Westerlies Affecting the Seasonal Variation of Water Vapor Transport over the Tibetan Plateau Induced by Tropical Cyclones in the Bay of Bengal

This study investigates the activity of tropical cyclones(TCs)in the Bay of Bengal(BOB)from 1979 to 2018 to discover the mechanism affecting the contribution rate to the meridional moisture budget anomaly(MMBA)over the southern boundary of the Tibetan Plateau(SBTP).May and October–December are the bimodal phases of BOB TC frequency,which decreases month by month from October to December and is relatively low in May.However,the contribution rate to the MMBA is the highest in May.The seasonal variation in the meridional position of the westerlies is the key factor affecting the contribution rate.The relatively southern(northern)position of the westerlies in November and December(May)results in a lower(higher)contribution rate to the MMBA.This mechanism is confirmed by the momentum equation.When water vapor enters the westerlies near the trough line,the resultant meridional acceleration is directed north.It follows that the farther north the trough is,and the farther north the water vapor can be transported.When water vapor enters the westerlies from the area near the ridge line,for Type-T(Type-R)TCs,water vapor enters the westerlies downstream of the trough(ridge).Consequently,the direction of the resultant meridional acceleration is directed south and the resultant zonal acceleration is directed east(west),which is not conducive to the northward transport of water vapor.This is especially the case if the trough or ridge is relatively south,as the water vapor may not cross the SBTP.

OBJECTIVE CLASSIFICATION OF THE TRACKS OF TROPICAL STORMS IN THE BAY OF BENGAL

An objective analysis of tropical cyclone tracks is performed, with which the tracks of 131 tropical storms(TSs) in 1972-2011 are separated into three types that move west-, north- and northwestward, denoted as Types A, B and C, respectively. Type A(21 TSs and 16% of total) has the origin in the southwestern Bay of Bengal, with the TS in a unimodal distribution as its seasonal feature, occurring mainly in autumn; 18 of the 21 TSs(taking up 90%) land mostly on the western Bay coast(west of 85°E); 5% of Type-A TSs attains the wind speed of >42.7 to 48.9 m/s. Type A has little or no effect on Tibet. Type B(74 TSs, 56.6% of the total) has its preferable origin in the central Bay of Bengal, with the TS in a bimodal distribution as its seasonal pattern. This type denotes the travel in the north in spring,with the landfall of 67 of the 74 TSs(accounting for 91%) mainly on the middle coast of the Bay(85° to 95°E), and19% of the TSs reaching the wind velocity of >42.7 to 48.9 m/s, which exert great effect on Tibet and it is this TS track that gives strong precipitation on its way through this region. Type C(36 TSs, 27.5% of the total) has its main origin in the southern part of the bay, and these TSs are formed largely in autumn, moving in the northwest direction,and 23 of the 36 TSs(64%) land mostly on the western Bay coast, lasting for a longer time, with almost no impact upon Tibet.

Statistical and Comparative Analysis of Tropical Cyclone Activity over the Arabian Sea and Bay of Bengal(1977-2018)

A statistical comparative analysis of tropical cyclone activity over the Arabian Sea (AS) and Bay of Bengal(BoB) has been conducted using best-track data and wind radii information from 1977 to 2018 issued by the Joint Typhoon Warning Center. Results show that the annual variation in the frequency and duration of tropical cyclones has significantly increased over time over the AS and insignificantly decreased over the BoB. The monthly frequency of tropical cyclones over the AS and the BoB shows a notable bimodal character, with peaks occurring in May and OctoberNovember, respectively. The maximum frequency of tropical cyclones occurs in the second peak as a result of the higher moisture content at mid-levels in the autumn. However, the largest proportion of strong cyclones (H1-H5 grades) occurs in the first peak as a result of the higher sea surface temperatures in early summer. Tropical cyclones over the AS break out later during the first peak and activity ends earlier during the second peak, in contrast with those over the BoB. This is related to the onset and drawback times of the southwest monsoon in the two basins. Tropical cyclones over the AS are mainly generated in the eastern basin, whereas in the BoB the genesis locations are meridionally (zonally) distributed in May-June (October-November) as a result of the seasonal movement of the low-level positive vorticity belt. The Arabian Sea is dominated by tropical cyclones that track west and northwest, accounting for about 74.6%of all the tropical cyclones there, whereas the tropical cyclones with a NE track account for only 25.4%. The proportions of the three types of tracks are similar in the BoB, with each accounting for about 33%of the tropical cyclones. The mean intensity and size of tropical cyclones over the AS are stronger and larger, respectively, than those over the BoB and the size of tropical cyclones over the North Indian Ocean in early summer is larger than that in the autumn. The asymmetrical structure of tropical cyclones over the North Indian Ocean is affected by topography and the longest radius of the 34 kt surface wind often lies in the eastern quadrant of the tropical cyclone circulation in both sea areas.

