The role of convection and low-level wind jets in the development of east Pacific easterly waves

Abstract:

Convective processes and low-level wind jets are important contributors to the development of east Pacific easterly waves. East Pacific easterly waves have been shown to form without pre-existing African easterly waves and are linked to topographically-forced convective activity in the Panama Bight, which is located off of the coast of northwest South America. A convective-permitting WRF model simulation is performed to analyze a vorticity budget for a Panama Bight mesoscale convective system (MCS) that develops into an easterly wave without African easterly wave support. Top-heavy stratiform structures, supported by top-heavy vertical motion profiles, appear to be important in the transition from an MCS to an easterly wave in WRF by intensifying and spatially growing a mid-level vortex via vortex stretching.

A comparison with ERA5 reanalysis and CMORPH precipitation shows that the evolution of the disturbance is reproduced reasonably well by the WRF model and that the behaviors of the Papagayo and Choco jets are captured. In particular, the Papagayo jet increases after the MCS forms and during the transition to an easterly wave. Contrasts in the vertical structures of vorticity and vertical motion between the model and reanalysis highlight different potential responses to the Papagayo jet and highlight the need for further study of the interactions between low-level wind jets and easterly waves. To this end, the influence of low-level wind jets on developing vorticity disturbances, such as easterly waves, is explored through two regional WRF simulations—a control simulation and a modified topography simulation. The modified simulation “fills in” the gaps in the Sierra Madre mountain range to effectively eliminate the Papagayo and Tehuantepec jets. Impacts on the mean-state are assessed and a vorticity-tracking algorithm is developed to investigate changes in vorticity track statistics and easterly wave structures between the simulations.