Heat content variability in the North Atlantic Ocean in ocean reanalyses

Warming of the North Atlantic Ocean from the 1950s to 2012 is analyzed on neutral density surfaces and vertical levels in the upper 2000m. Three reanalyses and two observational data sets are compared. The net gain of 5x10(22)J in the upper 2000m is roughly 30% of the global ocean warming over this period. Upper ocean heat content (OHC) is dominated in most regions by heat transport convergence without widespread changes in the potential temperature/salinity relation. The heat convergence is associated with sinking of midthermocline isopycnals, with maximum sinking occurring at potential densities sigma(0)=26.4-27.3, which contain subtropical mode waters. Water masses lighter than sigma(0)=27.3 accumulate heat by increasing their volume, while heavier waters lose heat by decreasing their volume. Spatially, the OHC trend is nonuniform: the low latitudes, 0-30 degrees N are warming steadily while large multidecadal variability occurs at latitudes 30-65 degrees N.