A mechanism for African monsoon breaks: Mediterranean cold air surges

Surges of cold air from the Mediterranean into northern Africa during the boreal summer are documented, and their influence on monsoon breaks is analyzed using Tropical Rainfall Measuring Mission rainfall estimates and reanalysis products. Between 1998 and 2006, 6-10 cold air surges occurred each summer, with low-level temperature anomalies ranging from less than -1 K to over -6 K. Composite analysis indicates that cold air surges over northern Africa persist for 2-10 days and travel equatorward at approximately 5.5 m s(-1), which is 0.5-1.5 m s(-1) faster than the observed climatological low-level meridional flow. Northern African cold surges have characteristics similar to surges observed elsewhere in the world, including a hydrostatically induced ridge of surface pressure and an amplified upper tropospheric ridge/trough pattern. The African cold surge is preceded by the passage of a shortwave trough and an intensification of the upper tropospheric subtropical westerly jet streak over the Mediterranean Sea. These events are associated with increased confluence in the jet entrance region over the central Mediterranean, an enhanced direct secondary circulation, subsidence, and low-level ageostrophic northerly flow over northeastern Africa. Composite analysis shows that the passage of a cold surge is associated with an enhancement in convective activity over southern Algeria, western Niger, northern Mali, and Mauritania 2 to 5 days before the surge reaches the eastern Sahel ( similar to 17.5 degrees N), when northeasterly flow channeled between the Atlas and Ahaggar Mountains strengthens and transports relatively moist air from the western Mediterranean and eastern North Atlantic over the region and increases moisture convergence over western Africa north of 20 degrees N. Over the eastern Sahel of Sudan and eastern Chad, the composite results reveal a break in convective activity and decrease in low-level convergence when the surge arrives that persists for about 6 days. These results offer great promise for improving the short-range prediction of rainfall over northern Africa.