Monsoon driven water mass exchange between the Bay of Bengal (BoB) 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 BoB 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 BoB to AS along the western boundary of the BoB during northeast monsoon. In the same way, reverses the phenomena during southwest monsoon, transporting high salinity warm water from AS to the BoB. 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.
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 cells 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 El Niño-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.