Seasonal Variation Of The Upper Ocean Circulation In The Eastern Tropical Indian Ocean And Its Water Exchange With The Bay Of Bengal

In this paper, we first analyze seasonal variations of upper ocean circulation in theeastern tropical Indian Ocean, and discuss the possible pathways of seasonalwater-exchange between the bay and outside ocean using the circulation patterns. Then,we analyze the thermohaline structure of the upper ocean, the spatial distribution ofArabian Sea water (ASW) and Bay of Bengal water (BBW) and its seasonal variation inthe eastern tropical Indian Ocean, basing on which together with the circulation patterns,the expansion pathways of the these two watermasses are further revealed. Finally, weselect6oN section in the bay mouth to analyze the water exchange processes betweenthe bay and outside ocean using the net salt flux and freshwater transport formulae.The results indicate that as driven by the Indian monsoon, the upper oceancirculation of the study area shows a significant seasonal variation. At the equator andsouth of it, the Wyrtki Jet and South Equatorial Countercurrent change periodically,while South Equatorial Current moves meridionally. At the north, it shows a seasonalreversal of the large-scale circulation. During the summer monsoon, a large-scaleanti-cyclonic circulation appears, which is composed by several currents such as thesoutheastward West Indian Coastal Current, the eastward Southwest Monsoon Current,northward East Indian Coastal Current. The circulation reverses to cyclonic one duringthe winter monsoon. In step with the variations of the large-scale circulation, the currentin the eastern bay mouth is southward (northward) in the southwest monsoon (northeastmonsoon). The directon of the current in the western bay mouth is almost opposite tothat in the east. This circulation patterns suggest that, the waters in the outer oceancould enter the bay through the western of the bay mouth during the summer monsoon,and the waters in the interior of the bay is bound to travel southward into the equatorialIndian Ocean via the eastern part. The water-exchange pattern in the study region isreverse during the winter monsoon.The result of watermass analysis shows that in the outer bay, BBW mainly appearsin the Arabian Sea, the equator and off Sumatra. Its distribution range changes seasonally. From November to the proceeding March, it mainly distributes in thesoutheast of Arabian Sea, while it appears at the equator and off Sumatra during Juneand October, which persists to the proceeding February. The distribution range of ASWat the equator changes semiannually. It extends eastward along the equator, as far as90oE in the May-June and October-the proceeding January. Besides, ASW also appearsin the bay at the most time (June-the proceeding April). The seasonal variations of thedistribution of the two watermasses are mainly determined by the upper oceancirculation. During November and the proceeding March, southward East India CoastalCurrent along the western boundary of the bay and the coast of Sri Lanka and thesouthwestward northeast monsoon current are two main pathways for transportingBBW from the interior bay to the southeast of Arabian Sea. During June and October,BBW is carried southward into the equator and off the Sumatra via the eastern baymouth by the continuously southward current between the bay and the equator. Thesemiannual variations of the ASW distribution range at the equator is related with thespring and fall Wyrtki Jet. There are two pathways for ASW to invade into the bay.During November and the proceeding April, ASW is carried into the bay through theeastern bay mouth by the northward flow located between the bay and the equator. It istransported into the bay by the summer monsoon current during June and September.The similar transport pattern of the two watermasses is revealed in the cruiseobservations during the southwest monsoon transition period,2011.The result of the net salt flux calculation shows that the salt exchange processbetween the bay and outside ocean is mainly controlled by the currents, and its directionis almost opposite between the east and west sides in the bay mouth. During thesouthwest monsoon (northeast monsoon), the net salt flux is northward (southward) inthe west while it is southward (northward) in the east. The annual outputs freshwaterfrom the bay through6oN section is0.1Sv, which is calculated using the freshwatertransport formula.