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

==== 6.3.1.3 Waves and Extreme Sea Levels ====

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AR5 also concluded that there is ''medium confidence'' that mean significant wave height has increased in the North Atlantic north of 45°N based on ship observations and reanalysis-forced wave model hindcasts. ESL events have increased since 1970, mainly due to a rise in mean sea levels (MSLs) over this period (Rhein et al., 2013 <sup>[[#fn:r248|248]]</sup> ). There is ''medium confidence'' that mid-latitude jets will move 1–2 degrees further poleward by the end of the 21st century under RCP8.5 in both hemispheres with weaker shifts in the NH. In the SH during austral summer, the poleward movement of the mid-latitude westerlies under climate change is projected to be partially offset by stratospheric ozone recovery. There is ''low confidence'' in projections of NH storm tracks particularly in the North Atlantic. Tropical expansion is ''likely'' to continue causing wider tropical regions and poleward movement of the subtropical dry zones (Collins et al., 2013 <sup>[[#fn:r249|249]]</sup> ). In the SH, it is ''likely'' that enhanced wind speeds will cause an increase in annual mean significant wave heights. Wave swells generated in the Southern Ocean may also affect wave heights, periods and directions in adjacent ocean basins. The projected reduction in sea ice extent in the Arctic Ocean (Holland et al., 2006 <sup>[[#fn:r250|250]]</sup> ) will increase wave heights and wave season length (Church et al., 2013 <sup>[[#fn:r251|251]]</sup> ).

Since AR5, new studies have shown observed changes in wave climate. Satellite observations from 1985 – 2018, showed small increases in significant wave height (+0.3 cm/year) and larger increases in extreme wave heights (90th percentiles), especially in the Southern (+1 cm/year) and North Atlantic (+0.8 cm/year) Oceans (Young and Ribal, 2019 <sup>[[#fn:r252|252]]</sup> ) as well as positive trends in wave height in the Arctic over 1992–2014 due to sea ice loss (Stopa et al., 2016 <sup>[[#fn:r253|253]]</sup> ; Thomson et al., 2016 <sup>[[#fn:r254|254]]</sup> ). Based on a wave reanalysis and satellite observations, Reguero et al. (2019) <sup>[[#fn:r255|255]]</sup> found that the global wave power, which represents the transport of the energy transferred from the wind into the sea surface motion, therefore including wave height, period and direction, has increased globally at a rate of 0.41% yr -1 between 1948 and 2008, with large variations across oceans. Long-term correlations are found between the increase in wave power and SSTs, particularly between the tropical Atlantic temperatures and the wave power in high southern latitudes, the most energetic region globally.

The results of several new global wave climate projection studies are consistent with those presented in IPCC AR5. Mentaschi et al. (2017) find up to a 30% increase in 100-year return level wave energy flux (the rate of transfer of wave energy) for the majority of coastal areas in the southern temperate zone, and a projected decrease in wave energy flux for most NH coastal areas at the end of the century in wave model simulations forced by six CMIP5 RCP8.5 simulations. The most significant long-term trends in extreme wave energy flux are explained by their relationship to modelled climate indices (Arctic Oscillation, ENSO and NAO). Wang et al. (2014b) assessed the climate change signal and uncertainty in a 20-member ensemble of wave height simulations, and found model uncertainty (inter-model variability) is significant globally, being about 10 times as large as the variability between RCP4.5 and RCP8.5 scenarios. In a study focussing on the western north Pacific wave climate, Shimura et al. (2015) <sup>[[#fn:r256|256]]</sup> associate projected regions of future change in wave climate with spatial variation of SSTs in the tropical Pacific Ocean. A review of 91 published global and regional scale wind-wave climate projection studies found a consensus on a projected increase in significant wave height over the Southern Ocean, tropical eastern Pacific ( ''high confidence'' ) and Baltic Sea ( ''medium confidence'' ), and decrease over the North Atlantic and Mediterranean Sea. They found little agreement between studies of projected changes over the Atlantic Ocean, southern Indian and eastern North Pacific Ocean and no regional agreement of projected changes to extreme wave height. It was noted that few studies focussed on wave direction change, which is important for shoreline response (Morim et al., 2018 <sup>[[#fn:r257|257]]</sup> ).

