Understanding The Observed Circulation

The simplest observed global characteristic of the atmosphere is that the tropics are much warmer than the poles. As discussed in Chapter 5, this is a straightforward consequence of the geometry of the Earth: the annually averaged incoming solar radiation per unit area of the Earth's surface is much greater at the equator than at the poles, a difference that is compounded by the fact that the polar regions are covered in ice and snow and therefore reflect much of the incoming radiation back to space. A little less obvious is the fact that the tropical regions actually receive more energy from the Sun than they emit back to space, whereas the converse is true in high latitudes. Since both regions are, on an annual average, in equilibrium, there must be a process acting to transport excess energy from the tropics to make up the deficit in high latitudes, as depicted schematically in Fig. 8.1 (left).

The implied transport of some 6 x 1015W (see Fig. 5.6) must be effected by the atmospheric circulation, carrying warm air poleward and cold air equatorward. (In fact, the ocean circulation also contributes, as discussed in Chapter 11.) As a result, the tropics are cooler, and polar regions warmer, than they would be in the absence of such transport. Thus, in this as in other respects, the atmospheric general circulation plays a key role in climate.

What are the motions that deliver the required transport? The gross features of the observed atmospheric circulation discussed in Chapter 5 are depicted in Fig. 8.2. The zonal flow is strongly westerly aloft in middle latitudes, a fact that we can now, following Section 7.3, understand as a straightforward consequence of the decrease of temperature with latitude. Surface winds are constrained to be weak by the action of friction near the ground, and thermal wind balance, Eq. 7-24, implies that a poleward decrease of temperature is necessarily accompanied by increasing westerly winds with height. Taken together, these two facts require a zonal flow in middle latitudes that increases from near zero at the ground to strong westerlies at altitude, as schematized in Fig. 7.19.

Angular Momentum Earths Circulation

FIGURE 8.1. Latitudinal transport of (left) energy and (right) angular momentum (mtm) implied by the observed state of the atmosphere. In the energy budget there is a net radiative gain in the tropics and a net loss at high latitudes; to balance the energy budget at each latitude, a poleward energy flux is implied. In the angular momentum budget the atmosphere gains angular momentum in low latitudes (where the surface winds are easterly) and loses it in middle latitudes (where the surface winds are westerly). A poleward atmospheric flux of angular momentum is thus implied.

FIGURE 8.1. Latitudinal transport of (left) energy and (right) angular momentum (mtm) implied by the observed state of the atmosphere. In the energy budget there is a net radiative gain in the tropics and a net loss at high latitudes; to balance the energy budget at each latitude, a poleward energy flux is implied. In the angular momentum budget the atmosphere gains angular momentum in low latitudes (where the surface winds are easterly) and loses it in middle latitudes (where the surface winds are westerly). A poleward atmospheric flux of angular momentum is thus implied.

Westerlies

FIGURE 8.2. Schematic of the observed atmospheric general circulation for annual-averaged conditions. The upper level westerlies are shaded to reveal the core of the subtropical jet stream on the poleward flank of the Hadley circulation. The surface westerlies and surface trade winds are also marked, as are the highs and lows of middle latitudes. Only the northern hemisphere is shown. The vertical scale is greatly exaggerated.

FIGURE 8.2. Schematic of the observed atmospheric general circulation for annual-averaged conditions. The upper level westerlies are shaded to reveal the core of the subtropical jet stream on the poleward flank of the Hadley circulation. The surface westerlies and surface trade winds are also marked, as are the highs and lows of middle latitudes. Only the northern hemisphere is shown. The vertical scale is greatly exaggerated.

Although the near-surface winds are weak—a few m s-1—they nevertheless exhibit a distinct spatial distribution, with the zonal component being easterly in the tropics and westerly in middle latitudes (Fig. 7.28). In the presence of surface friction, the atmosphere must therefore be losing angular momentum to the ground in middle latitudes and gaining angular momentum in the tropics. As Fig. 8.1 (right) illustrates, the angular momentum balance of the atmosphere thus requires a transport of westerly angular momentum from low to middle latitudes to maintain equilibrium.

Even though the west-to-east circulation in the upper troposphere is the dominant component of the large-scale atmospheric flow, it cannot be responsible for the required poleward transports of energy and angular momentum, for which north-south flow is needed. As we saw in Chapter 5 (Fig. 5.21) there is indeed a mean circulation in the meridional plane, which is dominated by the Hadley circulation of the tropical atmosphere with, on an annual average, mean upwelling near the equator, poleward flow aloft, subsidence in the subtropics, and equatorward return flow near the surface. This circulation transports energy and angular momentum poleward, as required, within the tropics; however, the meridional circulation becomes much weaker in middle latitudes and so cannot produce much transport there. We saw in Section 5.4.1 that most of the north-south flow in the extratropical atmosphere takes the form of eddies, rather than a mean overturning as in the tropics. As we will see, it is these eddies that produce the poleward transports in the extratropics.

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