Weather Regimes in the Euro-Atlantic and Mediterranean Sector, and Relationship with West African Rainfall over the 1989-2008 Period from a Self-Organizing Maps Approach

Weather regimes (WRs) have been defined over the Euro-Mediterranean region (15 degrees-70 degrees N, 60 degrees W-60 degrees E) from May to October using the daily sea level pressure, 700-hPa geopotential height, and specific humidity from the European Centre for Medium-Range Weather Forecasts Re-Analysis (ERA)-Interim over the 1989-2008 period. Computations are based on a neural network classification technique referred to as self-organizing maps, and the WRs produced can be used by the scientific community for comparison with other periods, projection onto model outputs, seasonal prediction, or teleconnection studies. The article particularly examines the relationship between WRs and West Africa (WA) rainfall, and the study's results suggest that changes in particular WR frequencies can account for a part of the WA's interannual rainfall variability. Thus, during anomalous wet (dry) years in WA rainfall, both more occurrences of WRs related to the negative (positive) summer North Atlantic Oscillation (NAO)-like pattern and fewer occurrences of WRs related to the positive (negative) summer NAO-like pattern are attested in July and August (SN- and SN+, respectively). This is associated with a zonal symmetric pattern, consistent along the midtroposphere-that is, a low pressure anomaly centered over 50 degrees N, 20 degrees W and Eurasia (Greenland) and a high pressure anomaly centered over Iceland (central Europe) for SN- (SN+). Another striking characteristic of SN- (SN+) are southeastward (southwestward) anomalous surface winds flowing from (to) the Atlantic Ocean at 20 degrees N; therefore able to enhance (weaken) wet convection. That sea surface temperature associated with SN-shows a warming of the Mediterranean in July and the opposite with SN+ in August suggests that temperature anomalies could be a precursor in the change of frequency of SN- and SN+.