Abstract
Abstract. A simple model of the thermohaline circulation (THC) is formulated, with the objective to represent explicitly the geostrophic force balance of the basinwide THC. The model comprises advective-diffusive density balances in two meridional-vertical planes located at the eastern and the western walls of a hemispheric sector basin. Boundary mixing constrains vertical motion to lateral boundary layers along these walls. Interior, along-boundary, and zonally integrated meridional flows are in thermal-wind balance. Rossby waves and the absence of interior mixing render isopycnals zonally flat except near the western boundary, constraining meridional flow to the western boundary layer. The model is forced by a prescribed meridional surface density profile. This two-plane model reproduces both steady-state density and steady-state THC structures of a primitive-equation model. The solution shows narrow deep sinking at the eastern high latitudes, distributed upwelling at both boundaries, and a western boundary current with poleward surface and equatorward deep flow. The overturning strength has a 2/3-power-law dependence on vertical diffusivity and a 1/3-power-law dependence on the imposed meridional surface density difference. Convective mixing plays an essential role in the two-plane model, ensuring that deep sinking is located at high latitudes. This role of convective mixing is consistent with that in three-dimensional models and marks a sharp contrast with previous two-dimensional models. Overall, the two-plane model reproduces crucial features of the THC as simulated in simple-geometry three-dimensional models. At the same time, the model self-consistently makes quantitative a conceptual picture of the three-dimensional THC that hitherto has been expressed either purely qualitatively or not self-consistently.
Highlights
The oceanic thermohaline circulation (THC), a key agent in Earth’s climate, has as yet denied researchers important insights into its dynamics
Attempts to capture THC dynamics in a reduced-complexity model (Marotzke et al, 1988; Wright and Stocker, 1991; Sakai and Peltier, 1995; Wright et al, 1995) have long disregarded at least one of two fundamental observations: that diapycnal mixing, one of the driving mechanism of the meridional overturning, is essentially confined to lateral boundaries (Munk, 1966; Wunsch and Ferrari, 2004) and that the western boundary current and, by implication, the zonally integrated thermohaline overturning are in geostrophic balance (Johns et al, 2005)
Our starting point is the assumption that upwelling from the abyss and, by implication, total THC strength are limited by the rate of diapycnal mixing (e.g. Munk, 1966; Wunsch and Ferrari, 2004)
Summary
The oceanic thermohaline circulation (THC), a key agent in Earth’s climate, has as yet denied researchers important insights into its dynamics. Attempts to capture THC dynamics in a reduced-complexity model (Marotzke et al, 1988; Wright and Stocker, 1991; Sakai and Peltier, 1995; Wright et al, 1995) have long disregarded at least one of two fundamental observations: that diapycnal mixing, one of the driving mechanism of the meridional overturning, is essentially confined to lateral boundaries (Munk, 1966; Wunsch and Ferrari, 2004) and that the western boundary current and, by implication, the zonally integrated thermohaline overturning are in geostrophic balance (Johns et al, 2005). Enhanced boundary mixing would make possible a realistic THC strength while accounting for the observed low diapycnal mixing rates in the ocean interior (e.g. Armi, 1978; Ledwell et al, 1993; Wunsch and Ferrari, 2004). Boundary mixing was not implemented into a primitiveequation model until Marotzke (1997), who found flow patterns very similar to the uniform-mixing case, except that vertical motion was entirely confined to lateral boundary
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