Anatomy of Great Plains Protracted Heat Waves (especially the 1980 U.S. summer drought)

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  • 1 Scripps Institution of Oceanography, University of California, San Diego, La Jolla, 92093
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Abstract

The protracted heat wave and drought of the Great Plains during summer 1980 was a manifestation of an abnormal form of the general circulation. An upper-level continental high developed rapidly over the Southern Plains in late May and persisted with only small changes throughout the summer. The associated subsidence, dry soil, lack of cloud and high insolation led to unrelenting heat.

Descriptions of several interlocking features are given followed by an attempt to ascribe the development and maintenance of the drought-producing cell to several physical factors.

Strong low-latitude westerlies (expanded circumpolar vortex) over the North Pacific, North America and North Atlantic during spring 1980 led to latitudinally depressed storm tracks relative to normal. Combined with normal seasonal forcing this anomalous pattern was stable in spring but unstable in summer after the climatologically dependable northward march of the westerlies. Moreover, normal spring to summer mid-tropospheric height changes, especially height changes from May to June, show maximum rises in the eastern Pacific, central U.S. Plains and central Atlantic—areas favoring the development of a 3-cell pattern characteristic of summer U.S. drought.

Once this strong Great Plains ridge was set up, it reinforced the normal June anticyclone over the Southern Plains through enhanced surface heating, by radiative factors (Charney effect), and possibly through high dust counts (Twomey-Squires effect).The newly emerged pattern of robust anticyclones stationed over the eastern North Pacific and Southern Plains seems to be an equilibrium state indicated by teleconnections characteristic of great droughts of earlier years, particularly 1952–54, and 1934 and 1936.

The transition over the United States from strong low-latitude westerlies in spring to an upper-level anticyclone in summer actually occurred in a brief period during the last week in May, when redistribution of vorticity from a rapidly developing east Pacific ridge spread downstream to the West Coast trough, to the Great Plains ridge, and then to the trough off the eastern seaboard.

Some speculations are offered that abnormally cold sea-surface temperatures generated and maintained by strong westerlies over the North Pacific in spring and in the antecedent winter may have helped to generate the strong Pacific anticyclone and upper-level ridge in late May by lowering the atmospheric drag coefficient over the very area where normal pressure changes in late spring also favor anticyclogenesis.

The failure of conventional long-range forecasts to predict this record-breaking heat wave calls for a reassessment of methods which rely almost entirely on long-period persistence and contingencies.

Abstract

The protracted heat wave and drought of the Great Plains during summer 1980 was a manifestation of an abnormal form of the general circulation. An upper-level continental high developed rapidly over the Southern Plains in late May and persisted with only small changes throughout the summer. The associated subsidence, dry soil, lack of cloud and high insolation led to unrelenting heat.

Descriptions of several interlocking features are given followed by an attempt to ascribe the development and maintenance of the drought-producing cell to several physical factors.

Strong low-latitude westerlies (expanded circumpolar vortex) over the North Pacific, North America and North Atlantic during spring 1980 led to latitudinally depressed storm tracks relative to normal. Combined with normal seasonal forcing this anomalous pattern was stable in spring but unstable in summer after the climatologically dependable northward march of the westerlies. Moreover, normal spring to summer mid-tropospheric height changes, especially height changes from May to June, show maximum rises in the eastern Pacific, central U.S. Plains and central Atlantic—areas favoring the development of a 3-cell pattern characteristic of summer U.S. drought.

Once this strong Great Plains ridge was set up, it reinforced the normal June anticyclone over the Southern Plains through enhanced surface heating, by radiative factors (Charney effect), and possibly through high dust counts (Twomey-Squires effect).The newly emerged pattern of robust anticyclones stationed over the eastern North Pacific and Southern Plains seems to be an equilibrium state indicated by teleconnections characteristic of great droughts of earlier years, particularly 1952–54, and 1934 and 1936.

The transition over the United States from strong low-latitude westerlies in spring to an upper-level anticyclone in summer actually occurred in a brief period during the last week in May, when redistribution of vorticity from a rapidly developing east Pacific ridge spread downstream to the West Coast trough, to the Great Plains ridge, and then to the trough off the eastern seaboard.

Some speculations are offered that abnormally cold sea-surface temperatures generated and maintained by strong westerlies over the North Pacific in spring and in the antecedent winter may have helped to generate the strong Pacific anticyclone and upper-level ridge in late May by lowering the atmospheric drag coefficient over the very area where normal pressure changes in late spring also favor anticyclogenesis.

The failure of conventional long-range forecasts to predict this record-breaking heat wave calls for a reassessment of methods which rely almost entirely on long-period persistence and contingencies.

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