Genesis of Easterly Waves over the Intra Americas Seas and tropical Eastern Pacific.

Easterly waves (EWs) are characteristic features of the tropical atmosphere during summer seasons. Over the Atlantic Ocean, EWs frequently serve as seeds for tropical cyclones. While the nature and variability of EWs over other tropical regions, such as northern Africa and the western Pacific, are well known, efforts towards increasing the understanding of EWs over the Intra Americas Seas (IAS) and the eastern Pacific have just been explored recently.

The westward propagation of EWs over the Atlantic, and their presence over the IAS and the eastern Pacific, has led to discussions about their origin. Though initially EWs over the eastern Pacific were considered to be a continuation of those over the Atlantic, some modeling studies have started to suggest that they could be generated in situ, primarily forced by the high terrain over the northern mountain ranges of South America and over the Panama bight. This hypothesis is in agreement with previous results of EW genesis over Africa. However, these studies also highlight the necessity of a pre-existing EW for further development downstream.

In this study, we first use a series of diagnostics to identify regions where EWs over the IAS and Central America are present, as well as points of enhanced forcing characterized by diabatic heating. With a clear idea of the mechanisms of forcing and support of EWs, a second part consists of using a spectral modeling approach, similar to Thorncroft et al. (2008), and considers different heating profiles.

Results show that the mountain region over Central America is more likely to be a genesis point of EWs over the eastern Pacific. Additionally, the growth and support of EWs once they are created, strongly depends on the environment. In particular, EWs tend to propagate for a longer time when the westerly phase of MJO is present. Finally, different heating profiles are examined. Contrary to regions over Africa, deep convection is not the best profile for genesis of EWs over the IAS and eastern Pacific. Instead a shallow convective heating profile is more likely to support EW genesis due to energy sources and warm SSTs found over the eastern Pacific region.