The Surface-Forced Overturning of the North Atlantic: Estimates from Modern Era Atmospheric Reanalysis Datasets

Estimates of the recent mean and time varying water mass transformation rates associated with North Atlantic surface-forced overturning are presented. The estimates are derived from heat and freshwater surface fluxes and sea surface temperature fields from six atmospheric reanalyses (JRA, NCEP-1, NCEP-2, ERA-I, CFSR and MERRA) together with sea surface salinity fields from two globally gridded data sets (World Ocean Atlas and EN3). The resulting twelve estimates of the 1979-2007 mean surface-forced streamfunction all depict a subpolar cell, with maxima north of 45°N, near σ =27.5 kg m-3, and a subtropical cell between 20°N and 40°N, near σ =26.1 kg m-3. The mean magnitude of the subpolar cell varies between 12-18 Sv, consistent with estimates of the overturning circulation from subsurface observations. Analysis of the thermal and haline components of the surface density fluxes indicates that large differences in the inferred low latitude circulation are largely due to the biases in reanalysis net heat flux fields, which range in the global mean from -13 W m-2 to 19 W m-2. The different estimates of temporal variability in the subpolar cell are well correlated with each other. This suggests the uncertainty associated with the choice of reanalysis product does not critically limit the ability of the method to infer the variability in the subpolar overturning. In contrast, the different estimates of subtropical variability are poorly correlated with each other, and only a subset of them capture a significant fraction of the variability in independently estimated North Atlantic Subtropical Mode Water volume.