Influence of Summer Monsoon on Asymmetric Bimodal Pattern of Tropical Cyclogenesis Frequency over the Bay of Bengal

The influence of summer monsoon on tropical cyclone (TC) genesis over the Bay of Bengal (BoB) is explored using an empirical genesis potential (GP) index. The annual cycle of cyclogenesis frequency over the BoB shows an asymmetric bimodal pattern with the maximum genesis number appearing in late October and the second largest in early May. The two peaks correspond to the withdrawal and onset of the BoB summer monsoon, respectively. The semimonthly GP index calculated without TC days over the BoB is consistent with TC genesis frequency, indicating that the index captures the monsoon-induced changes in the environment that are responsible for the seasonal variation of TC genesis frequency. Of the four environmental variables (i.e., low-level vorticity, mid-level relative humidity, potential intensity, and vertical wind shear) that enter into the GP index, the potential intensity makes the largest contribution to the bimodal distribution, followed by vertical wind shear due to small wind speed during the summer monsoon onset and withdrawal. The difference in TC genesis frequency between autumn and late spring is mainly owing to the relative humid-ity difference because a divergence (convergence) of horizontal moisture flux associated with cold dry northerlies (warm wet wester-lies) dominates the BoB in late spring (autumn).

Spatial and temporal distribution of storms and their activities over the Bay of Bengal

By statistically analyzing the storm data from the Joint Typhoon Warning Center(JTWC) over the Bengal Bay during the period 1945-2006,it is found that the yearly averaged tropical cyclone(TC) number over the Bay of Bengal is 8.12,which takes place in any month of the whole year;February and March have the fewest TC numbers.The TC numbers begin to increase starting in April and arrive at a peak in October.Differing from TC over the Bay of Bengal,the tropical storms(TS) over the Bay of Bengal has two peak periods,appear in May and in October or November,respectively.With regard to TS intensity,the super severe storm of H4 criterion appeared only one time during the period 1971-1986,but appeared eight times during the period 1987-2006.The monthly change of the original position,the averaged maintaining time,and the longest maintaining time of TS also have two peak values:They appear in April or May and in October or November,respectively.The peak value of the original position in October or November is bigger than that in April or May.The peak value of the average maintaining time and the longest main-taining time of the TS in April or May is bigger than that in October or November.TC landfall path is mostly moving toward northwest or west and accounts for 56.7 percent.The landfall path of the TS differs from that of TC in some respects.The main difference is that the numbers of the northwestern path and un-landfall of TS are less than those for TC,and the numbers of the western path and northeastern path of TS are greater than for TC.Because of the landfall TS in the north-east path has a peak,it and the beginning of the rainy season in Yunnan Province are closely related;it is on Yunnan Province's early summer precipita-tion that they have a great impact.

