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

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• 作者：F. Schloesser ; R. Furue ; J.P. McCreary ; A. Timmermann
• 刊名：Progress in Oceanography
• 出版年：January, 2014
• 年：2014
• 卷：120
• 期：Complete
• 页码：154-176
• 全文大小：3586 K

文摘

Recently, 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 study to include forcing by a zonal wind stress 蟿^x(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 蟻^{鈭?/sup>(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 蟿x 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 蟿^x and 蟻^{鈭?/sup> differ markedly from those forced only by 蟻鈭?/sup> 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鈥檚 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 鈥渟hortcuts鈥?for the overturning circulation; consequently, there are different rates for the convergence of upper-layer water near , and the export of deep water south of the Subpolar Gyre, , the latter being a better measure of large-scale MOC strength. When western-boundary upwelling occurs in our solutions, is independent of the diapycnal processes in the subpolar ocean.}