The Role of African topography in the South Asian Monsoon

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). Consistently, 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). However, in a recent study, Chakraborty et al. (2006) found that if the African topography is removed in a comprehensive general circulation model (GCM), the SAM strengthens, through strengthening of the oceanic westerlies and enhancement of the moisture convergence in the Indian region.

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, the SAM precipitation increases, consistent with the results in Chakraborty et al. (2006). Interestingly, our results also show that the cross-equatorial Somali Jet does weaken in the absence of the African topography, in agreement with previous studies. We analyze changes in the SAM precipitation in the context of the moisture budget, which shows that the increase in precipitation in the no-African topography experiment is primarily due to an increase in wind convergence. The dynamics of the cross-equatorial Somali jet is further investigated within the framework of the Potential Vorticity (PV) budget, to analyze how changes in friction and diabatic heating as topography is removed affect the PV tendency in this region.