Free and forced variability in the North Atlantic Ocean

The internal and forced low frequency variability in an isopycnic coordinate general circulation model of the North Atlantic Ocean is investigated. Numerical experiments performed under climatological buoyancy and momentum fluxes, reveal the existence of a 50 years oscillatory mode, emerging for low levels of internal dissipation and high meridional overturning rtes. The oscillation appears to be sustained by a feedback between the poleward propagation of surface thermal anomalies and deep convection in the subpolar gyre. The transition from a steady state to a multidecadal limit cycle solution, occurring for a narrow range of eddy mixing and meridional overturning strengths, suggests the presence of a Hopf bifurcation. In an additional set of numerical simulations, the ocean response to an idealised North Atlantic Oscillation (NAO)-like windstress is inspected as a function of the atmospheric forcing frequency and the strength of eddy mixing in the ocean. In order to elucidate the relationship between internal and forced variability, the NAO is modulated with two specific timescales, 10 and 50 years, the latter dominating the spectrum of the system’s internal variability. Different timescales of atmospheric variability select distinct sea surface temperature (SST) and large scale circulation patterns. When the NAO forcing frequency matches the natural frequency of the ocean, a lagged SST response is excited in the Gulf Stream extension region, which appears to be consistent with the spin-up 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 years modulation of the NAO, while an enhanced overturning response emerges under a 50 years NAO.