| Schott et al. (1990) observed that in the thermocline, an annual mean western boundary current of about 10 Sv flows across the equator into the north Indian Ocean “vorticity trap| nearly balancing the (rather well-defined) annual mean cross-equatorial Ekman transport. The observed inflow extends down to 500m where temperatures are about 10ZC. Vorticity constraints imply that much of it must return across the equator in the surface where temperatures are typically 26Z-28ZC. This requires water mass conversion to substantial depths, a large net surface heat flux into the northern Indian Ocean, and low Sea Surface Temperatures (SSTs). To explore the dynamics that allow the inflow to be so deep and cold, we examine the Indian Ocean overturning cell in an idealised model. This consists of a rectangular basin from 10ZN to 14ZS, with a top-to-bottom gap in the model's eastern boundary from 7ZS to 10ZS to simulate the Indonesian Throughflow. Wind stress is purely zonal and linear in distance from the equator. With such winds on a beta-plane, no Ekman pumping occurs anywhere, and Ekman transport and Sverdrup transport are spatially constant and equal everywhere. Temperature is relaxed strongly to the initial profile everywhere south of 10ZS. For steady winds inducing a constant southward Ekman-Sverdrup transport of 10 Sv, Kelvin and Rossby waves set up a near-zonal inflow through the gap that flows into a northward cross-equatorial western boundary current, which in turn feeds the northern boundary upwelling. The net inflow shallows progessively after onset of steady winds. Within a few months a near-steady state is reached near the northwest corner in which substantial diapycnal turbulent fluxes of heat occur. Flow to supply the Ekman-Sverdrup transport is balanced geostrophically by differences in pressure across the basin width. For northward geostrophic flow the requirement that the density in the northwest corner of the domain be convectively stable provide rules for estimating minimum geostrophic flow depths. For steady forcing, such rules provide rather accurate estimates of vertical profiles of geostrophic flow, which change little over a parameter range that spans a regime shift from dominance by horizontal diffusion to one of dominance by vertical diffusion. The depth of inflow and net surface heat flux are very nonlinear functions of the net magnitude V of Ekman-Sverdup transport. When seasonally-varying cross-equatorial Ekman transports are used that resemble observed Indian Ocean values, the lack of equilibrium causes annual mean flow to extend substantially deeper than when steady, annual mean Ekman transports are imposed. The annual mean heat absorbed in the northern Indian Ocean is therefore increased. Temperature profiles averaged over the basin as a whole (and hence the Indonesian Throughflow) react to changes in the northwest corner of the basin, with a timescale of about two years. |
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