East Pacific Easterly Waves: Interactions With Wind Jets in Current and Future Climate

East-to-west moving waves with wavelengths of about 1,000km are an important weather feature in the region spanning West Afria, the subtropical and tropical Atlantic, the Caribbean, and the low-latitude equatorial Pacific. These easterly waves (or EWs, meaning waves moving from the east) are best known as the precursors to hurricanes but they are also important rain-bearing systems in West Africa the Caribbean, and Central America.

Despite the importance of EWs their formation mechanisms and sensitivities to ambient conditions are still not fully understood.

One issue of particular interest for East Pacific EWs is their interaction with the Central American topography. Central America is known for its gap winds, strong winds that funnel through gaps in the American Cordillera, one at the Isthmus of Tehuantepec, another at the Gulf of Papagayo in northern Costa Rica, and a third that channels air through the mountains and into the Panama Bight.

Previous work by the Principal Investigator (PI) considered the role of the gap jets, in particular the horizontal shear of the jets, in promoting the generation of East Pacific EWs and reinvigorating EWs which have crossed Central America from the Caribbean.

Work conducted under this award would use eddy kinetic energy and vorticity budget calculations to elucidate the effects of the gap jets. Further work uses computer models to study the effects of gap winds by creating simulations in which the Central American topography is artifically altered to fill in the gaps and block the jets. In addition to the direct effects of the jets and associated wind shear on developing EWs the project considers the potential impact of the cooling of the ocean surface by strong surface winds.

Cooler sea surface temperatures (SSTs) can inhibit convection and precipitation, which could hinder the development of EWs.

Additional work examines the effects of climate change on gap flows and EWs. The PI's recent analysis of climate model simulations suggests that gap flows near the Gulf of Papagayo will strengthen as the world warms, and the strengthening could enhance EW formation. Work conducted here revisits this issue using the regional Weather Research and Forecasting (WRF) model, which can be run at considerably higher resolution to better resolve the gap topography and the mesoscale dynamics of EWs.

A further consideration is that EWs may shift away from the Central American coast, causing potentially serious reductions in rainfall and water resources in the region. Changes in the location of fully-developed EWs may be strongly influenced by the pattern of Pacific SST trends, and WRF simulations are used in combination with climate model simulations from the Community Earth System Model (CESM) to address the effects of SST trends.

The work has societal value due to the importance of EWs for hurricane formation and as rain-bearing weather systems for agriculture and water resources. In addition, the PI's group mentors undergraduate summer interns through the Earth System Modeling and Education Institute (ESMEI), a Research Experiences for Undergraduates (REU) program at Colorado State University.

The PI also works with the Bridge Program of the American Geophysical Union to recruit graduate students from diverse backgrounds. The project provides support and training to one graduate student, thereby developing the future workforce in this research area.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.