Modeling the three-dimensional upper ocean heat budget and subduction rate during the Subduction Experiment

Abstract

A global three-dimensional primitive equation general circulation model is used to estimate the upper ocean heat budget and subduction rates in the eastern North Atlantic during the period of the Subduction Experiment, June 1991 through July 1993. The seasonal cycle is dominated by one-dimensional processes throughout the subtropical gyre, i.e., a local balance between the net heat flux into the ocean through the surface and a vertical redistribution due to vertical mixing. However, closure of the heat budget on long timescales involves redistribution by Ekman transports (both horizonal and vertical), geostrophic advection, and vertical diffusion. Subgridscale parameterizations of eddy processes weakly restratify the upper ocean. The annual rate at which waters are subducted from the mixed layer into the permanent thermocline is estimated using both kinematic and thermodynamic methods. These subduction rates and patterns are generally consistent with each other and also with previous estimates of the large-scale subduction rate based on climatologies and models. Consideration of a finite thickness Ekman layer is shown to reduce the thermodynamic estimate of the subduction rate at low latitudes by O(50%), in much better agreement with the kinematic method. The effects of convergent eddy fluxes in the mixed layer and a diabatic thermocline on the subduction rate are also calculated and found to be small but not negligible. The seasonal and interannual evolution of the sea surface temperature and mixed layer depth in the model compare well with in situ measurements at five mooring locations taken as part of the Subduction Experiment. These results demonstrate that a global, low-resolution general circulation model forced with surface fluxes of heat and freshwater can accurately reproduce the evolution of the upper ocean thermal structure and provide a useful tool for the analysis of air-sea interaction and climate variability on seasonal to interannual timescales. Copyright 2000 by the American Geophysical Union.