Explaining Dynamics and Rapid Onset of the Somali Jet Through Its Kinetic Energy Budget


The low-level Somali jet is the primary mechanism of moisture transport for the South Asian monsoon. It precedes monsoon onset over India and shares its key characteristic features such as rapid intensification and slower retreat during seasonal evolution. This study analyzes the Kinetic Energy (KE) budget of Somali jet region using high spatiotemporal resolution reanalysis (ERA5) to explain these key features. The KE budget reveals that in the southern hemisphere, the easterly flow that ultimately feeds the jet exhibits a conventional Ekman balance, with KE generation balanced by frictional dissipation. A unique “advective balance” – balance between KE generation in the northward flow and its advection emerges as the jet begins to form near the equatorial region. The fully formed Somali jet exhibits a three-way balance between KE generation, its advection, and dissipation. A non-dimensional parameter - boundary-layer local Rossby number (Ro) characterizes the transitions across these regimes. The large Ro regime describes an advective balance under which KE-generating meridional winds become proportional to meridional pressure gradients yielding a nonlinear (quadratic) dependence of KE generation on pressure gradients. This nonlinear relation explains rapid onset of the jet as well as the asymmetry between rapid onset and slower retreat, leading us to propose a simple model for approximating the seasonal evolution of kinetic energy in the Somali jet given the evolution of pressure gradients. In summary, this work shows that Somali jet onset is closely tied to the seasonal evolution of Ro in the region where the advective boundary layer appears.