Numerical Simulation of the Somali Jet

Abstract

In this study we show that many of the observed features of the cross-equatorial low-level jet of the Arabian Sea, Indian Ocean and Somalia can be numerically simulated by including 1) the cast African and Madagascar mountains, 2) the beta effect and 3) a lateral forcing from the east around 75°E. This lateral forcing at 75°E is, in fact, a solution of another numerical model?one where the land-ocean contrast heating in the meridional direction is incorporated in much detail to simulate the zonally symmetric monsoons, essentially following Murakami et al. (1970). This zonally symmetric solution of a very long-term numerical integration from a state of rest exhibits many of the observed characteristics of the broad-scale monsoons at 75°E. This later solution is used as a lateral forcing for the low-level jet simulations over the Arabian Sea-Indian Ocean. The numerical model presented here is a one-level primitive equation model with a detailed bottom topography and a one-degree latitude grid size. Results of several controlled numerical experiments suppressing or including orography, the beta effect and the broad-scale lateral monsoon forcing at 75°E are discussed in this paper. When all the three above-mentioned parameters are included, features such as strong winds just downstream from the Madagascar mountains, an equatorial relative speed minimum, an intense jet off the Somali coast and a split of the jet over the northern Arabian Sea are simulated from an initial state of rest. The Ethiopian highland appears crucial for the simulation of the Somali coast strong winds; the Madagascar mountains are most important for the strong winds just downstream from Madagascar. The split in the jet over the Arabian Sea is analyzed as a barotropic instability problem. The beta effect is essential for the simulation of the observed geometry. Experiments with a weak broad-scale monsoon forcing at 75°E fall to produce strong winds over cast Africa. The implications of this forcing are analyzed in this paper and some relevant observations are presented.