Abstract
Quasi-biennial and long-term fluctuations in the latitude, longitude and pressure of the Azores High, Pacific High, Icelandic Low, and Aleutian Low have been determined for the period 1899–1967. Within the quasi-biennial framework, an increase in pressure of the Azores High is associated with an increase in pressure of the Pacific High and Aleutian Low, but a decrease in pressure of the Icelandic Low. Based on data for the period 1951–1967, there is a significant tendency for the Azores High to be relatively far west, and the pressure relatively low, at the time of quasi-biennial west wind maximum in the low tropical stratosphere, and for the Pacific High to be relatively far south at this same time. The latter appears in agreement with the finding that the centroid of the north polar vortex is displaced farther toward the Pacific Ocean at the time of quasi-biennial west wind maximum.
The most impressive long-term trends are the indicated 5° latitude northward displacement of the Pacific High and 6° longitude westward displacement of the Aleutian Low during the period 1905–1965. Both the Azores High and Icelandic Low attained their northernmost latitude about 1940, or near the time of surface temperature maximum in the Northern Hemisphere. The recent drought south of the Sahara is probably related to the southward (and eastward) displacement of the Azores High since that time. There is evidence that hurricane and typhoon frequency is greatest when the subtropical highs are relatively far north, and of weak intensity, and accordingly the recent sparsity of hurricanes may also be partly due to the southward shift of the Azores High.
