Editing IPCC:AR6/WGI/Chapter-2 (section)

===== 2.3.1.4.4 Surface wind and sea level pressure =====

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The AR5 concluded that surface winds over land had generally weakened. The ''confidence'' for both land and ocean surface wind trends was ''low'' owing to uncertainties in datasets and measures used. Sea level pressure (SLP) was assessed to have ''likely'' decreased from 1979–2012 over the tropical Atlantic and increased over large regions of the Pacific and South Atlantic, but trends were sensitive to the period analysed.

Terrestrial in situ wind datasets have been updated and the quality-control procedures have been improved, with particular attention to homogeneity and to better retaining true extreme values ( [[#Dunn--2012|Dunn et al., 2012]] , 2014, 2016). Global mean land wind speed (excluding Australia) from HadISD for 1979–2018 shows a reduction (stilling) of 0.063 m s <sup>–1</sup> per decade ( [[#Azorin-Molina--2019|Azorin-Molina et al., 2019]] ). Trends are broadly insensitive to the subsets of stations used. Although the meteorological stations are unevenly distributed worldwide and sparse in South America and Africa, the majority exhibit stilling particularly in the NH (Figure 2.19). Regionally, strong decreasing trends are reported in central Asia and North America (–0.106 and –0.084 m s <sup>–1</sup> per decade respectively) during 1979–2018 ( [[#McVicar--2012|McVicar et al., 2012]] ; [[#Vautard--2012|Vautard et al., 2012]] ; J. [[#Wu--2018|]] [[#Wu--2018|Wu et al., 2018]] ; [[#Azorin-Molina--2019|Azorin-Molina et al., 2019]] ). This stilling tendency has reversed after 2010 and the global mean surface winds have strengthened ( [[#Zeng--2019b|Zeng et al., 2019b]] ; [[#Azorin-Molina--2020|Azorin-Molina et al., 2020]] ), although the robustness of this reversal is unclear given the short period and interannual variability ( [[#Kousari--2013|Kousari et al., 2013]] ; [[#Kim--2015|Kim and Paik, 2015]] ; [[#Azorin-Molina--2019|Azorin-Molina et al., 2019]] ).

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[[File:26ea185ca5ced7098b8a78c982776822 IPCC_AR6_WGI_Figure_2_19.png]]

'''Figure 2.19''' '''|''' '''Trends in surface wind speed. (a)''' Station observed winds from the integrated surface database (HadISD v2.0.2.2017f); '''(b)''' Cross-Calibrated Multi-Platform wind product; '''(c)''' ERA5; and '''(d)''' wind speed from the Objectively Analyzed Air-Sea Heat Fluxes dataset, release 3 (OAFLUX, release 3). White areas indicate incomplete or missing data. Trends are calculated using OLS regression with significance assessed following AR(1) adjustment after [[#Santer--2008|Santer et al. (2008)]] ; ‘×’ marks denote non-significant trends. Further details on data sources and processing are available in the chapter data table (Table 2.SM.1).

Over the ocean, datasets demonstrate considerable disagreement in surface wind speed trends and spatial features ( [[#Kent--2013|Kent et al., 2013]] ). Global ocean surface winds from NOCv2.0 demonstrate upward trends of about 0.11 m s <sup>–1</sup> per decade (1979–2015) with somewhat smaller trends from WASwind for 1979–2011 ( [[#Azorin-Molina--2017|Azorin-Molina et al., 2017]] , 2019). The trends are consistent until 1998, but diverge thereafter. Both ERA5 and JRA-55 reanalyses show consistently increasing global marine wind speeds over 1979–2015, though flattening since 2000, whereas MERRA-2 agrees until 1998, but then exhibits increased variability and an overall decrease in the last two decades ( [[#Azorin-Molina--2019|Azorin-Molina et al., 2019]] ). This agrees with estimates by [[#Sharmar--2021|Sharmar et al. (2021)]] showing upward ocean wind trends from 1979 to 2000 which are consistent in ERA-Interim, ERA5 and MERRA-2, but disagree with CFSR trends for the same period. Over 2000–2019 all reanalyses show diverging tendencies. An updated multiplatform satellite database (comprising data from altimeters, radiometers, and scatterometers) from 1985–2018 shows small increases in mean wind speed over the global ocean, with the largest increase observed in the Southern Ocean ( [[#Young--2019|Young and Ribal, 2019]] ), consistent with signals in ERA-Interim, ERA5 and MERRA-2 ( [[#Sharmar--2021|Sharmar et al., 2021]] ). Overall, most products suggest positive trends over the Southern Ocean, western North Atlantic and the tropical eastern Pacific since the early 1980s.

The modern era reanalyses exhibit SLP increases over the SH subtropics with stronger increases in austral winter over 1979–2018. Over the NH, SLP increased over the mid-latitude Pacific in boreal winter and decreased over the eastern subtropical and mid-latitude North Atlantic in boreal summer. Discrepancies in the low-frequency variations during the first half of the 20th century exist in the centennial-scale reanalysis products ( [[#Befort--2016|Befort et al., 2016]] ). Overall, modern reanalysis datasets support the AR5 conclusion that there is no clear signal for trends in the strength and position of the permanent and quasi-permanent pressure centres of action since the 1950s. Instead, they highlight multi-decadal variations. Large-scale SLP is strongly associated with the changes in modes of variability ( [[#2.4|Section 2.4]] and Annex IV).

In summary, since the 1970s a worldwide weakening of surface wind has ''likely'' occurred over land, particularly marked in the NH, with ''low confidence'' in a recent partial recovery since around 2010. Differences between available wind speed estimates lead to ''low confidence'' in trends over the global ocean as a whole but with most estimates showing strengthening globally over 1980–2000 and over the last four decades in the Southern Ocean, western North Atlantic and the tropical eastern Pacific.

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