q 2001 American Meteorological Society Where Does Dense Water Sink? A Subpolar Gyre Example*

It is proposed that a dominant component of the downwelling limb of the thermohaline circulation takes place in regions where convective mixing is found adjacent to steep topography. A simple theoretical estimate of the overturning forced by such boundary convection is derived that depends only on the properties of the oceanic mixed layer along the boundary. Scaling estimates indicate that sinking forced by boundary convection is an order of magnitude greater than sinking in the open ocean resulting from large-scale dynamics or baroclinic instability of deep convective sites. Recent hydrographic observations in the Labrador Sea are used to estimate the downwelling due to these different mechanisms and support the notion that boundary sinking dominates. The theory compares well with the overturning rates diagnosed in a noneddy-resolving general circulation model over a wide range of parameters. As a direct consequence of these dynamics, the high-latitude hydrography and overturning circulation in the model are very sensitive to the presence of cyclonic rim currents. Lateral density advection by the rim currents in the subpolar gyre increases the stratification and limits the mixing near the boundaries, thus reducing the maximum downwelling. As a result, most of the high-latitude meridional heat transport is carried by the horizontal circulation instead of the overturning circulation. Such rim currents are found in different configurations of the model, including 1) a continental slope and standard diffusion parameters and 2) zero horizontal diffusion and a flat bottom. 1