-tropospheric me- the SCSSM onset from before and after the onset pentad in ridional temperature gradient (MTG, zT/zy) from winter to each epoch. Lower OLR corresponds to heavy rainfall and summer, which is defined by Mao et al. [34] and Liu et al. strong convection. In the late-onset years of 1979–1993, [31]. Specifically, the timing of the SCSSM onset is defined by before the SCSSM onset pentad (Figure 2(a)), a weak an- ticyclone is centered over the SCS, which makes the SCS be the moment when the pentadly area-averaged MTG in the mid-to-upper troposphere (500–200 hPa) changes from controlled by easterly wind. ,e active convection area is mainly located in the Bay of Bengal and Indochina Pen- negative to positive and remains positive for at least three ° ° pentads over the SCS (10–20 N, 110–120 E). ,en the first insula, extending to the south of China and the south of pentad from negative to positive phase is defined as the onset Japan. When the SCSSM onsets (Figure 2(b)), the developing of the SCSSM. ,ey suggest that MTG between 500 hPa and convection from the Bay of Bengal expands to the northern 200 hPa is more abrupt in assessing monsoon onset and part of the SCS along with weakening convection over the consistent with the main climatological characteristics of the southern Indian Ocean, indicating that the ITCZ stretched SCSSM onset, such as area-averaged zonal wind shear re- northwards [25]. Southwesterly wind from the Bay of Bengal versal, zonal wind reversal, and convection enhancement. reaches the SCS resulting in the anticyclone of the Northwest ,ere are 72 pentads per year. However, the 850 hPa zonal Pacific retreating eastward. After the onset (Figure 2(c)), the Advances in Meteorology 3 34.0 32.0 30.0 28.0 26.0 24.0 1978 1982 1986 1990 1994 1998 2002 2006 2010 2014 Figure 1: Time series of the MTG-defined SCSSM onset date with the red line denoting the mean epoch date. 10 m/s 10m/s 10m/s 40°N 40°N 40°N 30°N 30°N 30°N 20°N 20°N 20°N 10°N 10°N 10°N 0° 0° 0° 10°S 10°S 10°S 60°E 75°E 90°E 105°E 120°E 135°E 150°E 60°E 75°E 90°E 105°E 120°E 135°E 150°E 60°E 75°E 90°E 105°E 120°E 135°E 150°E 180 200 220 180 200 220 180 200 220 (a) (b) (c) 10m/s 10m/s 10m/s 40°N 40°N 40°N 30°N 30°N 30°N 20°N 20°N 20°N 10°N 10°N 10°N 0° 0° 0° 10°S 10°S 10°S 60°E 75°E 90°E 105°E 120°E 135°E 150°E 60°E 75°E 90°E 105°E 120°E 135°E 150°E 60°E 75°E 90°E 105°E 120°E 135°E 150°E 180 200 220 180 200 220 180 200 220 (d) (e) (f) Figure 2: Composite evolution of OLR (W/m , shadings) and 850 hPa wind (m/s, vectors) based on the SCSSM onset date in a typical year of the epoch 1979–93 (Top) and 1994–2016 (Bottom) from (a, d) one pentad before the monsoon onset (P − 1), (b, e) during the monsoon onset (P0), and (c, f) one pentad after the monsoon onset (P + 1). active convection over the Arabian Sea and Indochina Indonesian Maritime Continent and the eastern equator of Peninsula is expanded into the SCS. ,e convection in the Indian Ocean are expanded northward. ,e wind direction Bay of Bengal is significantly stronger than that in 1994–2016 over the SCS abruptly changes to westerly direction. After and the southwesterly wind in the SCS is more intense. the onset (Figure 2(f)), the convection over the Indonesian In early-onset years of 1994–2016, the convection in the Maritime Continent moves northward to the Philippines eastern equator of Indian Ocean is weaker than that in and enters south of the SCS, but the southwesterly wind in 1979–1993, before the onset pentad (Figure 2(d)). Another the SCS weakens. distinct convection area is over the Indonesian Maritime According to the above analysis, the convection source Continent. ,is convection is associated with tropical dis- favoring monsoon onset has changed since 1993/94. In turbances, such as easterly wave and MJO, which are fa- addition, it is clearly shown that the reversal and increment of low-level wind in SCS are more noticeable during vorable for the onset of the SCSSM [22]. When the SCSSM builds up (Figure 2(e)), both active convection over the 1979–1993 when the SCSSM onset is delayed. Figure 3(a) 4 Advances in Meteorology 8.0 3.0 6.0 2.0 4.0 1.0 2.0 0.0 0.0 –1.0 –2.0 –4.0 –2.0 1May 1Jun 1Jul 1Aug 1Sep 1Oct 1978 1982 1986 1990 1994 1998 2002 2006 2010 2014 OLR (P0) U (P1) – U (P0) (a) (b) ° ° ° ° Figure 3: (a) Time series of 5-day running mean 850 hPa zonal wind (m/s) averaged over the South China Sea (110 –120 E, 10 –20 N). ,e black, red, and blue curves are climatological years, typical late-onset years during 1979–1993, and typical early-onset years during 1994–2016, respectively. (b) Time series of the standardized OLR (red; W/m ) in the onset pentad and zonal wind growth intensity (U(P1) − ° ° ° ° U(P0); blue; m/s) over the SCS (110 –120 E, 10 –20 N) from 1979 to 2016. Table 1: Correlation coefficients between the anomalies of the shows the composite 850 hPa zonal wind evolution over the zonal wind growth intensity (U(P1) − U(P0)) and OLR over the SCS SCS for typical years and climatology. In 1979–1993, the in onset pentad and the SCSSM onset dates in the periods of monsoon builds up late, corresponding to the zonal wind 1979–93 and 1994–2016, respectively. turning positive late. On the other hand, the zonal wind U(P1) − U(P0) OLR (P0) growth is peaking faster in late June. However, in 1994–2016, 1979–1993 0.23 − 0.25 the monsoon builds earlier; the zonal wind turns positive in ∗ ∗ 1994–2016 0.38 − 0.43 early May and grows slowly. ,e first peak is in mid-June. ∗∗ ∗∗ 1979–2016 0.36 − 0.38 ,us, what is the relationship between monsoon onset date Values exceeding the 95% and 99% confidence levels are marked by a single and intensity? or double asterisk (∗ and ∗∗ ), respectively. Due to the change of the zonal wind in the SCS, we expressed the zonal wind growth intensity by zonal wind over the SCS after the SCSSM onset pentad minus the the relationship between zonal wind growth intensity and SCSSM onset pentad, which objectively depicts the abrupt OLR in the mid-1990s. ,is result illustrates that the late onset intensity of SCSSM. Time series of standardized OLR onset of the SCSSM is likely to correspond to a strengthened in onset pentad and zonal wind growth intensity in the SCS zonal wind and enhanced convection activity, which means shows that positive zonal wind growth intensity generally that the SCSSM onset intensity is stronger. ,erefore, the corresponds to negative OLR (Figure 3(b)). ,is means that later (earlier) the SCSSM onset is, the stronger (weaker) the a strong zonal wind intensity increase in the SCS generally intensity of the SCSSM onset is. Between 1979 and 1993 corresponds to an enhanced convection. ,erefore, the (1994 and 2016), the SCSSM onset is generally late (early), so convection and wind field in the SCS have obvious coupling the zonal wind growth intensity is stronger (more weakened) characteristics. ,ey show that the SCSSM onset process is a and convection is more active (suppressive) in the SCS. combination of dynamic and thermal. To reveal the rela- tionship between the SCSSM onset date and intensity, we calculated the correlation coefficients between the regional 3.2. Differences of the SCSSM Withdrawal Characteristics averaged OLR and the zonal wind growth intensity with the during the Two Epochs. ,e circulation evolution and SCSSM onset date in the period of 1979–1993 as well as in abrupt characteristics of the SCSSM onset of the two epochs are discussed above. After the SCSSM onset, the low-level the period of 1979–2016 (Table 1). ,e SCSSM onset date is significantly related to the SCSSM onset intensity. ,ere is a zonal winds appear with a two-peak pattern in summer positive correlation between the zonal wind growth intensity (Figure 3(a)). In the late-onset years of 1979–1993, the low- and the onset date of SCSSM, and there is a negative cor- level zonal wind over the SCS grew rapidly from early June relation between the OLR and the onset date of SCSSM. until it reached the first peak, and then it began to decrease. However, the correlation coefficient of two variations during Subsequently, the zonal wind grew again and reached a 1994–2016 is much higher and exceeds the 0.01 confidence second peak at the end of July. In the early-onset years of level, while that during the 1979–1993 epoch fails in passing 1994–2016, the zonal wind peaked firstly in mid-June, with a the 0.05 confidence level and this phenomenon is more weak intensity. ,en, in early July, it started to grow rapidly obvious during 1994–2016. However, different from Hu until the end of July. After summer, what changes have et al. [20] and Torrence and Webster [36], sliding correlation happened to the SCSSM withdrawal since 1993/94? To find the difference in SCSSM withdrawal character- between the SCSSM onset date with OLR and zonal wind growth intensity shows no significant interdecadal shift in istics between the two epochs, we must first figure out the Advances in Meteorology 5 From the composite results, the earlier the SCSSM onset, the climatological circulation evolution of 850 hPa wind and OLR during the SCSSM withdrawal. Figure 4 shows the later the SCSSM withdrawal after 1993. Now that the SCSSM withdrawal dates have been de- latitudinal-time and longitudinal-time cross section of low- level wind and OLR averaged from August 15 to the end of fined, we will explore the circulation evolution during the October. Different from the abrupt features of the SCSSM retreat period. Figure 6 shows the climatological composi- onset, it takes longer for the SCSSM to withdraw. ,e tions of low-level wind and OLR during the SCSSM with- southwesterly wind and convection retreat from the SCS do drawal period from 1979 to 2016. It can be seen that active not occur simultaneously during the SCSSM withdrawal. convection is located in the Bay of Bengal to the Northwest Meridionally, the southwesterly wind retreats completely Pacific before the SCSSM withdrawal pentad, and it retreats gradually from northeast to southwest (Figures 6(a) and from the northern SCS to the southern SCS in mid-Sep- tember, while the convection retreats gradually