Eulerian and eddy-induced meridional overturning circulations in the tropics

Inspired by recent measurements of the eddy-induced Meridional Overturning Circulation in the tropical North Pacific Ocean by Roemmich and Gilson, the authors analyze an oceanic general circulation model for its Eulerian and eddy-induced Meridional Overturning Circulations throughout the Tropics. The model representation for the mesoscale eddy-induced circulation is the parameterization by Gent and McWilliams, and there are also rectified contributions to the time-mean overturning circulation due to seasonal and interannual fluctuations. The eddy-induced circulation is similar in all tropical basins. It has a strength of about 10% of the Eulerian (mainly Ekman) circulation, and its contribution to the meridional heat flux is a similar fraction. The pattern of the meridional streamfunction is one of double cells in the vertical and antisymmetry about the equator. Near the equator there is downwelling above the undercurrent and upwelling below, with the return circulations closed within the upper 250 m and ±5° latitude. Away from the equator in each basin, there are overturning cells with flow in the opposite directions to those nearest the equator, which reach deeper into and through the main pycnocline as well as poleward into the subtropics. Similar to the wind-driven Eulerian Meridional Overturning Circulation, the seasonal cycle in the eddy-induced circulation has a magnitude comparable to the time-mean circulation, although for an entirely different dynamical reason associated with seasonal changes in the buoyancy field that are primarily diabatic. There is also a circulation anomaly during the 1997/98 El Niño-Southern Oscillation event that nearly cancels the time-mean, counterrotating, eddy-induced cells nearest the equator and surface. The rather good agreement between the measurements and the model solution gives support to the theory underlying the parameterization of eddy-induced circulation, and it indicates that the associated eddy transport coefficients are larger in the Tropics (i.e., ~2 × 10³ m² s^-1) than in middle and high latitudes.