Size Variations and Long-Wave Circulation within the January Northern Hemisphere Circumpolar Vortex: 1946–89

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  • 1 Department of Geography, Colgate University, Hamilton, New York
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Abstract

The close association between size variations in the Northern Hemisphere circumpolar vortex and surface and middle-troposheric thermal characteristics makes vortex measurement a valuable tool in monitoring and understanding climate change. Unfortunately, as with most hemispheric circulation indices, measures of total vortex size offer little insight into regional changes in the vortex. Traditional approaches to vortex size calculation, which are based upon planimeter measurements on a polar stereographic projection, limit the ability to examine regional contributions to the total vortex and cannot be used to assess specific linkages between vortex expansion and contraction and the broader class of long wave circulation phenomena. Furthermore, because the scale of the polar stereographic projection varies from one latitude to another, interannual variations in planimeter vortex size measurements are influenced somewhat by the position of the vortex relative to the North Pole.

Many of these problems are avoided if digital data sources are used to calculate vortex size. Digital data enable the calculation of actual earth surface area within longitudinal sectors of the vortex and provide a regional decomposition of total vortex that can he linked with variations in long wave circulation. In this study, digital 500-mb geopotential height data interpolated to a 5° latitude by 5° longitude grid were used to examine size variations in the January circumpolar vortex for the period 1946–89. Total January vortices were smaller than the 44-year average during the period 1946–64, after which larger than average vortices became more common. The last few years of the data record indicate that January vortices may be becoming more contracted again. These patterns of contraction and expansion are not reflective of all sectors of the vortex. Much of the vortex expansion after 1964 occurred in association with amplified troughing over the central Pacific Ocean and eastern North America/Atlantic Ocean. An inverse trend in regional vortex size within the western North American ridge indicates that more frequent occurrences of the positive Pacific–North American teleconnection pattern may be most responsible for the upward trend in total vortex size after 1964.

Abstract

The close association between size variations in the Northern Hemisphere circumpolar vortex and surface and middle-troposheric thermal characteristics makes vortex measurement a valuable tool in monitoring and understanding climate change. Unfortunately, as with most hemispheric circulation indices, measures of total vortex size offer little insight into regional changes in the vortex. Traditional approaches to vortex size calculation, which are based upon planimeter measurements on a polar stereographic projection, limit the ability to examine regional contributions to the total vortex and cannot be used to assess specific linkages between vortex expansion and contraction and the broader class of long wave circulation phenomena. Furthermore, because the scale of the polar stereographic projection varies from one latitude to another, interannual variations in planimeter vortex size measurements are influenced somewhat by the position of the vortex relative to the North Pole.

Many of these problems are avoided if digital data sources are used to calculate vortex size. Digital data enable the calculation of actual earth surface area within longitudinal sectors of the vortex and provide a regional decomposition of total vortex that can he linked with variations in long wave circulation. In this study, digital 500-mb geopotential height data interpolated to a 5° latitude by 5° longitude grid were used to examine size variations in the January circumpolar vortex for the period 1946–89. Total January vortices were smaller than the 44-year average during the period 1946–64, after which larger than average vortices became more common. The last few years of the data record indicate that January vortices may be becoming more contracted again. These patterns of contraction and expansion are not reflective of all sectors of the vortex. Much of the vortex expansion after 1964 occurred in association with amplified troughing over the central Pacific Ocean and eastern North America/Atlantic Ocean. An inverse trend in regional vortex size within the western North American ridge indicates that more frequent occurrences of the positive Pacific–North American teleconnection pattern may be most responsible for the upward trend in total vortex size after 1964.

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