The Role of Horizontal Boundaries in Parameter Sensitivity and Decadal-Scale Variability of Coarse-Resolution Ocean General Circulation Models

Abstract

Coarse-resolution f-plane and ?-plane frictional geostrophic models are used to study the response to restored surface buoyancies and fixed surface buoyancy fluxes. With restored surface buoyancies it is found that the overturning and meridional buoyancy transport generally follow the scaling from thermal wind and vertical advective?diffusive balance When maximum midbasin rather than maximum overall streamfunction is used as the metric, the sensitivity of overturning magnitude to vertical diffusivity agrees quite closely with that of a two-dimensional Rayleigh frictional model that follows an analogous scaling. This measure of meridional overturning, as well as the meridional buoyancy transport, also roughly follow the predicted f?1/3 scaling and are relatively insensitive to variations in ? and horizontal viscosity. The sensitivity experiments indicate that the coarse-resolution model overturning and thermodynamic structure are well characterized by the adjustment of a uniformly rotating viscous fluid to boundaries parallel to the surface forcing gradient. This adjustment is examined in more detail with linear and nonlinear models. In stratified regions, thermal wind currents normal to the coast initiate the adjustment by forcing pycnocline depth anomalies in the coastal (horizontal) Ekman layer. These anomalies propagate around the coast in the Kelvin wave direction, setting up geostrophic currents parallel to the coast. In a model initialized with a high-latitude baroclinic jet, a warm (cold) boundary signal spreads around the poleward (equatorward) part of the basin, initiating geostrophic currents connecting the flow onto the coast in the east with that away from the coast in the west to form two gyres. The warm coastal signal propagates slowly along the weakly stratified polar wall. In steady circulations, strong damping inhibits warm signal propagation, and the warm water on the poleward part of the eastern coast is forced to downwell. Self-sustaining decadal-scale oscillation is a robust feature of the models when forced with fixed buoyancy fluxes. This variability is inherently three-dimensional (it does not occur in a two-dimensional frictional model) and involves the periodic growth and decay of a baroclinic jet in the poleward eastern corner. Decay occurs when a jetlike disturbance, normal to the coast, propagates cyclonically around the basin replacing the cold water along the boundary with warm. Forcing of thermal wind currents normal to weakly stratified coasts and weak damping of the resulting propagating boundary disturbances are found to be conducive to oscillations.