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 idealized surface forcing anomalies that mimic the thermal and mechanical forcing associated with the North Atlantic Oscillation (NAO) is analyzed. Emphasis is given on the subpolar gyre and the mechanisms that drive the mass and heat exchanges between the two gyres. First, steady anomalies are applied. Surface heat flux anomalies induce density gradient anomalies at depth, which drive current anomalies reinforcing the north-eastward mean current separating the two gyres. The northward heat transport of these anomalous currents is 10 times stronger than that associated with the transport of temperature anomalies by the mean current. The heat transport anomalies between the two gyres balances the prescribed surface heat flux after 6–7 yr. A similar mechanism is at work when a decadal harmonic forcing is considered, but 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 and primarily enhances the response to the latter. These results suggest that the ocean circulation (both wind driven and thermohaline) is actively involved in the decadal variability, NAO related, of the surface layer at the western subtropical–subpolar gyre boundary.