from the SCS 6(b)) and then completely withdraws from the SCS, since the SCSSM withdrawal pentad and convection centers shift to until mid-October (Figure 4(a)). Different times for the wind and convection to retreat to the SCS can also be seen in the the Indonesia area (Figures 6(c) and 6(d)). Meanwhile, the low-level wind is already easterly before the SCSSM with- zonal variation (Figure 4(b)). ,is nonsimultaneous retreat of westerlies and convection in the SCS is due to the activities drawal pentad, but it is obvious that the northeasterly wind of TCs and the synoptic systems embedded in the basic flow has become stronger on the SCSSM withdrawal pentad. To of easterly winds, which can contribute to the active con- further objectively investigate the variability of circulation vection around the SCS [21]. However, the TC and other factors during the SCSSM withdrawal, the pentad-mean of synoptic scale systems are also greatly modulated by the 850 hPa zonal wind and OLR averaged over the SCS evo- WNP anticyclonic/cyclonic anomalies [35], which are lution are composited in Figure 7. ,ere appears to be a significant decrease in zonal wind and a weakened con- closely related to the SCSSM withdrawal. ,erefore, it is necessary to consider the progressive retreat convection in vection whose value rises remarkably from about 220 W/m the process of SCSSM withdrawal, especially in the deter- to around 240 W/m over the SCS in the SCSSM withdrawal mination of the SCSSM retreat date. pentad. Zonal wind and convection almost develop at the According to the above analysis, we use pentadly area- same time when the SCSSM onsets, but, during the SCSSM averaged MTG in the mid-to-upper troposphere retreat, the westerly wind and convection retreat from the (500–200 hPa) when it changes from positive to negative to SCS do not occur simultaneously [21]. It was zonal wind calculate the date of the SCSSM withdrawal. Considering the shifting to easterly wind firstly and then convection retreat inconsistency of convection and wind field retreat during the from the SCS (Figure 4). ,e time difference of them is about SCSSM withdrawal, the persistence of MTG turning nega- 20 days. ,e SCSSM withdrawal dates defined in this paper tive cannot be maintained for three continuous pentads; take into account not only the wind field but also OLR; therefore, the large-scale circulations such as wind field and therefore, the 850 hPa zonal wind in pentad (− 3) is already convection are also taken into account and every pentad in easterly wind. It reveals the “abrupt change” of the SCSSM which the MTG changes from positive to negative from onset and the “gradual change” of the SCSSM retreat from September is checked and corrected. Figure 5 shows the time the dynamics and thermal structure of atmosphere. ,us, the series of the SCSSM withdrawal pentad. ,e mean and characteristics of the wind field and convection during the standard deviation (SD) of the SCSSM retreat are pentads SCSSM withdrawal show that the retreat time defined by 55.3 (about October 8th) and 3.4, while the interannual MTG is reliable. However, based on the interdecadal change fluctuation of the SCSSM retreat is stronger than that of the of the SCSSM, what characteristics have changed on the onset, which is consistent with Luo and Lin [23] and Hu et al. SCSSM withdrawal? [37] who provided two different sets of SCSSM withdrawal To further study the differences of the SCSSM with- dates characteristics. ,e correlation between the SCSSM drawal before and after 1993, we composite the anomalies withdrawal dates defined by MTG and by Hu et al. [21] is 0.4, of 850 hPa wind and OLR during the SCSSM withdrawal which exceeds the 99% confidence level. We define pentads period for typical years in Figure 8. During the period of 53–57 as the withdrawal stages to describe the interannual 1979–1993, before the SCSSM withdrawal pentad, the low- variability of the SCSSM retreat process. However, the av- level and convection anomalies are not significant in the erage withdrawal time from 1979 to 1993 is pentad 54.5, SCS (Figure 8(a)). On the SCSSM retreat pentad, an en- while the average withdrawal time from 1994 to 2016 is hanced easterly wind anomaly and increased OLR pentad 55.9. ,e retreat time was later in the later epoch but anomaly, meaning weakened convection, appear over the it failed to pass the Mann–Kendall test. In Hu et al.’s work SCS and subtropical western North Pacific. ,e easterly [32], there was a significant delay of SCSSM withdrawal wind anomaly and weakened convection extend eastward between the periods of 1995–2005 and 2006–2016, but the to the Bay of Bengal (Figures 8(b) and 8(c)). Comparison of periods in this research were 1979–1993 and 1994–2016. ,is the difference before and after the SCSSM withdrawal may be because the study period is different. But the SCSSM pentad (Figure 8(d)) shows that the SCSSM withdrawal withdrawal also exhibits a delay after the mid-2000s if it was leads to remarkable increases in easterly wind from the SCS defined by our approach. For example, the average SCSSM to the Bay of Bengal, as well as an anomalous anticyclone withdrawal date from 1995 to 2005 was 54.4 pentads, and the over the western North Pacific, which accompanies a average SCSSM withdrawal date from 2006 to 2016 was 57.7 weakened convection over the east of Philippines and east pentads, which is almost consistent with Hu et al.’s result. of the SCS. 210 6 Advances in Meteorology 10 m/s 10 m/s 15Oct 20°N 4 4 1Oct 2 2 10°N 15Sep 0 0 0° 1Sep 210 200 1Sep 15Sep 1Oct 15Oct 70°E 80°E 90°E 100°E 110°E 120°E 130°E 140°E Contour from 200 to 220 by 10 Contour from 200 to 220 by 10 (a) (b) Figure 4: Latitude-time diagram of climatological 850 hPa wind (vector, m/s), southwesterly wind (shading, m/s), and OLR (contour, W/ 2 2 ° ° ° ° m ) (a) between 110 E and 120 E and (b) between 10 N and 20 N. ,e vector unit is 10 m/s and the shading and contour interval is 10 W/m . 62.0 60.0 58.0 56.0 54.0 52.0 50.0 48.0 1978 1982 1986 1990 1994 1998 2002 2006 2010 2014 Figure 5: ,e pentad of the SCSSM withdrawal dates (red line). ,e black lines denote the 5-year running average. 15m/s 15m/s 15m/s 15m/s 40°N 40°N 40°N 40°N 30°N 30°N 30°N 30°N 20°N 20°N 20°N 20°N 10°N 10°N 10°N 10°N 0° 0° 0° 0° 10°S 10°S 10°S 10°S 90°E 120°E 150°E 90°E 120°E 150°E 90°E 120°E 150°E 90°E 120°E 150°E 180 200 220 180 200 220 180 200 220 180 200 220 (a) (b) (c) (d) Figure 6: Composite evolution of OLR (W/m , shadings) and 850 hPa wind (m/s, vectors) based on the SCSSM withdrawal date from (a) two pentads before the monsoon withdrawal (P − 2), (b) one pentad before the monsoon withdrawal (P − 1), (c) during the monsoon withdrawal (P0), and (d) one pentad after the monsoon withdrawal (P + 1) for the period of 1979–2016. In the period of 1994–2016, before the SCSSM with- Bay of Bengal on the next pentad of the SCSSM withdrawal drawal pentad, strong westerly anomalies are found over (Figures 8(f) and 8(g)). Comparing the differences before the Bay of Bengal extending through the South China Sea to and after the SCSSM withdrawal pentad, the increased the east of the Philippines, which transports abundant easterly wind in the SCS is sharper than that in the period of warm moisture to favor the active convection over the SCS 1979–1993, and the convection over the Northwest Pacific, and Northwest Pacific (Figure 8(e)). On the withdrawal SCS, and the Bay of Bengal is weakened along with the pentad, the low-level wind abruptly transits to a north- SCSSM withdrawal, which is associated with a southward easterly anomaly in the SCS and persists into the next migration of the seasonal march of the ITCZ (Figure 8(h)). pentad, which is stronger than that in the period of It is suggested that the SCSSM withdrawal rate during the 1979–1993. On the other hand, a suppressed convection period of 1994–2016 is faster than that of the period of appears in the SCS and eventually extends eastward to the 1979–1993. 220 Advances in Meteorology 7 0.0 250 –2.0 –4.0 –6.0 –8.0 200 P (–3) P (–2) P (–1) P0 P (1) P (2) ° ° ° ° Figure 7: Pentad 850 hPa zonal wind (m/S) and OLR (W/m ) averaged over the South China Sea (110 –120 E, 10 –20 N) from three pentads before the SCSSM withdrawal (P − 3) to two pentads after the SCSSM withdrawal (P + 2). 5 m/s 7 m/s 5 m/s 5 m/s 30°N 30°N 30°N 30°N (a) (b) (c) (d) 20°N 20°N 20°N 20°N 10°N 10°N 10°N 10°N 0° 0° 0° 0° 10°S 10°S 10°S 10°S 75°E 90°E 105°E 120°E 135°E 150°E 75°E 90°E 105°E 120°E 135°E 150°E 75°E 90°E 105°E 120°E 135°E 150°E 75°E 90°E 105°E 120°E 135°E 150°E 5 m/s 5 m/s 5 m/s 7 m/s 30°N 30°N 30°N 30°N (e) (f ) (g) (h) 20°N 20°N 20°N 20°N 10°N 10°N 10°N 10°N 0° 0° 0° 0° 10°S 10°S 10°S 10°S 75°E 90°E 105°E 120°E 135°E 150°E 75°E 90°E 105°E 120°E 135°E 150°E 75°E 90°E 105°E 120°E 135°E 150°E 75°E 90°E 105°E 120°E 135°E 150°E –40 –30 –20 –10 0 10 20 30 40 Figure 8: Composite differences in OLR (W/m , shadings) and 850 hPa wind (m/s, vectors) based on th SCSSM withdrawal date in typical years of the epoch 1979–1993: (a) one pentad before the monsoon onset (P − 1), (b) during the monsoon onset (P0), (c) one pentad after the monsoon onset (P + 1) and the withdrawal stages (pentads 53 − 37), and (d) differences between after (P + 1) and before (P − 1) the monsoon withdrawal. (e–h) the same as (a–d) but for the epoch 1994–2016. In short, the distinct characteristics of the SCSSM are the different characteristics of the SCSSM in two epochs that, during the period of 1979–1993 (1994–2016), the and the rainfall distribution in Eastern China. Seasonal ° ° SCSSM onset is generally later (earlier) and the intensity of evolution of latitudinal (110 –120 E) averaged rainfall the SCSSM onset is stronger (weaker). On the other hand, between two epochs is shown in Figure 9. In the late-onset there is no obvious difference in the SCSSM retreat date, but years of 1979–1993, the maximum rainfall belt − 1 the speed of the SCSSM retreat is slower (faster) and the (>10 mm·day ), corresponding to the first rainy season in duration is longer (shorter). ,is raises the following Southern China, begins