Annual intensification of the Somali jet in a quasi-equilibrium framework: Observational composites

The annual intensification of the southwesterly Somali jet, which accompanies the onset of the Indian monsoon, is rapid compared to the evolution of the seasonal insolation forcing. We present a 27-year composite of the onset of this jet, based on reanalyzed winds, that shows the abrupt component of jet onset occurs over the Arabian Sea at least 1000 km east of the East African highlands, and is accompanied by large increases in both deep convection and baroclinic circulation. The core of the cross-equatorial jet, which is located over land adjacent to the East African highlands, evolves more slowly than the winds over the Arabian Sea. This topographically-bound flow accounts for about one quarter of the global cross-equatorial mass flux in the solsticial Hadley cell, while the flow that intensifies abruptly over the Arabian Sea accounts for another quarter.

The onset of the southwesterly jet is examined in a convective quasi-equilibrium framework. The change in low-level geopotential accompanying the onset of the jet is estimated to be consistent with an increase in boundary layer entropy over the Arabian Sea, which is in turn collocated with a wind-driven increase in ocean evaporation and an increase in deep-tropospheric ascent. These results suggest that a wind-evaporation feedback may be responsible for the abrupt nature of jet onset.