Editing IPCC:AR6/SROCC/Chapter-4 (section)

==== 4.2.2.3 Regional Sea Level Changes During the Instrumental Period ====

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Sea level does not rise uniformly. Observations from tide gauges and satellite altimetry (Figure 4.6) indicate that sea level shows substantial regional variability at decadal to multi-decadal time scales (e.g., Carson et al., 2017; Hamlington et al., 2018 <sup>[[#fn:r271|271]]</sup> ). These regional changes are essentially due to changing winds, air-sea heat and freshwater fluxes, atmospheric pressure loading and the addition of melting ice into the ocean, which alters the ocean circulation (Stammer et al., 2013 <sup>[[#fn:r272|272]]</sup> ; Forget and Ponte, 2015 <sup>[[#fn:r273|273]]</sup> ; Meyssignac et al., 2017b <sup>[[#fn:r274|274]]</sup> ). The addition of water into the ocean also change the geoid, alter the rotation of the Earth and deform the ocean floor which in turn change sea level (e.g., Tamisiea, 2011; Stammer et al., 2013 <sup>[[#fn:r275|275]]</sup> ).

Sea level is rising in all ocean basins ( ''virtually certain'' ; Legeais et al. 2018 <sup>[[#fn:r276|276]]</sup> ). Part of this regional sea level rise is due to global sea level rise of which a majority is attributable to anthropogenic greenhouse gas emissions ( ''high confidence'' ; Slangen et al. 2016 <sup>[[#fn:r277|277]]</sup> ). The remaining part of the regional sea-level rise in ocean basins is a combination of the response to anthropogenic GHG emissions and internal variability (e.g., Stammer et al. 2013; ''medium confidence'' ).

In the open ocean, the spatial variability and trends in sea level observed during the recent altimetry era or reconstructed over the previous decades are dominated by the thermal expansion of the ocean. In shallow shelf seas and at high latitudes (&gt;60°N and &lt;55°S), the effect of dynamic mass redistribution becomes important. At local scale, salinity changes can also generate sizeable changes in the ocean density similar to thermal expansion and lead to significant variability in sea level (Forget and Ponte, 2015 <sup>[[#fn:r278|278]]</sup> ; Meyssignac et al., 2017b <sup>[[#fn:r279|279]]</sup> ). On global average, the heat and freshwater fluxes from the atmosphere into the ocean are responsible for the total heat that enters the ocean and for the associated GMSL rise. At regional scale and local scale, both the ocean transport divergences caused by wind stress anomalies and the spatial variability in atmospheric heat fluxes are responsible for the spatial variability in thermal expansion and thus for most of the regional sea level departures around the GMSL rise (e.g., Stammer et al., 2013; Forget and Ponte, 2015 <sup>[[#fn:r280|280]]</sup> ).

Over the Pacific, the surface wind anomalies responsible for the sea level spatio-temporal variability are associated with the ENSO, Pacific Decadal Oscillation (PDO) and North Pacific Gyre Oscillation modes (Hamlington et al., 2013 <sup>[[#fn:r281|281]]</sup> ; Moon et al., 2013 <sup>[[#fn:r282|282]]</sup> ; Palanisamy et al., 2015 <sup>[[#fn:r283|283]]</sup> ; Han et al., 2017 <sup>[[#fn:r284|284]]</sup> ). In the Indian Ocean they are associated with the ENSO and Indian Ocean Dipole (IOD) modes (Nidheesh et al., 2013 <sup>[[#fn:r285|285]]</sup> ; Han et al., 2014 <sup>[[#fn:r286|286]]</sup> ; Thompson et al., 2016 <sup>[[#fn:r287|287]]</sup> ; Han et al., 2017 <sup>[[#fn:r288|288]]</sup> ). In particular, the PDO is responsible for most of the intensified SLR that has been observed in the western tropical Pacific Ocean since the 1990s (Moon et al., 2013 <sup>[[#fn:r289|289]]</sup> ; Han et al., 2014 <sup>[[#fn:r290|290]]</sup> ; Thompson and Mitchum, 2014 <sup>[[#fn:r291|291]]</sup> ). Several studies suggested that in addition to the PDO signal, warming of the tropical Indian and Atlantic Oceans enhanced surface easterly trade winds and thus also contributes to the intensified SLR in the western tropical Pacific (England et al., 2014 <sup>[[#fn:r292|292]]</sup> ; Hamlington et al., 2014 <sup>[[#fn:r293|293]]</sup> ; McGregor et al., 2014 <sup>[[#fn:r294|294]]</sup> ).

Over the Atlantic, the regional sea level variability at interannual to multi-decadal time scales, is generated by surface wind anomalies and heat fluxes associated with the North Atlantic Oscillation (NAO; Han et al., 2017 <sup>[[#fn:r295|295]]</sup> ) and also by ocean heat transport due to changes in the Atlantic Meridional Overturning Circulation (AMOC; McCarthy et al., 2015 <sup>[[#fn:r296|296]]</sup> ). Both mechanisms are not independent as heat fluxes and wind stress anomalies associated with NAO can induce changes in the AMOC (Schloesser et al., 2014 <sup>[[#fn:r297|297]]</sup> ; Yeager and Danabasoglu, 2014 <sup>[[#fn:r298|298]]</sup> ). In the Southern Ocean, the sea level variability is dominated by the SAM influence in particular in the Indian and Pacific sectors. The Southern Annular Mode (SAM) influence becomes weaker equator-wards in these sectors while the influence of PDO, ENSO and IOD increases (Frankcombe et al., 2015 <sup>[[#fn:r299|299]]</sup> ). In the southern ocean, the zonal asymmetry in westerly winds associated to the SAM, generates convergent and divergent transport in the Antarctic Circumpolar Current which may have contributed to the regional asymmetry of decadal sea level variations during most of the twentieth century (Thompson and Mitchum, 2014 <sup>[[#fn:r300|300]]</sup> ).

