A Model of the Asian Summer Monsoon.Part II: Cross-Equatorial Flow and PV Behavior

Abstract

The model developed and tested in Part I of this paper is used to investigate the mechanisms that sustain the low-level East African jet. The East African Highlands and a land/sea contrast in surface friction are shown to be essential for the existence and concentration of cross-equatorial flow. The question of inertial instability as air crosses the equator is addressed from a potential vorticity (PV) perspective. Surface friction and local diabatic heating provide mechanisms for material modification of PV and both are important for the maintenance of the jet. A compensation phenomenon that stabilizes the flow is suggested whereby reduced material tendencies of PV over the Indian Ocean are compensated for by increased modification over Africa. It is speculated that this stabilization may be a key ingredient in the monsoon circulation itself. However, in some cases there is intensification and increased variability of the wind speed maximum off the Somali coast, which may be an indication of reduced PV modification due to weaker material tendencies of PV. The tendency of the southern flank of the low-level monsoon inflow to turn anticyclonically over the Arabian Sea and to avoid India may also be a sign of weaker PV modification. The study identifies strong sensitivity of the East African jet to changes in heating over the southern Indian Ocean and highlights the importance of reliable data in this region for monsoon prediction.