LEE VORTICITY PRODUCTION BY TROPICAL MOUNTAIN RANGES

Abstract

Numerical simulations using the Penn State University/NCAR MM4 model are performed to examine a stably stratified, zonal easterly flow past large scale three-dimensional mountain ranges in a rotating, initially barotropic, atmosphere. Upstream blocking by the mountain range diverts the flow primarily to the south and around the mountain. Conservation of potential vorticity results in the formation of a horizontal jet at low levels south of the mountain. This jet is barotropically unstable and leads to a continuous production of synoptic scale vorticity maxima which separate from the mountain and propagate downstream. Numerical simulations using topography representative of the Sierra Madre in Mexico imply that this mechanism may be important in providing some of the initial disturbances which grow into tropical cyclones in the eastern North Pacific Ocean. The wave train produced in the simulations corresponds to waves with 3-7 day periods which have been identified observationally in the eastern North Pacific region. The sensitivity of this effect to the stability of the basic state and the upstream wind speed is investigated. Simulations are also performed which show that the Hoggar and Atlas mountains of west-central Africa block the low-level easterlies resulting in a barotropically unstable jet and a train of vorticity maxima which separate from the mountain and propagate downstream. The spacing of these disturbances is roughly 1600 km and they propagate to the east with a period of about 2.5 days. These characteristics correspond to those of observed waves in the Africa/Atlantic region. It will also be shown that the unique topography of north-central Africa results in a mid-tropospheric easterly jet which has a maximum between 0-10°E and 15-20°N. The location and magnitude of this jet correspond to the so-called African easterly jet which is usually attributed to the strong surface temperature gradients over the continent of Africa. The numerical simulations presented in this work suggest that the mechanical effect of the topography may provide a constant source of energy for the maintenance of the African easterly jet.