Response of a simplified oceanic general circulation model to idealized NAO-like stochastic forcing

The response of a simplified model of the subtropical and subpolar gyres to a mechanical stochastic forcing that mimics the North Atlantic Oscillation (NAO) is studied. An internal damped mode of oscillation with a 22-24-yr period is found, with a maximum amplitude in the northwestern part of the basin (subpolar gyre and intergyre area). Its mechanism is as follows. In response to the wind stress forcing, anomalous currents appear between 35degrees and 40degreesN. They create temperature anomalies near the western coast, which are in turn advected by the mean current into the subpolar gyre and in the convection area. These temperature anomalies affect the thermohaline circulation, which produces new temperature anomalies near 35degrees-40degreesN and thus appears as a key component of this damped periodic mode. Wind stress forcing acts as efficiently as heat flux at exciting this oscillation, which can persist if external forcing is strong enough.