North Atlantic climate variability in a hierarchy of ocean models

The focus of this study is on the oceanic processes important for the North Atlantic climate variability over a broad range of time scales. The approach is to employ a hierarchy of ocean models with increasing complexity.; On time scales of a few weeks and months and in a one-dimensional Lagrangian upper ocean model, sea surface temperature (SST) is most sensitive to surface heat flux perturbations. Strong end-of-winter sensitivity to heat flux from one year earlier exists off the east coast of the United States and in the sub-polar gyre, due to the reemergence of temperature anomalies during winter from the seasonal thermocline. The recurrence is weaker west of North Africa, because upper ocean seasonality is weaker there. The effects of wind stress and freshwater flux anomalies on SST are less important than heat flux changes but can be of either sign. They depend on the season and the vertical temperature profile at the perturbation time.; On interannual to decadal time scales and in a non-eddy-resolving model, SST variability exhibits a tripole pattern and significant spectral peaks at 5 and 10 years, which is mainly forced by the NAO associated air-sea heat flux perturbations. Ocean heat transport divergence changes induced by wind stress anomalies play a secondary role. The top 1000m upper ocean heat content varies on time scales of a few decades, however. In the subtropics, it is dominated by the wind stress driven thermocline displacement; in the subpolar North Atlantic, it is mainly regulated by buoyancy driven deep convection. Both are equally important in middle latitudes.; In a 1/6° eddy-resolving model, the mesoscale eddies do not change the large scale SST pattern on seasonal to short term interannual time scales. They do, however, generate a lot of highly time-varying fine scale structures, which seem to be associated the eddy heat transport divergence. The eddy heat transport is concentrated in the western boundary current region where most of the energetic eddies occur and shows significant northward transport along the Gulf Stream and part of the North Atlantic Current.