Contribution of the winter salinity barrier layer to summer ocean-atmosphere variability in the Bay of Bengal

It is found that the winter(December-February)barrier layer(BL)in the Bay of Bengal(BoB)acts as a dynamical thermostat,modulating the subsequent summer BoB sea surface temperature(SST)variability and potentially affecting the Indian summer monsoon(ISM)onset and associated rainfall variability.In the years when the prior winter BL is anomalously thick,anomalous sea surface cooling caused by intensified latent heat flux loss appears in the BoB starting in October and persists into the following year by positive cloud-SST feedback.During January-March,the vertical entrainment of warmer subsurface water induced by the anomalously thick BL acts to damp excessive cooling of the sea surface caused by atmospheric forcing and favors the development of deep atmospheric convection over the BoB.During March-May,the thinner mixed layer linked to the anomalously thick BL allows more shortwave radiation to penetrate below the mixed layer.This tends to maintain existing cold SST anomalies,advancing the onset of ISM and enhancing June ISM precipitation through an increase in the land-sea tropospheric thermal contrast.We also find that most of the coupled model intercomparison project phase 5(CMIP5)models fail to reproduce the observed relationship between June ISM rainfall and the prior winter BL thickness.This may be attributable to their difficulties in realistically simulating the winter BL in the BoB and ISM precipitation.The present results indicate that it is important to realistically capture the winter BL of the BoB in climate models for improving the simulation and prediction of ISM.

Seasonal influence of freshwater discharge on spatio-temporal variations in primary productivity, sea surface temperature, and euphotic zone depth in the northern Bay of Bengal

Ocean productivity is the foundation of marine food web,which continuously removes atmospheric carbon dioxide and supports life at sea and on land.Spatio-temporal variability of net primary productivity(NPP),sea surface temperature(SST),sea surface salinity(SSS),mixed layer depth(MLD),and euphotic zone depth(EZD) in the northern B ay of Bengal(BoB) during three monsoon seasons were examined in this study based on remote sensing data for the period 2005 to 2020.To compare the NPP distribution between the coastal zones and open BoB,the study area was divided into five zones(Z1-Z5).Results suggest that most productive zones Z2 and Zl are located at the head bay area and are directly influenced by freshwater discharge together with riverine sediment and nutrient loads.Across Z1-Z5,the NPP ranges from 5 315.38 mg/(m^(2)·d) to 346.7 mg/(m^(2)·d)(carbon,since then the same).The highest monthly average NPP of 5 315.38 mg/(m^(2)·d) in February and 5 039.36 mg/(m^(2)·d) in June were observed from Z2,while the lowest monthly average of 346.72 mg/(m^(2)·d) was observed in March from Z4,which is an oceanic zone.EZD values vary from 6-154 m for the study area,and it has an inverse correlation with NPP concentration.EZD is deeper during the summer season and shallower during the wintertime,with a corresponding increase in productivity.Throughout the year,monthly SST shows slight fluctuation for the entire study area,and statistical analysis shows a significant correlation among NPP,and EZD,overall positive between NPP and MLD,whereas no significant correlation among SSS,and SST for the northern BoB.Long-term trends in SST and productivity were significantly po sitive in head bay zones but negatively productive in the open ocean.The findings in this study on the distribution of NPP,SST,SSS,MLD,and EZD and their seasonal variability in five different zones of BoB can be used to further improve the management of marine resources and overall environmental condition in response to climate changes in BoB as they are of utmost relevance to the fisheries for the three bordering countries.

