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.

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.

Stock appraisal for Atlantic tripletail(Lobotes surinamensis;Bloch,1790)in the Bay of Bengal,Bangladesh

The Atlantic tripletail(Lobotes surinamensis)is a high revenue-generating fish species predominantly caught by mechanized artisanal fishers community and the most available member of its family in Bangladesh.This is a ground work of fish stock assessment study in the Bay of Bengal region to explore the life history parameters and associated biomass of this species,using three length-based approaches of TropFishR,the length-based Bayesian biomass estimation(LBB),and Froese’s length based indicators(LBIs).An almost homogenous body growth pattern(b=3.07;R^(2)=0.98)was observed in the length-weight relationship of tripletail.The life history parameters for tripletail,as determined by the von Bertalanffy Growth Function(VBGF)model,were L_(∞)=113.36 cm and k=0.51/a.The length converted catch curve(LCCC)yielded an estimation of the total mortality(Z=1.77/a),with the natural mortality estimated at(M=0.53/a)and the fishing mortality estimated at(F=1.24/a).But,the ratio of mortality(F/M=0.15)by LBB captured the non-fully exploited status of biomass(B/B_(MSY)=2.1).LBI analysis indicated that the tripletail fishery’s spawning stock biomass is greater than the target and limit reference points,indicating a healthy state of biomass.

Inter-annual and seasonal variations in hydrological parameters and its implications on chlorophyll a distribution along the southwest coast of Bay of Bengal

Seasonal and inter-annual variability of hydrological parameters and its impact on chlorophyll distribution was studied from January 2009 to December 2011 at four coastal stations along the southwest Bay of Bengal. Statistical analysis （principal component analysis （PCA）, two-way analysis of variance （ANOVA） and correlation analysis） showed the significant impact of hydrological parameters on chlorophyll distribution in the study area. The ranges of different parameters recorded were 23.8-33.8℃ （SST）, 4.00-36.00 （salinity）, 7.0-9.2 （pH）, 4.41-8.32 mg/L （dissolved oxygen）, 0.04-2.45 μmol/L （nitrite）, 0.33-16.10 μmol/L （nitrate）, 0.02-2.51 μmol/L （ammonia）, 0.04-3.32 μmol/L （inorganic phosphate）, 10.09-85.28 μmol/L （reactive silicate） and 0.04-13.8 μg/L （chlorophyll）. PCA analysis carried out for different seasons found variations in the relationship between physico-chemical parameters and chlorophyll in which nitrate and chlorophyll were positively loaded at PC1 （principal component 1） during spring inter-monsoon and at PC2 （principal component 2） during other seasons. Likewise correlation analysis also showed significant positive relationship between chlorophyll and nutrients especially with nitrate （~0.734）. Distribution of hydrobiological parameters between stations and distances was significantly varying as evidenced from the ANOVA results. The study found that the spatial and temporal distribution of chlorophyll was highly dependent on the availability of nutrients especially, nitrate in the southwest Bay of Bengal coastal waters.

Geochemistry of volcanic glass from Mahanadi offshore region,eastern continental margin of India:Constraints on the contribution of latest Toba super-eruption

The tephra layers in multiple sediment cores from the offshore region of the Mahanadi basin in the northern Bay of Bengal were investigated for possible volcanic sources. The glass shards from those tephra layers were studied for size distribution, texture, and elemental geochemistry to establish chronostratigraphic markers for regional and global Quaternary correlation. The textural features of fine-grained(silty) volcanic glasses suggest the distal source of these tephra deposits. Major element composition with elevated SiO_(2) contents ranging between75%–76% and dominance of K_(2)O(> 4.5%) over CaO(< 0.9%) suggest ashes have originated from siliceous rhyolitic melts, similar to the petrographic composition of tephra from the Toba volcano. The bulk trace element compositions of the same glass shards were comparable with those reported in the youngest Toba tephra reported elsewhere. Likewise, the LREE-dominated chondrite normalized REE profiles of tephra from the Mahanadi basin closely resemble the characteristic REE patterns in Toba ash from other parts of the Indian Ocean and thus confirmed the contribution of the youngest Toba super-eruption for this ash layers.

