A STUDY OF VARIATIONS OF THE GENERAL CIRCULATION

Abstract

Changes of the general circulation, especially the strength of the upper westerlies, over 28 months are investigated quantitatively.The role of the high troposphere, represented by the 300-mb level, is stressed and it is sought to determine whether the tropics have any measurable influence on middle-latitude weather. Computations cover the latitude belt 10°N to 70°N, at least for part of the periods studied. To bring out the fluctuations clearly, they are expressed in terms of deviations from seasonal and mean latitudinal gradients. These deviations amount to over 20 per cent of the mean circulation, even in the belt of strongest westerlies. The concern is not with very small values whose accuracy would be questionable. When the latitudinal distribution of deviations, especially those of momentum of the westerlies, is plotted against time, definite long-term trends appear. Bands of high and low westerly momentum gradually traverse the latitude circles, sometimes from north to south, but usually from south to north. Momentum maxima and minima hardly ever form in middle latitudes. They approach the jet-stream region from equator or pole and pass across it, continuing on their poleward or equatorward course. A synoptic example shows the effect of a traveling momentum maximum on the northern hemisphere during several stages of its motion. Coincident weather developments, such as typhoon formations and long-wave motion, are noted.