Decadal Changes in Interannual Dependence of the Bay of Bengal Summer Monsoon Onset on ENSO Modulated by the Pacific Decadal Oscillation

Interannual variations of the Bay of Bengal summer monsoon (BOBSM) onset in association with El Ni-o-Southern Oscillation (ENSO) are reexamined using NCEP1, JRA-55 and ERA20C atmospheric and Hadley sea surface temperature (SST) reanalysis datasets over the period 1900-2017. Decadal changes exist in the dependence of the BOBSM onset on ENSO, varying with the Pacific Decadal Oscillation (PDO). A higher correlation between the BOBSM onset and ENSO arises during the warm PDO epochs, with distinct late (early) onsets following El Ni-o (La Ni-a) events. In contrast, less significant correlations occur during the cold PDO epochs. The mechanism for the PDO modulating the ENSO-BOBSM onset relationship is through the variations in SST anomaly (SSTA) patterns. During the warm PDO epochs, the superimpositions of the PDO-related and ENSO-related SSTAs lead to the SSTA distribution of an El Ni-o (La Ni-a) event exhibiting significant positive (negative) SSTAs over the tropical central-eastern Pacific and Indian Ocean along with negative (positive) SSTAs, especially over the tropical western Pacific (TWP), forming a strong zonal interoceanic SSTA gradient between the TWP and tropical Indian Ocean. Significant anomalous lower tropospheric easterlies (westerlies) together with upper-tropospheric westerlies (easterlies) are thus induced over the BOB, favoring an abnormally late (early) BOBSM onset. During the cold PDO epochs, however, the superimpositions of PDO-related SSTAs with El Ni-o-related (La Ni-a-related) SSTAs lead to insignificant SSTAs over the TWP and a weak zonal SSTA gradient, without distinct circulation anomalies over the BOB favoring early or late BOBSM onsets.

An Analysis of the Characteristics of Monsoon Onset over the Bay of Bengal and the South China Sea in 2010

Based on NCEP/NCAR daily reanalysis and the Tropical Rainfall Measuring Mission data, the back- ground atmospheric circulation and the characteristics of meteorological elements during the period of the Bay of Bengal monsoon （BOBM） and the South China Sea （SCS） monsoon （SCSM） in 2010 are studied. The impacts of the BOBM onset on the SCSM onset and the relationship between the two monsoons are also analyzed. The two main results are as follows： （l） The BOBM onset obvi- ously occurs earlier than the SCSM onset in 2010, which is a typical onset process of the Asian monsoon. During the BOBM＇s onset, northward jump, and eastward expansion, convective precipitation and southwest winds occurred over the SCS, which resulted in the onset of the SCSM. （2） The relationship among strong convection, heavy rainfall, and vertical circulation configuration is obtained during the monsoon onsets over the BOB and SCS, and it is concluded that the South Asian High plays an important role in this period.

Cloud Condensation Nuclei over the Bay of Bengal during the Indian Summer Monsoon

The first measurements of cloud condensation nuclei （CCN） at five supersaturations were carried out onboard the research vessel ＂Sagar Kanya＂ （cruise SK-296） from the south to the head-bay of the Bay of Bengal as part of the Continental Tropical Convergence Zone （CTCZ） Project during the Indian summer monsoon of 2012. In this paper, we assess the diurnal variation in CCN distributions at supersaturations from 0.2% to 1% （in steps of 0.2%） and the power-law fit at supersaturation of 1%. The diurnal pattern shows peaks in CCN concentration （NccN） at supersaturations from 0.2% to 1% between 0600 and 0700 LST （local standard time, UTC＋0530）, with relatively low concentrations between 1200 and 1400 LST, followed by a peak at around 1800 LST. The power-law fit for the CCN distribution at different supersaturation levels relates the empirical exponent （k） of supersaturation （%） and the Nccy at a supersaturation of 1%. The Nccy at a supersaturation of 0.4% is observed to vary from 702 cm-3 to 1289 cm-3, with a mean of 961 ＋ 161 cm-3 （95% confidence interval）, representing the CCN activity of marine air masses. Whereas, the mean Nccy of 1628 ＋ 193 cm-3 at a supersaturation of 1% is higher than anticipated for the marine background. When the number of CCN spectra is 1293, the value of k is 0.57 ± 0.03 （99% confidence interval） and its probability distribution shows cumulative counts significant at k ≈ 0.55 ± 0.25. The results are found to be better at representing the features of the marine environment （103 cm-3 and k ≈ 0.5） and useful for validating CCN closure studies for Indian sea regions.