at the end of May (Figure 9(a)). question: what is the effect of this change on rainfall in Moreover, for the late-onset period, the SCSSM intensity China? is stronger, leading to more warm moisture being con- stantly transported into Southern China. ,e rainfall persists consistently until the beginning of June until the 3.3. Distinct Rainfall Distribution from the Onset to With- major rain-belt moves northward to the middle and lower drawal of the SCSSM during the Two Periods. Many previous reaches of the Yangtze River basin (around 30 N), which is studies have confirmed that the evolution of the SCSSM associated with the Meiyu front. By the end of July, the has a broad impact on the spatial and temporal distri- maximum rainfall belt moves over Southern China again, bution of rainfall in East Asia during the period from May which can be regarded as the beginning of the last flood to September. Particularly, the SCCSSM onset has been season in Southern China. ,e rainfall in Eastern China ° ° regarded as the beginning of the East Asian rainy season reduces from 35 N north to 25 N south from early Sep- [12, 38–41]. It is necessary to figure out the link between tember to early October. U850 hPa OLR 8 Advances in Meteorology 35°N 35°N 30°N 30°N 25°N 25°N 1May 1Jun 1Jul 1Aug 1Sep 1Oct 1Nov 1May 1Jun 1Jul 1Aug 1Sep 1Oct 1Nov 2468 10 2468 10 (a) (b) ° ° Figure 9: Latitude-time cross sections of 5-day running mean precipitation (mm/day) averaged between 110 E and 120 E in typical years of the epoch: (a) 1979–93 and (b) 1994–2016. In the period of 1994–2016, the SCSSM onset is generally by Mao and Chan [44], has been applied. ,at is, early. ,e first rainy season in South China begins at the I � ((v − v )/σ − (R − R))/σ , where v � (u + v)/ M SW SW v R SW √� beginning of May and continues intermittently to the end of 2, R is OLR, σ and σ are the standard deviations of v v R SW June until the major rain-belt moves northward to the and OLR, respectively, and v and R are the multiyear SW middle and lower reaches of the Yangtze River basin averages of v and OLR, respectively. ,e advantage of this SW (Figure 9(b)). ,e Meiyu period during 1994–2016 is shorter index is that it takes into account the dynamic and ther- than that during 1979–1993. By mid-July, the maximum modynamic characteristics of the monsoon. In this study, rainfall belt moves to the Southern China again and it de- the 30–80-day band-pass filtering of I over the SCS ° ° ° ° creases with Eastern China rainfall simultaneously from late (110 –120 E, 10 –20 N) is applied as the daily ISO index of September to early October, consistent with the faster and the SCSSM. Figure 11 shows the typical daily ISO evolution shorter retreat of the SCSSM. On the other hand, the two from April to November in the SCS between two epochs. A migrations of the rain-belt are closely related to the two-peak strong modulation of SCS ISOs on the SCSSM evolution is pattern of the zonal wind over the SCS in summer, which is observed. In the late-onset period of 1979–1993, the SCS ISO consistent with previous studies [42]. has been in a dry phase, postponing the monsoon onset due to low-frequency easterly wind anomalies and weakening convection until late May. ,e ISO then turns to the positive 4. Effects of ISO on the SCSSM Evolution from phase and develops to the maximum amplitude of the whole Onset to Withdrawal during the Two Periods monsoon period. ,e low-frequency convection and west- ,e above analyses have shown different characteristics of the erly wind are significantly strengthened and favor a strong SCSSM from onset to withdrawal before and after 1993/94 and SCSSM onset. ,e transformation time is very near pentad the effect of the SCSSM change on rainfall distribution. ,us, a 29 (May 26–31). ,ere are two active low-frequency oscil- question to be further addressed is, what are the mechanisms lations in summer, namely, “South China Sea monsoon surge,” which has been discussed in depth by Li et al. [46], responsible for this characteristic change? Several studies have discussed the importance of two dominant modes of ISOs on corresponding to the two rain-belt migrations. After Sep- the triggers of the SCSSM onset and withdrawal [43–46]. It is tember, the ISO gradually weakened and disappeared, found that 10–25-day and 30–80-day oscillations also control corresponding to the weakening and slowing retreat of the the SCSSM activity from May when the SCSSM onsets to SCSSM. October when the SCSSM retreats through power spectrum In the early-onset years of 1994–2016, the ISO of the analysis (fig. not shown). To detect the effects of ISO on the SCSSM turns to the positive phase around the beginning of SCSSM evolution from onset to withdrawal during the two May. ,e first positive peak value of ISO is smaller than that epochs, we calculated standard deviations of the 10–25- and in the period of 1979–1993, corresponding to a weaker SCSSM onset intensity. ,en there are three active low- 30–80-day filtered OLR from 15 May to 15 October, which cover the 5 months from climatological SCSSM onset to frequency oscillations from June to September, with stable withdrawal. Figure 10 shows the differences (1994–2016 to amplitude. In early October, the ISO transforms rapidly to 1979–1993) of mean standard deviation between the two the negative phase, contributing to the shorter, faster retreat epochs. ,e difference of the 30–80-day oscillation mode over of the SCSSM. It is inferred that the 30–80-day ISO was the SCS is significant and the strength of the 30–80-day os- crucial for the SCSSM from onset to withdrawal between two cillations is large in the SCS during the period of 1994–2016. epochs. ,us, the effect of 30–80-day ISO on the SCSSM evolution We have made a preliminary analysis for the effect of during the two periods will be studied further. local SCS 30–80-day ISO on the SCSSM evolution difference; To explore the 30–80-day ISO activities of the SCSSM, an how does each ISO propagate and where is the ISO prop- East Asian summer monsoon index (I ), which is proposed agation source? Figure 12 shows the zonal and meridional M Advances in Meteorology 9 30°N 30°N 20°N 20°N 10°N 10°N 0° 0° 10°S 10°S 75°E 90°E 105°E 120°E 135°E 150°E 75°E 90°E 105°E 120°E 135°E 150°E –4 –3 –2 –1 0 1 2 3 4 –4 –3 –2 –1 012 3 4 (a) (b) Figure 10: Difference in the Lanczos filtered OLR anomalies in (a) 10–25-day and (b) 30–80-day time scales from 15 May to 15 October between 1994–2016 and 1979–1993. Units are in W/m , and the dotted areas denote that the differences are significant at the 95% confidence level. have discovered that the delayed onset of SCSSM may result 0.60 from a La Niña event [47]. ,e early onset and delayed 0.40 retreat of the SCSSM can be ascribed to warm SSTanomalies 0.20 in the Philippine Sea [23, 30]. A warm SST in the tropical Indian Ocean during the previous winter [19] can delay the –0.00 SCSSM onset by suppressing the convection over the –0.20 western Pacific. It is also interesting to find that the spatial –0.40 pattern of the SST evolution exhibits specific interdecadal 4 5 6 7 8 9 10 11 characteristics during two periods. As shown in Figure 13 on the left, significant positive SSTanomalies appear around the Figure 11: Composite daily ISO index evolution from April to equatorial Eastern Pacific in the late-onset years of the November over the SCS. Red line is in a typical year of the epoch period of 1979–1993 in May and continue developing until 1979–93, and blue dots are in a typical year of the epoch 1994–2016. November. Meanwhile, the spatial pattern of the SST changes is similar to the horseshoe-shaped SST warming ° ° ° ° propagation of ISO along with 10 –20 N and 110 –120 E during El Niño events. In theory, the warm tropical eastern during the two periods. In the period of 1979–1993, the wet Pacific SST anomaly may induce an anticyclone anomaly in ISO propagates eastward from tropical Indian Ocean to the the northwest Pacific due to the excitation of ascending ° ° SCS along with 10 –20 N in early June and mid-August Rossby waves. ,is anticyclone anomaly connects to the (Figure 12(c)). Moreover, when ISO activity in the Indian low-level easterly anomaly in the SCS, which possibly Ocean is relatively strong, the propagation to the SCS is also postpones the onset of SCSSM [48]. In the early-onset years strong. In meridional direction, the positive ISO propagates of the period of 1994–2016, significant warm SST anomalies northward into the SCS. In September, there is ISO propa- appear in the Philippine Sea from April to September, and gating from Indian Ocean but not entering the SCS. Different the strongest warm SST appears in May and then gradually from the late-onset period, in the period of 1994–2016, the weakens. A few weak negative SST anomalies appear around first two wet ISOs propagate westward from the Northwest the equatorial Eastern Pacific (Figure 13 on the right). ,us, Pacific in May and June, and the last two propagate eastward the SST warming in the Philippine Sea from April to Sep- from the Indian Ocean in July and September. In zonal di- tember may be a crucial external force for the SCSSM rection, the ISOs also propagate northward into the SCS. evolution in the period of 1994–2016. However, a deeper ,erefore, the wet ISO in the Northwest Pacific propagating understanding of the mechanism of this SST change on the northwestward into the SCS may contribute to the SCSSM SCSSM evolution during two epochs calls for further study. evolution difference after 1993/94 and the SCSSM ISO in- tensity can be predicted by monitoring ISO anomalies over 6. Summary and Discussion the northwest Pacific and Indian oceans. In this study, we investigate the different characteristics and possible impact factors of the SCSSM evolution from onset 5. Effects of SSTAs onthe SCSSM Evolution from to withdrawal before and after 1993/94 when an interdecadal Onset to Withdrawal during the Two Periods change occurs. ,e results suggest that, during the period of 1979–1993, the SCSSM onsets later, corresponding to abrupt In addition to the effects