Editing IPCC:AR6/WGI/TS (section)

=== TS.2.3 Upper Air Temperatures and Atmospheric Circulation ===

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'''The effects of human-induced climate change have been clearly identified in observations of atmospheric temperature and some aspects of atmospheric circulation, and these effects are likely to intensify in the future. Tropospheric warming and stratospheric cooling are virtually certain to continue with continued net emissions of greenhouse gases. Several aspects of the atmospheric circulation have likely changed since the mid-20th century, and human influence has likely contributed to the observed poleward expansion of the Southern Hemisphere Hadley Cell and very likely contributed to the observed poleward shift of the Southern Hemisphere extratropical jet in summer. It is likely that the mid-latitude jet will shift poleward and strengthen, accompanied by a strengthening of the storm track in the Southern Hemisphere by 2100 under the high CO 2 emissions scenarios. It is likely that the proportion of intense tropical cyclones has increased over the last four decades and that this cannot be explained entirely by natural variability. There is low confidence in observed recent changes in the total number of extratropical cyclones over both hemispheres. The proportion of tropical cyclones that are intense is expected to increase (high confidence), but the total global number of tropical cyclones is expected to decrease or remain unchanged (medium confidence). Links to chapters 2.3, 3.3, 4.3, 4.4, 4.5, 8.3, 8.4, 11.7'''

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[[File:a3b98de4edea233dcdc3a218b972ddea IPCC_AR6_WGI_TS_Figure_10.png]]
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'''Figure TS.10 |''' '''Observed and projected upper air temperature and circulation changes.''' ''The intent of this figure is to visualize upper air temperature and circulation changes and the similarity between observed and projected changes.'' Upper panels: (left) Zonal cross-section of temperature trends for 2002–2019 in the upper troposphere region for the ROM SAF radio-occultation dataset. (Middle) Change in the annual and zonal mean atmospheric temperature (°C) in 2081–2100 in SSP1-2.6 relative to 1995–2014 for 36 Coupled Model Intercomparison Project Phase 6 (CMIP6) models. (right) the same in SSP3-7.0 for 32 models. Lower panels: (left) Long-term mean (thin black colour) and linear trend (colour) of zonal mean December–January–February (DJF) zonal winds for ERA5. (Middle) multi-model mean change in annual and zonal mean wind (m s <sup>–1</sup>) in 2081–2100 in SSP1-2.6 relative to 1995–2014 based on 34 CMIP6 models. The 1995–2014 climatology is shown in contours with spacing of 10 m s <sup>–1</sup>. (right) the same for SSP3-7.0 for 31 models. Links to chapters 2.3.1; Figures 2.12 and 2.18; 4.5.1; Figure 4.2.6

The troposphere has warmed since at least the 1950s, and it is ''virtually certain'' that the stratosphere has cooled. It is ''very likely'' that human-induced increases in GHGs were the main driver of tropospheric warming since 1979. It is ''extremely likely'' that anthropogenic forcing, both from increases in GHG concentrations and depletion of stratospheric ozone due to ozone-depleting substances, was the main driver of upper stratospheric cooling since 1979. It is ''very likely'' that global mean stratospheric cooling will be larger for scenarios with higher atmospheric CO <sub>2</sub> concentrations. In the tropics, since at least 2001 (when new techniques permit more robust quantification), the upper troposphere has warmed faster than the near-surface (''medium confidence'') (Figure TS.10). There is ''medium confidence'' that most CMIP5 and CMIP6 models overestimate the observed warming in the upper tropical troposphere over the period 1979–2014, in part because they overestimate tropical SST warming. It is ''likely'' that future tropical upper tropospheric warming will be larger than at the tropical surface. Links to chapters 2.3.1, 3.3.1, 4.5.1

