Editing IPCC:AR6/WGI/Chapter-10 (section)

==== 10.1.3.2 Internal Drivers of Regional Climate Variability ====

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Internal climate variability on seasonal to multi-decadal temporal scales is substantial at regional scales. This variability arises from internal modes of atmospheric and oceanic variability, intrinsically coupled climate modes, and may additionally be driven by processes other than those originating the modes. It also interacts with the response of the climate system to external forcing. The teleconnections associated with the modes are useful to understand the relationship between large and regional scales (Annex IV: Modes of Variability). A description of various large-scale modes of variability can be found in Chapters 2, 3 and 8, and in Annex IV, while their future projections are assessed in Chapter 4. The specificities of their regional influence are briefly discussed here. More details of their typical temporal scales and regional influences can be found in Annex IV.

Atmospheric modes of variability may have seasonally-dependent regional effects like the North Atlantic Oscillation (NAO) in European winter ( [[#Tsanis--2019|Tsanis and Tapoglou, 2019]] ) and summer ( [[#Bladé--2012|Bladé et al., 2012]] ; [[#Dong--2013|Dong et al., 2013]] ). Even though these modes are internal to the climate system, their variability can be affected by anthropogenic forcings. For instance, the SAM ( [[#Hendon--2014|Hendon et al., 2014]] ) is both internally driven ( [[#Smith--2017|Smith and Polvani, 2017]] ), but also affected by recent stratospheric ozone changes ( [[#Bandoro--2014|Bandoro et al., 2014]] ). The teleconnections between these modes of variability and surface weather often exhibit considerable non-stationarity ( [[#Hertig--2015|Hertig et al., 2015]] ).

Due to the large ocean heat capacity and their long temporal scales, multi-annual to multi-decadal modes of ocean variability such as the Pacific Decadal Variability (PDV; [[#Newman--2016|Newman et al., 2016]] ) and the Atlantic Multi-decadal Variability (AMV; [[#Buckley--2016|Buckley and Marshall, 2016]] ) are key drivers of regional climate change. In the case of the AMV both natural (volcanic) and anthropogenic (aerosol) external forcings are thought to be involved in its timing and intensity ( [[IPCC:Wg1:Chapter:Chapter-3#3.7.7|Section 3.7.7]] ). These modes not only affect nearby regions but also remote parts of the globe through atmospheric teleconnections ( [[#Meehl--2013|Meehl et al., 2013]] ; [[#Dong--2015|Dong and Dai, 2015]] ) and can act to modulate the influence of natural and anthropogenic forcings ( [[#Davini--2015|Davini et al., 2015]] ; [[#Ghosh--2017|Ghosh et al., 2017]] ; [[#Ménégoz--2018b|Ménégoz et al., 2018b]] ). The dynamics of the ocean modes is simultaneously affected by other modes of variability spanning the full range of spatial and temporal scales due to non-linear interactions (Figure 10.3; [[#Kucharski--2010|Kucharski et al., 2010]] ; [[#Dong--2018|Dong et al., 2018]] ). This mutual interdependence can result in changing characteristics of the connection over time ( [[#Gallant--2013|Gallant et al., 2013]] ; [[#Brands--2017|Brands, 2017]] ; [[#Dong--2017|Dong and McPhaden, 2017]] ), and of their regional climate impact ( [[#Martín-Gómez--2016|Martín-Gómez and Barreiro, 2016]] , 2017). As with atmospheric modes of variability, the regional influence of ocean modes of variability on regional climates can be seasonally dependent ( [[#Haarsma--2015|Haarsma et al., 2015]] ).

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