A statistical study on promoting effects of tropical cyclones over the Bay of Bengal on the South China Sea summer monsoon onset

The onset date of the South China Sea (SCS) summer monsoon (SCSSM) and the generation time of the first tropical cyclone (TC) over the Bay of Bengal (BoB) during late April and early June were significantly correlated with a correlation coefficient of 0.58 during 1979–2020. The composite analysis found that under the impact of BoB TCs, an enhanced southwesterly low‐level flow transported abundant moisture from the BoB to the northern SCS. Besides, the diabatic heating related to TC convection stimulated an anticyclonic anomaly in the upper troposphere over the southern Tibetan Plateau, which was conducive to the enhancement and expansion of the South Asian high (SAH) over the Indo‐China Peninsula. The stronger easterly outflows from the eastern periphery of the SAH overlapped with low‐level water vapor convergence over the northern SCS, enhancing the development of monsoon convection. Thus, more condensation heating warmed the tropospheric atmosphere and reversed the meridional temperature gradient over the SCS, implying the SCSSM onset.


| INTRODUCTION
The South China Sea (SCS) summer monsoon (SCSSM) onset indicates the seasonal reversal of the sea-land thermal contrast over the SCS and the advent of the rainy season over East Asia (Bombardi et al., 2019(Bombardi et al., , 2020Li & Zhang, 2009;Martin et al., 2019;Wang et al., 2022;Yan et al., 2021). During the onset process of the SCSSM, the range of the South Asian high (SAH) in the upper troposphere enlarges and extends into the SCS (Kueh & Lin, 2010; with the eastward retreat of the western pacific subtropical high (WPSH) in the middle-lower troposphere (He et al., 2006;Wang et al., 2019). In addition, an evident vertical easterly shear is observed over the SCS, as well as the seasonal reversal of the meridional temperature gradient (MTG) in the middle-upper troposphere Mao et al., 2004). In climatology, the onset of the SCSSM generally occurs in about Pentad 28 based on different monsoon onset definitions (Liu et al., 2015;Liu & Zhu, 2019;Zhu, 2005), while it exhibits an interannual variation under the impact of Tibetan Plateau (TP) forcing, ENSO event and intraseasonal oscillation (Feng et al., 2021;Hu et al., 2018Hu et al., , 2020Hu et al., , 2021Kajikawa & Wang, 2012;Li et al., 2019;Luo & Lin, 2017;Wang et al., 2016). On a synoptic timescale, tropical cyclones (TCs) over the SCS, the western North Pacific, and the Bay of Bengal (BoB) are important triggers for the SCSSM onset Huangfu et al., 2017;Kubota et al., 2017;Mao & Wu, 2008).
TCs over BoB exhibits a prominent bimodal feature (Fan et al., 2020;Gray, 1968;Mohapatra et al., 2017), with one peak in May, coinciding with the mean onset time of the SCSSM. Usually, the seasonal transition of the Asian monsoon first occurs over the eastern BoB, accompanied by a monsoon onset vortex (MOV) generating and developing over BoB (Wu et al., 2012). Previous studies suggested that the triggering mechanism for the onset of the BoB summer monsoon is closely related to the genesis and development of the MOV (Li et al., 2016;Liu et al., 2013;Wu et al., 2011Wu et al., , 2012. Regarding the relationship between the SCSSM and low pressure over BoB, Lau et al. (1998Lau et al. ( , 2000 observed that monsoon depressions occurred over BoB in the preonset stage of the SCSSM in both 1997 and 1998. Numerical simulations revealed that the condensational heating over BoB is vital for the formation of a large-scale circulation pattern that favors the establishment of SCSSM in 1998 (Chan et al., 2000;Liu et al., 2002;Wu et al., 2005). A study focusing on favorable conditions for MOV generation also suggested a close connection between the BoB MOV and the SCSSM during 1980 to 2009 (Zhu & He, 2013). A recent research showed that TC "Fani" over BoB boosted the onset of the SCSSM in 2019 by causing a northward shift of the SAH and providing upper-tropospheric pumping (Liu & Zhu, 2020).
