Dual influence of Atlantic and Pacific SST anomalies on the North Atlantic/Europe winter climate

Clustering analyses are used in both NCEP Reanalyses and model simulations to identify relationships between the low frequency variability of the zonal wind jet at 200hPa (U200) over the North Atlantic-Europe domain and the geographical distribution of the Sea Surface Temperature (SST) anomalies. For winter months over the 1959-1998 period, the U200 signature of the two phases of the North Atlantic Oscillation (NAO) is captured from NCEP in a cluster pair associated with the North Atlantic SST tripole. The classification also extracts a "La Nina" mode related to a strong ridge off Europe and reduced jet over the Azores. Model results confirm this dual oceanic association while exaggerating the ENSO influence. The role of the Atlantic SSTs is confirmed in model experiments where climatological SSTs are prescribed in the other basins. Tropical anomalies dominate the Atlantic SST signals and are linked to fluctuations of the jet consistently with the NAO phase.