Analysis of the Environment that Supports Easterly Waves over the Eastern Pacific and the Intra-Americas Sea in the Boreal Summer—A Potential Vorticity Perspective

Abstract

Abstract A review of the mean state over the tropical eastern Pacific (EPAC) and the Intra-Americas Sea (IAS) region is provided to assess the characteristics that impact the development and genesis of easterly waves (EWs). The EPAC–IAS region is characterized by complex topography, the Western Hemisphere warm pool, the ITCZ at 10°N, and predominant deep convection over the Panama Bight around 9°N, 78°W. A prominent easterly jet at 600 hPa of about 5.5 m s−1, is oriented approximately parallel to the Mexican coast. The jet is characterized by a strip of high potential vorticity (PV) on the cyclonic shear side and low PV on the anticyclonic side. This distribution of PV satisfies the necessary conditions for barotropic instability: the Charney–Stern condition, as well as the Fjørtoft condition. Together these conditions suggest the potential for barotropic growth of EWs over the EPAC region. The mean high PV region over the EPAC is created in association with two different populations of cloud/convection systems: stratiform and shallow, with the former being key for the creation of positive PV anomalies at midlevels. Evidence is also provided that suggests that the low PV region arises in association with sources of negative PV anomalies over the Sierra Madre region likely resulting from frequent dry convection. This is a key and novel result that is basic for the setting up of a negative meridional PV gradient and fundamental for the Charney–Stern condition associated with barotropic instability and growth of EWs. Significance Statement The tropical eastern Pacific is influenced by synoptic easterly waves that impact daily weather in the region and can trigger tropical cyclones. This research explores the nature of a midlevel jet that supports the development of easterly waves in this basin. The jet is established in association with moist convection over the ocean that leads to a midlevel potential vorticity maximum equatorward of the jet and, frequent dry convection over the Mexican Sierra Madre region that leads to a low-level potential vorticity minimum poleward of the jet. This finding highlights the need to better understand, and ultimately predict, these potential vorticity sources in order to better understand and predict the nature of the easterly wave developments in this region.