3-D Hydrographic Characteristics Of Tropical East Indian Ocean During The Monsoon Transition Period

As a monsoon ocean,the tropical East Indian Ocean(TEIO)is bounded by the Asian continent in the north,and has unique temperature,salinity and current structures.It plays crucial role in both the regional and global climate changes.The3 D characteristics of the circulation and the water mass distribution in this area need to be further investigated,especially that during the monsoon conversion period.Using in-situ cruise observations in March-May,2018,satellite remote sensing data,CMEMS monthly velocity data and WOA18 climatological data,this study analyzes the 3D structure of the currents and distribution of water masses in the TEIO during the monsoon conversion period with the spicity and volume chart method.The main conclusions are as follows:(1)Climatologically,the surface winds in the study area changes from northeasterly wind to southwesterly wind during the period of March-May,when the sea surface height,salinity,and temperature increase,and evident Kelvin wave signals appear at the equator and the eastern boundary area.The surface circulation is cyclonic in the north of the equator and south of the Bay of Bengal,but anticyclonic in the Bay area.The eastward flows dominate in the equator area,where the Wyrtki jets(WJs)are in the surface layer and Equatorial Undercurrent(EUC)in sub-surface layer,and the Equatorial Countercurrent(ECC)on the south.The WJs and the EUC have interannual variations of 2–6years due to the effects of the IOD and ENSO.The surface temperature in the TEIO is mainly determined by the solar radiation and the wind intensity,while the current advection affect the temperature variation in the subsurface layer.In the deeper layer,the temperature south of the equator is lower than that north of the equator.The thermocline thickness and the maximum buoyant frequency get decreased from March to May,while the thermocline depth increases gradually.The salinity in the surface and the sub-surface layers in the west is higher than that in the east,which is determined by the current fields,and the distribution of salinity in the the middle depths are similar with that of temperature.(2)During the observation time in 2018,the surface flow was weak and the WJs did not appear,while the EUC in the sub-surface layer was strong with maximum velocity reaching 0.9m/s.In the intermediate layer,southwestward flow crossing the equator appeared,and the zonal flows have opposite directions in two sides of the equator.At the east boundary area,alternative flow pattern appeared in the surface and the sub-surface layers due to the influences of the Kelvin wave,while polarward appeared along the boundary in the intermediate layer.In the Bay of Bengal,there were anticyclonic circulation during the observation with northeastward current at the bay mouth and strong southwestward current in the western boundary west of Sri Lanka.The surface temperature in the study area achieved 32? during the observation,with the mixing layer less than 50m.The middle depth of the thermocline is mostly distributed between 80-120m,with the thickness of about 70-150m,in which the thermocline thickness on the east part of the equator and the east boundary has the minimum value.The EUC and ECC have large influences on the thermocline structure around the equator.The stratification at the equator is the strongest with values achieving 10-2.4 s-1,and it gradually decreases from the equator to the polar sides.The salinity distribution in the surface and sub-surface layers were complicated.At the east boundary,Kelvin waves have large influences on the water temperature,salinity,thermocline and stratification intensity.(3)According to the water mass analysis with the ?-? and volume chart methods,nine water masses are identified in the TEIO,which are waters originating from the Bay of Bengal Water Mass(BBW),the Eastern Indian Ocean Water Mass(EIW),the Western Indian Ocean Surface Water Mass(WIW),the Asian– Australian Intercontinental Water Mass(AANW)and the Indian Ocean Central Water(NICW).The calculated thermodynamic distances indicate that the water mass in the mouth of the Bay of Bengal may originate from the WIW carried by the ECC.This provides further evidence that the ECC could cross in the equator to the Bay of Bengal.