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).

Role of Western Pacific Oceanic Variability in the Onset of the Bay of Bengal Summer Monsoon

The influence of the tropical Indo-Pacific Ocean heat content on the onset of the Bay of Bengal summer monsoon （BOBSM） onset was investigated using atmospheric data from the NCEP and ocean subsurface temperature data from the Japan Metorology Agency （JMA）. Results showed that the onset time of the BOBSM is highly related to the tropical Pacific upper ocean heat content （HC）, especially in the key region of the western Pacific warm pool （WPWP）, during the preceding winter and spring. When the HC anomalies in the WPWP are positive （negative）, the onset of the BOBSM is usually early （late）. Accompanied by the variation of the convection activity over the WPWP, mainly induced by the underlying ocean temperature anomalies, the Walker circulation becomes stronger or weaker. This enhances or weakens the westerly over the tropical Indian Ocean flowing into the BOB in the boreal spring, which is essential to BOBSM onset. The possible mechanism of influence of cyclonic/anti-cyclonic circulation over the northwestern tropical Pacific on BOBSM onset is also discussed.

Effects of monsoon onset vortex on heat budget in the mixed layer of the Bay of Bengal

We investigated the effects of monsoon onset vortex(MOV)on the mixed layer heat budget in the Bay of Bengal(BOB)in spring 2003 using the reanalysis datasets.The results suggest that the solar radiation flux penetrating the mixed layer and the existence of barrier layer are both able to modulate the effects of MOV on the evolution of sea surface temperature(SST)in the BOB.Prior to the formation of BOB MOV,the local SST raised quickly due to mass of solar radiation reaching the sea surface under the clear-sky condition.Meanwhile,since the mixed layer was shallow before the onset of the Asian summer monsoon(ASM),some solar radiation flux could penetrate to directly heat the deeper water,which partly offset the warming effect of shortwave radiation.On the other hand,the in-situ SST started to cool due to the upwelling of cold water when the MOV generated over the BOB,along with the rapidly increased surface wind speed and its resultant deeper mixed layer.As the MOV developed and moved northward,the SST tended to decrease remarkably because of the strong upward surface latent heat flux over the BOB ascribed to the wind-evaporation mechanism.However,the MOV-related precipitation brought more fresh water into the upper ocean to produce a thicker barrier layer,whose thermal barrier effect damped the cooling effect of entrainment upwelling on the decrease tendency of the BOB SST.In other words,the thermal barrier effect could slow down the decreasing trend of the BOB SST even after the onset of ASM,which facilitated the further enhancement of the MOV.

The cold pool of the Bay of Bengal and its association with the break phase of the Indian summer monsoon

During the summer monsoon season, strong coastal upwelling occurs along the southwest coast of India and at the southern tip of India, which cools the surface temperature of the waters around these regions. The summer monsoon current carries the upwelled cold waters into the Bay of Bengal and forms the ‘cold pool of the Bay of Bengal', with its core south of Sri Lanka and over the southcentral Bay of Bengal. The present study focuses on the intrusion of these cold waters into the south of the Bay of Bengal, its interannual variability, and its association with the surface wind during the break phase of the summer monsoon, when strong westerly surface winds flow south of 10°N.The authors hypothesize that the enhanced cooling in the cold pool region during monsoon spells is associated with the strong westerly wind stress there during the break spells of the monsoon.Seven cases of long break monsoon spells that occurred during the nine years from 2001 to 2009 are analyzed, and the results confirm our hypothesis.

