Top Of Moist Layer

Figure 11.4 A model of the areal (above) and vertical (below) structure of an easterly wave. Cloud is stippled and the precipitation area is shown in the vertical section. The streamline symbols refer to the areal structure, and the arrows on the vertical section indicate the horizontal and vertical motions.

Source: Partly after Malkus and Riehl (1964).

Conversely, there is divergence in the air moving southward ahead of the trough and curving anticyclonically. The true divergent zone is characterized by descending, drying air with only a shallow moist layer near the surface, while in the vicinity of the trough and behind it the moist layer may be 4500 m or more deep. When the easterly airflow is slower than the speed of the wave, the reverse pattern of low-level convergence ahead of the trough and divergence behind it is observed as a consequence of the potential vorticity equation. This is often the case in the middle troposphere, so that the pattern of vertical motion shown in Figure 11.4 is augmented.

The passage of such a transverse wave in the trades commonly produces the following weather sequence:

1 In the ridge ahead of the trough: fine weather, scattered cumulus cloud, some haze.

2 Close to the trough line: well-developed cumulus, occasional showers, improving visibility.

3 Behind the trough: veer of wind direction, heavy cumulus and cumulonimbus, moderate or heavy thundery showers and a decrease of temperature.

Satellite photography indicates that the classical easterly wave is less common than was once supposed. Many Atlantic disturbances show an 'inverted V' wave form in the low-level wind field and associated cloud, or a 'comma' cloud related to a vortex. They are often apparently linked with a wave pattern on the ITC further south. West African disturbances that move out over the eastern tropical Atlantic usually exhibit low-level confluence and upper-level diffluence ahead of the trough, giving maximum precipitation rates in this same sector. Many disturbances in the easterlies have a closed cyclonic wind circulation at about the 600 mb level.

It is difficult to trace the growth processes in wave disturbances over the oceans and in continental areas with sparse data coverage, but some generalizations may be made. At least eight out of ten disturbances develop some 2 to 4° latitude poleward of the equatorial trough. Convection is set off by convergence of moisture in the airflow, enhanced by friction, and maintained by entrainment into the thermal convective plumes (see Figure 11.3). Some ninety tropical disturbances develop during the June to November hurricane season in the tropical Atlantic, about one system every three to five days. More than half of these originate over Africa. According to N. Frank, a high ratio of African depressions in the storm total in a given season indicates tropical characteristics, whereas a low ratio suggests storms originating from cold lows and the baroclinic zone between Saharan air and cooler, moist monsoon air. Many of them can be traced westward into the eastern North Pacific. Out of an annual total of sixty Atlantic waves, 23 per cent intensify into tropical depressions and 16 per cent become hurricanes.

Developments in the Atlantic are closely related to the structure of the trades. In the eastern sectors of subtropical anticyclones, active subsidence maintains a pronounced inversion at 450 to 600 m (Figure 11.5). Thus the cool eastern tropical oceans are characterized by extensive, but shallow, marine stratocumulus, which gives little rainfall. Downstream the inversion weakens and its base rises (Figure 11.6) because the subsidence decreases away from the eastern part of the anticyclone and cumulus towers penetrate the inversion from time

Specific Humidity

Specific Humidity

Temperature (°C)

Figure 11.5 The vertical structure of trade wind air between the surface and 700 mb in the central equatorial Atlantic, 6 to 12 February 1969, showing air temperature (T), dew-point temperature (TD). The specific humidity can be read off the upper scale.

Temperature (°C)

Figure 11.5 The vertical structure of trade wind air between the surface and 700 mb in the central equatorial Atlantic, 6 to 12 February 1969, showing air temperature (T), dew-point temperature (TD). The specific humidity can be read off the upper scale.

Source: After Augstein et al. (1973, p. 104), by permission of the American Meteorological Society.

Figure 11.6 The height (in metres) of the base of the trade wind inversion over the tropical Atlantic.

Source: From Riehl (1954).

to time, spreading moisture into the dry air above. Easterly waves tend to develop in the Caribbean when the trade wind inversion is weak or even absent during summer and autumn, whereas in winter and spring subsidence aloft inhibits their growth, although disturbances may move westward above the inversion. Waves in the easterlies also originate from the penetration of cold fronts into low latitudes. In the sector between two subtropical high-pressure cells, the equatorward part of the front tends to fracture generating a westward-moving wave.

The influence of these features on regional climate is illustrated by the rainfall regime. For example, there is a late summer maximum at Martinique in the Windward Islands (15°N) when subsidence is weak, although some of the autumn rainfall is associated with tropical storms. In many trade wind areas, the rainfall occurs in a few rainstorms associated with some form of disturbance. Over a ten-year period, Oahu (Hawaii) had an average of twenty-four rainstorms per year, ten of which accounted for more than two-thirds of the annual precipitation. There is quite high variability of rainfall from year to year in such areas, since a small reduction in the frequency of disturbances can have a large effect on rainfall totals.

In the central equatorial Pacific, the trade wind systems of the two hemispheres converge in the equatorial trough. Wave disturbances may be generated if the trough is sufficiently far from the equator (usually to the north) to provide a small Coriolis force to begin cyclone motion. These disturbances quite often become unstable, forming a cyclonic vortex as they travel westward towards the Philippines, but the winds do not necessarily attain hurricane strength. The synoptic chart for part of the northwest Pacific on 17 August 1957 (Figure 11.7) shows three developmental stages of tropical low-pressure systems. An incipient easterly wave has formed west of Hawaii, which, however, filled and dissipated over the next twenty-four hours. A well-developed wave is evident near Wake Island, having spectacular cumulus towers extending above 9 km along the convergence zone some 480 km east of it (see Plate 26). This wave developed within forty-eight hours into a circular tropical storm with winds up to 20 m s-1, but not into a full hurricane. A strong, closed circulation situated east of the Philippines is moving northwestward. Equatorial waves may form on both sides of the equator in an easterly current located between about 5°N and S°. In such cases, divergence ahead of a trough in the northern

Figure 11.7 The surface synoptic chart for part of the northwest Pacific on 17 August 1957. The movements of the central wave trough and of the closed circulation during the following twenty-four hours are shown by the dashed line and arrow, respectively. The dashed L just east of Saipan indicates the location in which another low-pressure system subsequently developed. Plate 26 shows the cloud formation along the convergence zone just east of Wake Island.

Figure 11.7 The surface synoptic chart for part of the northwest Pacific on 17 August 1957. The movements of the central wave trough and of the closed circulation during the following twenty-four hours are shown by the dashed line and arrow, respectively. The dashed L just east of Saipan indicates the location in which another low-pressure system subsequently developed. Plate 26 shows the cloud formation along the convergence zone just east of Wake Island.

Source: From Malkus and Riehl (1964).

hemisphere is paired with convergence behind a trough line located further to the west in the southern hemisphere. The reader may confirm that this should be so by applying the equation for the conservation of potential vorticity, remembering that both f and Z operate in the reverse sense in the southern hemisphere.

Was this article helpful?

0 0
Renewable Energy Eco Friendly

Renewable Energy Eco Friendly

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable.

Get My Free Ebook


Post a comment