Editing IPCC:AR6/WGI/Chapter-7 (section)

====== Mid-latitude cloud amount feedback ======

Poleward shifts in the mid-latitude jets are evident since the 1980s ( [[IPCC:Wg1:Chapter:Chapter-2#2.3.1.4.3|Section 2.3.1.4.3]] ) and are a feature of the large-scale circulation change in future projections ( [[IPCC:Wg1:Chapter:Chapter-4#4.5.1.6|Section 4.5.1.6]] ). Because mid-latitude clouds over the North Pacific, North Atlantic and Southern Ocean are induced mainly by extratropical cyclones in the storm tracks along the jets, it has been suggested that the jet shifts should be accompanied by poleward shifts in the mid-latitude clouds, which would result in a positive feedback through the reduced reflection of insolation ( [[#Boucher--2013|Boucher et al., 2013]] ). However, studies since AR5 have revealed that this proposed mechanism does not apply in practice ( [[#Ceppi--2015|Ceppi and Hartmann, 2015]] ). While a poleward shift of mid-latitude cloud maxima in the free troposphere has been identified in satellite and ground-based observations ( [[#Bender--2012|Bender et al., 2012]] ; [[#Eastman--2013|Eastman and Warren, 2013]] ), associated changes in net CRE are small because the responses in high and low-clouds to the jet shift act to cancel each other ( [[#Grise--2016|Grise and Medeiros, 2016]] ; [[#Tselioudis--2016|Tselioudis et al., 2016]] ; [[#Zelinka--2018|Zelinka et al., 2018]] ). This cancellation is not well captured in ESMs ( [[#Lipat--2017|Lipat et al., 2017]] ), but the above findings show that the mid-latitude cloud feedback is not dynamically driven by the poleward jet shifts, which are rather suggested to occur partly in response to changes in high clouds (Y. [[#Li--2018|]] [[#Li--2018|Li et al., 2018]] ).

Thermodynamics play an important role in controlling extratropical cloud amount equatorward of about 50° latitude. Recent studies showed, using observed cloud controlling factors, that the mid-latitude low-cloud fractions decrease with rising SST, which also acts to weaken stability of the atmosphere unlike in the subtropics ( [[#McCoy--2017a|McCoy et al., 2017a]] ). ESMs consistently show a decrease of cloud amounts and a resultant positive SW feedback in the 30°–40° latitude bands, which can be constrained using observations of seasonal migration of cloud amount ( [[#Zhai--2015|Zhai et al., 2015]] ). Based on the qualitative agreement between observations and ESMs, the mid-latitude cloud amount feedback is assessed as positive with ''medium confidence.'' Following these emergent constraint studies using observations and CMIP5/6 models, the global contribution of net cloud amount feedback over 30°–60° ocean areas, covering 27% of the globe, is assessed at 0.09 ± 0.1 W m <sup>–2</sup> °C <sup>–1</sup> (one standard deviation), in which the uncertainty reflects potential errors in models’ low-cloud response to changes in thermodynamic conditions.

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