There appears to be a close linkage between BoB TCs and the SCSSM onset. It is crucial to clarify whether the occurrence of TCs over BoB is statistically and physically linked to the SCSSM onset. The relationship between these two events and the related physical processes were statistically investigated from 1979 to 2020 in this study, aiming at exploring new predictors for the SCSSM onset.

| DATA AND METHODS
The daily meteorological variables with a horizontal resolution of 2.5 Â 2.5 from the NCEP/NCAR reanalysis-2 dataset were utilized in this study. Convection was represented by the 1 Â 1 daily outgoing longwave radiation (OLR) from NOAA. The best tracks of TCs from 1979 to 2020 were provided by JTWC. The pentad values were the mean values for every 5 days, and there were 73 pentads per year. The MTG-defined SCSSM onset time was adopted in this study, specifically, the moment when the pentad area-averaged MTG in the mid-to-upper troposphere (500-200 hPa) changes from negative to positive and remains positive for at least 3 pentads over the SCS (10 -20 N, 110 -120 E) after pentad 21 , which was closely correlated with the times identified by the OLR or the 850 hPa zonal winds . The median day value in the onset pentad was treated as the onset date of the SCSSM. The atmospheric apparent heat source Q 1 and apparent moisture sink Q 2 were calculated following the method by Yanai et al. (1973). The isotach of zonal wind u = 0, with a positive meridional gradient of u, can be used to represent the ridge axis in the northern hemisphere. The two-tailed Student's t-test was used to evaluate the statistical significance of the anomalies.

| CORRELATION BETWEEN THE SCSSM ONSET AND BOB TCS
The onset time of the SCSSM from 1979 to 2020 was between Pentad 25 and Pentad 32 (blue line in Figure 1a), with an average date of Pentad 28. In the time period of Pentad 23-32 (April 21-June 10), a total of 27 years (accounting for 64% of the analyzed 42 years) were observed for the occurrence of BoB TCs (red and orange lines). The correlation coefficient between the onset date of the SCSSM and the generation date of the first TC in the same year reached 0.58, exceeding the 99% confidence level. The TC samples that were active during one pentad before or within the pentad of the SCSSM onset date were considered to have an impact on the SCSSM onset in the current year (as shown in Table 1 and red lines in Figure 1a). Accordingly, such TCs occurred for 21 years within the 42 years, accounting for 50%, with the average generation date of May 12, and an average duration of about 7 days, corresponding to the average SCSSM onset date of May 19. Averagely, the SCSSM onset date was about 7 days after the generation of BoB TC. Twenty of these 21 TCs were the first TCs generated after Pentad 23 in the current year, except for the TC sample in 1991. From the track distribution of these 21 TCs (Figure 1b), there were 19 TCs with the northeast track and only 2 TCs with the northwest track. In addition, about 57% (12 TCs) of the TC samples reached hurricane intensity category (H1-H5, V max > 63 kt), while the remaining 9 TCs (accounting for 43%) were tropical storm (TS, V max : 34-63 kt).
Reversal of MTG in the middle-upper troposphere over the SCS is an indicator of the SCSSM onset. The blue lines in Figure 1b denote the zero contour of MTG averaged in the middle-upper troposphere, with positive (negative) values in the south (north) of it, indicating the onset (or not) of the summer monsoon. In a pentad before the SCSSM onset (blue dashed line), convection over BoB developed actively, contributing to the tilt of MTG's zero contour toward the east-central region over BoB. Meanwhile, it fell back southward abruptly in the Indo-China Peninsula, and the MTG was still negative over the SCS. However, during the SCSSM onset pentad (blue solid line), the positive MTG area over BoB expanded to the east. Hence, the MTG over the Indo-China Peninsula and SCS transitioned from negative to positive, suggesting the onset of the SCSSM.
Based on the composite analysis, the following sections addressed the anomalous features and the possible mechanism during the SCSSM onset pentads of the above-mentioned 21 TC samples. The values in Pentad 28 during 1979-2020 as the climate mean were subtracted to analyze the anomalous features of the SCSSM onset under the influence of BoB TCs. As a supplement, the difference between the 21 years in which SCSSM onset was impacted by BoB TCs (TC years) minus the 21 years in which it was not affected by BoB TCs (non-TC years) was also considered, based on the actual SCSSM onset date.