Modulation of Low-Latitude West Wind on Abnormal Track and Intensity of Tropical Cyclone Nargis (2008) in the Bay of Bengal

Tropical cyclone （TC） Nargis （2008） made landfall in Myanmar on 02 May 2008, bringing a storm surge, major flooding, and resulting in a significant death toll. TC Nargis （2008） displayed abnormal features, including rare eastward motion in its late stage, rapid intensification before landing. Using reanalysis data and a numerical model, we investigated how a low-latitude westerly wind modulated TC Nargis’ （2008） track and provided favorable atmospheric conditions for its rapid intensification. More importantly, we found a possible counterbalance effect of flows from the two hemispheres on the TC track in the Bay of Bengal. Our analysis indicates that a strong westerly wind burst across the Bay of Bengal, resulting in TC Nargis’ （2008） eastward movement after its recurvature. This sudden enhancement of westerly wind was mainly due to the rapidly intensified mid-level cross-equatorial flow. Our results show that a high-pressure system in the Southern Hemisphere induced this strong, mid-level, cross-equatorial flow. During the rapid intensification period of TC Nargis （2008）, this strong and broad westerly wind also transported a large amount of water vapor to TC Nargis （2008）. Sufficient water vapor gave rise to continuously high and increased mid-level relative humidity, which was favorable to TC Nargis’ （2008） intensification. Condensation of water vapor increased the energy supply, which eventuated the intensification of TC Nargis （2008） to a category 4 on the Saffir-Simpson scale.

MESOSCALE ANALYSIS OF YUNNAN SUCCESSIVE HEAVY PRECIPITATION AROUSED BY THE STORM OVER THE BAY IN EARLY SUMMER

By using regular meteorological data, physical quantity fields, satellite pictures and Doppler radar echo data, we analyze the mesoscale features and the conditions of 4 successive heavy precipitation processes in Yunnan aroused by the storm over the Bay in the early summer. The results show that the life of the storm over the Bay is usual 2 or 3 days and the cloud top temperature of the storm is always below -65℃. The storm over the Bay affects Yunnan by mesoscale convective cloud clusters, cloud system in peripheral or weaken itse,fmoving to the northeast. The Tibetan Plateau shear lines and vortexes, NE-SW convergence channels and southwest wind convergence supply favorable circulation background and dynamical conditions. There are many common features about Doppler radar echoes, the flocculent echoes with intensity about 35-45 dBZ move to the east to produce successive precipitation in Yunnan, and the mesoscale features of southwest jet and wind veering with altitude not only are favorable to transport warm and moist airflow brought to the north by the storm over the Bay. but also are favorable to convective development.

ANALYSIS OF AMBIENT FIELDS AND SATELLITE IMAGERY CHARACTERISTICS OF EFFECT OF BAY OF BENGAL STORMS ON LOW- LATITUDE PLATEAU

Based on the composite analysis method, 12 rainstorms triggered by Bay of Bengal storms (shortened as B-storms hereafter) across the whole province of Yunnan were studied, and some interesting results of rain and circulation characteristics influenced by the storms were obtained for low-latitude plateau. Usually, when a rainstorm weather occurs in low-latitude plateau, the B-storm center locates in the central, east or north parts of the Bay of Bengal. At the same time, the subtropical high ridge moves to 15°N– 20°N and the west ridge point moves to the Indo-china Peninsula from the South China Sea and the low-latitude plateau is controlled by southwest air streams coming from the front of the trough and the periphery of the subtropical high. The southwest low-level jet stream from the east side of the bay storm has great effect on heavy rains. On the one hand, the southwest low-level jet stream is playing the role of transporting water vapor and energy. On the other hand, the southwest low-level jet stream is helpful to keep essential dynamical condition. From the analysis of the satellite cloud imagery, it is found that mesoscale convection cloud clusters will keep growing and moving into the low-latitude plateau to cause heavy rains when a storm forms in the Bay of Bengal.

Estimate of contribution of near-inertial waves to the velocity shear in the Bay of Bengal based on mooring observations from2013 to 2014

