Wave disturbances

Several types of wave travel westward in the equatorial and tropical tropospheric easterlies; the differences between them probably result from regional and seasonal variations in the structure of the tropical atmosphere. Their wavelength is about 2000 to 4000 km, and they have a life span of one to two weeks, travelling some 6 to 7° longitude per day.

The first wave type to be described in the tropics was the easterly wave of the Caribbean area. This system is quite unlike a mid-latitude depression. There is a weak pressure trough, which usually slopes eastward with height (Figure 11.4). Typically the main development of cumulonimbus cloud and thundery showers is behind the trough line. This pattern is associated with horizontal and vertical motion in the easterlies. Behind the trough, low-level air undergoes convergence, while ahead of it there is divergence (see Chapter 6B.1). This follows from the equation for the conservation of potential vorticity (cf. Chapter 9G), which assumes that the air travelling at a given level does not change its potential temperature (i.e. dry adiabatic motion; see Chapter 5A):

Ap where f = the Coriolis parameter, i = relative vorticity (cyclonic positive) and Ap = the depth of the tropospheric air column. Air overtaking the trough line is moving both poleward (f increasing) and towards a zone of cyclonic curvature (i increasing), so that if the left-hand side of the equation is to remain constant Ap must increase. This vertical expansion of the air column necessitates horizontal contraction (convergence).

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-<- SURFACE STREAMLINES

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