Kinetic Energy Generation in Cross‐Equatorial Flow and the Somali Jet.

Autor: Seshadri, Ashwin K., Dixit, Vishal
Předmět:
Zdroj: Journal of Geophysical Research. Atmospheres; 7/16/2022, Vol. 127 Issue 13, p1-18, 18p
Abstrakt: 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. Plain Language Summary: This study is motivated by a basic question: How exactly is the kinetic energy (KE) in the South Asian monsoonal cross‐equatorial flows and Somali Jet generated? While the mechanism of KE generation for midlatitude weather systems is well understood, a similar understanding for monsoons does not exist. The popular notion suggests that, in monsoons, the KE to drive circulations is derived from the latent heat released during the condensation process. We conducted a detailed KE budget for these flows using ERA5 reanalysis data to identify its sources and sinks. Furthermore, we propose a simple model to understand KE generation at any atmospheric level in terms of surface and tropospheric contributions. Our analysis identifies 3 "hot spots" that act as refueling stations for the monsoonal flow. Notably, all these hot spots lie in close proximity of orography and are significantly away from core monsoon regions where significant condensation occurs. We propose that KE to drive cross‐equatorial flows comes mainly from the interaction of the cross‐isobaric flows with surface features, while remote monsoon precipitation can control it only indirectly through its influence on the orientation of isobars. Key Points: KE generation in cross‐equatorial flow maximizes at "hot spots" near East African orography, Western Ghats, and Arabian SeaA decomposition of low‐level kinetic energy (KE) generation shows that contributions due to surface pressure gradients, adjusted for orography, are dominantThe dominant balance in the KE budget is between generation and its meridional advection near the jet [ABSTRACT FROM AUTHOR]
Databáze: Complementary Index