Time evolution of observed July-September SST-Sahel climate teleconnection with removed quasi-global effect (1900-2008)
Résumé
Using SST, precipitation and atmospheric information, this statistical study revisits the questions of the July-September SST-Sahel teleconnection variability after removing impact of quasi-global SSTs over the period 1900-2008. The eastern Mediterranean and the Indian Ocean dominate the relationship, both in terms of intensity and time stability, with significant values in 52% and 47% of years, respectively. More than two thirds of the rainy seasons classified as dry (wet) and 16/18 (12/15) of those classified as very dry (very wet) are concomitant of negative (positive) differences between the Mediterranean and the Indian Ocean. Correlations with the tropical Atlantic, the Nino area and the western Pacific region are generally lower and less robust, although, in some periods, they can be high with the southern tropical Atlantic. Telecconnection observed with continental precipitation and the 950 hPa moisture flux field confirmed these results. Positive SST differences between the eastern Mediterranean and the Indian Ocean are synchronous of in phase rainfall excess over the whole Sudan-Sahel due to a strengthening of the convergence between the north-easterly moisture transport from the eastern Mediterranean and the monsoon south-westerly moisture transport from the eastern equatorial Atlantic. This is associated with changes in the atmospheric circulation along the meridional and zonal planes, mainly (i) a subsidence departure from mid-levels above 10°N-18°N associated with air ascents above the Saharan thermal lows; (ii) upward anomalies on the western and eastern Sahel reinforcing the atmospheric ascents in upper levels; (iii) a low-level subsidence anomaly by 30°E-40°E in agreement with the Indian cooling weakening the normal uplifts; (iv) a reinforcement of the Tropical easterly Jet over 0° -20°E.
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