Interaction between the Subtropical and Equatorial Ocean Circulations: The Subtropical Cell

Abstract

Three versions of a 2 1/2-layer ocean model are used to study the subtropical cell (STC), a shallow, meridionalcirculation cell consisting of subtropical subduetion, equatorward advection of cool subsurface water into thetropics, upwelling at the equator, and poleward advection of warm surface water back to midlatitudes. Thethree versions are a steady-state analytic model, a numerical model with constant layer temperatures, and anumerical model with variable layer temperatures and active thermodynamics. Two different pammeterizationsof mixed-layer processes are utilized to determine how water moves between the lwo layers. In the simplerparameterization, entrainment and detrainment rates, we and wd, are specified so that the upper-layer thicknessh1 relaxes back to an externally prescribed thickness; in the other, they are related to the surface heat flux Q.in both versions detrainment is cut off at the latitude yd = 18° to prevent subduction from occurring in thetropics. Solutions are obtained in a rectangular basin that is symmetric about the equator. They are forced byidealized representations of observed zonal wind stress τx and Q fields, the latter used only for the thermodynamicmodel. The analytic solution provides a comprehensive, three-dimensional description ofthe STC and illustrates itsfundamental dynamics. First, it indicates that the strength of the STC depends only on the wind stress τx andCoriolis force f at the latitude yd; it is not related to the Ekman pumping velocity (τx/f)y over the subtropicalocean or to the strength of the equatorial wind field. Thus, the amount of subtropical water that upwells in thetropics is remotely forced by processes outside the tropics (along yd). Second, two types of water contributeroughly equally to the STC: unventilated water from the lower-layer western boundary current and ventilatedwater subducted in the subtropical ocean. Third, an internally determined streamline xe(y) determines whetherthe subtropical water approaches the equator entirely in the western boundary current or partly through theinterior ocean. Fourth, another streamline xb(y) defines the western edge of the equatorward branch of the STCand thereby determines the latitude at which the westward lower-layer flow bifurcates at the western boundary. Solutions to the constant-temperature numerical model corroborate the analytic results and illustrate thenature of boundary layers. Among other things, they demonstrate that the equatorial circulation is sensitive tothe equatorial wind through its influence on the location of tropical upwelling field we; in our control run forcedwith equatorial easterly winds, we occurs on the equator in the eastern ocean, and the lower-layer flow fielddevelops an equatorial undercurrent (EUC); in test solutions forced without equatorial winds, we, exists in anoff-equatorial band across the interior ocean and there is no EUC. It follows that local forcing by equatorialwinds is required for the existence of equatorial upwelling and the EUC in the control run. In the solution to, the thermodynamic model, the circulation is similar to that in the control run, except thath1 deepens markedly north of the line where Q changes sign to become negative. As a result, the total subductionin the subtropics increases by a factor of 2.1, and the source of all the water that contributes to the equatorwardbranch of the STC is subtropical subduction. In the tropics, the lower-layer temperature is maintained at a coolvalue by adveetion associated with the STC.