Editing IPCC:AR6/WGI/Chapter-8 (section)

==== 8.2.2.1 Thermodynamic Constraints on Atmospheric Moisture Fluxes ====

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A warming climate drives increases in atmospheric moisture and horizontal moisture transport from the divergent to the convergent portions of the atmospheric circulation (including storm systems, the tropical rain belt and monsoons) that on average amplifies existing precipitation minus evaporation (P–E) patterns ( [[#Held--2006|Held and Soden, 2006]] ). Increased latent heat transports in high latitudes also contribute to polar amplification of warming ( [[IPCC:Wg1:Chapter:Chapter-7#7.4.4.1|Section 7.4.4.1]] ). Although convergent parts of the atmospheric circulation are expected to become wetter (in terms of increasing P–E) and net evaporative regions drier (increasing E–P) these regions are not geographically and seasonally fixed and their location and timing are expected to alter ( [[#8.2.2.2|Section 8.2.2.2]] ). Atmospheric and ocean circulation changes overall decrease the amplification of P–E and salinity patterns. Paleoclimate evidence confirms that during the LGM zonal mean changes were roughly in agreement with thermodynamic expectations (G. [[#Li--2013|]] [[#Li--2013|Li et al., 2013]] ). However regional changes can be dominated by dynamics, including responses to the large Northern Hemisphere (NH) ice sheets (DiNezio and Tierney, 2013; T. [[#Bhattacharya--2017|]] [[#Bhattacharya--2017|Bhattacharya et al., 2017]] ; [[#Scheff--2017|Scheff et al., 2017]] ; [[#D’Agostino--2019|D’Agostino et al., 2019]] ; [[#Lowry--2019|Lowry and Morrill, 2019]] ) such that altered P–E patterns are not well described by thermodynamic drivers (Osteret al., 2015; [[#Lora--2018|Lora, 2018]] ; [[#Morrill--2018|Morrill et al., 2018]] ).

There is ''robust evidence'' and ''high agreement'' across thermodynamics, detailed modelling and observations that amplification of P–E patterns occurs over the oceans (Figure 8.5a) with an associated ‘fresh gets fresher, salty gets saltier’ signature in ocean salinity (Sections 2.3.3.2 and 3.5.2). This amplification is moderated by proportionally larger increases in subtropical ocean evaporation and weakening of the tropical circulation ( [[#8.2.2.2|Section 8.2.2.2]] ), an expectation supported by observations (Skliriset al., 2016) and process understanding (Yang andRoderick, 2019). Thermodynamics explain a smaller low latitude evaporation increase (1% °C <sup>–1</sup> ) than in high latitudes (5% °C <sup>–1</sup> ) with changes in surface radiation, boundary layer adjustments and ocean heat uptake playing a secondary role, based on idealized modelling ( [[#Siler--2019|Siler et al., 2019]] ). Increased evaporation from warmer oceans and lakes is exacerbated by the loss of surface ice in some regions (Bintanja and Selten, 2014; [[#Laîné--2014|Laîné et al., 2014]] ; W. [[#Wang--2018|]] [[#Wang--2018|]] [[#Wang--2018|]] [[#Wang--2018|]] [[#Wang--2018|]] [[#Wang--2018|Wang et al., 2018]] ; [[#Sharma--2019|Sharma et al., 2019]] ; [[#Woolway--2020|Woolway et al., 2020]] ). This can generate a more local moisture source for precipitation, for example in north-west Greenland during non-summer months since the 1980s ( [[#Nusbaumer--2019|Nusbaumer et al., 2019]] ), though moisture transport changes can counteract this effect ( [[#Nygård--2020|Nygård et al., 2020]] ). Ocean stratification due to heating of the upper layers through radiative forcing has been identified as a mechanism that further amplifies surface salinity patterns beyond the responses driven by water cycle changes alone (Zika et al., 2018) .

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'''Figure 8.5 |''' '''Zonally-averaged annual mean changes in precipitation minus evaporation (P–E) over (a) ocean and (b) land between the historical''' ( '''1995–2014''' ''') and''' '''SSP2-4.5''' ( '''2081–2100''' ''') CMIP6 simulations (blue lines, an average of the CanESM5 and MRI-ESM2-0 models).''' Dashed lines show estimated P–E changes using a simple thermodynamic scaling ( [[#Held--2006|Held and Soden, 2006]] ); dotted lines show estimates using an extended scaling ( [[#Byrne--2016|Byrne and O’Gorman, 2016]] ). All curves have been smoothed in latitude using a three grid-point moving-average filter. Further details on data sources and processing are available in the chapter data table (Table 8.SM.1).