A statistical analysis of mesoscale eddies in the Bay of Bengal from 22–year altimetry data

Eddy properties in the Bay of Bengal are studied from 22 a archiving, validation and interpretation of satellite oceanographic（AVISO） data using a sea level anomaly（SLA）-based eddy identification. A geographical distribution and an eddy polarity, an eddy lifetime and propagation distances, eddy origins and terminations,eddy propagation directions and trajectories, eddy kinetic properties, the evolution of eddy properties,seasonal and interannual variabilities of eddy activities are analyzed in this area. Eddies exist principally in the western Bay of Bengal and most of them propagate westward. The polarity distribution of eddies shows cyclones prefer to occur in the northwest and south of the Bay of Bengal, while anticyclones mainly occur in the east of the bay. Five hundred and sixty-five cyclones and 389 anticyclones with the lifetime that exceeds 30 d are detected during the 22 a period, and there is a preference for the cyclones for all lifetime and propagation distances. The kinetic properties of all observed eddies show the average amplitude of the cyclones is larger than that of the anticyclones, whereas that is opposite for average radius, and their average velocities are basically the same. Moreover, the evolution of eddies properties reveals that the eddies with a long lifetime that exceeds 90 d have a significant double-stage feature of the former 50 d growth period and the dying period after 50 d. For the seasonal variability of the eddies, the cyclones occur more often in spring while the anticyclones occur more often in summer. The analysis of long-lived eddy seasonal distributions shows that there is the obvious seasonal variation of the eddy activities in the Bay of Bengal. The interannual variability of an eddy number shows an obvious negative correlation with the EKE variation.

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.

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.

A Comparison of the Zooplankton Community in the Bay of Bengal and South China Sea During April-May, 2010

This study compares the structure of the zooplankton community in the Bay of Bengal(BoB) and South China Sea(SCS) during the period of spring inter-monsoon, 2010. A total of 215 species of zooplankton were identified, of which 187 species were present in the BoB and 119 in the SCS. Of the taxonomic groups recorded, Copepoda was the most diverse group in all samples followed by pelagic Tunicata, Siphonophorae and Chaetognatha. Flaccisagitta enflata, Cosmocalanus darwinii, Euchaeta larva, Macrura larva and Candacia truncata were predominant both in the BoB and SCS. Moreover, the distribution of some dominant species differed regionally, such as Cypridina dentata, Pleuromamma robusta and Mesosagitta decipiens only in the BoB, and Pleuromamma gracilis, Neocalanus gracilis and Eudoxoides spiralis in the SCS. The average zooplankton abundance was 33.37 ± 7.19 ind. m^(-3) in the BoB and 35.08 ± 2.07 ind. m-3 in the SCS. Copepoda was one of the most abundant groups in the Bo B and SCS. Based on multivariate analysis, it was possible to distinguish the zooplankton in the BoB and SCS communities at the similarity level of approximately 55%, and the dissimilarity was mainly due to C. dentata, P. robusta, M. decipiens, C. darwinii, N. gracilis and P. gracilis. The relationships between zooplankton and temperature, salinity and chlorophyll a were not statistically significant. Zooplankton community structure in the BoB and SCS was observed to be generally similar in terms of species composition and abundance, but the differences observed may be the result of species-specific geographical distribution and local hydrographic conditions.

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.

Longitudinal patterns of spring-intermonsoon phytoplankton biomass,species compositions and size structure in the Bay of Bengal

Vertical distributions of phytoplankton biomass, compositions and size structure were investigated during the spring-intermonsoon （April 22 to 30） of 2010 along transact 10°N of the Bay of Bengal, northern Indian Ocean. Surface phytoplankton biomass （Chl a） was （0.065±0.009） μg/L, being greater than 80% of which was contributed by pico-phytoplankton （〈3 μm）. The Chl a concen- tration vertically increased to the maximal values at deep chlorophyll maximum （DCM） layer that shoaled eastwards from 75 to 40 m. The Chl a biomass at DCM layer generally varied between 0.2 and 0.4 μg/L, reaching the maximum of 0.56 μg/L with micro-phytoplankton cells （~20 #m） accounting for 58% and nano- （3-20μm） or pico-cells for 15% and 27%, respectively. In particular, the cells concentration coupling well with phosphate level was observed at middle layer （75-100 m） of 87° to 89°E, dominated by micro-cells diatoms （e.g., Chaetoceros atlanticus v. neapolitana, Chaetoceros femur and Pseudonitzsehia sp.） and cyanobacteria （i.e., Trichodesmium hildebrandtii）, With the ceils concentration reached as high as 4.0 × 10^4 and 4.3 × 10^4 cells/L. At the rest of the trans- act however, dinoflagellates （e.g., Amphisdinium carterae and Prorocentrum sp.） were the dominant species, with the cells concentration varying from 0.3×10^3 to 6.8×10^3 cells/L. Our results also indicate that the regulation of large cells （micro-, nano-） on phytoplankton biomass merely occurred at DCM layer of the Bay.

