Easterly Waves And Tropical Cyclones

A large amount of heat is transported away from low latitudes by strong tropical cyclones - also known as hurricanes and typhoons. Tropical cyclones only develop over oceans and so it is difficult to study the atmospheric conditions associated with their formation. It is known, however, that they are triggered by small low pressure centres, as may occur in small vortices associated with the ITCZ. or in small depressions that have moved in from higher latitudes. Cyclones may also be triggered by linear low pressure areas that form at right angles to the direction of the Trade Winds and travel with them (Figure 2.14). These linear low pressure regions ('troughs') produce wave-like disturbances in the isobaric patterns, with wavelengths of the order of 2500 km: because they move with the easterly Trade Winds, they are known as easterly waves.

Easterly waves usually develop in the western parts of the large ocean basins, between about 5° and 20° N. They occur most frequently when the Trade Wind temperature inversion (Figures 2.2(b) and 2.12) is weakest, i.e. during the late summer, when the sea-surface, and hence the lower atmosphere in the Trade Wind belt, are at their warmest.

Figure 2.14 Schematic diagram of an easterly wave in the Northern Hemisphere. The black lines are isobars, with sea-level atmospheric pressure given in millibars. Wind direction is shown by the blue arrows. As the wave itself moves slowly westward, lower level air ahead of the 'axis' diverges, and dry air sinking from above to replace it leads to particularly fine weather. Behind the axis, moist air converges and rises, generating showers and thunderstorms (the main rainfall area is shown in greyish-blue).

Figure 2.14 Schematic diagram of an easterly wave in the Northern Hemisphere. The black lines are isobars, with sea-level atmospheric pressure given in millibars. Wind direction is shown by the blue arrows. As the wave itself moves slowly westward, lower level air ahead of the 'axis' diverges, and dry air sinking from above to replace it leads to particularly fine weather. Behind the axis, moist air converges and rises, generating showers and thunderstorms (the main rainfall area is shown in greyish-blue).

Ahead of the easterly wave's 'axis', the air flow at low level diverges, so air above sinks, strengthening and lowering the Trade Wind inversion and leading to particularly fine weather. Behind the axis (the 'trough' in atmospheric pressure), moist air converges and rises, temporarily destroying the Trade Wind inversion and generating showers and thunderstorms. Although only a small proportion of easterly waves give rise to cyclones, they are important because they bring large amounts of rainfall to areas that remain generally dry as long as air flow in the Trade Winds is unperturbed.

Figure 2.15 Satellite image of Hurricane 'Andrew' moving across the Gulf of Mexico in August 1992.

Figure 2.15 Satellite image of Hurricane 'Andrew' moving across the Gulf of Mexico in August 1992.

Intense tropical cyclones are not generated within about 5° of the Equator. Cyclonic (and anticyclonic) weather systems can only form where flow patterns result from an approximate balance between a horizontal pressure gradient force and the Coriolis force (Section 2.2.1) and, as mentioned earlier, the Coriolis force is zero at the Equator and very small within about 5° of it. Poleward of about 5° of latitude, positive feedback between the ocean and the atmosphere can transform a low pressure area into a powerful tropical cyclone, characterized by closely packed isobars encircling a centre of very low pressure (typically about 950mbar). Large pressure gradients near the centre of the cyclone cause air to spiral rapidly in towards the low pressure region (anticlockwise in the Northern Hemisphere, clockwise in the Southern Hemisphere), and wind speeds commonly reach 100-200 km hr~' (-30-60 m s~', see Table 2.2). Violent upward convection of warm humid air results in bands of cumulonimbus and thunderstorms, which spiral in to the core or 'eye' of the cyclone - an area of light winds and little cloud, surrounded by a wall of towering cumulonimbus (see Figures 2.15 and 2.16).

The energy that drives the cyclone comes from the release of latent heat as the water vapour in the rising air condenses into clouds and rain; the resultant warming of the air around the central region of the cyclone causes it to become less dense and to rise yet more, intensifying the divergent anticyclonic flow of air in the upper troposphere that is necessary for the cyclone to be maintained (Figure 2.16). Given their source of energy, it is not surprising that tropical cyclones occur only over relatively large areas of ocean where the surface water temperature is high.

anticvclonic diveraent flow at the toD of the traDOSDhere

cumulonimbus clouds

Figure 2.16 Schematic diagram of a tropical cyclone. Moist air spirals inwards and upwards at high speed, forming cumulonimbus clouds arranged in spiral bands which encircle the 'eye'. Meanwhile, in the eye itself (and between the bands), air subsides, warming adiabatically (red arrows), and there are light winds and little cloud.

In practice, the critical sea-surface temperature needed to generate the increased vertical convection which leads to extensive cumulonimbus development and rain and/or cyclone formation is about 27-29 °C. Why should this value be critical? One factor seems to be that the higher the temperature of an air mass the more moisture it can hold, and the greater the upward transfer of latent heat that can occur. Given the positive feedback of the system, a rise in temperature from 27 to 29 °C has a much greater effect on the overlying atmosphere than a rise from. say. 19 to 21 °C. The full answer to this question is, however, as yet unknown. Furthermore, because the local sea-surface temperature - strictly, the heat content of the uppermost 60 m of ocean - has such a marked effect on the rate of development of a tropical cyclone, predicting the intensity of such a system as it moves across expanses of ocean towards land is extremely difficult.

