Assessing Shallow Meridional Circulations over the East Atlantic ITCZ and West African Monsoon Regions

The shallow meridional circulation (SMC) is a vertical-meridional overturning circulation with return flow in the lower troposphere. SMCs occur in the tropical East Pacific, East Atlantic, and West Africa; however, the physical mechanisms producing the SMC in the East Pacific are not the same as in the East Atlantic and West African regions. This study aims to assess the vertical structure of the East Atlantic intertropical converegence zone (ITCZ) and West African monsoonal meridional circulations using the CloudSat and TRMM satellites, and data from four reanalyses (MERRA-2, ERA-Interim, ERA-5, and JRA-55) for the period 1998-2013.

This study estimates vertical profiles of latent heating using stratiform and deep convective precipitation from the TRMM precipitation radar (PR) and shallow precipitation from the more sensitive CloudSat radar. Diabatic heating (Q₁) is calculated from reanalyses as the residual of the thermodynamics equation. The meridional circulation linked to variations in the ITCZ and monsoon heating is analyzed using reanalysis winds.Preliminary results show that the SMC induced by shallow convection has strong seasonality and different large-scale forcing mechanisms. The East Atlantic SMC is located in the northern part of the ITCZ and is stronger during boreal spring (MAM). The return flow at 700-hPa seems to be associated with the southerly component of the anticyclonic circulation over West Africa (the Saharan high) and low-level pressure gradient modulated by the moderate meridional sea surface temperature gradient. The West African SMC, also stronger in MAM, is located north of the monsoon rainband and its shallow return flow penetrates through the latitude of the monsoon. Reanalyses show large discrepancies between each other in both regions. For example, MERRA2 (ERA-Interim) produces stronger and deeper (weaker and shallower) convection compared to the other reanalyses, which in turn impacts the strength and height of the resulting SMC.