A STUDY OF THE EFFECTS OF LARGE BLOCKING HIGHS ON THE GENERAL CIRCULATION IN THE NORTHERN-HEMISPHERE WESTERLIES

Abstract

The basic problems involved in the investigation of the effects of “blocking action” upon the circulation pattern of the northern-hemisphere westerlies are defined and the mode of attack outlined. By application of turbulence principles seasonal variations in lateral mixing and northward heat transport across the westerlies are related to variations in the extent of blocking action. Evidence is presented indicating that blocking action, lateral mixing, and northward heat transport are all greater in the winters of years when strong north–south thermal gradients exist and vice versa. The year-to-year variations in these quantities exhibit a long-period and relatively large amplitude climatic swing with a period of roughly 12–14 years. Considerable year-to-year variation in the radiative properties of the earth's surface and atmosphere, or in incoming radiation, is thus implied. An analysis of heat convergence through the lateral turbulence process shows in general the heart of the westerlies losing heat, particularly from the east coasts of the continents to the Aleutian and Icelandic Lows. This mechanism is apparently the means by which the large amount of heat picked up by the air from the oceans off the east coasts is dispersed. A distinction is made between the turbulent eddy sizes identified with unstable, migratory systems and those of blocking action, the latter representing dynamically quasistable systems capable of producing effects upon the general circulation of the westerlies at great distances both upstream and downstream and over a period of many days. Effects resulting from the intrusion of stable, persistent high-pressure cells into the normal westerly flow of the general circulation of the atmosphere in the characteristic manner of blocking action are investigated by means of empirical, semi-statistical procedures. These processes are found to cause the development of long stable wave patterns in the atmosphere downstream from the point of inception. These stationary patterns may occasionally exist for periods of as long as a month. The wave lengths are found to be longer than those generally considered representative of stationary wave patterns. Presumably this difference is due to the distorting effect of the larger amplitude of the long waves associated with the blocking action process. A theory of the formation of these blocking high cells is suggested based on the accumulation of heat in low latitudes and the necessity for the readjustment of the general circulation to redistribute this heat. One means of dissipation of the blocking cells is shown to be the formation of a wave pattern in the atmosphere which is out of phase with the stable wave pattern formed by the blocking high.