of internal atmospheric dynamic enhancement of zonal wind and convection activity, results mechanism, SST, an important external force, also plays an in a stronger SCSSM onset intensity. Delayed onset also important role in the SCSSM evolution. Previous studies 10 Advances in Meteorology 20°N 20°N 0.7 0.7 0.6 0.6 0.5 0.5 0.4 0.4 0.3 0.3 0.2 0.2 0.1 0.1 10°N 0 10°N 0 –0.1 –0.1 –0.2 –0.2 –0.3 –0.3 –0.4 –0.4 –0.5 –0.5 –0.6 –0.6 –0.7 –0.7 0 0 45 6 78 9 10 11 4 5 6 78 9 10 11 (a) (b) 140°E 140°E 0.7 0.7 0.6 0.6 130°E 130°E 0.5 0.5 0.4 0.4 0.3 0.3 120°E 120°E 0.2 0.2 0.1 0.1 110°E 0 110°E –0.1 –0.1 –0.2 –0.2 100°E 100°E –0.3 –0.3 –0.4 –0.4 –0.5 –0.5 90°E 90°E –0.6 –0.6 –0.7 –0.7 80°E 80°E 45 67 8 910 11 4 56 7 8 9 10 11 (c) (d) Figure 12: Latitude-time (left) and longitude-time (right) diagram of the daily SCSSM ISO propagation from April to November between ((a) and (c)) late-onset years of the period of 1979–1993 and ((b) and (d)) early-onset years of the period of 1994–2016. Apr May Jun Jul 40°N 40°N 40°N 40°N 30°N 30°N 30°N 30°N 20°N 20°N 20°N 20°N 10°N 10°N 10°N 10°N 0° 0° 0° 0° 10°S 10°S 10°S 10°S 45°E 90°E 135°E 180° 135°W 90°W 45°E 90°E 135°E 180° 135°W 90°W 45°E 90°E 135°E 180° 135°W 90°W 45°E 90°E 135°E 180° 135°W 90°W (a) (b) (c) (d) Aug Sep Oct Nov 40°N 40°N 40°N 40°N 30°N 30°N 30°N 30°N 20°N 20°N 20°N 20°N 10°N 10°N 10°N 10°N 0° 0° 0° 0° 10°S 10°S 10°S 10°S 45°E 90°E 135°E 180° 135°W 90°W 45°E 90°E 135°E 180° 135°W 90°W 45°E 90°E 135°E 180° 135°W 90°W 45°E 90°E 135°E 180° 135°W 90°W (e) (f) (g) (h) Apr May Jun Jul 40°N 40°N 40°N 40°N 30°N 30°N 30°N 30°N 20°N 20°N 20°N 20°N 10°N 10°N 10°N 10°N 0° 0° 0° 0° 10°S 10°S 10°S 10°S 45°E 90°E 135°E 180° 135°W 90°W 45°E 90°E 135°E 180° 135°W 90°W 45°E 90°E 135°E 180° 135°W 90°W 45°E 90°E 135°E 180° 135°W 90°W (i) (j) (k) (l) Figure 13: Continued. –0.5 –0.5 –0.5 –0.4 –0.4 –0.4 –0.3 –0.3 –0.3 –0.2 –0.2 –0.2 –0.1 –0.1 –0.1 0 0 0 0.1 0.1 0.1 0.2 0.2 0.2 0.3 0.3 0.3 0.4 0.4 0.4 0.5 0.5 0.5 –0.5 –0.5 –0.5 –0.4 –0.4 –0.4 –0.3 –0.3 –0.3 –0.2 –0.2 –0.2 –0.1 –0.1 –0.1 0 0 0 0.1 0.1 0.1 0.2 0.2 0.2 0.3 0.3 0.3 0.4 0.4 0.4 0.5 0.5 0.5 –0.5 –0.5 –0.5 –0.4 –0.4 –0.4 –0.3 –0.3 –0.3 –0.2 –0.2 –0.2 –0.1 –0.1 –0.1 0 0 0 0.1 0.1 0.1 0.2 0.2 0.2 0.3 0.3 0.3 0.4 0.4 0.4 0.5 0.5 0.5 –0.5 –0.5 –0.5 –0.4 –0.4 –0.4 –0.3 –0.3 –0.3 –0.2 –0.2 –0.2 –0.1 –0.1 –0.1 0 0 0 0.1 0.1 0.1 0.2 0.2 0.2 0.3 0.3 0.3 0.4 0.4 0.4 0.5 0.5 0.5 Advances in Meteorology 11 Aug Sep Oct Nov 40°N 40°N 40°N 40°N 30°N 30°N 30°N 30°N 20°N 20°N 20°N 20°N 10°N 10°N 10°N 10°N 0° 0° 0° 0° 10°S 10°S 10°S 10°S 45°E 90°E 135°E 180° 135°W 90°W 45°E 90°E 135°E 180° 135°W 90°W 45°E 90°E 135°E 180° 135°W 90°W 45°E 90°E 135°E 180° 135°W 90°W (m) (n) (o) (p) Figure 13: Composite monthly SSTAs (shading; (K)) from April to November between late-onset years of the period of 1979–1993 (left) and early-onset years of the period of 1994–2016 (right). ,ick solid lines surround the region exceeding the 90% confidence level, respectively. (a, i) Apr, (b, j) May, (c, k) Jun, (d, l) Jul, (e, m) Aug, (f, n) Sep, (g, o) Oct, and (h, p) Nov. results in a continuous, strong rain-belt appearing in SST and Philippine Sea temperature on the SCSSM onset in May and withdrawal in September [30, 32, 49]. ,e specific Southern China at the end of May. After the SCSSM onset, the low-level zonal winds appear with a two-peak pattern in physical mechanism for the impact of SST on the monsoon summer, and two abrupt increases of the zonal winds evolution from May to September should thus be studied correspond to the beginning of the Meiyu period in Yangtze further. On the other hand, air-sea interaction can provide River valley and the last flood season in Southern China, an additional northward propagation mechanism for 30–60- respectively. ,e monsoon retreats from September until day Northwest Pacific ISO and ISO source [50]. ,e in- October and exhibits an extended, gradual retreat, which fluence of SSTAs on the SCSSM ISO should also be studied as it may provide a basis for predicting the subsequent causes the withdrawal of the rain-belt to begin in September from north to south China and completely vacate at the SCSSM evolution. beginning of October. However, after 1993/94, the SCSSM onsets earlier and is Data Availability characterized by a weaker onset intensity and more rapid retreat. ,e preflood season advanced and lasted intermit- Daily precipitation data and OLR were derived from https:// tently throughout May. ,e rain-belt in Southern China www.esrl.noaa.gov/psd/data/gridded/. ,e HadISST data persisted for a longer time in summer. ,e monsoon started were from http://www.metoffice.gov.uk/hadobs/hadisst/ to retreat from October, resulting in the rapid retreat of the data/download.html. ,e ERA-Interim data were obtained rain-belt from the whole eastern China. from https://apps.ecmwf.int/datasets/data/interim-full-daily/ Further analyses indicate that the enhanced 30–80-day levtype�pl/. SCS ISO activities are an important contributor for SCSSM evolution change. ,ere are two active low-frequency os- Conflicts of Interest cillations over the SCS in summer during the late-onset period but three ISOs in the early-onset period. Prior to ,e authors declare that they have no conflicts of interest. 1993/94, the SCS ISO had been in a dry phase, which postponed the monsoon onset until late May. ,en a late and Acknowledgments strong onset is triggered by enhanced wet ISO propagating northeastward from the equatorial Indian Ocean. After ,is research was jointly supported by the National Key September, the ISO gradually weakened and disappeared, R&D Program of China (2019YFC1510004) and the Na- corresponding to the weakening and slowing retreat of the tional Natural Science Foundation of China (41975085). SCSSM. After 1993/94, the wet ISO from the Northwest Pacific propagated northwestward to the SCS in early May, References which modulated the mid-May onset of SCSSM. After October, the SCS ISO transformed rapidly to the negative [1] X. Shao, P. Huang, and R. H. 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Characteristics of the South China Sea Monsoon from the Onset to Withdrawal before and after 1993/94

Advances in Meteorology , Volume 2020 – Sep 16, 2020

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Copyright © 2020 Zhao Xiaofang and Wang Lijuan. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
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Abstract

Hindawi Advances in Meteorology Volume 2020, Article ID 8820460, 13 pages https://doi.org/10.1155/2020/8820460 Research Article Characteristics of the South China Sea Monsoon from the Onset to Withdrawal before and after 1993/94 1,2 1 Zhao Xiaofang and Wang Lijuan Key Laboratory of Meteorological Disaster, Ministry Education, Joint International Research Laboratory of Climate and Environment Change, Collaborative Innovation Center on Forecast and Evaluation of Meteorological Disasters, Nanjing University of Information Science and Technology, Nanjing 210044, China Wuhan Regional Climate Center, Wuhan 430074, China Correspondence should be addressed to Wang Lijuan; wljfw@163.com Received 8 April 2020; Revised 21 August 2020; Accepted 3 September 2020; Published 16 September 2020 Academic Editor: Enrico Ferrero Copyright © 2020 Zhao Xiaofang and Wang Lijuan. ,is is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. ,e characteristics and possible impact factors of the South China Sea summer monsoon (SCSSM) evolution from onset to withdrawal before and after 1993/94 are investigated using ERA-Interim, CPC rainfall, and OLR data. During the late-onset period of 1979–1993, the SCSSM was characterized by stronger onset intensity and a gradual withdrawal, resulting in a con- tinuous, strong preflood season in Southern China and a slower rain-belt retreat from north to south China in September. In addition, the rain-belt in the Yangtze River basin persisted much longer during summer. However, during the early-onset period in 1994–2016, the SCSSM is associated with a weaker onset intensity and comparatively faster retreat. ,e advanced preflood season lasted intermittently throughout May and the whole eastern China precipitation lasted until October when it retreated rapidly, making the rain-belt in Southern China persist for an extended duration. Further analysis indicates that a strong modulation of SCS intraseasonal oscillation (ISO) on the SCSSM evolution is observed. ,ere are two active low-frequency oscillations over the SCS in summer during the late-onset period but three during the early-onset period. ,e wet ISO in the Northwest Pacific propagating northwestward into the SCS and enhanced SCSSM ISO activity may contribute to the early onset and faster withdrawal after 1993/94. ,e effect of warm western Pacific sea surface temperatures (SST) on the SCSSM evolution is also discussed. interannual variability of the SCSSM onset. A warm (cold) El 1. Introduction Niño-Southern Oscillation (ENSO) event in the previous Asian monsoons are one of the main energy and moisture winter and the warm (cold) tropical Indian Ocean (TIO) in sources of the global atmosphere [1]. ,e SCSSM has a the spring can delay (advance) the SCSSM onset [16–19]. significant position in the Asian monsoon, forming a vital ,ere are fewer studies that focus on SCSSM withdrawal link between the East Asian and South Asian summer when compared to the SCSSM onset [20]. Hu et al. [21] monsoons [2–4]. Climatologically, the SCSSM onset is found that SCSSM withdrawal mainly occurred due to the generally accompanied by a continuous eastward retreat of westward intrusion of the WNP subtropical high, which is the western North Pacific (WNP) subtropical high, con- accompanied by the retreat of the weakening low-level in- vection enhancement, and the reversal of low-level zonal tertropical convergence zone (ITCZ) and rain-belt in the wind in the SCS [5, 6]. ,e synoptic-scale circulation sys- SCS-WNP. ,e factors contributing to the interannual tems, the disturbance of mid-latitude, and the ISO activities variability of the SCSSM withdrawal may include tropical can trigger SCSSM onset [7–15]. In addition, the sea surface cyclones (TCs) and ISO [22]. Luo and Lin [23] suggested that temperature anomaly (SSTA) is closely related to the El Niño can delay the monsoon onset and advance the 2 Advances in Meteorology SCSSM withdrawal, thus shortening the length of the wind and convection are the two common and significant summer monsoon season. indices for identifying the SCSSM onset [21, 35]. In order to Recent research has shown that the SCSSM onset date take low-level features for a more comprehensive consid- has advanced by about two weeks since 1994 [24–29]. eration into account the, we corrected the onset date by Kajikawa and Wang [25] suggested that the enhanced combining circulation and the onset date defined by the northwestward intraseasonal variability (ISV) and TC ac- National Climate Centre. ,e National Climate Centre tivity from WNP could be responsible for the interdecadal defines the SCSSM onset by the first pentad when 850 hPa change in the SCSSM onset. Xiang and Wang [30] indicated zonal wind in the SCS changes to westerly wind for two that the interdecadal advance of the Asian summer monsoon constant pentads stably and the average potential pseu- onset may be attributed to a grand La Niña-like pattern doequivalent temperature is greater than 340 K. As a result, change in the Pacific, while that in the South China Sea (SCS) we corrected the onset date of 1982, 1997, 2000, 2008, and is primarily determined by the abrupt SST warming near the 2009. ,e SCSSM withdrawal pentad for 1979–2016 is also Philippine Sea. Liu et al. [31] highlighted the impact of defined by MTG. ,e MTG-defined SCSSM onset and southern Indian Ocean (SIO) on the SCSSM onset ad- withdrawal originate from thermal differences between vancement after 1993. Many efforts have been made to north and south, which is the essence of monsoon forma- understand the cause driving interdecadal change of the tion. In order to extract the more obvious signal caused by SCSSM onset, and interdecadal change in the SCSSM the SCSSM interdecadal transition, the years with later than withdrawal after the mid-2000 has been found recently [32]. average onset are regarded as the typical late-onset years Considering the integrity of SCSSM evolution, particularly during 1979–1993, which are 1981, 1982, 1985, 1987, 1991, from onset to withdrawal, the following questions can be and 1993. ,e years with an earlier than average onset are raised: (1) What changes have happened in the SCSSM regarded as the typical early-onset years during 1994–2016, evolution from onset to withdrawal since 1993? (2) Previous which are 1994, 1996, 2000, 2001, 2002, 2005, 2006, 2008, studies have highlighted the role of SSTA and ISO during 2012, 2013, and 2014. this advanced SCSSM onset. What are the effects of ISO and SSTAs on the SCSSM evolution from onset to withdrawal in 3. The Decadal Shift of the SCSSM before different periods? ,ese questions will be answered in this and after 1993/94 paper. 3.1. Difference of the SCSSM Onset Characteristics during the Two Epochs. To show the reliability of the MTG-defined 2. Data and Methods SCSSM, the time series of the SCSSM onset date is plotted in ,e datasets used in this paper include (1) the daily mean Figure 1, which shows a significant advancement of the interpolated outgoing longwave radiation (OLR) data from SCSSM onset date after 1993/1994 and the SCSSM onset the National Oceanic and Atmospheric Administration time we defined is basically consistent with that defined by ° ° (NOAA) satellite with resolution of 2.5 latitude by 2.5 the National Climate Centre and its correlation is 0.58. ,e longitude from 1979 to 2016; (2) the daily mean ECMWF mean SCSSM onset occurs in about pentad 29 during Interim Re-Analysis dataset (ERA-Interim) with resolution 1979–1993, while during 1994–2016 it occurs in about ° ° of 2.5 latitude by 2.5 longitude from 1979 to 2016 (the pentad 27. ,e change is about two pentads and the standard variables used in this paper include temperature and three- deviation (STD) of the SCSSM onset time is 1.67. ,is dimensional wind fields); (3) the monthly mean Hadley interdecadal change of the onset date is consistent with previous studies [24–26] and passes the sliding t-test (not Centre sea surface temperature (SST) dataset for the period of 1979–2016 [33]; and (4) daily precipitation set from the shown). Based on the interdecadal change of SCSSM onset, CPC Unified Precipitation Project which is underway at the circulation and convection differences during the NOAA Climate Prediction Centre (CPC). SCSSM onset should be researched further. In the present study, the SCSSM onset is identified by the Figure 2 shows the composite OLR and 850 hPa winds of seasonal transition of the mid- to upper-tropospheric me- the SCSSM onset from before and after the onset pentad in ridional temperature gradient (MTG, zT/zy) from winter to each epoch. Lower OLR corresponds to heavy rainfall and summer, which is defined by Mao et al. [34] and Liu et al. strong convection. In the late-onset years of 1979–1993, [31]. Specifically, the timing of the SCSSM onset is defined by before the SCSSM onset pentad (Figure 2(a)), a weak an- ticyclone is centered over the SCS, which makes the SCS be the moment when the pentadly area-averaged MTG in the mid-to-upper troposphere (500–200 hPa) changes from controlled by easterly wind. ,e active convection area is mainly located in the Bay of Bengal and Indochina Pen- negative to positive and remains positive for at least three ° ° pentads over the SCS (10–20 N, 110–120 E). ,en the first insula, extending to the south of China and the south of pentad from negative to positive phase is defined as the onset Japan. When the SCSSM onsets (Figure 2(b)), the developing of the SCSSM. ,ey suggest that MTG between 500 hPa and convection from the Bay of Bengal expands to the northern 200 hPa is more abrupt in assessing monsoon onset and part of the SCS along with weakening convection over the consistent with the main climatological characteristics of the southern Indian Ocean, indicating that the ITCZ stretched SCSSM onset, such as area-averaged zonal wind shear re- northwards [25]. Southwesterly wind from the Bay of Bengal versal, zonal wind reversal, and convection enhancement. reaches the SCS resulting in the anticyclone of the Northwest ,ere are 72 pentads per year. However, the 850 hPa zonal Pacific retreating eastward. After the onset (Figure 2(c)), the Advances in Meteorology 3 34.0 32.0 30.0 28.0 26.0 24.0 1978 1982 1986 1990 1994 1998 2002 2006 2010 2014 Figure 1: Time series of the MTG-defined SCSSM onset date with the red line denoting the mean epoch date. 10 m/s 10m/s 10m/s 40°N 40°N 40°N 30°N 30°N 30°N 20°N 20°N 20°N 10°N 10°N 10°N 0° 0° 0° 10°S 10°S 10°S 60°E 75°E 90°E 105°E 120°E 135°E 150°E 60°E 75°E 90°E 105°E 120°E 135°E 150°E 60°E 75°E 90°E 105°E 120°E 135°E 150°E 180 200 220 180 200 220 180 200 220 (a) (b) (c) 10m/s 10m/s 10m/s 40°N 40°N 40°N 30°N 30°N 30°N 20°N 20°N 20°N 10°N 10°N 10°N 0° 0° 0° 10°S 10°S 10°S 60°E 75°E 90°E 105°E 120°E 135°E 150°E 60°E 75°E 90°E 105°E 120°E 135°E 150°E 60°E 75°E 90°E 105°E 120°E 135°E 150°E 180 200 220 180 200 220 180 200 220 (d) (e) (f) Figure 2: Composite evolution of OLR (W/m , shadings) and 850 hPa wind (m/s, vectors) based on the SCSSM onset date in a typical year of the epoch 1979–93 (Top) and 1994–2016 (Bottom) from (a, d) one pentad before the monsoon onset (P − 1), (b, e) during the monsoon onset (P0), and (c, f) one pentad after the monsoon onset (P + 1). active convection over the Arabian Sea and Indochina Indonesian Maritime Continent and the eastern equator of Peninsula is expanded into the SCS. ,e convection in the Indian Ocean are expanded northward. ,e wind direction Bay of Bengal is significantly stronger than that in 1994–2016 over the SCS abruptly changes to westerly direction. After and the southwesterly wind in the SCS is more intense. the onset (Figure 2(f)), the convection over the Indonesian In early-onset years of 1994–2016, the convection in the Maritime Continent moves northward to the Philippines eastern equator of Indian Ocean is weaker than that in and enters south of the SCS, but the southwesterly wind in 1979–1993, before the onset pentad (Figure 2(d)). Another the SCS weakens. distinct convection area is over the Indonesian Maritime According to the above analysis, the convection source Continent. ,is convection is associated with tropical dis- favoring monsoon onset has changed since 1993/94. In turbances, such as easterly wave and MJO, which are fa- addition, it is clearly shown that the reversal and increment of low-level wind in SCS are more noticeable during vorable for the onset of the SCSSM [22]. When the SCSSM builds up (Figure 2(e)), both active convection over the 1979–1993 when the SCSSM onset is delayed. Figure 3(a) 4 Advances in Meteorology 8.0 3.0 6.0 2.0 4.0 1.0 2.0 0.0 0.0 –1.0 –2.0 –4.0 –2.0 1May 1Jun 1Jul 1Aug 1Sep 1Oct 1978 1982 1986 1990 1994 1998 2002 2006 2010 2014 OLR (P0) U (P1) – U (P0) (a) (b) ° ° ° ° Figure 3: (a) Time series of 5-day running mean 850 hPa zonal wind (m/s) averaged over the South China Sea (110 –120 E, 10 –20 N). ,e black, red, and blue curves are climatological years, typical late-onset years during 1979–1993, and typical early-onset years during 1994–2016, respectively. (b) Time series of the standardized OLR (red; W/m ) in the onset pentad and zonal wind growth intensity (U(P1) − ° ° ° ° U(P0); blue; m/s) over the SCS (110 –120 E, 10 –20 N) from 1979 to 2016. Table 1: Correlation coefficients between the anomalies of the shows the composite 850 hPa zonal wind evolution over the zonal wind growth intensity (U(P1) − U(P0)) and OLR over the SCS SCS for typical years and climatology. In 1979–1993, the in onset pentad and the SCSSM onset dates in the periods of monsoon builds up late, corresponding to the zonal wind 1979–93 and 1994–2016, respectively. turning positive late. On the other hand, the zonal wind U(P1) − U(P0) OLR (P0) growth is peaking faster in late June. However, in 1994–2016, 1979–1993 0.23 − 0.25 the