Sensitivity of climate response to variations in freshwater hosing location

In a recent intercomparison of the response of general circulation models (GCMs) to high-latitude freshwater forcing (Stouffer et al., J Climate 19(8):1365–1387, 2006), a number of the GCMs investigated showed a localised warming response in the high-latitude North Atlantic, as opposed to the cooling that the other models showed. We investigated the causes for this warming by testing the sensitivity of the meridional overturning circulation (MOC) to variations in freshwater forcing location, and then analysing in detail the causes of the warming. By analysing results from experiments with HadCM3, we are able to show that the high-latitude warming is independent of the exact location of the additional freshwater in the North Atlantic or Arctic Ocean basin. Instead, the addition of freshwater changes the circulation in the sub-polar gyre, which leads to enhanced advection of warm, saline, sub-surface water into the Greenland–Iceland–Norwegian Sea despite the overall slowdown of the MOC. This sub-surface water is brought to the surface by convection, where it leads to a strong warming of the surface waters and the overlying atmosphere.