An idealized model study of the mass and heat transports between the subpolar and subtropical gyres

The response of an ocean model of the subtropical and subpolar gyres to ide alized surface forcing anomalies that mimic the thermal and mechanical forc ing associated with the North Atlantic Oscillation (NAO) is analyzed. Empha sis is given on the subpolar gyre and the mechanisms that drive the mass an d heat exchanges between the two gyres. First, steady anomalies are applied . Surface heat flux anomalies induce density gradient anomalies at depth, w hich drive current anomalies reinforcing the north-eastward mean current se parating the two gyres. The northward heat transport of these anomalous cur rents is 10 times stronger than that associated with the transport of tempe rature anomalies by the mean current. The heat transport anomalies between the two gyres balances the prescribed surface heat flux after 6-7 yr. A sim ilar mechanism is at work when a decadal harmonic forcing is considered, bu t the lag is shortened to 3-4 yr. The model response to a wind stress curl anomaly shows a similar behavior to that found with the surface heat flux a nd primarily enhances the response to the latter. These results suggest tha t the ocean circulation (both wind driven and thermohaline) is actively inv olved in the decadal variability, NAO related, of the surface layer at the western subtropical-subpolar gyre boundary.