On the Role of the African Topography in the South Asian Monsoon
The Somali jet, a strong low-level cross-equatorial flow concentrated in a narrow longitudinal band near the coast of Somalia, is a key feature of the South Asian monsoon (SAM) circulation. Previous work has emphasized the role of the East African highlands in strengthening and concentrating the jet. However, the fundamental dynamics of the jet remains debated, as does its relation to the SAM precipitation. In this study, numerical experiments with modified topography over Africa are conducted with the GFDL atmospheric model, version 2.1 (AM2.1), general circulation model (GCM) to examine the influence of topography on the Somali jet and the SAM precipitation. It is found that when the African topography is removed, the SAM precipitation moderately increases in spite of a weakening of the cross-equatorial Somali jet. The counterintuitive precipitation increase is related to lower-level cyclonic wind anomalies, and associated meridional moisture convergence, which develop over the Arabian Sea 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 and with weaker cross-equatorial flow, air particles originate from higher latitudes with larger background planetary vorticity and thus larger PV.
Additional Information© 2016 American Meteorological Society. Manuscript received 3 July 2015, in final form 11 May 2016. Published online: 25 July 2016. This work was partially supported by the Caltech Terrestrial Hazard and Reporting (THOR) Center. We thank Brian Hoskins, an anonymous reviewer, and Ming Cai for their helpful review and insightful suggestions. We also thank Zhiming Kuang, Ian Eisenman, andOlivier Pauluis for the helpful discussions. The numerical simulations and analyses were conducted on the Caltech's Division of Geological and Planetary Sciences CITerra high-performance computing cluster.
Published - jas-d-15-0182.1.pdf