The boreal springtime formation of the Somali Jet: dynamics of its seasonal evolution and diurnal cycle

While the Somali Jet has been widely studied during the boreal summer months (JJAS), the early stages of the jet's formation in April and May are not as well documented or understood. Riddle and Cook (2008) noted that starting in mid-April, a strong southerly cross-equatorial jet is present at 925 hPa along the coast of East Africa. This preliminary stage in the development of the Somali Jet is distinct from the fully-formed Somali Jet in that 1) it is confined to a narrow region along the coast and 2) it consists solely of a meridional branch which terminates over the Horn of Africa. In this study, we examine the dynamics associated with the springtime jet's seasonal evolution and diurnal cycle using the NCEP II reanalysis and a high-resolution regional model simulation. A momentum budget analysis based on the NCEP II climatology shows that the cross-equatorial flow along the east African coast is governed by the large-scale pressure difference between northern and southern hemispheres over the Arabian Sea basin. This north-south pressure gradient becomes positive in early March and increases gradually throughout the boreal spring, primarily due to strong surface heating over the Asian landmass to the north, and cooling over southern Africa. However, the springtime coastal jet does not acquire a jet-like structure until mid-April when the zonal land/sea pressure gradient between the Indian Ocean and the African continent disappears, eliminating the previously strong zonal (onshore) component to the flow. The zonal branch of the Somali jet forms over the Arabian Sea in June, when depressions over India and Africa merge to form the zonal monsoon trough. The timing of the zonal branch formation is coincident with the onset of the Indian monsoon. The diurnal cycle of the springtime Somali Jet is examined using a high resolution regional climate model and found to be controlled primarily by day/night changes in surface friction. This study provides a physical understanding of the dynamics controlling the formation of the Somali Jet in the boreal springtime, and lays the groundwork for further investigation into, for example, its interannual variability.