Dynamics of the Atlantic meridional overturning circulation. Part 2: Forcing by winds and buoyancy

Abstract

Recently, Schloesser et al. (2012) explored the dynamics of the descending branch of meridional overturning circulations (MOCs), by obtaining analytic solutions to a variable-density, 2-layer model (VLOM) forced only by a surface buoyancy flux. Key processes involved are the poleward thickening of the upper layer along the eastern boundary due to Kelvin-wave adjustments, the westward propagation of that coastal structure by Rossby waves, and their damping by mixing; the resulting zonal pressure gradient causes the surface MOC branch to converge into the northern basin near the eastern boundary.In this paper, we extend the Schloesser et al. (2012) study to include forcing by a zonal wind stress τx(y). Much of the paper is devoted to the derivation and analysis of analytic solutions to VLOM; for validation, we also report corresponding numerical solutions to an ocean general circulation model (OGCM). Solutions are obtained in a flat-bottom, rectangular basin confined to the northern hemisphere. The buoyancy forcing relaxes upper-ocean density to a prescribed profile ρ∗(y) that increases polewards until it becomes as large as the deep-ocean density at latitude y2; north of y2, then, the ocean is homogeneous (a 1-layer system). The wind stress τx drives Subtropical and Subpolar Gyres, and in our standard solution the latter extends north of y2. Vertical diffusion is not included in VLOM (minimized in the OGCM); consequently, the MOC is not closed by upwelling associated with interior diffusion, but rather by flow through the southern boundary of the basin (into a southern-boundary sponge layer in the OGCM), and solutions are uniquely determined by specifying the strength of that flow or the thermocline depth along the tropical eastern boundary.Solutions forced by τx and ρ∗ differ markedly from those forced only by ρ∗ because water flows across y2 throughout the interior of the Subpolar Gyre, not just near the eastern boundary. In some of our solutions, the strength of the MOC’s descending branch is determined entirely by this wind-driven mechanism, whereas in others it is also affected by Rossby-wave damping near the eastern boundary. Upwelling can occur in the interior of the Subpolar Gyre and in the western-boundary layer, providing “shortcuts” for the overturning circulation; consequently, there are different rates for the convergence of upper-layer water near y2,Mn, and the export of deep water south of the Subpolar Gyre, M, the latter being a better measure of large-scale MOC strength. When western-boundary upwelling occurs in our solutions, M is independent of the diapycnal processes in the subpolar ocean.