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An easterly wave is defined as a migratory wave-like disturbance of the subtropical easterlies or put more simply, a trough in the easterlies that moves from east to west. See figure 9-3-3. The wave is usually oriented from south-southwest to north-northeast on surface charts and lower-level upper-air charts in the Northern Hemisphere and from north-northwest to south-southeast in the Southern Hemisphere. The deformation of the isobars may be more pronounced at the 700-mb level than at the surface. Easterly waves lie within the boundaries of the tropical easterlies-between 25░N and 20░S. Easterly waves are classified as stable, neutral, and unstable.

Stable Easterly Waves

Stable waves slope east with height and have divergence ahead, and convergence to the rear,

Figure 9-3-3.ŚModel of an easterly wave, both vertical and horizontal.

of their troughs. The height of the moist layer is at a minimum approximately 20 miles ahead of the trough line, but it rises rapidly near the trough line and attains its maximum height in the region of strongest convergence behind the wave. Further to the east of this zone of maximum convergence, the top of the moist layer lowers again to its normal height. As was discussed earlier, the height of the moist layer plays a major role in the type of clouds and weather that occurs within the tropics.

To the west, ahead of a stable easterly wave, the characteristic cloud is cumulus, and the visibility is poor in haze. To the east, behind the wave, cumulus congestus and cum-ulonimbus with showers and thunderstorms are characteristic. The cumulonimbus are usually arranged in parallel bands. Layers of alto-cumulus, altostratus, and cirrus connect the cumulonimbus. Visibility is good except in the precipitation.

Near and to the rear of the trough line, temperatures may lower from the evaporation of falling rain and the vertical stretching of the atmosphere created by the low-level convergence. However, this cooling does not amount to more than 4░F to 6░F in 24 hours and does not extend above 15,000 feet. Ahead of and to the rear of the convective area, the temperature remains normal or may even be slightly higher than normal.

Surface pressure falls of 0.5 mb to 3.0 mb in 24 hours occur ahead of the trough, and rises of a similar magnitude occur behind. The greatest relative pressure changes usually occur between 10,000 and 15,000 feet. Between 25,000 and 35,000 feet, there is normally no pressure change.

Easterly waves usually do not move as fast as the current in which they are embedded. A stable wave usually moves at a moderate speed of approximately 10 knots.

Neutral and Unstable Easterly Waves

When an easterly wave becomes vertical or slopes westward with height, the wave is deepening (intensifying). When vertical, the wave is said to be neutral. The lines of cumulonimbus and areas of precipitation, which are found east of the axis of a stable wave, move west to a position along the trough axis.  

Neutral waves usually have a short life (often less than 24 hours) and they often revert to stable, but sometimes become unstable waves. When a wave slopes westward with height, the wave is said to be unstable. This type of wave has the most violent weather, and is often associated with the development of tropical cyclones that reach hurricane intensity. The showers and thunderstorms are found west, or ahead, of the trough line.

Satellite imagery and increased amounts of meteorological data have shown that the classical easterly wave does not occur as frequently as originally believed.

Learning Objective: Describe the Inter-tropical Convergence Zone with regard to its location, representation on streamline charts, associated weather, and seasonal shifts.

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