The Hadley Circulation has ''likely'' widened since at least the 1980s, predominantly in the Northern Hemisphere, although there is only ''medium confidence'' in the extent of the changes. This has been accompanied by a strengthening of the Hadley Circulation in the Northern Hemisphere (''medium confidence''). It is ''likely'' that human influence has contributed to the poleward expansion of the zonal mean Hadley cell in the Southern Hemisphere since the 1980s, which is projected to further expand with global warming (''high confidence''). There is ''medium confidence'' that the observed poleward expansion in the Northern Hemisphere is within the range of internal variability. Links to chapters 2.3.1, 3.3.3, 8.4.3

Since the 1970s, near-surface average winds have ''likely'' weakened over land. Over the ocean, near-surface average winds ''likely'' strengthened over 1980–2000, but divergent estimates lead to ''low confidence'' thereafter. Extratropical storm tracks have ''likely'' shifted poleward since the 1980s. There is ''low confidence'' in projected poleward shifts of the Northern Hemisphere mid-latitude jet and storm tracks due to large internal variability and structural uncertainty in model simulations. There is ''medium confidence'' in a projected decrease in the frequency of atmospheric blocking over Greenland and the North Pacific in boreal winter in 2081–2100 under the SSP3-7.0 and SSP5-8.5 scenarios. There is ''high confidence'' that Southern Hemisphere storm tracks and associated precipitation have migrated polewards over recent decades, especially in the austral summer and autumn, associated with a trend towards more positive phases of the Southern Annular Mode (SAM) (Section TS.4.2.2) and the strengthening and southward shift of the Southern Hemisphere extratropical jet in austral summer. In the long term (2081–2100), the Southern Hemisphere mid-latitude jet is ''likely'' to shift poleward and strengthen under the SSP5-8.5 scenario relative to 1995–2014, accompanied by an increase in the SAM (Section TS.4.2.2). It is ''likely'' that wind speeds associated with extratropical cyclones will strengthen in the Southern Hemisphere storm track for SSP5-8.5. There is ''low confidence'' in the potential role of Arctic warming and sea ice loss on historical or projected mid-latitude atmospheric variability. Links to chapters 2.3.1, 3.3.3, 3.7.2, 4.3.3, 4.4.3, 4.5.1, 4.5.3, 8.2.2, 8.3.2, Cross-Chapter Box 10.1

It is ''likely'' that the proportionof major (Category 3–5) tropical cyclones (TCs) and the frequency of rapid TC intensification events have increased over the past four decades. The average location of peak TC wind-intensity has ''very likely'' migrated poleward in the western North Pacific Ocean since the 1940s, and TC forward translation speed has ''likely'' slowed over the contiguous USA since 1900. It is ''likely'' that the poleward migration of TCs in the western North Pacific and the global increase in TC intensity rates cannot be explained entirely by natural variability '''.''' There is ''high confidence'' that average peak TC wind speeds and the proportion of Category 4–5 TCs will increase with warming and that peak winds of the most intense TCs will increase. There is ''medium confidence'' that the average location where TCs reach their maximum wind-intensity will migrate poleward in the western North Pacific Ocean, while the total global frequency of TC formation will decrease or remain unchanged with increasing global warming. Links to chapters 11.7.1

There is ''low confidence'' in observed recent changes in the total number of extratropical cyclones over both hemispheres. There is also ''low confidence'' in past-century trends in the number and intensity of the strongest extratropical cyclones over the Northern Hemisphere due to the large interannual-to-decadal variability and temporal and spatial heterogeneities in the volume and type of assimilated data in atmospheric reanalyses, particularly before the satellite era. Over the Southern Hemisphere, it is ''likely'' that the number of extratropical cyclones with low central pressures (&lt;980 hPa) has increased since 1979. The frequency of intense extratropical cyclones is projected to decrease (''medium confidence''). Projected changes in the intensity depend on the resolution of climate models (''medium confidence''). There is ''medium confidence'' that wind speeds associated with extratropical cyclones will change following changes in the storm tracks. Links to chapters 2.3.1, 3.3.3, 4.5.1, 4.5.3, 8.3.2, 8.4.2, 11.7.2

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