Section 2.2
Several modes of tropical wave disturbances in the tropospheric easterlies have been studied extensively: the westward- moving North African easterly waves (Burpee, 1972), the eastward-moving equatorial Africa waves (Johnson, 1964), the westward moving Eastern Atlantic easterly waves (Piersig, 1936), the Caribbean easterly waves (Dunn, 1940; Riehl, 1945), and the Pacific equatorial waves (Palmer, 1952). Westward propagating cloud patterns in the tropical Pacifc are also detected form satellite cloud pictures (Chang, 1970). These waves are usually observed in the zone between the equator and 30°N.
Figures 2.25 and 2.26 (Nieuwolt, 1977) show a top view and cross section of an idealized easterly wave in the Caribbean. These waves move westward and usually last from one week to several weeks. The wave speeds are 10 to 15 knots (about 5 to 7° latitude per day) and are usually slower than the easterly current in which they are embedded. The wave pattern can be seen in the streamline charts, with maximum amplitude in the layer between 500 to 700-hPa levels. The characteristic wavelength is of the order of 2,000 km. The characteristic periods of waves can vary from 2 to 5 days. The line of maximum wind shear has a somewhat north-to-south orientation. There is generally fair weather to the west of the trough line, where divergence, subsidence and drier air dominate the low troposphere. The moisture layer raises sharply near the trough, intense low tropospheric convergence and moisture, convective cloudiness, and rainfall prevail in and to the east of the trough. Cloud clusters, seen on satellite cloud imageries, are usually associated with the waves. A majority of these waves are cold-core, and the axis of the wave tilts toward the east with height. Some squalls and TCs are believed to develop from cloud clusters associated with the waves (Yanai, 1961, 1968).
The easterly waves on the daily weather map can have large deviation from the idealized model discussed above. Riehl (1967) studied a wave in the western Caribbean and found the wave was moving westward faster than the basic current. The convergence area was located at the trough. Frank (1969) found an inverted-V shaped cloud pattern associated with Caribbean easterly waves. These waves move westward at an average speed of about 16 knots (80 latitude per day). Riehl (1979) pointed out that the basic easterlies in a region may differ substantially from year to year. Therefore, there will be many waves in some years, such as in the Caribbean in 1969, and few waves in other years, such as 1972.
Atkinson (1971) offered a warning that the easterly-wave models mentioned above have been grossly overworked and misused, especially as an aid to tropical forecasting. It is now known that there are a wide variety of weather-producing systems. Some easterly waves in the lower tropospheric easterlies are the reflections of upper tropospheric disturbances (Yanai and Nitta, 1967; Yanai, et al., 1968).
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Chapter 2
