Oceanic forcing of the wintertime low-frequency atmospheric variability in the North Atlantic European Sector: A study with the ARPEGE model

The relationship between global sea surface temperatures (SSTs) and the North Atlantic-Europe (NAE) atmospheric circulation is investigated using an ensemble of eight simulations with the ARPEGE atmospheric global circulation model forced with prescribed SSTs over the 1948-97 period. The model mean state is first validated against NCEP reanalyses. The interannual SST-forced variability is then compared to the internal one using analysis of variance (ANOVA) techniques. Both components are maximum in winter over the Northern Hemisphere and the associated potential predictability shows weak but significant values located over the Icelandic low (IL) and the Azores high (AH). The North Atlantic oscillation (NAO) is found to be the leading internal variability mode over the NAE sector as shown by principal component analysis of a control simulation with climatological SSTs. The noise imprint dominates the forced response estimated from the ensemble mean. The latter is related first to the El Nino-Southern Oscillation (ENSO) activity. During warm (cold) events in the Pacific, the AH shows negative (positive) pressure anomalies and weakened opposition with the IL. The AH fluctuations exhibit a 3,7-yr peak and result from changes in the activity of the Atlantic Hadley cell and from the eastward extension of the Pacific North America teleconnection pattern. Eliassen-Palm diagnostics show that eddy-mean flow interaction acts to maintain the anomalous Atlantic stationary wave pattern as described by Fraedrich in a review based on observational results. The simulated ENSO-NAE connection is, however, too strong in the model and this dominance may be related to the simulated mean state biases. Second, the North Atlantic atmospheric forced signal is associated with the Atlantic SSTs. A tripole structure over the North Atlantic basin with maximum loading in its tropical branch is linked to the phase of the simulated NAO. A local Hadley cell mechanism associated with Rossby wave excitation over the Atlantic is suggested to explain tropical-midlatitude interactions in the model.