A Modeling Study of Easterly Waves and Their Intraseasonal Variability in the East Pacific

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Easterly waves (EWs) are wavelike atmospheric disturbances with a ridge-to-trough size of roughly 1,000km which propagate from east to west at low latitudes. EWs are important weather makers in much of the tropics and play a critical role in hurricane development. Much of the research has focused on African EWs, which along the easterly jet over the Sahel and propagate westward over the North Atlantic. The genesis of these waves is often explained in terms of jet instabilities particular to the easterly jet, yet similar EWs are found in the tropical eastern Pacific where the large-scale flow characteristics are very different. One explanation for the presence of EWs in the eastern equatorial Pacific is that they originated as African EWs, formed through processes associated with the African easterly jet, and subsequently propagated across the Atlantic and the Caribbean to reach the eastern Pacific. While there is some evidence to support this theory, previous work by the PI (see AGS-1347738) suggests that EWs can form in the tropical eastern Pacific despite the clear differences between this region and the Sahel.This project tests the hypothesis that east Pacific EWs are largely caused by upscale growth of topographically-generated disturbances downwind of the mountain ranges of Central America. Numerical simulation by the PI have shown that EW activity is dramatically reduced when the Central American topography is flattened, but these simulations were crude as the removal of topography has a variety of dramatic effects on the meteorology of the region including mean flow, cloudiness and precipitation. Simulations to be performed here would use a "nudging" technique to constrain the background flow despite modifications to topography and other factors so that a more focused study of wave generation by specific forcing mechanisms can be performed in a realistic setting. The work would use the Weather Research and Forecasting (WRF) model in a convection-permitting configuration. The work would also address the modulation of EWs by intraseasonal oscillations of the background flow. Additional work uses diagnostic techniques including vorticity and energy budgets to study EW generation and propagation in observational data.The work has societal relevance due to the key role of EWs in tropical meteorology and hurricane genesis. If EWs found in the eastern Pacific originated in the African easterly jet they may be more predictable than if they are generated locally by Central American topography, thus the results of this work have potential implications for operational forecasting. The modulation of EW activity by intraseasonal oscillations is also of practical interest given the long lead times of intraseasonal oscillations. In addition, the project provides support and training for a graduate student, thereby providing for the future workforce in this research area.