QUESTION 2 2 W ith the help of Figure 2,3 and bearing in mind what you have been reading uhout the conditions that favour the initiation and development of easterly waves and tropical cyclones, can you explain wh\ they occur more often in the Northern Hemisphere than the Southern Hemisphere?

Figure 2.17 The effect of a cyclone on the surface ocean. Near the centre of the storm -in fact, somewhat to the rear, as it is moving -surface water diverges and upwelling of deeper, cooler water occurs; some distance from the centre, the surface water that has travelled outwards converges with the surrounding ocean surface water and downwelling, or sinking, occurs. In between the zones of upwelling and downwelling, the displaced surface water mixes with cooler subsurface water. (How cyclonic winds lead to upwelling of subsurface water will be explained in Chapter 3.)

Figure 2.17 The effect of a cyclone on the surface ocean. Near the centre of the storm -in fact, somewhat to the rear, as it is moving -surface water diverges and upwelling of deeper, cooler water occurs; some distance from the centre, the surface water that has travelled outwards converges with the surrounding ocean surface water and downwelling, or sinking, occurs. In between the zones of upwelling and downwelling, the displaced surface water mixes with cooler subsurface water. (How cyclonic winds lead to upwelling of subsurface water will be explained in Chapter 3.)

Tropical cyclones also occur more often in the western than the eastern parts of the Atlantic and Pacific Oceans. This is because sea-surface temperatures are higher there, for reasons that will become clear in later Chapters.

The violent winds of tropical cyclones generate very large waves on the sea-surface. These waves travel outwards from the central region and as the cyclone progresses the sea becomes very rough and confused. The region where the winds are blowing in the same direction as that in which the cyclone is travelling is particularly dangerous because here the waves have effectively been blowing over a greater distance (i.e. they have a greater fetch). Furthermore, ships in this region are in danger of being blown into the cyclone's path.

Cyclones also affect the deeper structure of the ocean over which they pass. For reasons that will be explained in Chapter 3. the action of the wind causes the surface waters to diverge so that deeper, cooler water upwells to replace it (Figure 2.17). Thus not only are cyclones affected by sea-surface temperature, but they also modify it. so that their tracks are marked out by surface water with anomalously low temperatures, perhaps as much as 5 °C below that of the surrounding water.

Characteristic tracks followed by tropical cyclones as they move away from their sites of generation are shown in Figure 2.18. The path of a newly generated cyclone is relatively easy to predict, because it is largely determined by the general air flow in the surrounding atmosphere - compare the paths in Figure 2.18 with the average winds (for summer) in Figure 2.3. The fact that cyclones nearly always move polewards enhances the contribution that they make to the transport of heat from the tropics to higher latitudes.

If they move over extensive areas of land they begin to die away, as the energy conversion system needed to drive them can no longer operate. Their decay over land may be hastened by increased surface friction and the resulting increased variation of wind velocity with height (vertical wind shear) which inhibits the maintenance of atmospheric vortices. The lower temperatures of land masses (especially at night) may also play a part in their decay. The average lifespan of a tropical cyclone is about a week.

There is great interannual variation in the frequency of tropical cyclones. Over the past few decades there seems to have been an increase in their occurrence, but it is not clear whether this is a genuine long-term increase or part of a cyclical variation. Information about the number of cyclones occurring annually in the various oceanic regions over a 20-year period is given in Table 2.1.

Figure 2.18 Characteristic tracks followed by tropical cyclones, commonly called hurricanes (Atlantic and eastern North Pacific), typhoons (western North Pacific) or cyclones (India and Australia). Initially they move westwards, but then generally curve polewards and eastwards around the subtropical high pressure centres. The pink regions are those areas where the sea-surface temperature exceeds 27 °C in summer (mean values for September and March in the Northern and Southern Hemispheres, respectively).

Figure 2.18 Characteristic tracks followed by tropical cyclones, commonly called hurricanes (Atlantic and eastern North Pacific), typhoons (western North Pacific) or cyclones (India and Australia). Initially they move westwards, but then generally curve polewards and eastwards around the subtropical high pressure centres. The pink regions are those areas where the sea-surface temperature exceeds 27 °C in summer (mean values for September and March in the Northern and Southern Hemispheres, respectively).

Cyclones have a great impact on life in the tropics. They are largely responsible for late summer or autumn rainfall maxima in many tropical areas. The strong winds and large waves associated with them may cause severe damage to natural environments such as reefs; indeed, these catastrophic events play a major role in determining the patterns of distribution of species and life forms within such communities. They may lead to great loss of human life, particularly where there are large populations living near to sea-level, on the flood plains of major rivers or on islands. Quite apart from the high seas caused by the winds, the low atmospheric pressures within a cyclone may cause a storm surge - a local rise in sea-level of up to 5 m or more (see Table 2.2, overleaf). Storm surges can lead to widespread flooding of low-lying areas, and deaths from drowning, post-flood disruption and disease greatly exceed those caused by the winds. A storm surge contributed to the havoc caused by tropical cyclones in Orissa, on the north-west of the Bay of Bengal, in October-November 1999, and the cyclonic disaster which devastated The delta region of the Bay in May 1985 was largely the result of a storm surge further amplified by the occurrence of high tides.

Table 2.1 Numbers of tropical cyclones (> 33 m s_l sustained wind speed) occurring annually during a 20-year period (from 1968/69 to 1989/90) in various oceanic regions.

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