monsoon builds earlier; the zonal wind turns positive in ∗ ∗ 1994–2016 0.38 − 0.43 early May and grows slowly. ,e first peak is in mid-June. ∗∗ ∗∗ 1979–2016 0.36 − 0.38 ,us, what is the relationship between monsoon onset date Values exceeding the 95% and 99% confidence levels are marked by a single and intensity? or double asterisk (∗ and ∗∗ ), respectively. Due to the change of the zonal wind in the SCS, we expressed the zonal wind growth intensity by zonal wind over the SCS after the SCSSM onset pentad minus the the relationship between zonal wind growth intensity and SCSSM onset pentad, which objectively depicts the abrupt OLR in the mid-1990s. ,is result illustrates that the late onset intensity of SCSSM. Time series of standardized OLR onset of the SCSSM is likely to correspond to a strengthened in onset pentad and zonal wind growth intensity in the SCS zonal wind and enhanced convection activity, which means shows that positive zonal wind growth intensity generally that the SCSSM onset intensity is stronger. ,erefore, the corresponds to negative OLR (Figure 3(b)). ,is means that later (earlier) the SCSSM onset is, the stronger (weaker) the a strong zonal wind intensity increase in the SCS generally intensity of the SCSSM onset is. Between 1979 and 1993 corresponds to an enhanced convection. ,erefore, the (1994 and 2016), the SCSSM onset is generally late (early), so convection and wind field in the SCS have obvious coupling the zonal wind growth intensity is stronger (more weakened) characteristics. ,ey show that the SCSSM onset process is a and convection is more active (suppressive) in the SCS. combination of dynamic and thermal. To reveal the rela- tionship between the SCSSM onset date and intensity, we calculated the correlation coefficients between the regional 3.2. Differences of the SCSSM Withdrawal Characteristics averaged OLR and the zonal wind growth intensity with the during the Two Epochs. ,e circulation evolution and SCSSM onset date in the period of 1979–1993 as well as in abrupt characteristics of the SCSSM onset of the two epochs are discussed above. After the SCSSM onset, the low-level the period of 1979–2016 (Table 1). ,e SCSSM onset date is significantly related to the SCSSM onset intensity. ,ere is a zonal winds appear with a two-peak pattern in summer positive correlation between the zonal wind growth intensity (Figure 3(a)). In the late-onset years of 1979–1993, the low- and the onset date of SCSSM, and there is a negative cor- level zonal wind over the SCS grew rapidly from early June relation between the OLR and the onset date of SCSSM. until it reached the first peak, and then it began to decrease. However, the correlation coefficient of two variations during Subsequently, the zonal wind grew again and reached a 1994–2016 is much higher and exceeds the 0.01 confidence second peak at the end of July. In the early-onset years of level, while that during the 1979–1993 epoch fails in passing 1994–2016, the zonal wind peaked firstly in mid-June, with a the 0.05 confidence level and this phenomenon is more weak intensity. ,en, in early July, it started to grow rapidly obvious during 1994–2016. However, different from Hu until the end of July. After summer, what changes have et al. [20] and Torrence and Webster [36], sliding correlation happened to the SCSSM withdrawal since 1993/94? To find the difference in SCSSM withdrawal character- between the SCSSM onset date with OLR and zonal wind growth intensity shows no significant interdecadal shift in istics between the two epochs, we must first figure out the Advances in Meteorology 5 From the composite results, the earlier the SCSSM onset, the climatological circulation evolution of 850 hPa wind and OLR during the SCSSM withdrawal. Figure 4 shows the later the SCSSM withdrawal after 1993. Now that the SCSSM withdrawal dates have been de- latitudinal-time and longitudinal-time cross section of low- level wind and OLR averaged from August 15 to the end of fined, we will explore the circulation evolution during the October. Different from the abrupt features of the SCSSM retreat period. Figure 6 shows the climatological composi- onset, it takes longer for the SCSSM to withdraw. ,e tions of low-level wind and OLR during the SCSSM with- southwesterly wind and convection retreat from the SCS do drawal period from 1979 to 2016. It can be seen that active not occur simultaneously during the SCSSM withdrawal. convection is located in the Bay of Bengal to the Northwest Meridionally, the southwesterly wind retreats completely Pacific before the SCSSM withdrawal pentad, and it retreats gradually from northeast to southwest (Figures 6(a) and from the northern SCS to the southern SCS in mid-Sep- tember, while the convection retreats gradually from the SCS 6(b)) and then completely withdraws from the SCS, since the SCSSM withdrawal pentad and convection centers shift to until mid-October (Figure 4(a)). Different times for the wind and convection to retreat to the SCS can also be seen in the the Indonesia area (Figures 6(c) and 6(d)). Meanwhile, the low-level wind is already easterly before the SCSSM with- zonal variation (Figure 4(b)). ,is nonsimultaneous retreat of westerlies and convection in the SCS is due to the activities drawal pentad, but it is obvious that the northeasterly wind of TCs and the synoptic systems embedded in the basic flow has become stronger on the SCSSM withdrawal pentad. To of easterly winds, which can contribute to the active con- further objectively investigate the variability of circulation vection around the SCS [21]. However, the TC and other factors during the SCSSM withdrawal, the pentad-mean of synoptic scale systems are also greatly modulated by the 850 hPa zonal wind and OLR averaged over the SCS evo- WNP anticyclonic/cyclonic anomalies [35], which are lution are composited in Figure 7. ,ere appears to be a significant decrease in zonal wind and a weakened con- closely related to the SCSSM withdrawal. ,erefore, it is necessary to consider the progressive retreat convection in vection whose value rises remarkably from about 220 W/m the process of SCSSM withdrawal, especially in the deter- to around 240 W/m over the SCS in the SCSSM withdrawal mination of the SCSSM retreat date. pentad. Zonal wind and convection almost develop at the According to the above analysis, we use pentadly area- same time when the SCSSM onsets, but, during the SCSSM averaged MTG in the mid-to-upper troposphere retreat, the westerly wind and convection retreat from the (500–200 hPa) when it changes from positive to negative to SCS do not occur simultaneously [21]. It was zonal wind calculate the date of the SCSSM withdrawal. Considering the shifting to easterly wind firstly and then convection retreat inconsistency of convection and wind field retreat during the from the SCS (Figure 4). ,e time difference of them is about SCSSM withdrawal, the persistence of MTG turning nega- 20 days. ,e SCSSM withdrawal dates defined in this paper tive cannot be maintained for three continuous pentads; take into account not only the wind field but also OLR; therefore, the large-scale circulations such as wind field and therefore, the 850 hPa zonal wind in pentad (− 3) is already convection are also taken into account and every pentad in easterly wind. It reveals the “abrupt change” of the SCSSM which the MTG changes from positive to negative from onset and the “gradual change” of the SCSSM retreat from September is checked and corrected. Figure 5 shows the time the dynamics and thermal structure of atmosphere. ,us, the series of the SCSSM withdrawal pentad. ,e mean and characteristics of the wind field and convection during the standard deviation (SD) of the SCSSM retreat are pentads SCSSM withdrawal show that the retreat time defined by 55.3 (about October 8th) and 3.4, while the interannual MTG is reliable. However, based on the interdecadal change fluctuation of the SCSSM retreat is stronger than that of the of the SCSSM, what characteristics have changed on the onset, which is consistent with Luo and Lin [23] and Hu et al. SCSSM withdrawal? [37] who provided two different sets of SCSSM withdrawal To further study the differences of the SCSSM with- dates characteristics. ,e correlation between the SCSSM drawal before and after 1993, we composite the anomalies withdrawal dates defined by MTG and by Hu et al. [21] is 0.4, of 850 hPa wind and OLR during the SCSSM withdrawal which exceeds the 99% confidence level. We define pentads period for typical years in Figure 8. During the period of 53–57 as the withdrawal stages to describe the interannual 1979–1993, before the SCSSM withdrawal pentad, the low- variability of the SCSSM retreat process. However, the av- level and convection anomalies are not significant in the erage withdrawal time from 1979 to 1993 is pentad 54.5, SCS (Figure 8(a)). On the SCSSM retreat pentad, an en- while the average withdrawal time from 1994 to 2016 is hanced easterly wind anomaly and increased OLR pentad 55.9. ,e retreat time was later in the later epoch but anomaly, meaning weakened convection, appear over the it failed to pass the Mann–Kendall test. In Hu et al.’s work SCS and subtropical western North Pacific. ,e easterly [32], there was a significant delay of SCSSM withdrawal wind anomaly and weakened convection extend eastward between the periods of 1995–2005 and 2006–2016, but the to the Bay of Bengal (Figures 8(b) and 8(c)). Comparison of periods in this research were 1979–1993 and 1994–2016. ,is the difference before and after the SCSSM withdrawal may be because the study period is different. But the SCSSM pentad (Figure 8(d)) shows that the SCSSM withdrawal withdrawal also exhibits a delay after the mid-2000s if it was leads to remarkable increases in easterly wind from the SCS defined by our approach. For example, the average SCSSM to the Bay of Bengal, as well as an anomalous anticyclone withdrawal date from 1995 to 2005 was 54.4 pentads, and the over the western North Pacific, which accompanies a average SCSSM withdrawal date from 2006 to 2016 was 57.7 weakened convection over the east of Philippines and east pentads, which is almost consistent with Hu et al.’s result. of the SCS. 210 6 Advances in Meteorology 10 m/s 10 m/s 15Oct 20°N 4 4 1Oct 2 2 10°N 15Sep 0 0 0° 1Sep 210 200 1Sep 15Sep 1Oct 15Oct 70°E 80°E 90°E 100°E 110°E 120°E 130°E 140°E Contour from 200 to 220 by 10 Contour from 200 to 220 by 10 (a) (b) Figure 4: Latitude-time diagram of climatological 850 hPa wind (vector, m/s), southwesterly wind (shading, m/s), and OLR (contour, W/ 2 2 ° ° ° ° m ) (a) between 110 E and 120 E and (b) between 10 N and 20 N. ,e vector unit is 10 m/s and the shading and contour interval is 10 W/m . 