A simple and self-consistent geostrophic-force-balance model of the thermohaline circulation with boundary mixing

[摘要] A simple model of the thermohaline circulation (THC) is formulated,with the objective to represent explicitly the geostrophic forcebalance of the basinwide THC. The model comprises advective-diffusivedensity balances in two meridional-vertical planes located at theeastern and the western walls of a hemispheric sector basin. Boundarymixing constrains vertical motion to lateral boundary layers alongthese walls. Interior, along-boundary, and zonally integratedmeridional flows are in thermal-wind balance. Rossby waves and theabsence of interior mixing render isopycnals zonally flat except nearthe western boundary, constraining meridional flow to the westernboundary layer. The model is forced by a prescribed meridional surfacedensity profile.

This two-plane model reproduces both steady-state density andsteady-state THC structures of a primitive-equation model. Thesolution shows narrow deep sinking at the eastern high latitudes,distributed upwelling at both boundaries, and a western boundarycurrent with poleward surface and equatorward deep flow. Theoverturning strength has a 2/3-power-law dependence onvertical diffusivity and a 1/3-power-law dependence on theimposed meridional surface density difference. Convective mixing playsan essential role in the two-plane model, ensuring that deep sinkingis located at high latitudes. This role of convective mixing isconsistent with that in three-dimensional models and marks a sharpcontrast with previous two-dimensional models.

Overall, the two-plane model reproduces crucial features of the THC assimulated in simple-geometry three-dimensional models. At the sametime, the model self-consistently makes quantitative a conceptualpicture of the three-dimensional THC that hitherto has been expressedeither purely qualitatively or not self-consistently.

[学科分类] 海洋学与技术