Meridional Overturning Transports at 30°S in the Indian and Pacific Oceans in 2002-2003 and 2009

Hydrographic sections at 30ºS in the Indian and Pacific Oceans have been repeated in 2002-03 and 2009. For each period, an inverse model is applied to the box formed by the two sections. The model imposes mass conservation for individual layers, defined by isoneutral surfaces, and the whole water column, using surface Ekman transport and several transport constraints for specific ranges of longitudes and depths. As a result, the velocities at the reference layer for each station pair are estimated. Vertical velocities and vertical diffusivities are estimated assuming a mass conservation from the residual of the mass transport per layer from the inverse model and a steady-state heat balance, respectively. The most robust difference between 2002 and 2009 in the Indian Ocean is that the Subantarctic Mode Water (SAMW) and thermocline waters are shifted to lower densities in 2009 reflecting the upward shift in the Indian’s overturning transport from 2002 to 2009. The northward inflow in 2002 is principally in the central Indian Ocean with little or slightly southward transport in the western basins. In contrast, the northward inflow in 2009 is principally in the westernmost Mozambique and Madagascar Basins, with little northward transport in the central basins. The Indian upper ocean transport is similar in north years. The transport in the Pacific Ocean differs markedly in the two years. The difference is accompanied by a large difference in the mid-depth and bottom layers transports but in little difference in net overturning transport. The net southward heat transport across 32ºS for the combined Indo-Pacific section is -0.6 to -0.7±0.2 PW in 2002-03, consistent with previous estimates based on earlier 1987/1993 WOCE sections. The heat transport was more than doubled for 2009, to -1.3 to -1.5±0.1 PW. However, the vertical structure of the overturning in 2009 was much more similar to that of the 1987/1993 WOCE sections. Diapycnal diffusivity and velocity are strongly enhanced near the ocean bottom; relatively high Indian Ocean diffusivity extends farther up in water column than in Pacific, as does larger upwelling vertical velocity. The higher diffusivity may thus enable the extent of Indian Ocean overturning up to lower density and shallower depths than in the Pacific, where the overturn is concentrated deeper.