Non-local topographic influences on deep convection: An idealized model for the Nordic Seas

Issues relevant to the water mass transformation and circulation in the Nordic Seas are explored using a series of idealized numerical model calculations. The essential aspects of the model configuration are: a marginal sea with two sub-basins separated by a ridge; a sill separating the marginal sea from the open ocean; a region of sloping topography surrounding the marginal sea; surface heat loss; and eddy-resolving physics. A calculation with enhanced topographic slope along the eastern boundary, as is found in the Lofoten Basin, produces a hydrography and circulation that are similar to that observed in the Nordic Seas, including two poleward warm currents, a mid-depth thermocline in the eastern basin (the model’s Lofoten Basin), and deep convection in the western basin (the model’s Greenland Sea). It is shown that the key elements to the mid-ocean poleward warm current and the mid-depth thermocline in the eastern basin are: an asymmetry in the eddy heat flux from the boundary into the basin interior, here related to a region of steep bottom topography that makes the eastern boundary current more unstable than the western boundary current, and a mid-ocean ridge that partially, but not completely, inhibits exchange between eastern and western basins. This exchange can take place by baroclinic eddy fluxes or by flow through deep gaps in the ridge. The basin-scale hydrography and circulation in this idealized Nordic Seas are controlled to a large degree by eddy fluxes, which are themselves strongly influenced by regional features of the bottom topography that are located far from the convection sites.