Tropical Easterly Waves in the Atlantic and East Pacific and their connections to the West African and Central American Monsoons

Tropical easterly waves (TEWs) are synoptic-scale inverted, Rossby Wave-like disturbances that propagate westward across the tropics. Globally, there are two main regions of high TEW activity: the tropical Atlantic and East Pacific. In the Atlantic basin, TEWs form over eastern Africa and are supported by the African Easterly Jet (AEJ) prior to advection over the eastern Atlantic while the East Pacific region is particularly interesting in that TEWs can propagate across the Atlantic basin and form locally through convective processes. TEWs also propagates through two distinct monsoonal regions: West Africa and Central America. Studies have shown that the onset dates of these monsoons have significant impacts on the location of the Intertropical Convergence Zone (ITCZ) and the strength and location of the AEJ in the case of the West African Monsoon. This study aims to analyze if there is a relationship between track density of TEWs in different reanalysis datasets with the variations of the West African and Central American Monsoons.

TEWs were identified using an objective tracking algorithm which used 5 criteria: (1) zonal winds are less than 2.5 m/s, (2) the curvature vorticity anomaly exceeded the 70th percentile of the climatology, (3) the advection of curvature vorticity is equal to 0 s^-2, (4) the TEW lasted for at least 2 days, and (5) the median speed of the TEW is -25 m/s to -2 m/s (i.e., it maintained a westward propagation). Six different reanalysis datasets were analyzed at 850 mb, 700 mb, and 600 mb across a domain extending from 100°W to 30°E and 0° to 25°N. These datasets included the European Centre for Medium-Range Weather Forecasting (ECMWF) Reanalysis v5 (ERA5) from 1986-2022 and the National Centers for Environmental Prediction-Department of Energy Reanalysis II (NCEP-R2) from 1986-2001 along with the Climate Forecasting System Reanalysis (CFS-R) from 1979-2010, ECMWF Reanalysis – 40 years (ERA-40) from 1965-2001, ECMWF Reanalysis – Interim (ERA-Interim) from 1979-2010, and NCEP/National Center for Atmospheric Research (NCAR) Reanalysis I from 1974-2010 with the latter four taken from the National Center for Environmental Information’s Tropical Easterly Wave Climatology, Version 1. For the monsoon rainfall variations, we used the Global Precipitation Mission (GPM) Integrated Multi-Satellite Retrievals for GPM Version 7 (IMERGv7) dataset.

Current results show TEW track density at 850 mb and 700 mb in ERA5 has some weak positive correlation at 850 mb and negative correlation at 700 mb near the coast of Africa with the onset of the West African Monsoon, but the strength and location of the AEJ has stronger positive correlations just off the coast of Africa (with AEJ strength) at both pressure levels and surprisingly even across the eastern Caribbean and northern Central America (with AEJ latitude). Our hypothesis is that the West African Monsoon can influence the strength of the TEWs through the strength and location of the AEJ which in turn have an impact on their tracks across the Atlantic. Similarly, we find comparatively stronger negative relationships between the TEW activity and the variations in the Central American Monsoon onset, especially over the main development region in the Atlantic basin.