Easterly waves

From Glossary of Meteorology
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easterly wave

It is a wave within the broad easterly current and moves from east to west, generally more slowly than the current in which it is embedded. Although best described in terms of its wavelike characteristics in the wind field, it also consists of a weak trough of low pressure. Easterly waves do not extend across the equatorial trough. To the west of the trough line in an easterly wave over the ocean, there is generally found divergence, a shallow moist layer, and exceptionally fine weather. The moist layer rises rapidly near the trough line; in and to the east of the trough line intense convergence, much cloudiness, and heavy rain showers prevail. This asymmetric weather pattern may be greatly distorted by orographic and diurnal influences if the wave passes over land areas. Easterly waves occasionally intensify into tropical cyclones.
Compare equatorial wave.