Near-inertial motions contribute most of the velocity shear in the upper ocean.In the Bay of Bengal(BoB),the annual-mean energy flux from the wind to near-inertial motions in the mixed layer in 2013 is dominated by tropical cyclone(TC)processes.However,due to the lack of long-term observations of velocity profiles,our knowledge about interior near-inertial waves(NIWs)as well as their shear features is limited.In this study,we quantified the contribution of NIWs to shear by integrating the wavenumber-frequency spectra estimated from velocity profiles in the upper layers(40-440 m)of the southern Bo B from April 2013 to May 2014.It is shown that the annual-mean proportion of near-inertial shear out of the total is approximately 50%,and the high contribution is mainly due to the enhancement of the TC processes during which the near-inertial shear accounts for nearly 80%of the total.In the steady monsoon seasons,the near-inertial shear is dominant to or at least comparable with the subinertial shear.The contribution of NIWs to the total shear is lower during the summer monsoon than during the winter monsoon owing to more active mesoscale eddies and higher subinertial shear during the summer monsoon.The Doppler shifting of the M_(2)internal tide has little effect on the main results since the proportion of shear from the tidal motions is much lower than that from the near-inertial and subinertial motions.

Anomalies of Precipitation over China on Days with Tropical Cyclone Activity over the Bay of Bengal: Role of Moisture Transport

Tropical cyclones over the Bay of Bengal(BoBTC)affect the precipitation over China,with distinct seasonal and daily variabilities.This study quantitatively examines the daily standardized precipitation anomalies(SPAs)over China on the days with BoBTC activities(storm-days)and related circulations,based on rainfall measurements at surface meteorological stations and ECMWF reanalysis data on a 0.25°×0.25°resolution during 1979-2019.Significant positive SPA is found over the stations in the two adjacent regions around BoB(Southwest China in May/November and southern Tibetan Plateau in October)and three distant regions(Southeast China and the northeastern boundary of the Qinghai-Tibet Plateau in May,and central North China in October).The SPA distributions are remarkably consistent with the integrated water vapor transport(IVT)anomalies.Enhanced IVT is found associated with the interaction between southwesterly(southerly)of the BoBTC circulation and low-level monsoonal flow in May(midlevel westerly in winter months).The probabilities of extreme precipitation(EP)occurrences over the above regions all increase on storm-days.For adjacent regions,EP is significantly correlated with the northward IVT anomalies to the east of BoBTC circulation,which strengthen the water vapor input through the southern border.Such IVT anomalies are stronger in May,benefited by the deep monsoonal southwesterlies than those in November.For distant regions,EP is more closely related to the IVT anomaly extending back from BoB.Enhanced moisture from BoB concentrates along a local low-level convergence line over Southeast China,being further facilitated by coexistence of the BoBTC depression and midlevel westerly trough in midlatitudes.Our results highlight the interactions between BoBTCs and local weather systems that influence the general precipitation anomalies and occurrence of EP over China,especially over distant regions.

Synergistic Effects of Bay of Bengal Tropical Cyclones and Tibetan Plateau Vortices on Water Vapor Transport over the Tibetan Plateau in Early Summer

The Bay of Bengal(BoB)tropical cyclones(TCs)and the Tibetan Plateau vortices(TPVs)are two crucial weather systems influencing the Tibetan Plateau(TP).Their synergistic effects can lead to widespread heavy precipitation events on the TP.In this study,we employ the Hybrid Single-Particle Lagrangian Integrated Trajectory(HYSPLIT)model to track the trajectory of water vapor transport during three large-scale precipitation events on the TP under the combined influence of BoB TCs and TPVs.The results indicate that low-level water vapor from the BoB under the influence of BoB TCs was cyclonically entangled into the cyclonic circulation,lifted and transported northward by southwesterly flow to the southeastern part of the TP,which contributes to the moistening of the entire troposphere there.Additionally,convergence of the cyclonic circulation of the TPVs on the northern TP further transports water vapor collected in the southeastern TP northward,conducive to the maintenance and development of precipitation systems,thus inducing widespread heavy precipitation events over the TP.

