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

==== 9.6.1.3 Regional Sea Level Change in the Satellite Era ====

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Regional sea level changes are resolved by both tide gauge and satellite altimetry observations ( [[#Hamlington--2020a|Hamlington et al., 2020a]] ). Altimeters have the advantage of quasi-global coverage but are limited to a period (1993–present) in which the forced trend response is just emerging on regional scales ( [[#9.6.1.4|Section 9.6.1.4]] ). An analysis of the local altimetry error budget to estimate 90% confidence intervals on regional sea level trends and accelerations reports that 98% of the ocean surface has experienced significant sea level rise over the satellite era ( [[#Prandi--2021|Prandi et al., 2021]] ). The same study finds that sea level accelerations display a less uniform pattern, with an east–west dipole in the Pacific, a north–south dipole in the Southern Ocean and in the North Atlantic, and 85% of the ocean surface experiencing significant sea level acceleration or deceleration, above instrumental and post-processing noise. Longer records are available from tide gauges, albeit with variable coverage by basin. Regional departures from GMSL rise are primarily driven by ocean transport divergences that result from wind stress anomalies and spatial variability in atmospheric heat and freshwater fluxes ( [[#9.2.4|Section 9.2.4]] ).

The SROCC ( [[#Oppenheimer--2019|Oppenheimer et al., 2019]] ) noted the occurrence of large multiannual sea level variations in the Pacific, associated with the Pacific Decadal Oscillation (PDO) in particular, and involving the El Niño Southern Oscillation (ENSO), North Pacific Gyre Oscillation (NPGO) and Indian Ocean Dipole (IOD; Annex IV; [[#Royston--2018|Royston et al., 2018]] ; [[#Hamlington--2020b|Hamlington et al., 2020b]] ). There was intensified sea level rise during the 1990s and 2000s, with 10-year trends exceeding 20 mm yr <sup>–1</sup> in the western tropical Pacific Ocean, while sea level trends were negative on the North American west coast. During the 2010s, the situation reversed, with western Pacific sea level falling at more than 10 mm yr <sup>–1</sup> ( [[#Hamlington--2020b|Hamlington et al., 2020b]] ). For the Atlantic Ocean, SROCC described regional sea level variability as being driven primarily by wind and heat flux variations associated with the North Atlantic Oscillation (NAO) and heat transport changes associated with Atlantic Meridional Overturning Circulation (AMOC) variability ''.'' During periods of subpolar North Atlantic warming, winds along the European coast are predominantly from the south and may communicate steric anomalies onto the continental shelf, driving regional sea level rise, with the reverse during periods of cooling ( [[#Chafik--2019|Chafik et al., 2019]] ). High rates of sea level rise in the North Indian Ocean are accompanied by a weakening summer South Asian monsoon circulation ( [[#Swapna--2017|Swapna et al., 2017]] ).

The Arctic ocean is typically excluded from global sea level studies, owing to the uncertainties associated with resolving sea level in ice-covered regions, strong variations in gravitational, rotational, and deformational (GRD) effects, and uncertain glacial isostatic adjustment (GIA) estimates (Box 9.1). Spanning 1991–2018, a ''very likely'' sea level rise of 1.16–1.81 mm yr <sup>–1</sup> is observed ( [[#Rose--2019|Rose et al., 2019]] ). Since SROCC, the forced response in regional sea level varies in time with the relative influence of different forcing agents ( [[#Fasullo--2020|Fasullo et al., 2020]] ).

The SROCC estimated regional sea level changes from combinations of the various contributions to sea level change from CMIP5 climate model outputs, allowing comparison with satellite altimeter and tide gauge observations. Closure of the regional sea level budget is complicated by the fact that regional sea level variability is larger than GMSL variability. Also, there are more processes that need to be considered, such as vertical land movement and ocean dynamical changes (Box 9.1). A number of observation-based studies have focused on specific areas, such as the Mediterranean ( [[#García--2006|García et al., 2006]] ), the South China Sea ( [[#Feng--2012|Feng et al., 2012]] ), the east coast of the USA ( [[#Frederikse--2017|Frederikse et al., 2017]] ; [[#Piecuch--2018|Piecuch et al., 2018]] ), the North Atlantic basin ( [[#Kleinherenbrink--2016|Kleinherenbrink et al., 2016]] ) and the north-western European continental shelf seas ( [[#Frederikse--2016|Frederikse et al., 2016]] ). Studies using tide gauge data and observation-based estimates of the contributions find that, while local agreement is not yet possible, the observational sea level budget can be closed on a basin scale ( [[#Slangen--2014b|Slangen et al., 2014b]] ; [[#Frederikse--2016|Frederikse et al., 2016]] , 2018, 2020b). A budget analysis for the GRACE era found that the budget closes in some, but not all, coastal regions: substantial parts of the sea level change signal in the North Atlantic could not be explained by steric or barystatic changes ( [[#Rietbroek--2016|Rietbroek et al., 2016]] ). This is in agreement with other work comparing climate model estimates to 20th-century tide gauge observations ( [[#Meyssignac--2017|Meyssignac et al., 2017]] ), where the majority of local spatial variability is determined by the ocean dynamic component. Vertical land movement is another major cause of local spatial variability in sea level change and, for instance, relevant for oceanic islands ( [[#Forbes--2013|Forbes et al., 2013]] ; [[#Martínez-Asensio--2019|Martínez-Asensio et al., 2019]] ). In summary, the regional sea level budget, using either observations or models, can currently only be closed on basin scales ( ''medium confidence'' ), with large uncertainties remaining on smaller scales ''.''

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