Monsoonal surges are plumes of moisture that move from the tropics and trigger thunderstorms over the poleward fringes of the North American monsoon. Predicting the onset and evolution of these surges is challenging. Previous research on the mechanisms of surges has focused on the tropical forcing. This work is based on an alternative hypothesize that breaking Rossby waves amplify the monsoonal upper-tropospheric high and cause an abrupt poleward shift in the "barrier" between the extratropics and tropics. This provides a conduit for poleward surges of moisture into the desert southwest.
It is hypothesized that Rossby wave breaking leads to mesoscale forcing that drives Gulf surges in two ways. First, the rapid poleward shift in the jet mountain-valley heating contrasts to gain dynamical control over the interior western United States. This creates a favorable environment to flux warm air near the surface into the Great Basin. Surface heating increases the lower-middle tropospheric thermal contrasts and leads to the development of a thermal low. Secondly, wave breaking injects filaments of extratropical cyclonic air into the subtropics leading to the formation of an inverted trough. The inverted trough is consequently wrapped westward around the ridge toward the Gulf of California. As this inverted trough propagates anticyclonically around the upper-level high, moist lower tropospheric air is forced poleward toward the surface low associated with the heated plateaus over the Great Basin. The confluence of these responses to wave breaking opens the midlatitude interior western North America to poleward intrusions of tropical moisture that cause widespread storm activity.
These hypotheses will be tested through an observational diagnostic study and a modeling study. The modeling study will employ simulations from an adaptive grid numerical model, a framework suited to capture the nonlinearities of wave breaking and the ensuing mesoscale response. A set of simulations will be performed for case studies from the North American Monsoon Experiment (NAME). Sensitivity runs will be conducted with and without extratropical dynamics, as well as with and without tropical disturbances to examine the importance of the extratropics in providing conditions favorable to Gulf surge events.
Broader impacts of the study derive from the importance of the North American monsoon circulation for water resources, fire weather frequency, energy availability, and land use. By making use of the predictable nature of the midlatitude flow, this project could lead to improved operational predictions of monsoonal surges in the southwestern United States.
