The Intertropical Convergence

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The tendency for the trade wind systems of the two hemispheres to converge in the equatorial (low-pressure) trough has already been noted (see Chapter 7B). Views on the exact nature of this feature have been subject to continual revision. From the 1920s to the 1940s, the frontal concepts developed in mid-latitudes were applied in the tropics, and the streamline confluence of the northeast and southeast trades was identified as the intertropical front (ITF). Over continental areas such as West Africa and South Asia, where in summer hot, dry continental tropical air meets cooler, humid equatorial air, this term has some limited applicability (Figure 11.1). Sharp temperature and moisture gradients may occur, but the front is seldom a weather-producing mechanism of the mid-latitude type. Elsewhere in low latitudes, true fronts (with a marked density contrast) are rare.

Recognition of the significance of wind field convergence in tropical weather production in the 1940s and 1950s led to the designation of the trade wind convergence as the intertropical convergence zone (ITCZ). This feature is apparent on a mean streamline map, but areas of convergence grow and decay, either in situ or within disturbances moving westward (see Plates 1 and 24), over periods of a few days. Moreover, convergence is infrequent even as a climatic feature in the doldrum zones (see Figure 7.13). Satellite data show that over the oceans the position and intensity of the ITCZ varies greatly from day to day.

The ITCZ is predominantly an oceanic feature where it tends to be located over the warmest surface waters. Hence, small differences of sea-surface temperature may cause considerable changes in the location of the ITCZ. A sea-surface temperature of at least 27.5°C seems to provide a threshold for organized convective activity; above this temperature organized convection is essentially competitive between different regions potentially available to form part of a continuous ITCZ. The convective rainfall belt of the ITCZ has very sharply defined latitudinal limits. For example, along the West African coast the following mean annual rainfalls are recorded:

In other words, moving southwards into the ITCZ, precipitation increases by 440 per cent in a meridional distance of only 330 km.

As climatic features, the equatorial trough and the ITCZ are asymmetric about the equator, lying on average to the north. They also move seasonally away from the equator (see Figure 9.1) in association with the

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TC cloud bands ITF over land

TC cloud bands ITF over land

Figure 11.1 The position of the equatorial trough (intertropical convergence zone or intertropical front in some sectors) in February and August. The cloud band in the southwest Pacific in February is known as the South Pacific convergence zone; over South Asia and West Africa the term monsoon trough is used.

Sources: After Saha (1973), Riehl (1954) and Yoshino (1969).

Streamline Convergence

Figure 11.2 Illustrations of (A) streamline convergence forming an intertropical convergence (ITC) and South Pacific convergence zone (SPCZ) in February, and (B) the contrasting patterns of monsoon trough over West Africa, streamline convergence over the central tropical North Atlantic, and axis of maximum cloudiness to the south for August.

Sources: (A) C. S. Ramage, personal communication (1986). (B) From Sadler (1975a).

Figure 11.2 Illustrations of (A) streamline convergence forming an intertropical convergence (ITC) and South Pacific convergence zone (SPCZ) in February, and (B) the contrasting patterns of monsoon trough over West Africa, streamline convergence over the central tropical North Atlantic, and axis of maximum cloudiness to the south for August.

Sources: (A) C. S. Ramage, personal communication (1986). (B) From Sadler (1975a).

thermal equator (zone of seasonal maximum temperature). The location of the thermal equator is related directly to solar heating (see Figures 11.2 and 3.11), and there is an obvious link between this and the equatorial trough in terms of thermal lows. However, if the ITC were to coincide with the equatorial trough then this zone of cloudiness would decrease incoming solar radiation, reducing the surface heating needed to maintain the low-pressure trough. In fact, this does not happen. Solar energy is available to heat the surface because the maximum surface wind convergence, uplift and cloud cover is commonly located several degrees equatorward of the trough. In the Atlantic (Figure 11.2B), for example, the cloudiness maximum is distinct from the equatorial trough in August. Figure 11.2 illustrates regional differences in the equatorial trough and ITCZ. Convergence of two trade wind systems occurs over the central North Atlantic in August and the eastern North Pacific in February. In contrast, the equatorial trough is defined by easterlies on its poleward side and westerlies on its equatorward side over West Africa in August and over New Guinea in February.

The dynamics of low-latitude atmosphere-ocean circulations are also involved. The convergence zone in the central equatorial Pacific moves seasonally between about 4°N in March to April and 8°N in September, giving a single pronounced rainfall maximum in March to April. This appears to be a response to the relative strengths of the northeast and southeast trades. The ratio of South Pacific/North Pacific trade wind strength exceeds 2 in September but falls to 0.6 in April. Interestingly, the ratio varies in phase with the ratio of Antarctic-Arctic sea ice areas; Antarctic ice is at a maximum in September when Arctic ice is at its minimum. The convergence axis is often aligned close to the zone of maximum sea-surface temperatures, but is not anchored to it. Indeed, the SST maximum located within the equatorial counter-current (see Figure 7.29) is a result of the interactions between the trade winds and horizontal and vertical motions in the ocean-surface layer.

Aircraft studies show the complex structure of the central Pacific ITCZ. When moderately strong trades provide horizontal moisture convergence, convective cloud bands form, but the convergent lifting may be insufficient for rainfall in the absence of upper-level divergence. Moreover, although the southeast trades cross the equator, the mean monthly resultant winds between 115° and 180°W have, throughout the year, a more southerly component north of the equator and a more northerly one south of it, giving a zone of divergence (due to the sign change in the Coriolis parameter) along the equator.

In the southwestern sectors of the Pacific and Atlantic Oceans, satellite cloudiness studies indicate the presence of two semi-permanent confluence zones (see Figure 11.1). These do not occur in the eastern South Atlantic and South Pacific, where there are cold ocean currents. The South Pacific convergence zone (SPCZ) shown in the western South Pacific in February (summer) is now recognized as an important discontinuity and zone of maximum cloudiness (see Plate 24). It extends from the eastern tip of Papua New Guinea to about 30°S, 120°W. At sea-level, moist northeasterlies, west of the South Pacific subtropical anticyclone, converge with southeasterlies ahead of high-pressure systems moving eastward from Australia/New Zealand. The low-latitude section west of 180° longitude is part of the ITCZ system, related to warm surface waters. However, the maximum precipitation is south of the axis of maximum sea-surface temperature, and the surface convergence is south of the precipitation maximum in the central South Pacific. The southeastward orientation of the SPCZ is caused by interactions with the mid-latitude westerlies. Its southeastern end is associated with wave disturbances and jet stream clouds on the South Pacific polar front. The link across the subtropics appears to reflect upper-level tropical mid-latitude transfers of moisture and energy, especially during subtropical storm situations. Hence the SPCZ shows substantial short-term and interannual variability in its location and development. The interannual variability is strongly associated with the phase of the Southern Oscillation (see p. 145). During the northern summer the SPCZ is poorly developed, whereas the ITCZ is strong all across the Pacific. During the southern summer the SPCZ is well developed, with a weak ITCZ over the western tropical Pacific. After April the ITCZ strengthens over the western Pacific, and the SPCZ weakens as it moves westward and equatorward. In the Atlantic, the ITCZ normally begins its northward movement in April to May, when South Atlantic sea-surface temperatures start to fall and both the subtropical high-pressure cell and the southeast trades intensify. In cold, dry years this movement can begin as early as February and in warm, wet years as late as June.

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