Kinetic Energy Generation in Cross-Equatorial Flow and the Somali Jet

Abstract

In response to north-south pressure gradients set by the annual march of the Sun, a cross-equatorial flow that turns to become a low-level Somali Jet at around 10°N is established in the lower troposphere over the Indian Ocean. This flow plays a fundamental role in the Indian monsoon. A mechanistic understanding of drivers of this flow is lacking. Here, we present a seasonal-mean analysis of the kinetic energy (KE) budget of the low-level flow using high spatiotemporal resolution ERA5 reanalysis to identify sources and sinks of KE. We find that the largest KE generation occurs around East African orography where the Somali Jet forms while significant KE is also generated over the Western Ghats and the Madagascar Island (“hot spots”). These regions are distant from core monsoon precipitation regions, suggesting that local circulations driven by condensation do not directly produce the bulk of KE during monsoons. A unique KE balance supports the generation of the Somali Jet, with KE generation balanced by nonlinear KE advection as it forms. Over oceans, KE generation occurs mainly due to cross-isobaric meridional winds in the boundary layer (BL). In contrast, over the East African highlands and Western Ghats, KE generation maximizes just above the BL and mainly occurs due to the interaction of flow with orography. We propose a simple decomposition of lower tropospheric KE generation into contributions from surface pressure, orography, and free-tropospheric gradients that corroborate the important role played by surface pressure gradients once adjusted for effects of orography.