| Convective activity
In a pentad before the SCSSM onset, significant negative OLR anomalies emerged over BoB and south to the Yangtze River, as seen in Figure 2a. In contrast, positive OLR anomalies appeared in the southwestern TP, western North Pacific, and southeastern SCS, where monsoon convection was suppressed. The mean MTG in the middle-upper troposphere exhibited a significant positive anomaly in the southern BoB and a negative anomaly in the SCS (Figure 2b). The ridge line at 200 hPa (solid red line) of the SAH was located north of the climate mean position (dashed red line) in the west of the Indo-China Peninsula, while located southward in the SCS. Moreover, the ridge line at 500 hPa (solid purple line) of the WPSH broke earlier than the climate means (dashed purple line) over BoB as a result of TC convection in this pentad. The first splitting of the subtropical high belt over BoB is an essential precursor for establishing the SCSSM (He & Liu, 2016).
In the SCSSM onset pentad, OLR in the SCS turned to negative anomalies (Figure 2c), indicating that the monsoon convection was no longer suppressed here. The MTG's significant positive anomalies center expanded from BoB to the Indo-China Peninsula and the northern SCS as shown in Figure 2d, in which the ridge line at 200 hPa (solid red line) was located northward of its climate mean position (dashed red line), suggesting that the location of SAH was northward of the climate field. The ridge line at 500 hPa (solid purple line) retreated southward in the SCS compared with the previous pentad. Such the northward dip of the ridge surface demonstrates the establishment of the SCSSM circulation pattern. Concisely, monsoon convection deepened over the SCS under

| Monsoon circulation
The anomalous distribution of the atmospheric circulation at the upper-, middle-, and lower-troposphere suggests that the upper-level divergence anomalies took place over BoB in a pentad before the SCSSM onset (Figure 3a), where the vertical ascend movement enhanced ( Figure 3b) and more abundant water vapor was converged (Figure 3c). A striking anticyclonic anomaly existed from BoB to the western TP in the upper troposphere, which favored the northward movement of the SAH (Wei et al., 2019). As shown in Figure 3a, the 16,680 gpm geopotential height was much broader under the impact of BoB TCs (solid line) than climate mean (dashed line), covering the northwestern SCS with a divergence anomaly. In the middle and lower troposphere, there was an evident cyclonic anomaly over BoB. The position of WPSH at 500 hPa was further west than the climatology and controlled the SCS. This favored the anomalous meridional transport of water vapor flux from the eastern BOB to the southern TP and South China in the lower troposphere. In this pentad, the ascending movement and moisture flux convergence were weakened in the SCS, while they were enhanced in South China.
In the onset pentad of the SCSSM, the anticyclonic anomaly over the southern TP expanded northwards ( Figure 3d). Meanwhile, the SAH developed northeastwards toward the SCS, leading to the anomalous divergence and the transition of the westerly wind to the northeasterly wind in the upper troposphere over the SCS. The ridge line of WPSH over the SCS retreated southward to a certain degree and abnormal northerly occurred over the northwestern edge of SCS (Figure 3e), while it still prevailed the westerly wind flow from the WPSH periphery. The southern branch trough in the northern BoB at 700 hPa was much deeper with stronger southerly than the climate mean (Figure 3f), resulting in the prominent anomalous convergence center of the water vapor flux over South China and the northern SCS. And the vertical easterly shear over the SCS was increased. The anomalies of upper-level divergence and lower-level water vapor convergence in the northern SCS were beneficial to the development of the monsoon convection. In addition, similar circulation features can be found in the field of differences between 21 years affected by BoB TCs minus 21 years unaffected by BoB TCs based on the corresponding SCSSM onset pentad (Figure omitted).