The Onset of the Monsoon over the Bay of Bengal:The Year-to-Year Variations

In situ buoy observation data spanning four years(2008-2011) were used to demonstrate the year-to-year variations of the monsoon onset processes in the Bay of Bengal(BoB).A significant early(late) monsoon onset event in 2009(2010) was analyzed in detail.It is found that the year-to-year variations of monsoon onset can be attributed to either the interannual variability in the BoB SST or the irregular activities of the intra-seasonal oscillation(ISO).This finding raises concern over the potential difficulties in simulating or predicting the monsoon onset in the BoB region.This uncertainty largely comes from the unsatisfactory model behavior at the intra-seasonal time scale.

Zooplankton Distribution in Coastal Water off Gopalpur,North-Western Bay of Bengal

An interannual study on zooplankton abundance, biomass, and species composition was carried out during different seasons in two local coastal water types off Gopalpur, north-western Bay of Bengal. Although, Type-1 was observed with higher zooplankton abundance in comparison to Type-2, pattern of variation followed similar seasonal trends in both water types during individual years. Well pronounced seasonality was observed in zooplankton distribution. Zooplankton community was composed of 217 holoplankton and 22 meroplankton. The holoplankton community was predominated by copepod in terms of species diversity and abundance followed by hydrozoa, tintinnida, malacostraca, gastropoda, chaetognatha and chordata. The meroplankton were represented by larval forms viz. bivalve veliger, brachyuran zoea larvae, caridean larvae, copepod nauplii, fish egg and gastropod veliger. Dominance of copepod species viz. Acrocalanus longicornis, Paracalanus aculeatus and Paracalanus parvus were observed frequently in both water types. The species richness was higher in Type-1 in comparison to Type-2 during both the years. Salinity regimes and availability of phytoplankton prey influenced the distribution and species composition of zooplankton assemblage.

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.

Geochemistry of core sediments from the southeastern Bay of Bengal:Inferences on weathering and early diagenetic changes

A sediment core(ABP24/05),collected at a water depth of 3520 m from the southeastern Bay of Bengal was studied to determine the change in chemical weathering during the last glacial to deglacial periods and the factors of sedimentary environment which controlled earliest diagenetic changes in the sediment after its deposition.High ratios of K/Rb,Ti/Al and Zr/Rb during~45 to~18 cal kyr B.P.in the core sediments may be attributed to the stronger physical erosion and turbidity currents activity during this period.This might have brought a higher quantity of unaltered minerals to the study area.Low ratios of K/Rb,Zr/Rb,and Ti/Al and increase of SiO_(2)/TiO_(2),Rb/Al and Cs/Al from~18 cal kyr B.P.to present may be indicating an increase in the rate of chemical weathering during this period.The time of increased chemical weathering in the study area is consistent with deglaciation warming in the tropical Indian Ocean and strengthening of river runoff into the Andaman Sea.Climate change during the interglacial period by increased solar insolation thereby strengthened the summer monsoon which might have led to intensified chemical weathering in the source region since~18 cal kyr B.P.The low organic carbon(OC),high Mn/Al,Fe/Al and the Mn-oxides minerals precipitation indicate prevailing of oxic conditions during~11 cal kyr B.P.in the core sediments,which is contradictory to suboxic conditions developed in the deep ocean sediments in the western Bay of Bengal and the equatorial Indian Ocean.The low terrigenous influx and export of less OC to the bottom sediments might have created a favorable condition for the formation of Mn-oxides in the study area during Holocene.

Numerical study of the seasonal salinity budget of the upper ocean in the Bay of Bengal in 2014

Impact factors on the salinity budget, especially the eddy salt fluxes and smaller-scale diffusive salt fluxes for the upper 50 m of the Bay of Bengal(BoB) in 2014 are investigated using a box model based on the Regional Ocean Modeling System(ROMS) daily outputs. The model results reproduce that the precipitation and river runoff s are the dominant factors modulating the sharp salinity decrease during the summer monsoon season. The analysis shows that the salinity increase after the summer monsoon is mostly due to the meridional advective and diffusive salt fluxes. The vertical advective salt flux, which is sensitive to the different signals of the wind stress curl, plays an important role in balancing the salinity change induced by the meridional advective salt flux during both the summer and winter monsoon seasons. Distinctive spatial mesoscale structures are presented in the eddy salt flux throughout the year, and their contributions are sizeable(over 30% in the meridional direction and about 10%–30% in the vertical direction). The meridional eddy salt flux is larger in the monsoon seasons than that in the inter-monsoon seasons, and in a positive pattern near the western boundary during the winter monsoon and autumn inter-monsoon. The vertical eddy salt flux makes an important contribution to the salinity budget, especially along the coastal area and around the Andaman and Nicobar Islands. The vertical eddy salt flux becomes large when a tropical cyclone passes the area.