Forecasting of tropical cyclones ASANI(2022)and MOCHA(2023)over the Bay of Bengal-real time challenges to forecasters

This study examines the track and intensity forecasts of two typical Bay of Bengal tropical cyclones(TC)ASANI and MOCHA.The analysis of various Numerical Weather Prediction(NWP)model forecasts[ECMWF(European Centre for Medium range Weather Forecast),NCEP(National Centers for Environmental Prediction),NCUM(National Centre for Medium Range Weather Forecast-Unified Model),IMD(India Meteorological Department),HWRF(Hurricane Weather Research and Forecasting)],MME(Multi-model Ensemble),SCIP(Statistical Cyclone Intensity Prediction)model,and OFCL(Official)forecasts shows that intensity forecasts of ASANI and track forecasts of MOCHA were reasonably good,but there were large errors and wide variation in track forecasts of ASANI and in intensity forecasts of MOCHA.Among all model forecasts,the track forecast errors of IMD model and MME were least in general for ASANI and MOCHA respectively.Also,the landfall point forecast errors of IMD were least for ASANI,and the MME and OFCL forecast errors were least for MOCHA.No model is found to be consistently better for landfall time forecast for ASANI,and the errors of ECMWF,IMD and HWRF were least and of same order for MOCHA.The intensity forecast errors of OFCL and SCIP were least for ASANI,and the forecast errors of HWRF,IMD,NCEP,SCIP and OFCL were comparable and least for MOCHA up to 48 h forecast and HWRF errors were least thereafter in general.The ECMWF model forecast errors for intensity were found to be highest for both the TCs.The results also show that although there is significant improvement of track forecasts and limited or no improvement of intensity forecast in previous decades but challenges still persists in real time forecasting of both track and intensity due to wide variation and inconsistency of model forecasts for different TC cases.

Can the tropical storms originated from the Bay of Bengal impact the precipitation and soil moisture over the Tibetan Plateau?

This study investigates the impacts of tropical storms originated from the Bay of Bengal(BOBTSs) on the precipitation and soil moisture over the Tibetan Plateau(TP) in April–June(AMJ) and September–December(SOND) during 1981–2011 based on the best track dataset provided by Joint Typhoon Warning Centre(JTWC). Results indicate that there are about 1.35 BOBTSs influence the TP in each year and most of them occurred in May and October, and the BOBTSs in AMJ influence the TP with larger extension and higher latitudes than those in SOND. The maximum regional precipitation induced by the BOBTSs accounts for more than 50% for the total precipitation in the corresponding month and about 20% for the season. Further analysis reveals that the surface soil moisture anomalies induced by the BOBTSs can persist only 20–25 days in AMJ, and the case is also true for the snow depth in SOND. Numerical simulations by using the regional climate model of Weather Research and Forecasting(WRF) suggest that the soil moisture anomalies in the sub-surface can last 2 months whereas for the surface it can persist only about 20 days, which agrees well with the observation analysis. Overall, the effect of the preceding BOBTSs on the snow depth and soil moisture anomalies over the TP cannot maintain to summer, and there is no robust connection between the BOBTSs and summer precipitation anomalies in East China. Moreover, since the mid-1990 s, the spring rainfall induced by the BOBTSs over the TP seems to be enhanced to a certain degree because of the intensified BOBTSs.

Southwesterly Water Vapor Transport Induced by Tropical Cyclones over the Bay of Bengal during the South Asian Monsoon Transition Period

Based on best-track,outgoing longwave radiation,sea surface temperature,and reanalysis data during 1979-2018,statistical and composite analyses were performed to investigate characteristics of the southwesterly water vapor transport(WVT)induced by Bay of Bengal(BoB)tropical cyclones(TCs)during the South Asian summer monsoon(SASM)transition period.The results show that the BoB TCs mainly occur several days before/after the date of SASM onset(retreat)in May(October-November),thereby imposing an important impact on southwesterly WVT to China in those two periods.The WVT is significant in the middle and lower troposphere during the bimodal peak periods of BoB TCs,with large values over the east-central BoB,southeastern Tibetan Plateau,and Southwest and South China.The WVT is located more northward at 500 hPa than at 700 hPa,reaching close to 35°N and covering the southeastern Tibetan Plateau owing to weakening of the plateau’s blocking effect at upper levels.The BoB TCs mainly increase the northward and eastward WVT anomalies compared to the climatological mean.Furthermore,the large southerly WVT anomalies are located in the lower troposphere in low-latitude areas,while the large westerly WVT anomalies appear mainly in the middle troposphere in high-latitude areas.This indicates an enhanced WVT channel where the southwesterly moisture jet first climbs northwards to the southeastern Tibetan Plateau and then turns eastwards to East China under the influence of the BoB TCs.Besides,the southwesterly WVT during the TC period in May is stronger and more widespread than that in October-November,being about twice the latter in value.However,their maximum contributions to the climatological average do not differ much,with maximums of 12%and19%occurring in Southwest China during the bimodal periods,respectively,implying an important role played by the BoB TCs in the WVT.

