Influence of North Atlantic SST and Sea-ice Anomalies on the NAO: Importance of Vertical Stability

In winter the atmospheric vertical stability is at a minimum over the relatively warm ocean surrounding Iceland, which is usually taken as the northern center of the NAO. Deep convection typically sets in at the end of November when vigorous warming from the ocean overturns more or less the whole troposphere in the area (T. Jonsson 2000, personal communications). Some Icelandic meteorologists refer to this as the onset of the ``Atlantic monsoon''. Given this backdrop, we examine the structure of the response in an atmospheric GCM (CCM3) to North Atlantic SST anomalies of both signs and different amplitudes, and sea-ice anomalies. Warm SST anomalies and removal of sea ice give by far the largest amplitude response in all fields, and in particular the strongest projection on the NAO. We relate this difference in response to differences in static stability in each case. In the warming/sea-ice removal cases, the response is deep (equivalent barotropic). The response in the cooling/sea-ice increase cases is considerably shallower so that the signal of changed surface fluxes does not get carried deep into the atmosphere. Here, we shall primarily concentrate on the vertical structure of the response.