As for GMSL, net regional sea level changes can be estimated from a combination of the various contributions to sea level change. The contributions from dynamic sea level, atmospheric loading, glacier mass changes and ice sheet SMB can be derived from CMIP5 climate model outputs either directly or through downscaling techniques (Perrette et al., 2013 <sup>[[#fn:r301|301]]</sup> ; Kopp et al., 2014 <sup>[[#fn:r302|302]]</sup> ; Slangen et al., 2014a <sup>[[#fn:r303|303]]</sup> ; Bilbao et al., 2015 <sup>[[#fn:r304|304]]</sup> ; Carson et al., 2016 <sup>[[#fn:r305|305]]</sup> ; Meyssignac et al., 2017a <sup>[[#fn:r306|306]]</sup> ). The contributions from groundwater depletion, reservoir storage and dynamic ice sheet mass changes are not simulated by coupled climate models over the 20th century and have to be estimated from observations. The sum of all contributions, including the GIA contribution, provides a modelled estimate of the 20th century net regional sea level changes that can be compared with observations from satellite altimetry and tide-gauge records (see Figure 4.6).

In terms of interannual to multi-decadal variability, there is a general agreement between the simulated regional sea level and tide gauge records, over the period 1900–2015 (see inset figures in Figure 4.6). The relatively large, short-term oscillations in observed sea level (black lines in insets in Figure 4.6), which are due to the natural internal climate variability, are included in general within the modelled internal variability of the climate system represented by the blue shaded area (5–95% uncertainty). But, as for GMSL, climate models tend to systematically underestimate the observed sea level trends from tide gauge records, particularly in the first half of the 20th century. This underestimation is explained by a bias identified in modelled Greenland SMB, and glacier ice loss around Greenland in the early 20th century (see Section 4.2.2.2.6; Slangen et al., 2017b <sup>[[#fn:r307|307]]</sup> ). The correction of this bias improves the agreement between the spatial variability in sea level trends from observations and from climate models (see Figure 4.6). Climate models indicate that the spatial variability in sea level trends observed by tide-gauge records over the 20th century is dominated by the GIA contribution and the thermal expansion contribution over 1900–2015. Locally all contributions to sea level changes are important as any contribution can cause significant local deviations. Around India for example, groundwater depletion is responsible for the low 20th century SLR (because the removal of groundwater mass generated a local decrease in geoid that made local SLR slower; Meyssignac et al., 2017c <sup>[[#fn:r308|308]]</sup> )

These results show the ability of models to reproduce the major 20th century regional sea level changes due to GIA, thermal expansion, glacier mass loss and ice sheet SMB. This is tangible progress since AR5. But some doubts remain regarding the ability of climate models to reproduce local variations such as the glaciers and the Greenland SMB contributions to sea level in the region around the southern tip of Greenland (Slangen et al., 2017b <sup>[[#fn:r309|309]]</sup> ) or such as the thermal expansion in some eddy active regions (Sérazin et al., 2016 <sup>[[#fn:r310|310]]</sup> ). Because of these doubts there is still ''medium confidence'' in climate models to project future regional sea level changes associated with thermal expansion, glacier mass loss and ice sheet SMB. Coupled climate models have not simulated the other contributions to 20th century sea level, including the growing ice sheet dynamical contribution and land water storage changes.

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'''Figure 4.6'''

<span id="figure-4.6-20th-century-simulated-regional-sea-level-changes-by-coupled-climate-models-and-comparison-with-a-selection-of-local-tide-gauge-time-series.-in-the-upper-left-corner-map-of-changes-in-simulated-relative-sea-level-rsl-for-the-period-19011920-to-19962015-estimated-from-climate-model-outputs.-insets-observed-rsl-changes-black"></span>
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'''Figure 4.6 | 20th century simulated regional sea level changes by coupled climate models and comparison with a selection of local tide gauge time series. In the upper left corner: map of changes in simulated relative sea level (RSL) for the period 1901–1920 to 1996–2015 estimated from climate model outputs. Insets: Observed RSL changes (black […]'''
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[[File:ce92a975877f501626ca4d1003740770 IPCC-SROCC-CH_4_6-3000x1993.jpg]]

Figure 4.6 | 20th century simulated regional sea level changes by coupled climate models and comparison with a selection of local tide gauge time series. In the upper left corner: map of changes in simulated relative sea level (RSL) for the period 1901–1920 to 1996–2015 estimated from climate model outputs. Insets: Observed RSL changes (black lines) from selected tide gauge stations for the period 1900–2015. For comparison, the estimate of the simulated RSL change at the tide gauge station is also shown (blue plain line for the model estimates and blue dashed line for the model estimates corrected for the bias in glaciers mass loss and Greenland surface mass balance (SMB) over 1900–1940, see Section 4.2.2.2.6). The relatively large, short-term oscillations in observed local sea level (black lines) are due to the natural internal climate variability. For Mediterranean tide gauges, that is, Venice and Alexandria, the local simulated sea level has been computed with the simulated sea level in the Atlantic ocean at the entrance of the strait of Gibraltar following (Adloff et al., 2018). Tide gauge records have been corrected for vertical land motion (VLM) not associated with GIA where available, that is, for New York, Balboa and Lusi. Updated from Meyssignac et al. (2017b) to mimic RSL as good as possible.

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