There are great differences between surface and upper winds at the tropical latitudes (Figure 12.10), with easterly Trade winds surmounted by westerlies, notably in winter. The explanation is as follows. The Trades tend towards the ITCZ (Figure 12.1), where there is a chain of centres of convergence associated with convective storms and these lift air into the upper atmosphere. The raised air increases the upper-level pressure locally, which creates winds poleward as irregular 'anti-Trades'. These vary considerably with season and longitude. For instance, the upper-level winds diverge in all directions over the west equatorial Pacific warm pool (Section 11.2). The antiTrades are deflected by the Coriolis force to become patchy upper westerlies much interrupted by other winds, especially over the continents.
The Coriolis deflection prevents the antiTrades reaching further towards the pole than about 30° latitude, so they bank up there, and the extra air creates the belt of subtropical highs at sea-level seen in Figure 12.1 and Figure 12.7. The accumulated air aloft gradually cools by radiation loss to space, and therefore contracts, leaving room for more air, whose extra weight leads to the relatively high pressures at surface level. The consequent subsidence in areas where surface pressures are high (Note 12.G) replaces air that spirals out near the surface to create the Trades once more. Thus a cycle is completed, defining what is called a Hadley cell, named after George Hadley (1685-1768). It involves ascent at the ITCZ and subsidence in the belt of highs which lies (in the southern hemisphere) at about 35°S in summer and 30°S in winter. There is a similar low-latitude meridional circulation north of the equator, and the two Hadley cells extend and contract with the annual swing of the Sun's path (Figure 12.11). The cell extends over the equator in winter, towards the ITCZ in the summer hemisphere, and the circulation is several times stronger than in the other (summertime) cell.
The north-south parts of a Hadley cell's circulation occur at the same time and latitudes as the east-west flows shown in Figure 12.10. The Hadley cell involves simply the average of the meridional components of the real winds. The cells are secondary circulations, so-called because they are weaker than the primary zonal circulation around the Earth, shown in Figure 12.10. On the other hand, they are important in
carrying warm moist air to the ITCZ, leading to condensation in the convective updraughts there, accompanied by the release of latent heat, which is then carried polewards by the upper limb of the Hadley cell. This lessens the difference between polar and equatorial temperatures.
Hadley circulations are driven chiefly by the solar energy absorbed in the high rate of evaporation from tropical oceans (Figure 4.11). Further energy comes from the reduced loss of longwave radiation to space (Note 2.C), due to the coldness of the high tops of equatorial cumulonimbus.
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