Circulations in the vertical and horizontal planes

There are two possible ways in which the atmosphere can transport heat and momentum. One is by circulation in the vertical plane as indicated in Figure 7.18, which shows three meridional cells in each hemisphere. The low-latitude Hadley cells were considered to be analogous to the convective circulations set up when a pan of water is heated over a flame and are referred to as thermally direct cells. Warm air near the equator was thought to rise and generate a low-level flow towards the equator, the earth's rotation deflecting these currents, which thus form the northeast and southeast trades. This explanation was put forward by G. Hadley in 1735, although in 1856 W. Ferrel pointed out that the conservation of angular momentum would be a more effective factor in causing easterlies, because the Coriolis force is small in low latitudes. Poleward counter-currents aloft would complete the low-latitude cell, according to the above scheme, with the air sinking at about 30° latitude as it is cooled by radiation. However, this scheme is not entirely correct. The atmosphere does not have a simple heat source at the equator, the trades are not continuous around the globe (see Figure 7.13) and poleward upper flow occurs mainly at the western ends of the subtropical high-pressure cells aloft.

Figure 7.18 shows another thermally direct (polar) cell in high latitudes with cold dense air flowing out from a polar high pressure. The reality of this is doubtful, but in any case it is of limited importance to the general circulation in view of the small mass involved. It is worth noting that a single direct cell in each hemisphere is not possible, because the easterly winds near the surface would slow down the earth's rotation. On average the atmosphere must rotate with the earth, requiring a balance between easterly and westerly winds over the globe.

The mid-latitude Ferrel cell in Figure 7.18 is thermally indirect and would need to be driven by the other two. Momentum considerations indicate the necessity for upper easterlies in such a scheme, yet aircraft and balloon observations during the 1930s to 1940s demonstrated the existence of strong westerlies in the upper troposphere (see A.3, this chapter). Rossby modified the three-cell model to incorporate this fact, proposing that westerly momentum was transferred to middle latitudes from the upper branches of the cells in high and low latitudes. Troughs and ridges in the upper flow could, for example, accomplish such horizontal mixing.

These views underwent radical amendment from about 1948 onwards. The alternative means of

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Figure 7.19 The poleward transport of energy, showing the importance of horizontal eddies in mid-latitudes.

Figure 7.19 The poleward transport of energy, showing the importance of horizontal eddies in mid-latitudes.

transporting heat and momentum - by horizontal circulations - had been suggested in the 1920s by A. Defant and H. Jeffreys but could not be tested until adequate upper-air data became available. Calculations for the northern hemisphere by V. P. Starr and R. M. White at the Massachusetts Institute of Technology showed that in middle latitudes horizontal cells transport most of the required heat and momentum polewards. This operates through the mechanism of the quasi-stationary highs and the travelling highs and lows near the surface acting in conjunction with their related wave patterns aloft. The importance of such horizontal eddies for energy transport is shown in Figure 7.19 (see also Figure 3.27). The modern concept of the general circulation therefore views the energy of the zonal winds as being derived from travelling waves, not from meridional circulations. In lower latitudes, however, eddy transports are insufficient to account for the total energy transport required for energy balance. For this reason the mean Hadley cell is a feature of current representations of the general circulation, as shown in Figure 7.20. The low-latitude circulation is recognized as being complex. In particular, vertical heat transport in the Hadley cell is effected by giant cumulonimbus clouds in disturbance systems associated with the equatorial trough (of low pressure), which is located on average at 5°S in January and at 10°N in July (see Figure 11.1). The Hadley cell of the winter hemisphere is by far the most important, since it gives rise to low-level transequatorial flow into the summer hemisphere. The traditional model of global circulation with twin cells, symmetrical about the equator, is found only in spring/ autumn.

