Diurnal variations of the meridional overturning circulations over West Africa during the premonsoon and monsoon seasons
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AbstractDiurnal variations in the shallow and deep meridional overturning over West Africa during the premonsoon (AprilMay) and monsoon (JulySeptember) seasons were analyzed using ECMWF Reanalysis 5th Generation (ERA5) reanalysis and Tropical Rainfall Measuring Mission (TRMM)/Global Precipitation Measurement (GPM) satellite radar data from 1998 to 2019. We found that a daytime sea breeze and two nocturnal lowlevel jets have varying impacts on the diurnal cycle of convection and the meridional circulations over West Africa, depending on the season. During the premonsoon, the zonal rain belt is centered over the Gulf of Guinea and the sea breeze initiates intense deep convection (and thus deep overturning) over the coastal region in the afternoon that propagates northward into the early evening. The nocturnal Sahel/Sahara lowlevel jet (SLLJ) then drives lowlevel convergence into the West African heat low, producing a strong, dry shallow overturning over land at night. During the monsoon, the rain belt is centered over land (N) and the sea breeze is forced by weaker temperature gradients, producing less intense convection that does not propagate as far inland as during the premonsoon. However, organized convection inland maintains deep daytime overturning. At night, a coastal lowlevel jet strengthens northward moisture transport from the Gulf of Guinea into the active monsoon region after the sea breeze dissipates, helping maintain convective systems with large stratiform components and continued deep meridional overturning over land. The nocturnal SLLJ still drives lowlevel convergence into the West African heat low, but the dry shallow meridional overturning is farther north and weaker than in the premonsoon. While ERA5 winds and moistureflux convergence are generally consistent with diurnal variations in the TRMM/GPM precipitation, ERA5 precipitation is not. The reanalysis indicates significant biases in the timing and magnitude of rainfall over the Gulf of Guinea and West Africa in each season, with the convective and largescale rain fields both contributing to the disparate precipitation patterns.
