Mechanisms of North Atlantic wintertime sea surface temperature anomalies

The authors address the question: What are the oceanic mechanisms that control North Atlantic sea surface temperature (SST) anomalies? The approach is to examine the sensitivity dynamics of a non-eddy-resolving North Atlantic Ocean general circulation model (GCM) using its adjoint. The adjoint GCM yields the sensitivity of end-of-winter SSTs to the prior ocean state and prior air-sea forcing over a seasonal cycle. Diagnosis of the sensitivity results identifies the oceanic mechanisms involved in controlling SST anomalies. The most effective way to alter SST is to change the local, contemporaneous air-sea heat flux. Wind stress and freshwater perturbations are ineffective over one year. Upstream, wintertime heat flux anomalies can cause SST fluctuations in the following winter but heat flux anomalies during summer weakly affect subsequent end-of-winter SSTs. The dominant mechanism is the end-of-winter detrainment of warmer or colder water and its subsequent entrainment downstream into the mixed layer the next winter. This process is more effective in the midlatitude and subpolar North Atlantic where deep winter mixed layers occur, than in the tropical-subtropical regions, which are characterized by a shallow mixed layer and a weak seasonal cycle. Mean-flow advection in the seasonal thermocline of the North Atlantic Current is moderately important in the subpolar gyre. Dynamical mechanisms, such as planetary waves and anomalous currents, are much less important over one year. The GCM results indicate that internal ocean anomalies forced by remote heat fluxes do affect SST variability. But, overall, contemporaneous winter heat flux anomalies are 3-30 times more effective at causing SST anomalies than heat flux anomalies from the previous winter. The loss of sensitivity to prior air-sea fluxes suggests that North Atlantic SST fluctuations are thus primarily a response to local, recent forcing.