Annual and interannual variations of sea-level anomaly in the Bay of Bengal and the Andaman Sea

Annual and interannual variations of sea-level anomaly （SLA） in the Bay of Bengal and the Andaman Sea are investigated using altimeter - derived SLA data from 1993 to 2003. It is found that the SLA annual variation in the study area can be divided into three phases with distinctive patterns. During the southwest monsoon （May -September）, positive SLA presents in the equatorial region and extends northward along the eastern boundary of the bay, and the SLA distribution in the interior bay appears to be high in the east and low in the west with two cyclonic ceils developing in the north and south of the western bay respectively, between which an anticyclonic cell exists. During the early northeast monsoon （ October - December） , the whole bay is dominated by a large cyclonic cell with the pattern of high SLA in the east and low in the west still retained, and the SLA distribution outside the bay is changed in response to the reversal of the Indian Monsoon Current （IMC） in November. During the late northeast monsoon （January -April） , a large anticyclonic cell of SLA develops in the bay with negative SLA prevailing in the equatorial region and extending northward along the eastern boundary of bay. Therefore, the SLA distribution in the interior bay reverses to be high in the west and low in the east. It is suggested that the SLA annual variation in the bay is primarily driven by the local wind stress curl, involving Sverdrup balance while the abrupt SLA variation during the peak of northeast monsoon may be partly caused by the semiannual fluctuation of wind in the equatorial region. This fast adjustment in the interior bay is induced by the upwelling coastal Kelvin wave excited by the decay of Wyrtki jet during December through January. Besides the annual variation, in the bay, there are obvious SLA fluctuations with the periods of 2 and 3 - 7 a, which are driven by the interannual variability of large - scale wind field in the equatorial region. The coastal Kelvin wave also provides an important link for the SLA interannual variation between the equatorial region and the interior bay. It is found that the E1 Nino -Southern Oscillation （ENSO） -induced influence on the SLA interannual variation in the interior bay is stronger than the Indian Ocean dipole （IOD） with the associated pattern of low sea-level presenting along the periphery of the bay and high sea-level in the northeast of Sri Lanka.

Observations of contrasted glacial-interglacial dissolution of foraminifera above the lysocline in the Bay of Bengal,northeastern Indian Ocean

Site U1446(19°50’N,85°44’E,at water depth 1430 m)was drilled during Expedition 353(Indian monsoon rainfall)of the International Ocean Discovery Program(IODP).It is located in the Mahanadi offshore basin,on the northern Bay of Bengal.Sedimentation rates and contents of biocarbonates are high at this relatively shallow site.Using a micropaleontological approach,we examined planktonic and benthic foraminifera in the upper around 40 m of this site,spanning the last around 190 ka.A striking feature of the foraminiferal record is the occurrence of strong but varying dissolution although the site is located well above the modern lysocline.Such strong dissolution has never been reported in this area.We estimated the flux of foraminifera and quantified the ratio of benthic foraminifera over total foraminifera(benthic/total foraminifera)along with the foraminifer fragmentation index in order to characterize past changes in this above-lysocline dissolution.This study reveals a clear glacialinterglacial contrast,with a stronger dissolution during marine isotope stages(MISs)1 and 5 than during MISs 2–4 and 6.Such a difference in preservation is likely to have a strong impact on geochemical proxies measured on foraminifera.Our new observations call for an in-depth study of the causes of such above-lysocline dissolution in the region,and an evaluation of its impact on the foraminifera-based proxies used for paleoenvironmental reconstruction.

Distribution and generic composition of culturable marine actinomycetes from the sediments of Indian continental slope of Bay of Bengal

