The influence of MJO-driven land-atmosphere feedbacks on the West African monsoon

Abstract:

Previous studies have shown that the Madden-Julian Oscillation (MJO) influences precipitation rates across West Africa and the dynamics of the West African monsoon. A better understanding of the land surface response to different phases of the MJO will support stakeholders that rely on forecasts of land surface characteristics. In a semi-arid region such as the Sahel, MJO-driven fluctuations in rainfall can perturb land surface fluxes via soil moisture variations. In principle, the surface flux response may feedback onto the atmosphere, though this question is yet to be addressed. Using earth-observational data including land surface temperature, soil moisture, and vegetation optical depth, we demonstrate a strong and spatially coherent response of the land surface to the MJO.

As seen in other semi-arid regions, the land surface response to precipitation anomalies influences land-atmosphere interactions. Decreased precipitation during MJO phases six and seven are associated with a warmer, drier surface and a reduction in total vegetation content. The surface response to changes in precipitation affects the partitioning of the surface energy balance with a favouring of surface sensible heat flux over latent heat flux. Surface warming promotes the development of a heat low circulation anomaly across the Sahel which influences zonal variations in meridional moisture fluxes across the West African monsoon and the likelihood of extreme convection. The atmospheric response to changes in the land surface associated with the MJO highlights the importance of land-atmosphere feedbacks on the dynamics of the West African monsoon. As the MJO varies on intra-seasonal timescales, fluctuations in land-atmosphere feedbacks across the Sahel are a key source of predictability for sub-seasonal weather forecasts.