The Meirdional Structure And Seasonal Dynamics Of Meridional Currents And Heat Transports At Central Equatorial Indian Ocean

RAMA ADCP current data near0°,80.5°E from August2008to August2012areanalyzed in this study. The annual mean and seasonal cycle of meridional currents andmass transports at the central equatorial Indian Ocean are presented with strongobservational evidences. To investigate the dynamics of meridional currents in theupper equatorial ocean, a derivation from the vorticity equation is made and atheoretical framework is created to explain its mechanism and physical meaning. Thetheoretical framework is well supported by a simple experiment in Regional OceanModels (ROMs). The meridional heat transports in the upper ocean are also evaluatedin the annual scale.According to the study, annual mean westerlies at the equator drive meridionalconvergences at surface and divergences at subsurface with the cores at70-75m incentral Indian Ocean, which form the reversed equatorial part of the Subtropical Cell(STC) pattern compared with the Pacific and Atlantic Ocean in the annual mean state.A shallow equatorial roll preculiar in Indian Ocean is overlaid on the reversed STCpattern at the equator, shifting its cores northward. The meridional structure ofmeridional currents is quite different from previous research which indicated thatsouthward Ekman drifts dominated on both sides of the equator at central equatorialIndian Ocean.The seasonal cycle of meridional currents goes through totally particular phasesin boreal spring and fall. In April and October, the meridional structure of currents issimilar to the annual mean state with convergences at surface and divergences atsubsurface along80.5°E section. In May to June and November, strong meridionaldivergences occur in the upper ocean. The divergences and convergences dominate the annual mean of meridional currents instead of the Sverdrup transport duringboreal summer as previous studies indicated. It is worth noting that the cores of thereversed STC pattern and the divergences in boreal spring are in the northernHemisphere while in boreal fall are in the southern Hemisphere.The physical meaning of the derivation from the vorticity equation explains thedynamics of meridional currents in the upper equatorial ocean. Previous studiesassume a wind field antisymmetric about the equator. But it’s only one component ofthe real wind field in Indian Ocean. The other component is a westerlywind symmetric about the equator. The former component dominates boreal summerand winter while the later component dominates boreal spring and fall. According tothese two parts together, the meridional mass transport is comprised of twocomponents too. One is wind-curl driven Sverdrup transports, and the other isgenerated to balance the difference between the pressure gradient force and the windstress symmetric about the equator. In the ideal model, the later component equals tothe change of SSH over time. Via the derivation from vorticity equation in the realocean, the balance can be described in another version: The input of PotentialVorticity (PV) from the curl of wind stress plus shear stress at a certain depth z drivesone part of the meridional transport. The stretching and pulling of the liquidcolumn led by the change of SSH over time plus vertical velocity at depth z drives theother part via the change of the vortex flux.Hence, the difference between Sverdrup transports and real meridional transportsare well explained by the increase of the SSH from May to June and the decreasefrom November to December. From the theoretical framework, the structure ofmeridional velocities is easy to understand. Sverdrup transports led by opposite windstress to a great extent are balance in the long time average. Hence, the annual meanmeridional currents are dominated by the wind field in boreal spring and fall insteadof summer. The reversed STC pattern in boreal spring and boreal fall is explained bythe westerly wind bursts in April and October while the divergences are driven by thewestward pressure gradient force during the extinction of the westerlies in May andNovember. The hemispheric asymmetries of the convergences and divergences are due to the shallow equatorial rolls led by the Sverdrup transports in the upper oceanand wind driven currents at surface which are always against the Sverdrup transport.The analysis of the annual variation seperately in2009,2010and2011is alsomade to preliminarily investigate the influence of ENSO and IOD on the meridionalcurrents at central equatorial Indian Ocean. It is found that the easterlies anomalyduring boreal winter when El Nino occurs affects the meridional currents inNovember differently from the easterlies anomaly during boreal fall when positiveIOD event occurs. It is inferred that ENSO enhances the divergences in Novemberwhile IOD removes it. Further research is needed to study the relations betweenENSO/IOD and meridional currents at central equatorial Indian Ocean.The mechanism of meridional mass and heat transport are easy to understand onthe basis of the theory too. Opposite wind stress in boreal summer and winter drivesopposite Sverdrup transports and heat transports at central equatorial Indian Ocean.Southward mass and heat transports dominate in July to September while Northwardmass and heat transports dominate in December to March. Strong divergences inboreal spring and fall generate annual mean northward mass and heat transport ataround2.0°N, which is different from previous conclusions. The zonal integratedcross2.5°S heat transport in Indian Ocean is evaluated to be-0.81PW to-1.48PWfrom the ADCP data. The negative meridional gradient of heat transports at centralequatorial Indian Ocean generates a trend to lose heat in the annual mean state.The results of the research to some extent develop the dynamics of circulationand enrich the understanding of the meridional heat transports in the upper tropicalIndian Ocean. It greatly contributes to the future research on water mass horizontaldistribution and heat exchanges between hemispheres etc.