In addition to changes in the amount of meandering, and connected with them, changes occur in the strength of the main wind flow each year in the round of the seasons, from one spell of weather to another and from one climatic epoch to another. And when the upper westerly winds are most strongly developed around the globe, with only modest waves in the pattern of the circumpolar flow, the wave-length or spacing of these waves increases. The spacing also increases if and when the main stream is displaced towards higher latitudes without change of strength. Changes in the spacing of the troughs and ridges in the pattern of the circumpolar vortex mean changes in the positions around the hemisphere that are affected by extensions of the cold polar and warm subtropical regimes respectively. They also mean changes in the positions at which the cyclonic disturbances develop and the frequency, and speed, with which their rain belts and often stormy surface winds are steered along various paths. In this way great differences may arise between dry and wet, warm and cold weather prevailing in a given season or a given climatic epoch in different parts of the same latitude zone around the Earth. The situation depends on the wave-length and resulting longitude positions 'favoured' by the troughs and ridges in the flow of the upper winds.
It is the intimate relationship between the circumpolar vortex and the steering of the surface weather systems, together with the fact that its pattern is basically simple, and an entity subject to variations which are also of simply recognized types, that makes it possible to reconstruct global weather patterns of the past from fragmentary and scattered information just here and there around each hemisphere. From fossil evidence of the gross temperature distribution prevailing at the surface of the oceans and over land in any past epoch, we can reconstruct in outline the prevailing features of the circumpolar vortex and hence of the large-scale wind and ocean circulations at the surface.
Interrelationships between the northern and southern hemispheres also need to be studied, both as regards the large-scale wind circulation and other aspects of the climatic regime. It is a noticeable feature of figs. 4a and b (pp. 26-7) and 6a and b (p. 32) that in the present epoch the mean wind circulation over the southern hemisphere, with its glaciated continent in the high latitudes, is stronger than that over the northern hemisphere. On the other hand, the northern hemisphere circulation not infrequently develops much more 'meridional' patterns than are seen over the southern hemisphere: these are liable at times to push the interhemispheric convergence zone (the meteorological equator) far south across the equator over a narrow range of longitudes into Brazil, southern Africa or Australia, evidently assisted by its being drawn into the convection system developed over the heated continents. We thus find some evidence of impacts either way of the circulation over one hemisphere on that over the other. In the case of longterm changes of the climatic regime, some curious features come to light which have not received the attention they deserve and are not yet widely known or understood. Thus, although the whole Earth experienced the last ice age and now enjoys the present interglacial period, the timing of the changes shows some important differences between north and south. And within the last thousand years, the development of what has been reasonably called the Little Ice Age seems to have affected the whole Earth, as has the twentieth-century recovery from it; but when the ice on the Arctic seas extended farthest south, particularly in the Atlantic sector, all the climatic zones seem to have shifted south, including the storm activity of the Southern Ocean and the Antarctic fringe. This apparently broke up much of the Antarctic sea ice, enabling Captain Cook in the 1770s and Weddell in 1823 to sail farther south than ships have usually been able to reach in this century. The southward extension of open water would presumably result in some mildening of the regime not only over the ocean but some way into the interior of Antarctica, and this just when the world in general north of about 40 °S was experiencing a notably cold regime. Amongst the evidence which builds up this picture, at that time the winter rains failed to reach so far north over Chile. And radiocarbon dating of abandoned penguin rookeries on the Antarctic coast near 77 1/2 °S, in the southernmost part of of the Ross Sea, suggests that there were periods of milder climate there about AD 1250-1450 and 1670-1840. These periods include the sharpest phases of development of the Little Ice Age climate in the northern hemisphere.
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