Characteristics And Mechanisms Of The Intraseasonal And Interannual Variability Of Sea Surface Salinity In The Bay Of Bengal And Its Surrounding Areas

Based on SMAP(Soil Moisture Active Passive)and SODA(Simple Ocean Data Assimilation)salinity data,combined with OAFlux(Objectively Analyzed Air-Sea Heat Flux)sea surface heat flux and other model data,this work explores the characteristics and mechanisms of the intraseasonal and interannual variability of sea surface salinity(SSS)in the Bay of Bengal(BOB)and its surrounding areas.The characteristics and mechanisms of interannual variability of SSS in the BOB are investigated by SODA model data.First,the impact of El Ni?o-Southern Oscillation(ENSO)on SSS in the BOB is divided into two main patterns: the northeast monsoon pattern(October-March)and the following southwest monsoon pattern(May-October).The influence of the northeast monsoon type is mainly located at the equator,east of the bay estuary and the Andaman Sea,showing a significant negative correlation and reaching its peak in November-January.The significant impact(negative correlation)of the following year’s southwest monsoon pattern is mainly located in the southern part of the BOB and reaches its peak in July-September of the following year.The composite analysis of El Ni?o shows that the SSS during the peak period of the Northeast monsoon is generally negative,and the salinity advection transport is the key reason.Under the effect of anticyclonic circulation,the low salinity water in the northern part of the BOB is transported southwards,extending to the eastern part of the estuary and the Andaman Sea,and then expanding westwards along the western current in the equator,resulting in a low salinity during the peak period.During the peak period of the following southwest monsoon type,the SSS in the central and southern part of the BOB shows negative anomalies,while the northern part of the BOB shows positive anomalies.During the same period,the distribution of freshwater flux plays an important role in determining the spatial variability of salinity characteristics in the Bay.In addition,the western branch of the Sri Lankan cold eddy contributes to the southward transport of low salinity water from the northern part of the Bay.The results of the La Ni?a composite analysis show that during the peak period of the northeast monsoon type,SSS in the central and southern part of the BOB shows positive anomalies and negative anomalies in the northern part of the BOB.Changes in sea surface freshwater flux during the same period explain the decrease in salinity in the northern part of the BOB and the increase in salinity on the western side of the bay.Subsequently,the high salinity on the western side is transported eastwards to the eastern part of the BOB estuary with the anomalously strong eastward current,which then extends to the Andaman Sea along the anomalously strong northward current.This resulted in anomalously strong positive anomalies in SSS near the equator,the eastern side of the Bay estuary and the Andaman Sea.SSS is generally positively anomalous during the southwest monsoon type peak in the following year.The spatial pattern of sea surface salinity in the BOB is mainly in responding to freshwater flux.In addition,the role of horizontal advection along the southwest monsoon current path in southeastern Sri Lanka is also important.The results of mixed layer salinity budget show that the salinity advection contributes significantly to the negative(positive)salinity anomaly during the northeast monsoon type peak of the El Ni?o(La Ni?a)event,while the freshwater flux determines the spatial pattern of the salinity trend during the southwest monsoon type peak of the following year,and the advection term also plays a more important contribution in some of the areas.Then,the impact of Indian Ocean Dipole(IOD)on the interannual variability of SSS in the BOB is characterized by two different phases: Phase I-occurs at the beginning of the IOD event(March-June);Phase II-occurs at the peak of the IOD event(August-December).In Phase I,the effect of IOD on SSS is mainly in the area north of 10?N in the BOB,showing a significant negative correlation,and reaches its peak in April-June.In Phase II,the significant effect of IOD on SSS(negative correlation)is mainly located from eastern Sumatra to the Andaman Sea,and reaches its peak in September-November.During the peak of Phase I of the positive IOD year,the distribution of salinity in the BOB north of 10?N shows a negative anomaly.The negative freshwater flux in the northern part of the BOB leads to a decrease in SSS in the northern part of the BOB,indicating that freshwater flux contributes significantly to the interannual variability of SSS in this area.In addition,salinity advection also plays a role.During the peak of Phase II in positive IOD years,the SSS from eastern Sumatra to the Andaman Sea shows negative anomalies,with the salinity advection transport process being the main reason.Finally,by using SMAP remote sensing data,this study also analyses the characteristics and mechanisms of the intraseasonal variability of SSS in the Southeastern Arabian Sea(SEAS).The results show that the intraseasonal variability of SSS in the SEAS is very significant,especially the strongest intraseasonal signal in SSS,which is located along the northeast monsoon current(NMC)path south of the Indian Peninsula.There are remarkable seasonal differences in intraseasonal SSS variability,which is very weak in spring and summer and much stronger in autumn and winter.This strong intraseasonal variability in autumn and winter is closely related to the Madden-Julian Oscillation(MJO)event during this period.The northeast wind anomaly in the Bay of Bengal(BOB)associated with the active MJO phase strengthens the East India Coastal Current(EICC)and NMC and consequently induces more BOB low-salinity water to enter the SEAS,causing strong SSS fluctuations.In addition,MJO-related precipitation further amplifies the intraseasonal variability of SSS in SEAS.Based on budget analysis of the mixed layer salinity using HYCOM data,it is shown that horizontal salinity advection(especially zonal advection)dominates the intraseasonal variability of mixed layer salinity and that surface freshwater flux has a secondary role.