A Planetary Boundary-Layer Model for the Somali Jet

Abstract

In this paper we investigate a baroclinic boundary layer of cross-equatorial flow over the Arabian Sea with a fine-mesh (200 m) vertical resolution numerical model. The dynamical model utilizes a prescribed vertically varying pressure field from observations. The vertical variation of eddy diffusivity is parameterized. Long-term steady state on a meridional-vertical plane is obtained via integrations of the numerical model starting from an initial Ekman balance. The principal results of the study include a realistic simulation of the vertical structure of the cross-equatorial flow. The balance of forces and boundary-layer transitions between strong cross-equatorial flow, the low-level jet and an ITCZ over the northern Arabian Sea are analyzed in some detail. We note that an advective boundary layer is dominant across the equator, and the core of the low-level jet is located toward the poleward edge of the advective boundary layer. North of this region a gradual increase of upward motion is noted and this region is identified as the Intertropical Convergence Zone (ITCZ). The ITCZ occurs in a transition zone between an advective and a quasi-Ekman boundary layer.