Disturbances along the Arizona Monsoon Boundary

Abstract

The possibility of disturbances along the boundary between two air masses in the southwestern United States and northwestern Mexico during the summer is examined. This boundary is an approximately north-south confluent zone resulting from the circulation pattern around Pacific- and Atlantic-based subtropical highs. Consequently, the boundary is distinguished by relatively warmer, moister air to the east and cooler, drier air to the west. This ?Arizona monsoon boundary? is so named because of its association with a seasonal flow reversal in Arizona leading to an increase, during the summer, in monthly rainfall there. This study is intended to investigate whether disturbances similar to those found along midlatitude fronts are a possibility, since wind shears along this boundary are comparable to those found in middle latitude fronts. Thus, we examine the barotropic/baroclinic stability of the boundary. In addition, we will examine an example of a wave that apparently developed on the boundary in early July 1984. The results of the linear calculations show that the monsoon boundary is indeed unstable with maximum growth rates between 0.4 and 0.6 d?1 at a wavelength near 1200 km. The energy source for the most unstable waves is baroclinic, and the waves are barotropically damped. The structure of these waves is typical for short baroclinic wave. The maximum amplitude in geopotential perturbation is at the surface, while the most unstable wave is confined below 700 mb. The observed wave has its maximum amplitude at tropopause level, although its horizontal scale and growth rate were similar to the most unstable linear wave. This is energetically very different (i.e., barotropic) from the most unstable linear wave, since the amplitude of the geopotential perturbation is greatest where the horizontal sheer is largest.