We saw in Section 5.2 that because of the pole-equator temperature gradient, isobaric surfaces slope down from equator to pole, inducing a horizontal pressure gradient at upper levels. There is thus a pressure gradient force aloft, directed from high pressure to low pressure, which is from warm latitudes to cold latitudes, as seen in Fig. 5.1. We might expect air to move down this pressure gradient. Hadley5 suggested one giant


FIGURE 5.19. The circulation envisaged by Hadley (1735) comprising one giant meridional cell stretching from equator to pole. Regions where Hadley hypothesized westerly (W) and easterly (E) winds are marked.


FIGURE 5.19. The circulation envisaged by Hadley (1735) comprising one giant meridional cell stretching from equator to pole. Regions where Hadley hypothesized westerly (W) and easterly (E) winds are marked.

meridional cell with rising motion in the tropics and descending motion at the pole, as sketched schematically in Fig. 5.19. One might expect rings of air circling the globe to be driven poleward by pressure gradient forces. As they contract, conserving the angular momentum imparted to them by the spinning Earth, westerly (W—► E) winds will be induced (see detailed discussion in Section 8.2.1). At the poles Hadley imagined

5George Hadley (1685—1768). British meteorologist who was the first to recognize the relationship between the rotation of the Earth and the atmospheric circulation (in particular, the trade winds). Hadley presented his theory in 1735. The pattern of meridional circulation in tropical zones, called the Hadley circulation (or a Hadley cell—see Fig. 5.19) is named after him.

that the rings would sink and then expand outward as they flow equatorward below, generating easterly winds, as marked on the figure.

As attractive as this simple circulation may seem, we shall see that this picture of a single meridional cell extending from equator to pole is not in accord with observations.

5.4.1. Distribution of winds

Wind velocity is, of course, a vector with components u = (u, v, w) in the eastward, northward, and upward directions. The vertical component is, except in the most violent disturbances, very much smaller than the horizontal components (a consequence, among other things, of the thinness of the atmosphere), so much so that it cannot usually be directly measured but must be inferred from other measurements.

Mean zonal winds

The typical distribution of zonal-mean zonal wind, u, in the annual mean and at the solstices (December, January, February [DJF]; and June, July, August [JJA]) is shown in Fig. 5.20. Except close to the equator, the zonal-mean winds are eastward (i.e., in meteorological parlance, westerly) almost everywhere. The stronger winds are found at the core of the subtropical jets, the strongest of which is located near 30° latitude in the winter hemisphere at 200 mbar, at a height of about 10 km with, on average, a speed6 of around 30 m s-1. A weaker jet of about 20 ms-1 is located near 45°

in the summer hemisphere. The easterlies observed in the tropics are much weaker, especially in northern winter.

Note that the winds are much weaker near the ground, but show the same pattern; westerlies are poleward of about 30° and easterlies equatorward thereof. The low-level easterlies (which, as we shall see, are actually north-easterlies in the northern hemisphere and south-easterlies in the southern hemisphere) are known as the ''trade winds'', a term that comes from the days of sailing ships when, together with the westerlies and south-westerlies of higher latitudes, these winds allowed ships to complete a circuit of the North Atlantic and thus to trade easily between Europe and North America.7 We shall see later, in Chapter 10, that this pattern of surface winds and the attendant stress is a primary driver of ocean circulation.

Mean meridional circulation

Figure 5.21 shows the zonal-mean circulation of the atmosphere in the meridional plane, known as the meridional overturning circulation, whose sense is marked by the arrows. Note the strong seasonal dependence. In DJF air rises just south of the equator and sinks in the subtropics of the northern hemisphere, around 30° N. (Conversely, in JJA air rises just north of the equator and sinks in the subtropics of the southern hemisphere.) We thus see strong upward motion on the summer side of the equator, where the warm surface triggers convection and rising motion, and strong descent on the winter side of the equator. In the annual mean we thus see two

6As we shall see, the jet actually wiggles around, both in longitude and in time, and so is smoothed out in the averaging process. Typical local, instantaneous maximum speeds are closer to 50 m s-1.

Matthew Fountaine Maury (1806—1873). U.S. Naval officer and oceanographer, the founder of the U.S. Naval Observatory, inventor of a torpedo, and pioneer of wind and current charts. Maury was the first to systematically study and map ocean currents and marine winds and recommend routes for sea clippers to take advantage of winds and currents.

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