Diurnal Variations in the Meridional Overtuning Circulation over West Africa and the Associated Convective Cloud Structures

Recent work by Huaman et al. (2023) has shown that there is a strongly linked diurnal cycle in low-level northerly winds, large-scale vertical motion, and rainfall over West Africa before and during the active monsoon season. This presentation expands upon this work by quantifying the diurnal changes in the meridional overturning circulation (MOC) and the associated three-dimensional convective cloud structures over the West African region during the pre-monsoon (April - June) and monsoon (July - September) seasons from 2014 - 2021. The location, strength, and vertical structure of the MOC were evaluated based on stream function fields calculated with MERRA-2 data. GPM Dual-frequency Precipitation Radar (DPR) observations were used to classify precipitating clouds into shallow convective, deep convective, and stratiform categories.

In both seasons, the MOC shows a strongly slanted vertical structure with a shallow (below 800 hPa) nose over land that deepens toward the coast. The shallow nose is closest to the coast at noon and rapidly transitions north as the day progresses (about 5° during the pre-monsoon and 15° during the active monsoon). This transition is associated with the development of shallow convective clouds. The nose becomes less distinct in the evening as deep convective and stratiform rain becomes more predominant. The diurnal evolution of the MOC is more gradual during the pre-monsoon season and appears to be initiated by deepening convection associated with the sea breeze and maintained by a nocturnal low-level jet. The diurnal evolution of the MOC during the active monsoon is much more abrupt with the strongest deepening in the late evening/early morning associated with organized convective systems far inland. These results suggest a strong interaction between the depth of convection and the large-scale meridional circulation over large (hundreds of km) latitudinal bands and over short (diurnal) time scales.

To further study these interactions in a GCM framework, we assess the ability of CAM5 to produce the West African MOC and its associated convective structures. For the latter, the vertical reflectivity distributions from the DPR convective and stratiform rain types were compared to CAM5 convective and large-scale rain reflectivity structures using a modification of the Cloud Feedback Model Intercomparison Project (CFMIP) Observation Simulator Package (COSP). Initial results show that CAM5 produces a MOC with a similar magnitude to MERRA-2, but with a northern boundary much farther north and with too extreme of a vertical slant. The timing of the CAM5 convective and large-scale vertical reflectivity distributions misalign with peak times from the DPR, potentially explaining the incorrect MOC structure.