Best Pressure Level/Latitude Band to Track Easterly Waves Over Different Regions Using NCEP–NCAR Reanalysis

Tropical easterly waves form over Africa and move westward over West Africa, the Atlantic, and sometimes as far as the Caribbean and East Pacific regions. These waves are often identified via fluctuations in meridional wind, relative vorticity, and/or by tracking mesoscale convective systems via satellite IR brightness temperatures. Over Africa, the waves tend to be associated with a northerly track with peak amplitude at 850 hPa and a southerly track that peaks at 700 hPa. These tracks tend to merge over the Atlantic. Given the complicated structure of easterly waves, this study seeks to determine the best pressure level and latitude band to identify and track easterly waves over different regions.

This study uses National Centers for Environmental Prediction–National Center for Atmospheric Research reanalysis meridional wind data over a region stretching from 130°W–20°E and from 5–25°N. A 3–7-day bandpass filter is applied to 700- and 850-hPa winds prior to the waves being separated into northerly, trough, southerly, and ridge phases. This phase separation is performed over various latitude bands (i.e., 5–15°N, 5–20°N, and 15–25°N). Then the filtered meridional wind values in each phase are correlated with the corresponding unfiltered wind values. These correlations are not only calculated over the full analysis domain but also over smaller longitude bands, including the East Pacific (130°–95°W), Caribbean (95°­–70°W), West Atlantic (70°–40°W), East Atlantic (40°–15°W), and Africa (15°W–20°E). The hypothesis is that a higher correlation implies that the corresponding pressure level and latitude band is more appropriate for tracking easterly waves using meridional wind over a given region.

Early results of this work will be available soon. Given previous work on the structure of easterly waves, we expect that the pressure level/latitude band most appropriate for tracking easterly waves will vary from region to region. For example, 850-hPa meridional winds in the northern latitude band (15–25°N) may be more important for wave identification over Africa and less important farther west over the ocean where the northern wave track tends to weaken.