Characteristics of Precipitation in China Associated with Tropical Cyclones over the Bay of Bengal

In comparison with the number of studies into the impact on precipitation in China of tropical cyclones(TCs)over the western North Pacific,investigation of the effect of TCs in the Bay of Bengal(BoB)on precipitation in China is lacking.In this study,precipitation in China associated with TCs over the BoB was divided into direct rainfall induced by TC cloud clusters and indirect rainfall related to the long-distance transport of TC water vapor.We partitioned the BoB TC-related rainfall that occurred during 2000–2018 in China and analyzed its statistical features.It was found that 40 of the 67(60%)TCs that occurred over the BoB exerted influence on rainfall in China.Direct rainfall was mainly distributed over the southeastern Tibetan Plateau and Southwest China,while indirect rainfall was distributed widely with two high-value centers:one over Yunnan Province and the other over the area south of the middle–lower reaches of the Yangtze River.The highest mean daily rainfall amount of direct TC precipitation appeared in northern Yunnan Province and southeastern Tibet,while that of indirect TC precipitation occurred eastward of 110°E.During the bimodal period of peak occurrence of BoB TCs in May and October–November,the annual mean amount,intensity,and number of days of rainfall in China related to BoB TCs were generally greater in May,e.g.,the mean daily rainfall amount was twice that in October–November,except at stations in southeastern Tibet.In comparison with the BoB TCs that induced heavy rainfall in China in early summer,the TCs in autumn had a more southwestward mean position and a more asymmetric structure,with the long axis oriented in the northeast–southwest direction.Heavy rainfall induced by BoB TCs occurred mainly over southeastern Tibet and provinces south of the middle–lower reaches of the Yangtze River in early summer and over Yunnan and Sichuan provinces in autumn,mostly in response to circulation patterns of a“northern trough with a southern TC”and of“convergence of two high pressure systems,”respectively.

ROLE OF SEA SURFACE TEMPERATURE IN MODULATING LIFE CYCLE OF TROPICAL CYCLONES OVER BAY OF BENGAL

Sea surface temperature(SST) varies significantly in the presence of tropical cyclones(TCs). Using fixed SST throughout the integration of high resolution TC models is general practice in research and operational endeavor over the North Indian Ocean. The present study is to assess the impact of updating realistic SST in TC lifetime on track, intensity and rainfall of TCs. The Hurricane Weather Research Forecast(HWRF) model of single domain with 9 km resolution is used. A total of 31 forecast cases are considered from 6 TCs during 2007-16 with unique track and intensity characteristics. A set of two numerical experiments are done without(CNTL) and with 6-hourly SST update(SST) in TC lifetime.Mean track and intensity errors show that there is an improvement of 3–41% in track during 12–120 h forecast length for SST run. The SST runs improve landfall position and time prediction by 20% and 33% respectively. The cross track error of SST run is comparable(44 km) with average errors available for this basin(34 km);and along track errors are improved by 60% as compared to CNTL as well as average errors of the basin. The model is biased to overestimate a weaker TC and underestimate a stronger TC, however, the bias is reduced in SST run by 5–51%. The analyses of wind, enthalpy flux and warm core structures provide insight for realistic intensity prediction of SST run unlike CNTL. Rainfall intensity and radial distribution is also improved in SST run. Thus, this study highlights the significance of ocean coupling with TC models to advance forecast guidance.

Water vapor image of snow storm process over southern Tibet

The northward movement of the storm over Bengal Bay was the main weather system producing heavy snow over southern Tibet in Nov. 1995. The effect of the Tibetan Plateau on the track of the storm over Bengal Bay and its cloud system was discussed by analyzing the GMS-5 water vapor image. It is estimated from this discussion that the altitude of the Plateau obstruct effect on synoptic systern can reach up to 300 hPa.

