Editing IPCC:AR6/WGI/Chapter-4 (section)

===== 4.5.1.6.1 Sea level pressure =====

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The AR5 assessed that mean sea level pressure is projected to decrease in high latitudes and to increase in mid-latitudes. Such a pattern is associated with a poleward shift in the storm track and an increase in the annular mode index. This broad pattern is also found in CMIP6 models (Figure 4.25). Under SSP1-2.6, the pattern in sea level pressure change resembles that for SSP3-7.0, but the amplitudes are small compared to internal variability in 20-year means (Figure 4.25). One exception is found in the SH mid-latitudes, where pressure robustly increases in SSP3-7.0 in both austral summer and winter, but shows no robust change in SSP1-2.6. This is ''likely'' attributable to the larger GHG forcing in SSP3-7.0 compared to SSP1-2.6, which contributes to a poleward shift of the SH mid-latitude circulation and becomes relatively more important than the effect of ozone recovery which drives an equatorward shift in the circulation (see [[#4.5.3.1|Section 4.5.3.1]] on the Southern Annular Mode; [[#Barnes--2013|Barnes and Polvani, 2013]] ; [[#Barnes--2014|Barnes et al., 2014]] ; [[#Bracegirdle--2020b|Bracegirdle et al., 2020b]] ). The poleward shift in SH mid-latitude circulation in SSP3-7.0 ''likely'' contributes to the wetting trend at high southern latitudes (Figure 4.25).

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'''Figure 4.25 |''' '''Long-term change of seasonal-mean sea level pressure.''' Displayed are projected spatial patterns of multi-model mean change in '''(top)''' December–January–February (DJF) and '''(bottom)''' June–July–August (JJA) mean sea level pressure (hPa) in 2081–2100 relative to 1995–2014, for '''(left)''' SSP1-2.6 and '''(right)''' SSP3-7.0. The number of models used is indicated in the top right of the maps. No overlay indicates regions where the change is robust and ''likely'' emerges from internal variability, that is, where at least 66% of the models show a change greater than the internal-variability threshold ( [[#4.2.6|Section 4.2.6]] ) and at least 80% of the models agree on the sign of change. Diagonal lines indicate regions with no change or no robust significant change, where fewer than 66% of the models show change greater than the internal-variability threshold. Crossed lines indicate areas of conflicting signals where at least 66% of the models show change greater than the internal-variability threshold but fewer than 80% of all models agree on the sign of change. Further details on data sources and processing are available in the chapter data table (Table 4.SM.1).

As was found in AR5, several regional sea level pressure features stand out from the zonal-mean change. Sea level pressure markedly decreases in north-eastern North America and north-eastern Asia in boreal winter. In boreal summer, sea level pressure robustly decreases in the Mediterranean and the Middle-East, a decrease that has been linked to a large-scale heat low forced by the amplified warming of the region ( [[#Haarsma--2009|Haarsma et al., 2009]] ). It is ''likely'' that sea level pressure will increase across the south-western North America and Central America in boreal summer under SSP3-7.0 due to an intensification of the eastern North Pacific subtropical summer high ( [[#Li--2012|Li et al., 2012]] ) and a weakening of the North American monsoon ( [[#4.5.1.5|Section 4.5.1.5]] ; [[#Pascale--2017|Pascale et al., 2017]] ; [[#Wang--2020|Wang et al., 2020]] ). These changes in circulation are connected to drying across the eastern subtropical Pacific and Central America regions (Figure 4.24).

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