Actinomycetes population from continental slope sediment of the Bay of Bengal was studied. Samples were collected during two voyages of FORV Sagar Sampada in 2004 （May-June） and 2005 （July） respectively from 11 transects （each transect had ca. 200 m, 500 m, and 1 000 m depth stations）. The physicochemical parameters of overlying water, and sediment samples were also recorded. The actinomycete population ranged from 5.17 to 51.94 CFU/g dry sediment weight and 9.38 to 45.22 CFU/g dry sediment weight during the two cruises respectively. No actinomycete colony was isolated from stations in 1 000 m depth. Two-way analysis of variance showed significant variation among stations （ANOVA two-way, P〈0.05）, but no significance was found between the two cruises （ANOVA two-way, P〈0.05）. Populations in stations in 500 m depth in both cruises were higher than that of 200 m depth stations with statistically insignificant difference （ANOVA two-way, P〉0.05）. Three actinomycetes genera were identified. Streptomyces was found to be the dominating one in both the cruises, followed by Micromonospora, and Actinomyces. The spore of Streptomyces isolates showed the abundance in spiral spore chain. Spore surface was smooth. Multiple regression analysis revealed that the influencing physico-chemical factors were sediment pH, sediment temperature, TOC, porosity, salinity, and pressure. The media used in the present study was prepared with seawater. Thus, they may represent an autochthonous marine flora and deny the theory of land runoff carriage into the sea for adaptation to the salinity of the seawater and sediments.

Major and trace element geochemistry of the mid-Bay of Bengal surface sediments: implications for provenance

The major and trace elements in 110 surface sediment samples collected from the middle of the Bay of Bengal（mid-Bay of Bengal） are analyzed to investigate provenance. Si levels are highest, followed by Al, and the distributions of these two elements are identical. The average CIA＊（chemical index of alteration） value is 72.07,indicating that the degree of weathering of the sediments in the study area is intermediate between those of sediments of the Himalayan and Indian rivers. Factor analyses and discrimination function analyses imply that the two main provenances are the Himalayan and the Indian continent. The inverse model calculation of the Tinormalized element ratios of the Bay of Bengal sediments indicate an estimated average contribution of 83.5%and 16.5% from the Himalayan and peninsular Indian rivers to the study area, respectively. The Himalayan source contributes more sediment to the eastern part of the study area, whereas the western part receives more sediment from the Indian Peninsula than did the eastern part. The primary mechanisms for deposition of sediments in the study area are the transport of Himalayan matter by turbidity currents and river-diluted water and the transport of Indian matter to the study area by a surface circulation in the Bay of Bengal, particularly the East India Coastal Current.

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.

Intensity of the Trough over the Bay of Bengal and Its Impact on the Southern China Precipitation in Winter

In this study, the intensity of the trough over the Bay of Bengal (BBT) and its association with the southern China precipitation, the Madden-Julian Oscillation (MJO) and the Rossby wave propagation along the African-Asian subtropical Jet stream (AASJ) are investigated on the intraseasonal time scale. The results show that the intensity of the BBT affects the southern China precipitation more directly and to a greater degree than the MJO. The peak amplitude of the BBT tended to occur in phase-3 of the MJO. The strong BBT was substantially modulated by the Rossby wave propagation along the AASJ, which was triggered by the anomalous upstream circulation similar to the pattern of the North Atlantic Oscillation (NAO). Therefore, from the perspective of medium- and extended-range weather forecasts, the NAO- like pattern may be regarded as a precursory signal for the strong BBT and thus the southern China precipitation.

Upper ocean high resolution regional modeling of the Arabian Sea and Bay of Bengal

In this paper, effort is made to demonstrate the quality of high-resolution regional ocean circulation model in realistically simulating the circulation and variability properties of the northern Indian Ocean(10°S–25°N,45°–100°E) covering the Arabian Sea(AS) and Bay of Bengal(BoB). The model run using the open boundary conditions is carried out at 10 km horizontal resolution and highest vertical resolution of 2 m in the upper ocean.The surface and sub-surface structure of hydrographic variables(temperature and salinity) and currents is compared against the observations during 1998–2014(17 years). In particular, the seasonal variability of the sea surface temperature, sea surface salinity, and surface currents over the model domain is studied. The highresolution model's ability in correct estimation of the spatio-temporal mixed layer depth(MLD) variability of the AS and BoB is also shown. The lowest MLD values are observed during spring(March-April-May) and highest during winter(December-January-February) seasons. The maximum MLD in the AS(BoB) during December to February reaches 150 m (67 m). On the other hand, the minimum MLD in these regions during March-April-May becomes as low as 11–12 m. The influence of wind stress, net heat flux and freshwater flux on the seasonal variability of the MLD is discussed. The physical processes controlling the seasonal cycle of sea surface temperature are investigated by carrying out mixed layer heat budget analysis. It is found that air-sea fluxes play a dominant role in the seasonal evolution of sea surface temperature of the northern Indian Ocean and the contribution of horizontal advection, vertical entrainment and diffusion processes is small. The upper ocean zonal and meridional volume transport across different sections in the AS and BoB is also computed. The seasonal variability of the transports is studied in the context of monsoonal currents.

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.