The Role of African topography on the South Asian Monsoon precipitation and circulation

The Somali cross-equatorial jet is estimated to contribute up to half of the mass flux crossing the equator during the Asian monsoon season. Previous studies have argued that the Somali jet is strengthened by the East African Highlands, which act as a wall and accelerate the flow (e.g., Krishnamurti et al. 1976, Sashegyi and Geisler 1987). Besides, observational studies have shown a positive correlation between the strength of the Somali jet and the South Asian Monsoon (SAM) precipitation (e.g., Findlater 1969, Halpern and Woiceshyn 2001). These imply that the existence of the topography would relate to a stronger SAM. However, in a more recent study, Chakraborty et al. (2002) found that if the African topography is removed in a comprehensive general circulation model (GCM), the SAM strengthens. In this study, we use the GFDL AM2.1 GCM to conduct experiments with and without topography in Africa, to further examine its influence on the cross-equatorial Somali jet and the SAM. We find that when the African topography is removed, while the cross-equatorial Somali jet weakens, the SAM precipitation increases, consistent with the results in Chakraborty et al. (2002). The counter-intuitive precipitation increase is related to lower-level cyclonic wind anomalies, and associated moisture convergence, that develop over the Indian Ocean in the absence of the African topography. Potential Vorticity (PV) budget analyses along particle trajectories show that this cyclonic anomaly primarily arises because, in the absence of the blocking effect by the African topography, air particles originate from higher latitude with larger background planetary vorticity.