Editing IPCC:AR6/SROCC/Chapter-6 (section)

==== 6.3.1.2 Extratropical Cyclones and Blocking ====

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ETCs form in the mid-latitudes of the North Atlantic, North Pacific and Southern Oceans, and the Mediterranean Sea. The storm track regions are characterised by large surface equator-to-pole temperature gradients and baroclinic instability, and jet streams influence the direction and speed of movement of ETCs in this region. The thermodynamic response of the atmosphere to CO 2 tends to have opposing influences on storm tracks; surface shortwave cloud radiative changes increase the equator-to-pole temperature gradient whereas longwave cloud radiative changes reduce it (Shaw et al., 2016 <sup>[[#fn:r221|221]]</sup> ). AR5 concluded that the global number of ETCs is not expected to decrease by more than a few percent due to anthropogenic change. The Southern Hemisphere (SH) storm track is projected to have a small poleward shift, but the magnitude is model dependent (Christensen et al., 2013). AR5 also found a ''low confidence'' in the magnitude of regional storm track changes and the impact of such changes on regional surface climate (Christensen et al., 2013 <sup>[[#fn:r223|223]]</sup> ).

A ‘blocking’ event is an extratropical weather system in which the anticyclone (region of high pressure) becomes quasi-stationary and interrupts the usual westerly flow and/or storm tracks for up to a week or more (Woollings et al., 2018 <sup>[[#fn:r224|224]]</sup> ). Recent attention has focused on whether Arctic warming is linked to increased blocking and mid-latitude weather extremes (Barnes and Screen, 2015 <sup>[[#fn:r225|225]]</sup> ; Francis and Skific, 2015 <sup>[[#fn:r226|226]]</sup> ; Francis and Vavrus, 2015 <sup>[[#fn:r227|227]]</sup> ; Kretschmer et al., 2016 <sup>[[#fn:r228|228]]</sup> ), such as drought in California due to sea ice changes that cause a reorganisation of tropical convection (Cvijanovic et al., 2017 <sup>[[#fn:r229|229]]</sup> ), cold and snowy winters over Europe and North America (Liu et al., 2012 <sup>[[#fn:r230|230]]</sup> ; Cohen et al., 2018 <sup>[[#fn:r231|231]]</sup> ), extreme summer weather (Tang et al., 2013 <sup>[[#fn:r232|232]]</sup> ; Coumou et al., 2014 <sup>[[#fn:r233|233]]</sup> ) and Balkan flooding (Stadtherr et al., 2016 <sup>[[#fn:r234|234]]</sup> ). Studies suggest how blocking may influence arctic sea ice extent (Gong and Luo, 2017 <sup>[[#fn:r235|235]]</sup> ) and various pathways whereby Arctic warming could influence extreme weather (Barnes and Screen, 2015 <sup>[[#fn:r236|236]]</sup> ) such as reducing the equator to pole temperature gradient, slowing the jet stream thereby increasing its meandering behaviour (Röthlisberger et al., 2016 <sup>[[#fn:r237|237]]</sup> ; Mann et al., 2017 <sup>[[#fn:r238|238]]</sup> ) or causing it to split (Coumou et al., 2014 <sup>[[#fn:r239|239]]</sup> ), changing local dynamics in the vicinity of the sea ice edge (Screen and Simmonds, 2013 <sup>[[#fn:r240|240]]</sup> ) or weakening the stratospheric polar vortex (Cohen et al., 2014 <sup>[[#fn:r241|241]]</sup> ). However, sensitivity to choice of methodology (Screen and Simmonds, 2013 <sup>[[#fn:r242|242]]</sup> ) and large internal atmospheric variability masks the detection of such links in past records, and climate change can lead to opposing effects on the mid-latitude jet stream response leading to large uncertainty in future changes (Barnes and Polvani, 2015 <sup>[[#fn:r243|243]]</sup> ; Barnes and Screen, 2015 <sup>[[#fn:r244|244]]</sup> ).

New studies of future storm track behaviour in the NH, include Harvey et al. (2014) <sup>[[#fn:r245|245]]</sup> who find that the future changes to upper and lower tropospheric equator-to-pole temperature differences by the end of the century in a CMIP5 multi-model RCP8.5 ensemble are not well correlated and the lower temperature gradient dominates the summer storm track response whereas both upper and lower temperature gradients play a role in winter. In the northern North Atlantic storm track region, projected changes are found to be more strongly associated with changes in the lower rather than upper tropospheric equator-to-pole temperature difference (Harvey et al., 2015). In the SH, Harvey et al. (2014) find equator-to-pole temperature differences in the upper and lower troposphere in the future climate across a multi-model ensemble are well correlated with a general strengthening of the storm track. The total number of ETCs in a CMIP5 GCM multi-model ensemble decreased in the future climate, whereas the number of strong ETCs increased in most models and in the ensemble mean (Grieger et al., 2014 <sup>[[#fn:r246|246]]</sup> ). This was associated with a general poleward shift related to both tropical upper tropospheric warming and shifting meridional SST gradients in the Southern Ocean. The poleward movement of baroclinic instability and associated storm formation over the observational period due to external radiative forcing, is projected to continue, with associated declining rainfall trends in the mid-latitudes and positive trends further polewards (Frederiksen et al., 2017 <sup>[[#fn:r247|247]]</sup> ).

A number of new studies have found links between Arctic amplification, blocking events and various types of weather extremes in NH mid-latitudes in recent decades. However, the sensitivity of results to analysis technique and the generally short record with respect to internal variability means that at this stage there is ''low confidence'' in these connections. Consistent with the AR5, projected changes to NH storm tracks exhibit large differences between responses, causal mechanisms and ocean basins and so there remains ''low confidence'' in future changes in blocking and storm tracks in the NH. The storm track projections for the SH remain consistent with previous studies in indicating an observed poleward contraction and a continued strengthening and southward contraction of storm tracks in the future ( ''medium confidence'' ).

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