Easterly Waves in the Intra-Americas Sea Region

Initiation of hurricanes in the tropical northeast Pacific is strongly associated with tropical easterly wave activity in the region, as a majority of hurricanes in the tropical northeast Pacific are spawned from these disturbances. While several factors influencing the strength and number of waves in the tropical northeast Pacific each year have been suggested, including barotropic instability, orography, and free propagation from the Atlantic, the relative importance of these factors and the large-scale modulation of them remain topics of active research. In this study, the formation of tropical easterly waves within the Caribbean and tropical northeast Pacific is revisited using ERA Interim fields in the Intra-Americas Sea region for May-November 1989-2007. Analysis of the conversion terms in the eddy kinetic energy equation in the vicinity of the Caribbean Low-Level Jet (CLLJ), and the extension of this jet into the tropical northeast Pacific known as the Papagayo Jet indicate a significant source of barotropic energy in the western Caribbean and eastern Pacific. These source regions are also coincident with track density and genesis maxima. Atlantic and Caribbean tropical storm formation is also found to be significantly anti-correlated with CLLJ strength, while eastern Pacific storm formation is correlated with jet strength. These results indicate that periods of strong easterly flow are associated with a greater frequency of easterly waves and storms in the Pacific, with the opposite effect in the Atlantic. The MJO is also a known factor in contributing to tropical storm formation in the eastern Pacific. Further investigation is required to understand the time scales and relative importance of the CLLJ and MJO mechanisms for enhancing synoptic activity and cyclogenesis in the IAS region. The smaller vorticity structures on the lee side of the Sierra Madre seen in our study also require further investigation to understand the possible role of the orography in setting the scale of EWs in this region and/or initiating such disturbances due to interactions of Atlantic zonal flow and EWs with the terrain, as suggested by previous studies. The authors are currently pursuing these topics further.