Informal seminar: Trends in the tropospheric general circulation since 1979
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Atmospheric general circulation changes since 1979 have been studied using ERA5. Changes in near-tropopause winds have been the main focus, but related changes in temperature, wind and other variables throughout the troposphere have also been considered. Several comparisons between ERA5 and JRA-55, and between ERA5 and the observations it assimilated, have been made.
Among the trends with high statistical significance is a little-discussed strengthening and latitudinal expansion of the tropical upper-tropospheric easterlies. Better-known are the strengthening of the lower-tropospheric easterlies over the tropical Pacific and strengthening of the westerlies around southern mid-latitudes. The common interpretation of the latter as a shift of the Southern Annular Mode to a more positive state is questioned. Weakening of the wintertime upper-level westerly flow associated with the Arctic amplification of warming appears to be more local than has been argued by some: the wintertime westerlies strengthen in northern middle latitudes. The jet-stream region over the eastern North Atlantic and western Europe shifts southward.
Extreme jet-stream winds tend to increase over the North Atlantic. Net kinetic energy also increases, mostly associated with sub-monthly variability along the mid-latitude storm tracks and over the tropical Pacific. Available potential energy changes less. Geopotential height shows a distinct pattern of change in the stationary northern-hemispheric long-wave structure. Surface pressure increases over the North Pacific and southern mid-latitudes, and decreases over the Arctic Ocean and offshore of Antarctica.