Editing IPCC:AR6/WGI/Chapter-9 (section)

==== 9.2.3.6 Coastal Systems and Marginal Seas ====

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Beyond the world’s coastlines lie the shoreline, shallow estuaries, continental shelves, and deeper fjords and slopes, where depths increase rapidly from the shelves to the deep-ocean floor. It is more difficult to transport fluid across (rather than along) the shelf-break or slope ( [[#Brink--2016|Brink, 2016]] ), and estuaries and shelves have complex circulations and mixing, leading to indirect connections between the inner shelves and coastlines and offshore conditions. Coastal processes link to large-scale metrics of climate and regional effects, from changing rivers and estuaries, melt and runoff to deep water, to how changes offshore affect regional and coastal conditions.

Shelf-deep ocean exchanges involve eddying, tidal, or turbulent motions and small-scale topography such as submarine canyons; high-resolution observations and models are needed to capture these effects ( [[#Greenberg--2007|Greenberg et al., 2007]] ; [[#Capet--2008|Capet et al., 2008]] ; [[#Allen--2009|Allen and Durrieu de Madron, 2009]] ; [[#Colas--2012|Colas et al., 2012]] ; [[#Trotta--2017|Trotta et al., 2017]] ). Example coastal processes that introduce uncertainty into large-scale projections are exchange of CDW across the Antarctic shelf-break, which affects AABW formation and Antarctic ice-shelf–ocean interaction (Sections 9.2.2.3 and 9.2.3.2; [[#Stewart--2013|Stewart and Thompson, 2013]] , 2015), river and estuarine plumes and their responses to water level and hydrology change ( [[#Banas--2009|Banas et al., 2009]] ; [[#Sun--2017|Sun et al., 2017]] ), fjord dynamics linked to glacial outflows ( [[#Straneo--2015|Straneo and Cenedese, 2015]] ; [[#Torsvik--2019|Torsvik et al., 2019]] ), and changing formation of water masses in marginal seas ( [[#Kim--2001|Kim et al., 2001]] ; [[#Greene--2007|Greene and Pershing, 2007]] ; [[#Giorgi--2008|Giorgi and Lionello, 2008]] ; [[#Renner--2009|Renner et al., 2009]] ). Downscaling projections to the local level allows process detail ( [[#Foreman--2014|Foreman et al., 2014]] ; [[#Mathis--2014|Mathis and Pohlmann, 2014]] ; [[#Meier--2015|Meier, 2015]] ; [[#Tinker--2016|Tinker et al., 2016]] ). Some processes can only be simulated when coastal models are forced by larger-scale models of the atmosphere, cryosphere, or hydrosphere ( [[#Seo--2007|Seo et al., 2007]] , 2008; [[#Somot--2008|Somot et al., 2008]] ; [[#Oerder--2015|Oerder et al., 2015]] ; [[#Renault--2016|Renault et al., 2016]] ; Y. [[#Zhang--2016|]] [[#Zhang--2016|]] [[#Zhang--2016|Zhang et al., 2016]] ; [[#Wåhlin--2020|Wåhlin et al., 2020]] ), including the addition of tides ( [[#Janeković--2012|Janeković and Powell, 2012]] ; [[#Timko--2013|Timko et al., 2013]] ; [[#Tinker--2015|Tinker et al., 2015]] ; [[#Pickering--2017|Pickering et al., 2017]] ; [[#Hausmann--2020|Hausmann et al., 2020]] ). Due to coastal process complexity and small scale, linking the effects of coastal ocean changes to global ocean changes requires high-resolution modelling ( [[#Holt--2017|Holt et al., 2017]] , 2018), two-way nesting, or local mesh refinement ( [[#Fringer--2006|Fringer et al., 2006]] ; [[#Zhang--2008|Zhang and Baptista, 2008]] ; [[#Mason--2010|Mason et al., 2010]] ; [[#Dietrich--2012|Dietrich et al., 2012]] ; [[#Hellmer--2012|Hellmer et al., 2012]] ; [[#Ringler--2013|Ringler et al., 2013]] ; Q. [[#Wang--2014|]] [[#Wang--2014|Wang et al., 2014]] ; [[#Zängl--2015|Zängl et al., 2015]] ; Y.J. [[#Zhang--2016|]] [[#Zhang--2016|]] [[#Zhang--2016|Zhang et al., 2016]] ; [[#Soto-Navarro--2020|Soto-Navarro et al., 2020]] ). Coarse climate models and HighResMIP models do not represent some coastal phenomena such as cross-shelf exchanges and sub-mesoscale eddies, which require 1 km or finer resolution. Thus, there is ''low confidence'' in projecting centennial scale coastal climate change where regional downscaling or refinement is lacking. There is ''high confidence'' in the ability of regional coupled models to improve coastal climate change process understanding and provide regional information ( [[IPCC:Wg1:Chapter:Chapter-12#12.4|Section 12.4]] ), but many sites globally await such projections.

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