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RESEARCH PRODUCT

The role of the eastern Mediterranean sea on the WAM in 2003 using the WRF regional model

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International audience; There is empirical evidence that sea surface temperatures (SST) exert a prominent role on the WAM, particularly over the Atlantic ocean where they can act to modify the location of the ITCZ and moisture advection. However, the role of the Mediterranean sea has been studied more recently. Rowell (2003) has first shown that the basin could play a significant role on the monsoon in modifying the moisture transport and convergence of humidity. Using GCM simulations, he demonstrated that warmer (colder) than normal SST on the Mediterranean sea is associated with a more/less intense monsoon in the Sahel region. A simple composite analysis between the SST and the Sahel July-August-September (JAS) rainfall shows significant relationships with the eastern Mediterranean basin. Let us recall also that in JAS 2003, warmer SST over the whole basin has been shown to be associated with rainfall excess (Jung et al., 2006). Using the Weather Regional Forecast model (WRF) developed by the NCAR, we defined 2 sensitivity experiments by adding/subtracting anomalies in the eastern basin for the year 2003 in order to design warm/cold SST forcing fields. Our results exhibit significant positive rainfall anomalies over the eastern Sahel when the basin is warmer than normal. The response to the warming can be resumed as follows. The moisture is brought in the low levels from the Mediterranean sea to the Sahara and the eastern Sahel. The additional moisture fuels the convection inside the ITCZ that leads to increased moisture convergence sustained by the moisture flux from the Atlantic, consistent with the positive feedback mechanism proposed by Rowell (2003). The main vertical motions within the ITCZ are located east of Greenwich where the rainfall surplus occurs. To the West of the primary meridian, the vertical ascent in the heat low is significantly enhanced and the associated mid levels high pressure reinforced. This is accompanied by a northward migration of the ITCZ and of the heat low. A subseasonal analysis shows that the precipitation positive anomalies then developed are found to prevail until 15 days after the onset of the rainy season at the end of June.