Thermohaline and wind forcing of a circumpolar channel with blocked geostrophic contours

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The Antarctic Circumpolar Current is governed by unique dynamics.Because the latitude belt of Drake Passageis not zonally bounded by continents,the Sverdrup theory does not apply to theAntarctic Circumpolar Current.However, most of the geostrophic contours are blocked atDrake Passage, which provides an important dynamic constraintfor the vorticity equation of the depth averaged flow.This study addresses the effects of thermohaline and wind forcingon the large scale transportof a circumpolar current with blocked geostrophic contours.Various numerical experiments with three different idealized model geometrieswere conducted.Based on the results and theoretical argumentswe promote an indirect wind effect on thecircumpolar current:while the direct effects of the wind in driving the circumpolar currentthrough a vertical transfer of the applied wind stress are of minor importance,the wind does substantially influence the circumpolar current transportthrough its effects on the density field.This indirect wind effect is discussed in two steps.Firstly, at the latitudes of the circumpolar current and longitudes wherethe geostrophic contours are blocked, the meridional gradient of the masstransport streamfunction is to leading order balanced by the meridional gradientof the baroclinic potential energy.This balance implies that the total transportis too leading order baroclinic and that the deep transport is small.For this statement, some theoretical arguments are offered.Secondly, a simplified analytical model is used to obtain the distributionof the baroclinic potential energy.Assuming an advective-diffusive balance for the densitiesin the deep downwelling northern branch of the Deacon Cell,this model reproduces the qualitative dependence of thecircumpolar current transport on the imposed wind and thermohalineforcing as well as on the turbulent diffusivities.

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Borowski, D. , Gerdes, R. and Olbers, D. (2002): Thermohaline and wind forcing of a circumpolar channel with blocked geostrophic contours , Journal of Physical Oceanography, Vol. 32, No. 9, pp, pp. 2520-2540 .

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