International Pacific Research Center, University of Hawaii, Honolulu, Hawaii, USA.

The ocean response to an idealised North Atlantic Oscillation-like wind stress is evaluated as a function of the atmospheric forcing frequency. In order to elucidate the relationship between internal and forced variability, the NAO is modulated with two specific timescales, 10 and 50 years, which characterise the spectrum of the system’s internal variability. Different timescales of atmospheric variability select distinct sea surface temperature (SST) and large scale circulation patterns. Under a 50 year NAO forcing period, a lagged SST response is excited in the Gulf Stream extension region, which is consistent with the spinup of the gyre circulation. The thermohaline circulation varies in phase with the NAO and shows a strong sensitivity to the forcing frequency: a dipole mode of the overturning is excited by a 10 year modulation of the NAO, while an enhanced overturning response emerges under a 50 year NAO. With low enough lateral mixing the ocean exhibits an irregular response to a regular NAO-like forcing.