An Evaluation of the Advanced Dvorak Technique (9.0) for the tropical cyclones over the North Indian Ocean

The Advanced Dvorak Technique(ADT)is used by tropical cyclone prediction centres around the world to accurately evaluate the intensity of tropical cyclones(TCs)from meteorological operational satellites.The algorithm development team has introduced new improvements to the objective ADT to further extend its capabilities and accuracy.A study has therefore undergone to evaluate the new edition of ADT(9.0)based on all the North Indian Ocean Tropical cyclones during 2018,2019 and 2020(Total 15 No.).It is found that ADT(9.0)performed well with the conformity of IMD’s best track T.No estimates.ADT is reasonably good in estimating the intensity for T≥4.0(VSCS to SuCS)and overestimate the intensity for T≤3.5(CS/SCS).

亚洲夏季风爆发前后西北太平洋和孟加拉湾热带气旋活动统计特征

亚洲夏季风爆发始于孟加拉湾,然后向中国南海和印度次大陆扩展,其过程约持续1个月。各地区夏季风爆发时间呈明显的年际变化。利用热带气旋资料和气象再分析资料,统计了1951—2010年孟加拉湾和中国南海夏季风爆发前后西北太平洋热带气旋、孟加拉湾气旋风暴活动和夏季风爆发的关系。结果表明,在孟加拉湾夏季风爆发过程中,共有36a出现孟加拉湾气旋风暴,并且夏季风爆发偏早年出现风暴的几率最高,为80%。在孟加拉湾夏季风爆发偏早、正常和偏晚3种类型中,孟加拉湾风暴活动频率高峰期多出现在夏季风爆发前后几天内。并且在孟加拉湾风暴活动频率高峰出现前期,西北太平洋热带气旋最先出现活动频率高峰。孟加拉湾夏季风爆发前有40%—50%的年份西北太平洋出现热带气旋活动,其中,夏季风爆发偏早年,爆发前西北太平洋热带气旋活跃的时间偏早(4月第2候),且多活动在中国南海和菲律宾附近;爆发正常年,西北太平洋热带气旋活跃的时间为4月第4候,多活动在略偏东的海域;爆发偏晚年,西北太平洋热带气旋活跃的时间为5月初,活动区域最偏东。中国南海夏季风爆发过程中,60a中共有29a西北太平出现热带气旋,其中爆发偏早和正常年出现热带气旋的频率较高,并且热带气旋多出现在爆发当日和爆发后一段时间。整体来看,亚洲夏季风爆发前,西北太平洋热带气旋活动频率最先开始增强,然后孟加拉湾风暴开始活跃并伴随着孟加拉湾夏季风爆发,夏季风爆发偏早和正常年,孟加拉湾夏季风爆发后,西北太平洋热带气旋再次增强,中国南海夏季风爆发。

孟加拉湾热带风暴活动路径的客观分类

利用热带气旋路径客观分析方法，将1972—2011年间共131个孟加拉湾热带风暴的移动路径分为3类：西行类（A类）、北行类（B类）和西北行类（C类）。类型A主要生成于孟加拉湾西南部，占全部TS的16％（21次），季节分布为单峰型，主要在秋季，有90％（18次）登陆，登陆点大部分为孟加拉湾海岸西段（85°E以西），有5％达到2级飓风以上强度，对西藏基本没有影响；类型B主要生成于孟加拉湾中部，约占全部TS的56．6％（74次），是主要的风暴移动路径，呈双峰型季节分布，是春季Ts的主要路径类型，有91％（67次）登陆，登陆点大部分为盂加扎湾海岸中段（85～95°E），有19％达到2级飓风以上强度，对西藏影响最大，是造成西藏强降水的主要TS路径类型；类型C主要生成于孟加拉湾南部，约占全部Ts的27．5％（36次），主要形成于秋季，是冬季Ts的主要路径类型，有64％（23次）登陆，大部分在西段登陆，持续时间较长，对西藏影响很小。