Longitudinally, the Hadley cells are linked with the monsoon regimes of the summer hemisphere. Rising air over South Asia (and also South America and Indonesia) is associated with east-west (zonal) outflow, and these systems are known as Walker circulations (pp. 145-6). The poleward return transport of the meridional Hadley cells takes place in troughs that extend into low latitudes from the mid-latitude westerlies. This tends to occur at the western ends of the upper tropospheric subtropical high-pressure cells. Horizontal mixing predominates in middle and high latitudes, although it is also thought that there is a weak indirect mid-latitude cell in much reduced form (Figure 7.20). The relationship of the jet streams to regions of steep meridional temperature gradient has already been noted (see Figure 7.7). A complete explanation of the two wind maxima and their role in the general circulation is still

Figure 7.20 General meridional circulation model for the northern hemisphere in winter.

Figure 7.21 Schematic illustrations of suggested processes that form/ maintain the northern subtropical anti-cylones in summer. (A) Boxes where summer heat sources are imposed in the atmospheric model; (B) Pattern of resultant stationary planetary waves (solid/dashed lines denote positive/ negative height anomalies) (Chen et al., 2001); (C) Schematic of the circulation elements proposed by Hoskins (1996). Monsoon heating over the continents with descent west- and poleward where there is interaction with the westerlies. The descent leads to enhanced radiative cooling acting as a positive feedback and to equator-ward motion; the latter drives Ekman ocean drift and upwelling.

Sources: From P. Chen et al. (2001) J. Atmos. Sci., 58, p.1832, Fig. 8(a); and from B. J. Hoskins (1996) Bull. Amer. Met. Soc. 77, p. 1291, Fig. 5. Courtesy of the American Meteorological Society.

lacking, but they undoubtedly form an essential part of the story.

In the light of these theories, the origin of the subtropical anticyclones that play such an important role in the world's climates may be re-examined. Their existence has been variously ascribed to: (1) the piling up of poleward-moving air as it is increasingly deflected eastward through the earth's rotation and the conservation of angular momentum; (2) the sinking of poleward currents aloft by radiational cooling; (3) the general necessity for high pressure near 30° latitude separating approximately equal zones of east and west winds; or to combinations of such mechanisms. An adequate theory must account not only for their permanence but also for their cellular nature and the vertical inclination of the axes. The above discussion shows that ideas of a simplified Hadley cell and momentum conservation are only partially correct. Moreover, recent studies rather surprisingly show no relationship, on a seasonal basis, between the intensity of the Hadley cell and that of the subtropical highs. Descent occurs near 25°N in winter, whereas North Africa and the Mediterranean are generally driest in summer, when the vertical motion is weak.

Two new ideas have recently been proposed (Figure 7.21). One suggests that the low-level subtropical highs in the North Pacific and North Atlantic in summer are remote responses to stationary planetary waves generated by heat sources over Asia. In contrast to this view of eastward downstream wave propagation, another model proposes regional effects from the heating over the summer monsoon regions of India, West Africa and southwestern North America that act upstream on the western and northern margins of these heat sources. The Indian monsoon heating leads to a vertical cell with descent over the eastern Mediterranean, eastern Sahara Desert and the Kyzylkum-Karakum Desert. However, while the ascending air originates in the tropical easterlies, Rossby waves in the mid-latitude westerlies are thought to be the source of the descending air and this may provide a link with the first mechanism. Neither of these arguments addresses the winter subtropical anticyclones. Clearly, these features await a definitive and comprehensive explanation.

It is probable that the high-level anticyclonic cells that are evident on synoptic charts (these tend to merge on mean maps) are related to anticyclonic eddies that develop on the equatorward side of jet streams. Theoretical and observational studies show that, as a result of the latitudinal variation of the Coriolis parameter, cyclones in the westerlies tend to move poleward and anticyclonic cells equatorward. Hence the subtropical anticyclones are constantly regenerated. There is a statistical relationship between the latitude of the subtropical highs and the mean meridional temperature gradient (see Figure 7.11); a stronger gradient causes an equatorward shift of the high pressure, and vice versa. This shift is evident on a seasonal basis. The cellular pattern at the surface clearly reflects the influence of heat sources. The cells are stationary and elongated north-south over the northern hemisphere oceans in summer, when continental heating creates low pressure and also the meridional temperature gradient is weak. In winter, on the other hand, the zonal flow is stronger in response to a greater meridional temperature gradient, and continental cooling produces east-west elongation of the cells. Undoubtedly, surface and highlevel factors reinforce one another in some sectors and tend to cancel each other out in others.