Significant developments have taken place since the AR5 to model storm surges and tides at the global scale. An unstructured global hydrodynamic modelling system has been developed with maximum coastal resolution of 5 km (Verlaan et al., 2015 <sup>[[#fn:r258|258]]</sup> ) and used to develop a global climatology of ESLs due to the combination of storm surge and tide (Muis et al., 2016 <sup>[[#fn:r259|259]]</sup> ). A global modelling study finds that under SLR of 0.5–10 m, changes to astronomical tidal mean high water exceed the imposed SLR by 10% or more at around 10% of coastal cities when coastlines are held fixed. When coastal recession is permitted a reduction in tidal range occurs due to changes in the period of oscillation of the basin under the changed coastline morphology (Pickering et al., 2017 <sup>[[#fn:r260|260]]</sup> ). A recent study on global probabilistic projections of ESLs considering MSL, tides, wind-waves and storm surges shows that under RCP4.5 and RCP8.5, the global average 100-year ESL is ''very likely'' to increase by 34–76 cm and 58–172 cm, respectively between 2000 – 2100 (Vousdoukas et al., 2018 <sup>[[#fn:r261|261]]</sup> ). Despite the advancements in global tide and surge modelling, using CMIP GCM multi-model ensembles to examine the effects of future weather and circulation changes on storm surges in a globally consistent way is still a challenge because of the ''low confidence'' in GCMs being able to represent small scale weather systems such as TCs. To date only a small number of higher resolution GCMs are able to produce credible cyclone climatologies (e.g., Murakami et al., 2012) although this will probably improve with further GCM development and increases to GCM resolution (Walsh et al., 2016 <sup>[[#fn:r262|262]]</sup> ).

The role of austral winter swell waves on ESL have been investigated in the Gulf of Guinea (Melet et al., 2016 <sup>[[#fn:r263|263]]</sup> ) and the Maldives (Wadey et al., 2017 <sup>[[#fn:r264|264]]</sup> ). Multivariate statistical analysis and probabilistic modelling is used to show that flood risk in the northern Gulf of Mexico is higher than determined from short observational records (Wahl et al., 2016 <sup>[[#fn:r265|265]]</sup> ). In Australia, changes in ESLs were modelled using four CMIP5 RCP8.5 simulations (Colberg et al., 2019 <sup>[[#fn:r266|266]]</sup> ). On the southern mainland coast, the southward movement of the subtropical ridge in the climate models led to small reductions (up to 0.4 m) in the modelled 20-year (5% probability of occurring in a year) storm surge. Over the Gulf of Carpentaria in the north, changes were largest and positive during austral summer in two out of the four models in response to a possible eastward shift in the northwest monsoon. Synthetic cyclone modelling was used to evaluate probabilities, interannual variability and future changes of extreme water levels from tides and TC-induced storm surge (storm tide) along the coastlines of Fiji (McInnes et al., 2014) and Samoa (McInnes et al., 2016 <sup>[[#fn:r268|268]]</sup> ). Higher resolution modelling for Apia, Samoa incorporating waves highlights that although SLR reduces wave setup and wind setup by 10–20%, during storm surges it increases wave energy reaching the shore by up to 200% (Hoeke et al., 2015 <sup>[[#fn:r269|269]]</sup> ).

In the German Bight, Arns et al. (2015) show that under SLR, increases in extreme water levels occur due to a change in phase of tidal propagation; which more than compensates for a reduction in storm surge due to deeper coastal sea levels. Vousdoukas et al. (2017) <sup>[[#fn:r271|271]]</sup> develop ESL projections for Europe that account for changes in waves and storm surge. In 2100, increases of up to 0.35 m relative to the SLR projections occur towards the end of the century under RCP8.5 along the North Sea coasts of northern Germany and Denmark and the Baltic Sea coast, whereas little to negative change is found for the southern European coasts.

In the USA, Garner et al. (2017) combine downscaled TCs, storm surge models, and probabilistic SLR projections to assess flood hazard associated with changing storm characteristics and SLR in New York City from the pre-industrial era to 2300. Increased storm intensity was found to compensate for offshore shifts in storm tracks leading to minimal change in modelled storm surge heights through 2300. However, projected SLR leads to large increases in future overall flood heights associated with TCs in New York City. Consequently, flood height return periods that were ∼ 500y (0.2% probability of occurring in a given year) during the pre-industrial era have fallen to ∼ 25y (4% probability of occurring annually) at present and are projected to fall to ∼ 5y (20% probability of occurring annually) within the next three decades.

In summary, new studies on observed wave climate change from 1985–2018 showed small increases in significant wave height of +0.3 cm/year and larger increases in 90th percentile wave heights of +1 cm/year in the Southern Ocean and +0.8 cm/year in the North Atlantic ocean ( ''medium confidence'' ). Sea ice loss in the Arctic has also increased wave heights over the period 1992–2014 ( ''medium confidence'' ). Global wave power has increased over the last six decades with differences across oceans related to long-term correlations with SST ( ''low confidence'' ). Future projections indicate an increase of the mean significant wave height across the Southern Ocean and tropical eastern Pacific ( ''high confidence'' ) and Baltic Sea ( ''medium confidence'' ) and decrease over the North Atlantic and Mediterranean Sea under RCP8.5 ( ''high confidence'' ). Extreme waves are projected to increase in the Southern Ocean and decrease in the North Atlantic and Mediterranean Sea under RCP4.5 and RCP8.5 ( ''high confidence'' ). There is still limited knowledge on projected wave period and direction. For coastal ESLs, new studies at the regional to global scale have generally had a greater focus on multiple contributing factors such as waves, tides, storm surges and SLR. At the global scale, probabilistic projections of extreme sea levels considering these factors projects the global average 100- year ESL is very likely to increase by 34–76 cm and 58–172 cm, under RCP4.5 and RCP8.5, respectively between 2000–2100.

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