Variation in Size and Location of the 300 mb North Circumpolar Vortex Between 1963 and 1975

Abstract

The time variation in size and location of the 300 mb north circumpolar vortex has been estimated from mean monthly polar stereographic maps for the years 1963?75, both by planimetering the area north of height contours in the main belt of westerlies and by determining the colatitude of these contours at 30° longitude intervals and squaring the average value. It is shown that the vortex areas determined by the two independent techniques are in excellent agreement, but that the vortex-center locations (based on equalization of arms) determined by the two techniques are in only fair agreement. The change in vortex area appears mainly to reflect a change in mean tropospheric temperature, but with some modification caused by a change in mean surface pressure in temperate latitudes. Since 1970 the yearly averaged vortex area has increased by almost 2%, but that increase apparently ended in 1975, suggesting that the cooling in north temperate latitudes has also ended. During this same five-year period the winter vortex contracted, implying that temperate latitudes are becoming more temperate. There has been a striking tendency for the polar vortex to he contracted at the time of quasi-biennial west wind maximum at 50 mb in the tropics, and it is estimated that the mid-latitude troposphere is warmer by a few tenths of a degree Celsius at this time. During the past few years the polar vortex has moved unusually far (based on only 13 years of record) into the Eastern Hemisphere, a result largely of an anomalous displacement of the winter vortex into this hemisphere. Considered is the relation of United States surface temperature to vortex size and location, and the possibility of climate foreshadowing by assuming persistence in the vortex trends.