Rapid intensification of Somali Jet kinetic energy prior to monsoon onset

Abstract

<p>The onset of the South Asian summer monsoon is characterized by low-level cross-equatorial flow in the western Indian ocean. This flow turns eastward and becomes a zonally oriented jet off the East African coast at around 10<sup>°</sup>N, which is called the Somali Jet. The Somali jet is an important factor in the monsoon onset over the Indian region and transports moisture from the Arabian sea, playing a key role in South Asian summer monsoon rainfall. The kinetic energy (KE) of the jet has an increase that is much more rapid (a few days) than the evolution of solar insolation forcing (over a month). With the help of high-resolution reanalysis data, we explore the factors responsible for this rapid increase in kinetic energy. Using calculations of the KE budget we find that KE generation, from the scalar product of geopotential gradient and horizontal winds, has a high correlation with KE itself, and furthermore shows a rapid increase at the time of jet onset. The major contribution of this KE generation comes from the meridional component (-v∂Φ/∂y) , and is confirmed by a decomposition of generation based on EOF analysis. We demonstrate that a dominant balance between the KE generation and KE advection exists, suggesting that the boundary layer at the location of the highest KE generation is advective in nature. Furthermore, we observe that high KE generation occurs in the regime where the local Rossby number is close to 1. The meridional wind (v) is, to a good approximation, linearly proportional to the meridional component of geopotential gradient (∂Φ/∂y), and the latitude at which this relationship between v and ∂Φ/∂y is the strongest coincides with the location of the jet strength maximum (around 10<sup>°</sup>N). This strong relationship and consequent abrupt increase of the KE generation diminishes as we ascend the troposphere. Together these findings give rise to an unconventional boundary layer dynamics view of the Somali jet.</p>