The sensitivity of the West African monsoon circulation to intraseasonal soil moisture feedbacks

Intraseasonal soil moisture variability has the potential to feed back onto the West Africa monsoon circulation through its influence on surface turbulent fluxes and planetary boundary-layer characteristics. Using satellite observations and an atmospheric reanalysis, we investigate intraseasonal soil moisture-atmosphere feedbacks triggered by large-scale dynamics within the West African monsoon. Surprisingly, even though the surface response across the Sahel to strong convection is short-lived (days) and precipitation accumulations are spatially and temporally heterogeneous, a coherent regional-scale surface response to intraseasonal variability is observed. This surface response then feeds back onto the West African monsoon circulation. For example, during a dry intraseasonal event, Sahelian surface soil moisture significantly decreases, which elevates surface temperatures by 1.5 degrees C and shifts the monsoon circulation southward by approximately 1.5 degrees latitude. Also, during a wet event the surface moistens and cools which leads to a northward monsoon shift. Alongside a low-level wind response, the African Easterly Jet (AEJ) also responds to surface changes due to variations in the meridional temperature gradient. For example, an increased temperature gradient during a dry event intensifies and shifts the AEJ southward. The combined response of low-level monsoon westerlies and the AEJ impacts low-level shear and characteristics of strong convection. Elevated low-level shear during a dry event promotes an intensification of deep convection across southern West Africa. This study provides new insight into the sensitivity of the West African monsoon circulation to intraseasonal soil moisture feedbacks and encourages similar research in other regions. An improved understanding and model representation of soil moisture-atmosphere feedbacks has the potential to improve forecasts beyond daily time-scales and enhance early warning systems.

Automated summary

Intraseasonal soil moisture variability can feed back onto the West Africa monsoon circulation through its influence on surface turbulent fluxes and planetary boundary-layer characteristics. Surprisingly, even though the surface response across the Sahel to strong convection is short-lived, a coherent regional-scale surface response to intraseasonal variability is observed, which then influences the West African monsoon circulation.