Lee vorticity Production by Large-Scale Tropical Mountain Ranges. Part II: A Mechanism for the Production of African Waves

Abstract

A mechanism that acts to produce vorticity in the lee of large-scale mountain ranges embedded in an easterly flow in a stably stratified rotating atmosphere is investigated as it applies to the production of westward-propagating African waves. Three-dimensional numerical simulations of a dry, adiabatic flow using the PSU/NCAR MM4 model show that the Hoggar and Atlas Mountains of west-central Africa block low-level easterly flow, resulting in a barotropically unstable jet that is associated with the continuous production of lee vortices, which separate from the mountain and propagate downstream. The wavelength of the disturbances is roughly 1600 km, and they propagate to the west with a period of about 2.5 days. These characteristics correspond to those of observed waves in the Africa-Atlantic region. It is also shown that flow interaction with the topography of north-central Africa results in a midtropospheric easterly jet having a maximum wind speed near 10 ms?1 between 15° and 20°N, 0° and 10°E. The location and magnitude of this jet correspond to the so-called African easterly jet, which is usually attributed to strong surface temperature gradients over the continent of Africa. The numerical simulations suggest that vertically propagating internal gravity waves associated with flow over the topography may provide a constant source of zonal momentum for the maintenance of the midtropospheric African easterly jet.