Weather Regimes in the Euro-Atlantic and Mediterranean sector and relationship with West African rainfall over the period 1989-2008 from a Self-Organizing Maps Approach
Résumé
Weather Regimes (WR) have been defined over the Euro-Mediterranean region [60°W-60°E; 15°N-70°N] from May to October season using the daily Sea Level Pressure, 700 hPa geopotential height and specific humidity from ERA-interim over 1989-2008 period. Computations are based on a neural network lassification technique referred to as Self Organizing Maps and the WR produced can be used by the community for comparison with other periods, projection onto model outputs, seasonal prediction or teleconnection studies. The article particularly examines the relationship between WR and West African (WA) rainfall and our results suggest that changes in particular WR frequencies can account for part of WA interannual rainfall variability. Thus during anomalous wet (dry) years in WA, both higher frequencies of occurrence of WR related to negative (positive) summer NAO-like pattern and less frequent WR related to positive (negative) summer NAO-like pattern are attested in July and August (hereafter SN- and SN+). This is associated with a zonal symmetric pattern, consistent along the middle troposphere, i.e. low pressure anomaly centered over 50°N-20°W and Eurasia (Greenland) and high pressure anomaly centered over Iceland (central Europe) for SN- (SN+). Another striking characteristic of SN- (SN+) is southeastward (southwestward) surface anomalous winds flowing from (to) the Atlantic Ocean at 20°N and 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, suggesting that temperature anomalies could be a precursor in the change of frequency of SN- and SN+.
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