Since AR5, numerous studies have confirmed that changes in P–E with warming over land cannot be interpreted simply as a ‘wet regions get wetter, dry regions get drier’ response ( [[#Chadwick--2013|Chadwick et al., 2013]] ; [[#Greve--2014|Greve et al., 2014]] ; [[#Roderick--2014|Roderick et al., 2014]] ; [[#Byrne--2015|Byrne and]] [[#O’Gorman--2015|O’Gorman, 2015]] ; [[#Scheff--2015|Scheff and Frierson, 2015]] ). Firstly, P–E is a simplistic diagnostic of the water cycle that inadequately describes ‘dryness’ or aridity ( [[#Fu--2014|Fu and Feng, 2014]] ; [[#Roderick--2014|Roderick et al., 2014]] ; [[#Greve--2015|Greve and Seneviratne, 2015]] ; [[#Scheff--2015|Scheff and Frierson, 2015]] ; [[#Greve--2019|Greve et al., 2019]] ; [[#Vicente-Serrano--2020|Vicente-Serrano et al., 2020]] ). Secondly, terrestrial P–E is generally positive and balanced by surface runoff and percolation into subsurface soils and aquifers (Figure 8.1). As a result, the simple thermodynamic scaling (Figure 8.5b) predicts that P–E over land will become more positive (wetter) with warming ( [[#Greve--2014|Greve et al., 2014]] ; [[#Roderick--2014|Roderick et al., 2014]] ; [[#Byrne--2015|Byrne and]] [[#O’Gorman--2015|O’Gorman, 2015]] ). This is not necessarily true, however, in the dry seasons and regions where terrestrial water is lost to the atmosphere and exported ( [[#Sheffield--2013|Sheffield et al., 2013]] ; [[#Kumar--2015|Kumar et al., 2015]] ; [[#Keune--2019|Keune and Miralles, 2019]] ). Thirdly, regional P–E patterns over land are affected by changes in atmospheric circulation, oceanic moisture supply and land surface feedbacks. As the land warms more than oceans, spatial gradients in temperature and relative humidity influence moisture supply and reduce P–E over some land regions, such as southern Chile and Argentina around 30°S – 50°S as captured by an extended thermodynamic scaling (Figure 8.5b). Drying of soils can be amplified by vegetation responses ( [[#Berg--2016|Berg et al., 2016]] ; [[#Byrne--2016|Byrne and O’Gorman, 2016]] ; [[#Lambert--2017|Lambert et al., 2017]] ) but limited by atmospheric circulation feedbacks ( [[#Zhou--2021|Zhou et al., 2021]] ). Changes in soil moisture and rainfall intensity (Sections 8.2.3.2 and 8.2.3.3) can alter the partitioning of precipitation between evaporation and runoff, further complicating terrestrial P–E responses ( [[#Short%20Gianotti--2020|Short Gianotti et al., 2020]] ).

The strong physical basis for regionally and seasonally dependent responses of P–E and the expectation for an increasing contrast between wet and dry seasons and weather regimes is supported by ''high agreement'' across multiple observational and CMIP5/CMIP6 modelling studies ( [[#Liu--2013|Liu and Allan, 2013]] ; [[#Kumar--2015|Kumar et al., 2015]] ; [[#Polson--2017|Polson and Hegerl, 2017]] ; [[#Ficklin--2019|Ficklin et al., 2019]] ; [[#Deng--2020|Deng et al., 2020]] ; [[#Schurer--2020|Schurer et al., 2020]] ). Increased moisture transports into storm systems, monsoons and high latitudes increase the intensity of wet events ( [[#8.2.3.2|Section 8.2.3.2]] ), while stronger atmospheric evaporative demand with warming ( [[#Scheff--2014|Scheff and Frierson, 2014]] ; [[#Vicente-Serrano--2018|Vicente-Serrano et al., 2018]] ; [[#Cook--2019|Cook et al., 2019]] ) is an important mechanism for intensifying dry events ( [[#8.2.3.3|Section 8.2.3.3]] ) and decreasing soil moisture over many subtropical land regions. However, aridification is modulated regionally by poleward migration of the subtropical dry zones and an increasing land – ocean temperature contrast that drives declining relative humidity ( [[#8.2.2.2|Section 8.2.2.2]] ).

To summarize, increased moisture transport from evaporative oceans to high precipitation regions of the atmospheric circulation will drive amplified P–E and salinity patterns over the ocean ( ''high confidence'' ) while more complex regional changes are expected over land. Greater warming over land than ocean alters atmospheric circulation patterns and on average reduces continental near-surface relative humidity which along with vegetation feedbacks can contribute to regional decreases in precipitation ( ''high confidence'' ). Based on an improved understanding of thermodynamic drivers since AR5 and multiple lines of evidence, there is ''high confidence'' that very wet or dry seasons and weather patterns will intensify in a warming climate such that wet spells become wetter and dry spells drier.

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