| Diabatic heating
The BoB TCs produced great diabatic heating to change the atmospheric circulation and thermal structure during the SCSSM onset. In a pentad before the SCSSM onset (Figure 4a), the maximum value of diabatic heating over BoB was located near 500 hPa, and the corresponding drying rate was positive. Therefore, the atmosphere over BoB was warmed by the latent heat of condensation released by TC precipitation. Nevertheless, there was no obvious convection and diabatic heating observed in the SCS. There was a significant diabatic heating anomaly related to TC convection in BoB (Figure 4b). The potential vorticity (PV) increased below the maximum heating layer and decreased above it, that is, the positive PV anomaly was below 250 hPa, while it was a negative anomaly above 250 hPa, which strengthened the anticyclonic circulation and favored the development of SAH. The upper troposphere in the SCS was the center of positive PV anomaly, which was related to the southward intrusion of high PV from high latitudes to the SCS by the northwesterly steering flow along the northeast side F I G U R E 2 Anomalous fields of (a, c) outgoing longwave radiation (shading, WÁm À2 ) and (b, d) MTG averaged between 500 and 200 hPa (shading, 10 À6 KÁm À1 ) under the influence of BoB TCs in (a, b) a pentad before the SCSSM onset and (c, d) a pentad during the SCSSM onset. Hereafter, anomalies exceeding the 95% confidence level are stippled. The purple (red) solid and dashed lines in (a) and (b) represent the ridge lines at 500 (200) hPa under the impact of TC samples and climate means, respectively. of SAH. This was conducive to the development of lowlevel cyclonic circulation over the SCS. However, the middle troposphere in the SCS was still controlled by WPSH, with anomalous descending motion, vertical westerly shear, and weak convective activities. In addition, in the field of differences between 21 TC years and 21 non-TC years (Figure 4c), we can find a similar distribution as shown in Figure 4b, and the positive PV anomaly over the SCS was stronger and more favorable to induce low-level cyclonic vortices.
In the SCSSM onset pentad, the denser contours of drying rates in BoB illustrated that the latent heat of moisture condensation led to larger warming and enhanced diabatic heating (Figure 4d). The vertical wind shear over the SCS was transformed into easterly shear, and the latent heat of convective condensation was released to heat the middle and upper troposphere, with the warming center at 400 hPa. As is seen from the anomalous field in Figure 4e, the maximum positive anomaly of the diabatic heating in BoB was located at 200 hPa. And in the field of differences between 21 TC years and 21 non-TC years (Figure 4f), the diabatic heating differences in BoB were significantly positive above 300 hPa as well as between 850 and 400 hPa at 90 -100 E, with the evident ascend movement. In both Figure 4e,f, the negative PV anomaly above 200 hPa extended eastward to the SCS, which was advantageous to the enhancement and eastward expansion of the SAH. It strengthened the divergence and easterly wind anomalies in the upper level over the SCS to intensify the pumping effect. The convective stability in the northern SCS decreased with the southward retreat of WPSH, favoring the convergence of low-level water vapor flux and thus the occurrence of convection. As a result, the anomalous diabatic heating with a large value between 500 and 150 hPa in the SCS promoted the reversal of the MTG in the middle and upper troposphere and boosted the onset of the SCSSM.

| SUMMARY AND DISCUSSION
TC activities over BOB may provide a new precursor to the SCSSM onset on a synoptic timescale. The onset dates of the SCSSM were significantly correlated to the generation of the first BOB TC during late April and early June from 1979 to 2020. On average, the SCSSM onset date was about 7 days after the generation date of BoB TCs, F I G U R E 3 Horizontal distribution of the anomalous circulation in (a-c) a pentad before the SCSSM onset and (d-f) the pentad during the SCSSM onset. (a, d) Anomalies of divergence (shading, 10 À6 Ás À1 ) and winds (vectors, mÁs À1 ) at 100 hPa. (b, e) anomalies of vertical velocity (shading, 10 À2 PaÁs À1 ) and horizontal winds at 500 hPa (vectors, mÁs À1 ). (c, f) anomalies of water vapor flux (gÁcm À1 ÁhPa À1 Ás À1 ) and its divergence (10 À7 gÁcm À2 ÁhPa À1 Ás À1 ) at 700 hPa. Black dashed (solid) contours in the left, middle, and right columns are the 16,680-, 5880-, and 3130-gpm geopotential heights in the climatology (under the impact of the BoB TCs).
which most with northward tracks. The primary physical process was sketched in Figure 5.