Monsoonal impact on circulation pathways in the Indian Ocean

Monsoon driven water mass exchange between the Bay of Bengal(Bo B) and Arabian Sea(AS) is the common experience. However, it is not yet firmly confirmed that the exchange pathway is either passing through southern tip of Sri Lanka or Palk Strait. Local circulation patterns impact the pathways followed by the East Indian Coastal Currents(EICC) that drive exchange, thereby modulating mixing and water mass transformation in the Bay of Bengal around Sri Lanka. In this study, observations from surface drifters were incorporated with the satellite derived data to understand the monsoonal impact on circulation patterns in the Indian Ocean. This was the first multi-national scientific effort which was conducted in the Bo B and AS during 2013 to 2015 to understand the monsoonal impact on circulation patterns in the complex region. The results indicated that seasonally reversing monsoonal currents of southern Sri Lanka, traced by the wintertime freshwater export pathways of the EICC. The deflection of monsoon currents running along the east coast of Sri Lanka by forming cyclonic and anti-cyclonic eddies, which influence the mixing and stirring associated with these flows. Results further indicate the low salinity cold water flows from the Bo B to AS along the western boundary of the Bo B during northeast monsoon. In the same way, reverses the phenomena during southwest monsoon, transporting high salinity warm water from AS to the Bo B. This maintain the bay status which occurred due to freshwater influx from large rivers and high saline water from AS. However, no evidences were observed for the exchange through Palk Strait during the study.Also, there are some mis-matches in in-situ and remotely sensed measurements which imply the necessity of systematic observation system for the complex region as an alternative approach.

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.

Role of mesoscale eddies on atmospheric convection during summer monsoon season over the Bay of Bengal: A case study

Our study aims to understand the variability of oceanic mesoscale eddies during contrasting(2009 and 2013)monsoon seasons and the role of such eddies on atmospheric deep convection over the Bay of Bengal(BoB).Oceanic eddies are detected and tracked using sea surface height anomalies(SSHA),by employing the Okubo-Weiss parameter eddy detection method.Significant differences in the SSHA and eddy activity are evident during the contrasting monsoon years.During the year 2013(2009),anticyclonic eddies are predominant(absent)in the eastern BoB and longer(shorter)lifespans of cyclonic eddies are observed in the northern and western BoB.Analysis of time-longitude SSHA and zonal wind stress reveals the presence of strong(weak)equatorial downwelling Kelvin waves,coastal Kelvin waves and Rossby waves in the BoB during 2013(2009)Indian summer monsoon(ISM).The variability of eddies in the BoB during contrasting monsoons is attributed by the remote forcing effect of planetary scale waves.Our study is extended to investigate the associated atmospheric deep convection over the regions of cyclonic and anticyclonic eddies.Lag-correlations demonstrates that SSHA leads the outgoing longwave radiation by four days over anticyclonic eddy regions.Findings from the present study provide new insight into the internal dynamics of the ocean.

