The Role of Midlatitude Circulations in Triggering Extratropical Convection during the 2004 North American Monsoon

The purpose of this paper is to document the first stage of a three-stage process that leads to a strong monsoonal surge of water vapor from the Gulf of Mexico to the southwestern U.S. During this first stage, Rossby Wave Breaking (RWB) occurs over the western part of North America resulting in a large and positively-tilted ridge of high pressure flanked by a southwestward propagating polar and subtropical jet streak. As the first wave within the polar jet moves downstream from the RWB region, the southwestward-propagating jet streaks phase with northerly flow around the large ridge over northern Mexico and the southwestern U.S. The divergence within the left front quadrants of both jet streaks promotes ascent and adiabatic expansion resulting in a cold pool over the Northern Rio Grande River Valley. This cold pool contracts in scale producing a mid-upper tropospheric frontal zone. As the cold pool propagates southeastwards towards the Gulf of Mexico it results in the dynamical destabilization in the region over the southeastern part of Texas and coastal Louisiana. Moisture transported from the Pacific at mid-levels into this region of cold pool propagation further enhances the favorability for mesoscale convective system development. This large area of convection then establishes a massive area of outflow aloft that both turns the flow northwestward and injects moisture from the subtropics into the mid-troposphere thus establishing the environment for the stage II upstream transport of monsoonal moisture around the base of the monsoonal high pressure area.