62.0 60.0 58.0 56.0 54.0 52.0 50.0 48.0 1978 1982 1986 1990 1994 1998 2002 2006 2010 2014 Figure 5: ,e pentad of the SCSSM withdrawal dates (red line). ,e black lines denote the 5-year running average. 15m/s 15m/s 15m/s 15m/s 40°N 40°N 40°N 40°N 30°N 30°N 30°N 30°N 20°N 20°N 20°N 20°N 10°N 10°N 10°N 10°N 0° 0° 0° 0° 10°S 10°S 10°S 10°S 90°E 120°E 150°E 90°E 120°E 150°E 90°E 120°E 150°E 90°E 120°E 150°E 180 200 220 180 200 220 180 200 220 180 200 220 (a) (b) (c) (d) Figure 6: Composite evolution of OLR (W/m , shadings) and 850 hPa wind (m/s, vectors) based on the SCSSM withdrawal date from (a) two pentads before the monsoon withdrawal (P − 2), (b) one pentad before the monsoon withdrawal (P − 1), (c) during the monsoon withdrawal (P0), and (d) one pentad after the monsoon withdrawal (P + 1) for the period of 1979–2016. In the period of 1994–2016, before the SCSSM with- Bay of Bengal on the next pentad of the SCSSM withdrawal drawal pentad, strong westerly anomalies are found over (Figures 8(f) and 8(g)). Comparing the differences before the Bay of Bengal extending through the South China Sea to and after the SCSSM withdrawal pentad, the increased the east of the Philippines, which transports abundant easterly wind in the SCS is sharper than that in the period of warm moisture to favor the active convection over the SCS 1979–1993, and the convection over the Northwest Pacific, and Northwest Pacific (Figure 8(e)). On the withdrawal SCS, and the Bay of Bengal is weakened along with the pentad, the low-level wind abruptly transits to a north- SCSSM withdrawal, which is associated with a southward easterly anomaly in the SCS and persists into the next migration of the seasonal march of the ITCZ (Figure 8(h)). pentad, which is stronger than that in the period of It is suggested that the SCSSM withdrawal rate during the 1979–1993. On the other hand, a suppressed convection period of 1994–2016 is faster than that of the period of appears in the SCS and eventually extends eastward to the 1979–1993. 220 Advances in Meteorology 7 0.0 250 –2.0 –4.0 –6.0 –8.0 200 P (–3) P (–2) P (–1) P0 P (1) P (2) ° ° ° ° Figure 7: Pentad 850 hPa zonal wind (m/S) and OLR (W/m ) averaged over the South China Sea (110 –120 E, 10 –20 N) from three pentads before the SCSSM withdrawal (P − 3) to two pentads after the SCSSM withdrawal (P + 2). 5 m/s 7 m/s 5 m/s 5 m/s 30°N 30°N 30°N 30°N (a) (b) (c) (d) 20°N 20°N 20°N 20°N 10°N 10°N 10°N 10°N 0° 0° 0° 0° 10°S 10°S 10°S 10°S 75°E 90°E 105°E 120°E 135°E 150°E 75°E 90°E 105°E 120°E 135°E 150°E 75°E 90°E 105°E 120°E 135°E 150°E 75°E 90°E 105°E 120°E 135°E 150°E 5 m/s 5 m/s 5 m/s 7 m/s 30°N 30°N 30°N 30°N (e) (f ) (g) (h) 20°N 20°N 20°N 20°N 10°N 10°N 10°N 10°N 0° 0° 0° 0° 10°S 10°S 10°S 10°S 75°E 90°E 105°E 120°E 135°E 150°E 75°E 90°E 105°E 120°E 135°E 150°E 75°E 90°E 105°E 120°E 135°E 150°E 75°E 90°E 105°E 120°E 135°E 150°E –40 –30 –20 –10 0 10 20 30 40 Figure 8: Composite differences in OLR (W/m , shadings) and 850 hPa wind (m/s, vectors) based on th SCSSM withdrawal date in typical years of the epoch 1979–1993: (a) one pentad before the monsoon onset (P − 1), (b) during the monsoon onset (P0), (c) one pentad after the monsoon onset (P + 1) and the withdrawal stages (pentads 53 − 37), and (d) differences between after (P + 1) and before (P − 1) the monsoon withdrawal. (e–h) the same as (a–d) but for the epoch 1994–2016. In short, the distinct characteristics of the SCSSM are the different characteristics of the SCSSM in two epochs that, during the period of 1979–1993 (1994–2016), the and the rainfall distribution in Eastern China. Seasonal ° ° SCSSM onset is generally later (earlier) and the intensity of evolution of latitudinal (110 –120 E) averaged rainfall the SCSSM onset is stronger (weaker). On the other hand, between two epochs is shown in Figure 9. In the late-onset there is no obvious difference in the SCSSM retreat date, but years of 1979–1993, the maximum rainfall belt − 1 the speed of the SCSSM retreat is slower (faster) and the (>10 mm·day ), corresponding to the first rainy season in duration is longer (shorter). ,is raises the following Southern China, begins at the end of May (Figure 9(a)). question: what is the effect of this change on rainfall in Moreover, for the late-onset period, the SCSSM intensity China? is stronger, leading to more warm moisture being con- stantly transported into Southern China. ,e rainfall persists consistently until the beginning of June until the 3.3. Distinct Rainfall Distribution from the Onset to With- major rain-belt moves northward to the middle and lower drawal of the SCSSM during the Two Periods. Many previous reaches of the Yangtze River basin (around 30 N), which is studies have confirmed that the evolution of the SCSSM associated with the Meiyu front. By the end of July, the has a broad impact on the spatial and temporal distri- maximum rainfall belt moves over Southern China again, bution of rainfall in East Asia during the period from May which can be regarded as the beginning of the last flood to September. Particularly, the SCCSSM onset has been season in Southern China. ,e rainfall in Eastern China ° ° regarded as the beginning of the East Asian rainy season reduces from 35 N north to 25 N south from early Sep- [12, 38–41]. It is necessary to figure out the link between tember to early October. U850 hPa OLR 8 Advances in Meteorology 35°N 35°N 30°N 30°N 25°N 25°N 1May 1Jun 1Jul 1Aug 1Sep 1Oct 1Nov 1May 1Jun 1Jul 1Aug 1Sep 1Oct 1Nov 2468 10 2468 10 (a) (b) ° ° Figure 9: Latitude-time cross sections of 5-day running mean precipitation (mm/day) averaged between 110 E and 120 E in typical years of the epoch: (a) 1979–93 and (b) 1994–2016. In the period of 1994–2016, the SCSSM onset is generally by Mao and Chan [44], has been applied. ,at is, early. ,e first rainy season in South China begins at the I � ((v − v )/σ − (R − R))/σ , where v � (u + v)/ M SW SW v R SW √� beginning of May and continues intermittently to the end of 2, R is OLR, σ and σ are the standard deviations of v v R SW June until the major rain-belt moves northward to the and OLR, respectively, and v and R are the multiyear SW middle and lower reaches of the Yangtze River basin averages of v and OLR, respectively. ,e advantage of this SW (Figure 9(b)). ,e Meiyu period during 1994–2016 is shorter index is that it takes into account the dynamic and ther- than that during 1979–1993. By mid-July, the maximum modynamic characteristics of the monsoon. In this study, rainfall belt moves to the Southern China again and it de- the 30–80-day band-pass filtering of I over the SCS ° ° ° ° creases with Eastern China rainfall simultaneously from late (110 –120 E, 10 –20 N) is applied as the daily ISO index of September to early October, consistent with the faster and the SCSSM. Figure 11 shows the typical daily ISO evolution shorter retreat of the SCSSM. On the other hand, the two from April to November in the SCS between two epochs. A migrations of the rain-belt are closely related to the two-peak strong modulation of SCS ISOs on the SCSSM evolution is pattern of the zonal wind over the SCS in summer, which is observed. In the late-onset period of 1979–1993, the SCS ISO consistent with previous studies [42]. has been in a dry phase, postponing the monsoon onset due to low-frequency easterly wind anomalies and weakening convection until late May. ,e ISO then turns to the positive 4. Effects of ISO on the SCSSM Evolution from phase and develops to the maximum amplitude of the whole Onset to Withdrawal during the Two Periods monsoon period. ,e low-frequency convection and west- ,e above analyses have shown different characteristics of the erly wind are significantly strengthened and favor a strong SCSSM from onset to withdrawal before and after 1993/94 and SCSSM onset. ,e transformation time is very near pentad the effect of the SCSSM change on rainfall distribution. ,us, a 29 (May 26–31). ,ere are two active low-frequency oscil- question to be further addressed is, what are the mechanisms lations in summer, namely, “South China Sea monsoon surge,” which has been discussed in depth by Li et al. [46], responsible for this characteristic change? Several studies have discussed the importance of two dominant modes of ISOs on corresponding to the two rain-belt migrations. After Sep- the triggers of the SCSSM onset and withdrawal [43–46]. It is tember, the ISO gradually weakened and disappeared, found that 10–25-day and 30–80-day oscillations also control corresponding to the weakening and slowing retreat of the the SCSSM activity from May when the SCSSM onsets to SCSSM. October when the SCSSM retreats through power spectrum In the early-onset years of 1994–2016, the ISO of the analysis (fig. not shown). To detect the effects of ISO on the SCSSM turns to the positive phase around the beginning of SCSSM evolution from onset to withdrawal during the two May. ,e first positive peak value of ISO is smaller than that epochs, we calculated standard deviations of the 10–25- and in the period of 1979–1993, corresponding to a weaker SCSSM onset intensity. ,en there are three active low- 30–80-day filtered OLR from 15 May to 15 October, which cover the 5 months from climatological SCSSM onset to frequency oscillations from June to September, with stable withdrawal. Figure 10 shows the differences (1994–2016 to amplitude. In early October, the ISO transforms rapidly to 1979–1993) of mean standard deviation between the two the negative