Variability of the Southwest Monsoon since the Last 25 000 Years and Their Possible Causes: Role of Northern Hemisphere versus Tibetan Plateau

High-precision, clay sediment oxygen and hydrogen isotopes analyses of Pleistocene-Holocene deep-sea sediments from the Bay of Bengal （BOB） are presented for the first time. Our study shows that the major source of sediments in the study area, since the last-25 000 years, is likely to be the Higher Himalayan crystalline rocks. Further, the study of these stable isotope data displays the variation of southwest monsoon （SWM） in the BOB region since the last-25 000 years and the cause behind the variation has been interpreted. The δ 18 O values of the clay sediments are compared with δ 18 O values of the BOB seawater. This comparison shows that the clay sediment δ 18 O values of the studied sediment cores temporally vary along with the changes in strength of the SWM. Based on the changes in the clay sediment δ 18 O values of the studied sediment, we evaluate the variance in the SWM since the last 25 000 years in the BOB. Our results are consistent with previous work in the region based on other proxies. To evaluate the factors influencing the intensity of the SWM since the last glacial maxima, we conducted comparative analyses of the studied clay sediment δ 18 O values with δ 18 O values in the Greenland ice cores （GISP2） and Tibetan ice cores （Guliya）. The results from this comparative study show that large-scale changes in the intensity of the SWM since 25 000 years are affected by the climate oscillations of the Northern Hemisphere, but rapid and abrupt fluctuations in the SWM seem to be controlled by the amount of snow cover in the Tibetan Plateau.

孟加拉湾南部冷池的极端事件及其机理初析

本研究利用多源卫星遥感数据对比分析了2018年和2015年孟加拉湾南部冷池的极端异常事件。孟加拉湾南部冷池早期出现在5月,8月和9月强度达到最大,10月开始减弱,随后逐渐消失。数据显示2018年和2015年冷池海表温度距平值分别为-0.55℃和0.43℃,是1993—2018年26年中冷池降温最强和最弱的年份。2018年孟加拉湾南部冷池风应力距平值为0.02 N/m2,而2015年距平值为-0.01 N/m2。2018年Ekman抽吸速度距平在冷池区域为正,2015年为负。经混合层热收支计算得到,造成冷池产生的主要原因是平流效应,且2018年与2015年的夏季(6—8月)降温平流项分别占49.2%和80.7%。冷池极端事件主要发生在6月,2018年6月冷池降温是2015年的2.76倍,并且海表净热通量项和平流项的异常对冷池极端事件的产生起了重要作用。

孟加拉湾西南季风与南海热带季风的气候特征比较

本文运用NCAR/NCEP再分析数据和APHRO_MA_V1003R1降水数据,对比分析了孟加拉湾西南季风和南海热带季风的气候特征异同以及对降水分布的影响,得到如下结论:(1)孟加拉湾西南季风比南海热带季风爆发更早、强度更强、持续时间更久、向北推进更北.(2)孟加拉湾西南季风建立过程缓慢,主要是索马里越赤道西南气流的逐渐加强和热带印度洋ITCZ(赤道辐合带)的逐渐北移;而南海热带季风建立过程迅速,主要是东亚大槽的一次替换过程伴随西太平洋副热带高压的突然东撤和热带西太平洋ITCZ的突然北跳.(3)孟加拉湾西南风纬向分量较强,季风建立前后主要变化在于偏西风的强度;而南海西南风经向分量较强,季风建立后风向突然逆转,东南风由于副高东撤而迅速被西南风取代.(4)孟加拉湾西南季风撤退较快,而南海季风则撤退较慢.(5)根据季风进程将夏季风期划分为季风发展期(5月)、强盛期(6—8月)和减退期(9—10月).其间对流活跃区的发展和推进、季风槽的位置以及对应降水区域均有明显差异.(6)在夏季风期,孟加拉湾和南海经度上分别存在着由ITCZ北抬引起的、在季风槽对流活跃区上升而在南北两侧下沉的、南北对称分布的季风经向次级环流.由于孟加拉湾和青藏高原强大热源的存在,孟加拉湾上升区南北跨度比南海的更大;孟加拉湾经圈环流更加稳定,而南海经圈环流的南北摆动更明显;孟加拉湾上升中心区比南海的偏北;在季风减退期,由于南海ITCZ撤退较慢,其上升区比孟加拉湾上升区偏北.