Under the impact of BoB TCs, an enhanced southwesterly low-level flow transports abundant water vapor from the BoB to the northern SCS (arrows in the below panel of Figure 5). And along with the southward retreat of the WPSH (pink contours), the moisture channel shifts southward, and the stability of atmospheric stratification in the northern SCS is weakened, which favors the development of monsoon convection. Meanwhile, TCs activities release abundant diabatic heating over BoB. As a Gill response to the tropical diabatic heating (Gill, 1980), an upper-level anticyclonic anomaly is induced over the southern TP (arrows in the upper panel of Figure 5). It contributes to the northward and eastward expansion of the SAH in the Indo-China Peninsula and the Indian Peninsula (orange contours). Subsequently, the upper-level westerly winds on the east side of the SAH turn into strong easterly winds over the SCS, forming vertical easterly shear and stronger upperlevel divergence (blue shading), leading to the development of local convection. The latent heat released by convection condensation thus warms the northern SCS (red shading) to promote a positive MTG with the cooler air in southern SCS. The change of MTG value in the SCSSM monitoring area from negative to positive marks the onset of the SCSSM.
In summary, this work conducted a statistical and composite analysis of the relationship between the SCSSM onset and BoB TCs, and a significant correlation between them was revealed. Compared to the possible triggering mechanism of TC over the WNP for the SCSSM onset (Chen et al., 2022), TC over the BoB is farther away from the SCS and its effect on the SCSSM onset is indirect. As revealed by many previous studies, ENSO is the dominant factor controlling the SCSSM onset (Chen et al., 2022;Martin et al., 2019;Zhu & Li, 2017). During 1979-2020, there were 9 years (1979, 1990, 1994, 2002, 2004, 2006, 2009, 2013, and 2017) of the SCSSM onset unaffected by ENSO events. The partial correlation coefficient between the generation date of the first BoB TC and the corresponding onset date of the SCSSM of these nine non-ENSO years was 0.88 at the 99% confidence level, highlighting the important role of the BoB TCs. Furthermore, some other potential factors that may influence the onset of SCSSM were not considered sufficiently due to the limitation of the aim and space of the present study. For instance, during the F I G U R E 4 Averaged pressure-longitudinal cross sections at 10 -20 N of (a, d) diabatic heating rate Q 1 /c p (shading, KÁday À1 ), drying rate Q 2 /c p (contours, K day À1 ), and zonal circulation (vectors, u Â 500 w) under the impact of BoB TCs (the upper column) in a pentad before the SCSSM onset and (the lower column) in the SCSSM onset pentad, and the corresponding (b, e) anomalous field and (c, f) differences of 21 TC years minus 21 non-TC years based on the corresponding SCSSM onset pentad for diabatic heating rate Q 1 /c p (shading, KÁday À1 ), potential vorticity (contours, 10 À2 ÁPVU) and zonal circulation (vectors, u Â 500 w). The SCSSM monitoring area (10 -20 N, 110 -120 E) is indicated between the green lines.
activities of BoB TCs, the latent heat related to BoB monsoon convection and the activities of TCs over the SCS (there are simultaneous TC activities over SCS and BoB in May 1979and 2004, and June 2018, may also have an impact on the SCSSM onset. To further understand the specific physical process of the SCSSM onset that is affected by the BoB TCs and other influencing factors, more integral work is worth carrying out in the future, including multi-scale case analysis and numerical experiments.
F I G U R E 5 Schematic diagram of the promoting effect of BoB TCs on the SCSSM onset. In the upper panel, the arrows and shading denoted the wind anomaly (mÁs À1 ) and divergence filed (10 À6 Ás À1 ) at 100 hPa in the SCSSM onset pentad under the impact of BoB TCs, respectively. The same in the below panel, but for water vapor flux anomaly (gÁcm À1 ÁhPa À1 Ás À1 ) at 700 hPa and the vertically integrated diabatic heat <Q 1 > (WÁm À2 ), respectively. The orange (pink) dashed and solid contours denoted the 16,680-(5880-) gpm geopotential heights at 100 (500) hPa under the impact of BoB TCs in a pentad before and during the SCSSM onset, respectively.