Variations of three to eight weeks' duration are observed in the strength of the zonal westerlies, averaged around the hemisphere. They are rather more noticeable in the winter months, when the general circulation is strongest. The nature of the changes is illustrated schematically in Figure 7.23. The mid-latitude westerlies develop waves, and the troughs and ridges become accentuated, ultimately splitting up into a cellular pattern with pronounced meridional flow at certain longitudes. The strength of the westerlies between 35° and 55°N is termed the zonal index; strong zonal westerlies are representative of a high index, and marked cellular patterns occur with a low index (see Plate 15). A relatively low index may also occur if the westerlies are well south of their usual latitudes and, paradoxically, such expansion of the zonal circulation pattern is associated with strong westerlies in lower latitudes than usual. Figures 7.24 and 7.25 illustrate the mean 700-mb contour patterns and zonal wind speed profiles for two contrasting months. In December 1957, the westerlies were stronger than normal north of 40°N, and the troughs and ridges were weakly developed, whereas in February 1958 there was a low zonal index and an expanded circumpolar vortex, giving rise to strong low-latitude westerlies. The 700-mb pattern shows very weak subtropical highs, deep meridional troughs and a blocking anticyclone off Alaska (see Figure 7.25A). The cause of these variations is still uncertain, although it would appear that fast zonal flow is unstable and tends
Figure 7.23 The index cycle. A schematic illustration of the development of cellular patterns in the upper westerlies, usually occupying three to eight weeks and being especially active in February and March in the northern hemisphere. Statistical studies indicate no regular periodicity in this sequence. (A) High zonal index. The jet stream and the westerlies lie north of their mean position. The westerlies are strong, pressure systems have a dominantly east-west orientation, and there is little north-south airmass exchange. (B) and (C) The jet expands and increases in velocity, undulating with increasingly larger oscillations. (D) Low zonal index. The latter is associated with a complete breakup and cellular fragmentation of the zonal westerlies, formation of stationary deep occluding cold depressions in lower mid-latitudes and deep warm blocking anticyclones at higher latitudes. This fragmentation commonly begins in the east and extends westward at a rate of about 60° of longitude per week.
Source: After Namias; from Haltiner and Martin (1957).
to break down. This tendency is certainly increased in the northern hemisphere by the arrangement of the continents and oceans.
Detailed studies are now beginning to show that the irregular index fluctuations, together with secondary circulation features, such as cells of low and high pressure at the surface or long waves aloft, play a major role in redistributing momentum and energy. Laboratory experiments with rotating 'dishpans' of water to simulate the atmosphere, and computer studies using numerical models of the atmosphere's behaviour, demonstrate that a Hadley circulation cannot provide an adequate mechanism for transporting heat polewards. In consequence, the meridional temperature gradient increases and eventually the flow becomes unstable in the Hadley mode, breaking down into a number of cyclonic and anticyclonic eddies. This phenomenon is referred to as baroclinic instability. In energy terms, the potential energy in the zonal flow is converted into potential and kinetic energy of eddies. It is also now known that the kinetic energy of the zonal flow is derived from the eddies, the reverse of the classical picture, which viewed the disturbances within the global wind belts as superimposed detail. The significance of atmospheric disturbances and the variations of the circulation are becoming increasingly evident. The mechanisms of the circulation are, however, greatly complicated by numerous interactions and feedback processes, particularly those involving the oceanic circulation discussed below.
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