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The twentieth century African easterly waves in reanalysis systems and IPCC simulations, from intra-seasonal to inter-annual variability

Ruti, Paolo M., Dell'Aquila, Alessandro
Climate dynamics 2010 v.35 no.6 pp. 1099-1117
El Nino, Sahel, climate models, coasts, cold, surface temperature, troposphere, variance, Western Africa
The nature of intraseasonal and interannual variability of African easterly waves (AEWs) in IPCC-CMIP3 global simulations is investigated in comparison with 40-year NCEP and ERA40 products. AEWs are a major source of atmospheric variability over the Sahel and particularly over West Africa. An accurate representation of AEWs dynamics is therefore an important precondition to skilled climate predictions and seasonal forecasts for this area. We describe the synoptic features of these disturbances and we illustrate a statistical link, at interannual timescale, between Sea Surface Temperature and AEWs activity. At intraseasonal time scale, the models exhibit a wide variety of behaviours in reproducing the synoptic features of the disturbances, namely AEWs amplitude and pattern. It is possible to classify the models into two groups, one localizing intense variability well inside the continental area, and the other exhibiting a weaker variance mostly placed over West Africa. Concerning the inter-annual variability, we point out a statistical link between SST anomalies and AEWs activity that reveals a strong influence of the ENSO events on the variability of the disturbances over the Guinean coasts. Warm/cold ENSO events occur in conjunction with suppressed/enhanced AEWs through upper tropospheric teleconnection bridge. Only two model running at significantly different vertical and horizontal resolution (INM and INGV-SGX) are able to reproduce this mechanism in accordance to what observed in the global reanalysis systems. Our result introduces non-negligible caveats with respect to the ability of most of CMIP3 models in obtaining reliable description of AEWs.