Just as Hadley circulations represent major meridional (i.e. north-south) components of the atmospheric circulation, so Walker circulations represent the large-

A High Phase SOI

B Low Phase SOI

Pacific Oceanic Meridional Overturn Cell

Figure 7.22 Schematic cross-sections of the Walker circulation along the equator (based on computations of Y. Tourre (1984)) during the high (A) and low (B) phases of the Southern Oscillation (SO). The high (low) phases correspond to non-ENSO (ENSO) patterns (see p. 146). In the high phase there is rising air and heavy rains over the Amazon basin, central Africa and Indonesia, western Pacific. In the low phase (ENSO 1982-83) pattern the ascending Pacific branch is shifted east of the date-line and elsewhere convection is suppressed due to subsidence. The shading indicates the topography in exaggerated vertical scale.

Figure 7.22 Schematic cross-sections of the Walker circulation along the equator (based on computations of Y. Tourre (1984)) during the high (A) and low (B) phases of the Southern Oscillation (SO). The high (low) phases correspond to non-ENSO (ENSO) patterns (see p. 146). In the high phase there is rising air and heavy rains over the Amazon basin, central Africa and Indonesia, western Pacific. In the low phase (ENSO 1982-83) pattern the ascending Pacific branch is shifted east of the date-line and elsewhere convection is suppressed due to subsidence. The shading indicates the topography in exaggerated vertical scale.

Source: Based on K. Wyrtki (by permission of the World Meteorological Organization 1985).

scale zonal (i.e. east-west) components of tropical airflow. These zonal circulations are driven by major east-west pressure gradients that are set up by differences in vertical motion. On one hand, air rises over heated continents and the warmer parts of the oceans and, on the other, air subsides over cooler parts of the oceans, over continental areas where deep high-pressure systems have become established, and in association with subtropical high-pressure cells. Sir Gilbert Walker first identified these circulations in 1922 to 1923 through his discovery of an inverse correlation between pressure over the eastern Pacific Ocean and Indonesia. The strength and phase of this so-called Southern Oscillation is commonly measured by the pressure difference between Tahiti (18°S, 150°W) and Darwin, Australia (12°S, 130°E). The Southern Oscillation Index (SOI) has two extreme phases (Figure 7.22):

• positive when there is a strong high pressure in the southeast Pacific and a low centred over Indonesia with ascending air and convective precipitation;

• negative (or low) when the area of low pressure and convection is displaced eastward towards the Date Line.

Positive (negative) SOI implies strong easterly trade winds (low-level equatorial westerlies) over the central-western Pacific. These Walker circulations are subject to fluctuations in which an oscillation (El Nino-Southern Oscillation: ENSO) between high phases (i.e. non-ENSO events) and low phases (i.e. ENSO events) is the most striking (see Chapter 11G.1):

1 High phase (Figure 7.22A). This features four major zonal cells involving rising low-pressure limbs and accentuated precipitation over Amazonia, central Africa and Indonesia/India; and subsiding high-pressure limbs and decreased precipitation over the eastern Pacific, South Atlantic and western Indian Ocean. During this phase, low-level easterlies strengthen over the Pacific and subtropical westerly jet streams in both hemispheres weaken, as does the Pacific Hadley cell.

2 Low phase (Figure 7.22B). This phase has five major zonal cells involving rising low-pressure limbs and accentuated precipitation over the South Atlantic, the western Indian Ocean, the western Pacific and the eastern Pacific; and subsiding high-pressure limbs and decreased precipitation over Amazonia, central Africa, Indonesia/India and the central Pacific. During this phase, low-level westerlies and high-level easterlies dominate over the Pacific, and subtropical westerly jet streams in both hemispheres intensify, as does the Pacific Hadley cell.

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