phase, contributing to the shorter, faster retreat epochs. ,e difference of the 30–80-day oscillation mode over of the SCSSM. It is inferred that the 30–80-day ISO was the SCS is significant and the strength of the 30–80-day os- crucial for the SCSSM from onset to withdrawal between two cillations is large in the SCS during the period of 1994–2016. epochs. ,us, the effect of 30–80-day ISO on the SCSSM evolution We have made a preliminary analysis for the effect of during the two periods will be studied further. local SCS 30–80-day ISO on the SCSSM evolution difference; To explore the 30–80-day ISO activities of the SCSSM, an how does each ISO propagate and where is the ISO prop- East Asian summer monsoon index (I ), which is proposed agation source? Figure 12 shows the zonal and meridional M Advances in Meteorology 9 30°N 30°N 20°N 20°N 10°N 10°N 0° 0° 10°S 10°S 75°E 90°E 105°E 120°E 135°E 150°E 75°E 90°E 105°E 120°E 135°E 150°E –4 –3 –2 –1 0 1 2 3 4 –4 –3 –2 –1 012 3 4 (a) (b) Figure 10: Difference in the Lanczos filtered OLR anomalies in (a) 10–25-day and (b) 30–80-day time scales from 15 May to 15 October between 1994–2016 and 1979–1993. Units are in W/m , and the dotted areas denote that the differences are significant at the 95% confidence level. have discovered that the delayed onset of SCSSM may result 0.60 from a La Niña event [47]. ,e early onset and delayed 0.40 retreat of the SCSSM can be ascribed to warm SSTanomalies 0.20 in the Philippine Sea [23, 30]. A warm SST in the tropical Indian Ocean during the previous winter [19] can delay the –0.00 SCSSM onset by suppressing the convection over the –0.20 western Pacific. It is also interesting to find that the spatial –0.40 pattern of the SST evolution exhibits specific interdecadal 4 5 6 7 8 9 10 11 characteristics during two periods. As shown in Figure 13 on the left, significant positive SSTanomalies appear around the Figure 11: Composite daily ISO index evolution from April to equatorial Eastern Pacific in the late-onset years of the November over the SCS. Red line is in a typical year of the epoch period of 1979–1993 in May and continue developing until 1979–93, and blue dots are in a typical year of the epoch 1994–2016. November. Meanwhile, the spatial pattern of the SST changes is similar to the horseshoe-shaped SST warming ° ° ° ° propagation of ISO along with 10 –20 N and 110 –120 E during El Niño events. In theory, the warm tropical eastern during the two periods. In the period of 1979–1993, the wet Pacific SST anomaly may induce an anticyclone anomaly in ISO propagates eastward from tropical Indian Ocean to the the northwest Pacific due to the excitation of ascending ° ° SCS along with 10 –20 N in early June and mid-August Rossby waves. ,is anticyclone anomaly connects to the (Figure 12(c)). Moreover, when ISO activity in the Indian low-level easterly anomaly in the SCS, which possibly Ocean is relatively strong, the propagation to the SCS is also postpones the onset of SCSSM [48]. In the early-onset years strong. In meridional direction, the positive ISO propagates of the period of 1994–2016, significant warm SST anomalies northward into the SCS. In September, there is ISO propa- appear in the Philippine Sea from April to September, and gating from Indian Ocean but not entering the SCS. Different the strongest warm SST appears in May and then gradually from the late-onset period, in the period of 1994–2016, the weakens. A few weak negative SST anomalies appear around first two wet ISOs propagate westward from the Northwest the equatorial Eastern Pacific (Figure 13 on the right). ,us, Pacific in May and June, and the last two propagate eastward the SST warming in the Philippine Sea from April to Sep- from the Indian Ocean in July and September. In zonal di- tember may be a crucial external force for the SCSSM rection, the ISOs also propagate northward into the SCS. evolution in the period of 1994–2016. However, a deeper ,erefore, the wet ISO in the Northwest Pacific propagating understanding of the mechanism of this SST change on the northwestward into the SCS may contribute to the SCSSM SCSSM evolution during two epochs calls for further study. evolution difference after 1993/94 and the SCSSM ISO in- tensity can be predicted by monitoring ISO anomalies over 6. Summary and Discussion the northwest Pacific and Indian oceans. In this study, we investigate the different characteristics and possible impact factors of the SCSSM evolution from onset 5. Effects of SSTAs onthe SCSSM Evolution from to withdrawal before and after 1993/94 when an interdecadal Onset to Withdrawal during the Two Periods change occurs. ,e results suggest that, during the period of 1979–1993, the SCSSM onsets later, corresponding to abrupt In addition to the effects of internal atmospheric dynamic enhancement of zonal wind and convection activity, results mechanism, SST, an important external force, also plays an in a stronger SCSSM onset intensity. Delayed onset also important role in the SCSSM evolution. Previous studies 10 Advances in Meteorology 20°N 20°N 0.7 0.7 0.6 0.6 0.5 0.5 0.4 0.4 0.3 0.3 0.2 0.2 0.1 0.1 10°N 0 10°N 0 –0.1 –0.1 –0.2 –0.2 –0.3 –0.3 –0.4 –0.4 –0.5 –0.5 –0.6 –0.6 –0.7 –0.7 0 0 45 6 78 9 10 11 4 5 6 78 9 10 11 (a) (b) 140°E 140°E 0.7 0.7 0.6 0.6 130°E 130°E 0.5 0.5 0.4 0.4 0.3 0.3 120°E 120°E 0.2 0.2 0.1 0.1 110°E 0 110°E –0.1 –0.1 –0.2 –0.2 100°E 100°E –0.3 –0.3 –0.4 –0.4 –0.5 –0.5 90°E 90°E –0.6 –0.6 –0.7 –0.7 80°E 80°E 45 67 8 910 11 4 56 7 8 9 10 11 (c) (d) Figure 12: Latitude-time (left) and longitude-time (right) diagram of the daily SCSSM ISO propagation from April to November between ((a) and (c)) late-onset years of the period of 1979–1993 and ((b) and (d)) early-onset years of the period of 1994–2016. Apr May Jun Jul 40°N 40°N 40°N 40°N 30°N 30°N 30°N 30°N 20°N 20°N 20°N 20°N 10°N 10°N 10°N 10°N 0° 0° 0° 0° 10°S 10°S 10°S 10°S 45°E 90°E 135°E 180° 135°W 90°W 45°E 90°E 135°E 180° 135°W 90°W 45°E 90°E 135°E 180° 135°W 90°W 45°E 90°E 135°E 180° 135°W 90°W (a) (b) (c) (d) Aug Sep Oct Nov 40°N 40°N 40°N 40°N 30°N 30°N 30°N 30°N 20°N 20°N 20°N 20°N 10°N 10°N 10°N 10°N 0° 0° 0° 0° 10°S 10°S 10°S 10°S 45°E 90°E 135°E 180° 135°W 90°W 45°E 90°E 135°E 180° 135°W 90°W 45°E 90°E 135°E 180° 135°W 90°W 45°E 90°E 135°E 180° 135°W 90°W (e) (f) (g) (h) Apr May Jun Jul 40°N 40°N 40°N 40°N 30°N 30°N 30°N 30°N 20°N 20°N 20°N 20°N 10°N 10°N 10°N 10°N 0° 0° 0° 0° 10°S 10°S 10°S 10°S 45°E 90°E 135°E 180° 135°W 90°W 45°E 90°E 135°E 180° 135°W 90°W 45°E 90°E 135°E 180° 135°W 90°W 45°E 90°E 135°E 180° 135°W 90°W (i) (j) (k) (l) Figure 13: Continued. –0.5 –0.5 –0.5 –0.4 –0.4 –0.4 –0.3 –0.3 –0.3 –0.2 –0.2 –0.2 –0.1 –0.1 –0.1 0 0 0 0.1 0.1 0.1 0.2 0.2 0.2 0.3 0.3 0.3 0.4 0.4 0.4 0.5 0.5 0.5 –0.5 –0.5 –0.5 –0.4 –0.4 –0.4 –0.3 –0.3 –0.3 –0.2 –0.2 –0.2 –0.1 –0.1 –0.1 0 0 0 0.1 0.1 0.1 0.2 0.2 0.2 0.3 0.3 0.3 0.4 0.4 0.4 0.5 0.5 0.5 –0.5 –0.5 –0.5 –0.4 –0.4 –0.4 –0.3 –0.3 –0.3 –0.2 –0.2 –0.2 –0.1 –0.1 –0.1 0 0 0 0.1 0.1 0.1 0.2 0.2 0.2 0.3 0.3 0.3 0.4 0.4 0.4 0.5 0.5 0.5 –0.5 –0.5 –0.5 –0.4 –0.4 –0.4 –0.3 –0.3 –0.3 –0.2 –0.2 –0.2 –0.1 –0.1 –0.1 0 0 0 0.1 0.1 0.1 0.2 0.2 0.2 0.3 0.3 0.3 0.4 0.4 0.4 0.5 0.5 0.5 Advances in Meteorology 11 Aug Sep Oct Nov 40°N 40°N 40°N 40°N 30°N 30°N 30°N 30°N 20°N 20°N 20°N 20°N 10°N 10°N 10°N 10°N 0° 0° 0° 0° 10°S 10°S 10°S 10°S 45°E 90°E 135°E 180° 135°W 90°W 45°E 90°E 135°E 180° 135°W 90°W 45°E 90°E 135°E 180° 135°W 90°W 45°E 90°E 135°E 180° 135°W 90°W (m) (n) (o) (p) Figure 13: Composite monthly SSTAs (shading; (K)) from April to November between late-onset years of the period of 1979–1993 (left) and early-onset years of the period of 1994–2016 (right). ,ick solid lines surround the region exceeding the 90% confidence level, respectively. (a, i) Apr, (b, j) May, (c, k) Jun, (d, l) Jul, (e, m) Aug, (f, n) Sep, (g, o) Oct, and (h, p) Nov. results in a continuous, strong rain-belt appearing in SST and Philippine Sea temperature on the SCSSM onset in May and withdrawal in September [30, 32, 49]. ,e specific Southern China at the end of May. After the SCSSM onset, the low-level zonal winds appear with a two-peak pattern in physical mechanism for the impact of SST on the monsoon summer, and two abrupt increases of the zonal winds evolution from May to September should thus be studied correspond to the beginning of the Meiyu period in Yangtze further. On the other hand, air-sea interaction can provide River valley and the last flood season in Southern China, an additional northward propagation mechanism for 30–60- respectively. ,e monsoon retreats from September until day Northwest Pacific ISO and ISO source [50]. ,e in- October and exhibits an extended, gradual retreat, which fluence of SSTAs on the SCSSM ISO should also be studied as it may provide a basis for predicting the subsequent causes the withdrawal of the rain-belt to begin in September from north to south China and completely vacate at the SCSSM evolution. beginning of October. However, after 1993/94, the SCSSM onsets earlier and is Data Availability characterized by a weaker onset intensity and more rapid retreat. ,e preflood season advanced and lasted intermit- Daily precipitation data and OLR were derived from https:// tently throughout May. ,e rain-belt in Southern China www.esrl.noaa.gov/psd/data/gridded/. ,e HadISST data persisted for a longer time in summer. ,e monsoon started were from http://www.metoffice.gov.uk/hadobs/hadisst/ to retreat from October, resulting in the rapid retreat of the data/download.html. ,e ERA-Interim data were obtained rain-belt from the whole eastern China. from https://apps.ecmwf.int/datasets/data/interim-full-daily/ Further analyses indicate that the enhanced 30–80-day levtype�pl/. SCS ISO activities are an important contributor for SCSSM evolution change. ,ere are two active low-frequency os- Conflicts of Interest cillations over the SCS in summer during the late-onset period but three ISOs in the early-onset period. 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Advances in MeteorologyHindawi Publishing Corporation

Published: Sep 16, 2020

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