Trends in the tropospheric general circulation from 1979 to 2022

• Main Author: Simmons, Adrian John
• Format: Text
• Language: English
• Published: 2022
• Subjects: Arctic; Arctic Ocean; Pacific; Antarc*; Antarctica; North Atlantic
• Online Access: https://doi.org/10.5194/wcd-2022-19; https://wcd.copernicus.org/preprints/wcd-2022-19/

Atmospheric general circulation changes since 1979 are examined using the ERA5 reanalysis. Maps based on linear trends and time series for specific areas are presented. Attention is concentrated on monthly, seasonal and annual means, but shorter-timescale variability is also considered, including extremes. Changes in near-tropopause winds are the main focus, but related changes in temperature, wind and other variables throughout the troposphere are discussed. Middle- and upper-tropospheric warming is larger in the subtropics and outer tropics than in the deep tropics, except over the Pacific. This is linked with a strengthening and meridional expansion of the tropical easterlies that has received little previous attention. Warming over several mid-latitude and subtropical land areas comes close to matching the large warming of the Arctic. Westerly upper-level winds in general weaken over the Arctic in winter, but strengthen in northern middle latitudes, contrary to arguments based on circulation changes due solely to amplified Arctic warming. The jet-stream region over the eastern North Atlantic and western Europe shifts southward. Westerlies strengthen in a band stretching south-eastwards from the tropical western Pacific to southern Australia, and in the polar-jet-stream region that surrounds Antarctica. Extreme jet-stream winds 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. There are increases in surface pressure over the North Pacific and southern mid-latitudes, and decreases over the Arctic Ocean and offshore of Antarctica. Several comparisons are made between ERA5 and the JRA-55 reanalysis, and between ERA5 and the observations it assimilated. They show reassuring agreement, but some regional differences require further investigation.