Rising extreme sea levels in the German Bight under enhanced $${{\mathrm {CO}}_2}$$ levels: a regionalized large ensemble approach for the North Sea

Abstract

We quantify the change in extreme high sea level (ESL) statistics in the German Bight under rising CO 2 concentrations by downscaling a large ensemble of global climate model simulations using the regionally coupled climate system model REMO-MPIOM. While the model setup combines a regionally high resolution with the benefits of a global ocean model, the large ensemble size of 32 members allows the estimation of high return levels with much lower uncertainty. We find that ESLs increase with atmospheric CO 2 levels, even without considering a rise in the background sea level (BSL). Local increases of up to 0.5 m are found along the western shorelines of Germany and Denmark for ESLs of 20-50 years return periods, while higher return levels remain subject to sampling uncertainty. This ESL response is related to a cascade of an enhanced large-scale activity along the North Atlantic storm belt to a subsequent local increase in predominantly westerly wind speed extremes, while storms of the major West-Northwest track type gain importance. The response is seasonally opposite: summer ESLs and the strength of its drivers decrease in magnitude, contrasting the response of the higher winter ESLs, which governs the annual response. These results have important implications for coastal protection. ESLs do not only scale with the expected BSL rise, but become even more frequent, as preindustrial 50-year return levels could be expected to occur almost every year by the end of the century. The magnitude of the relative change in ESL statistics is hereby up to half of the expected rise in BSL, depending on the location. Changes in the highest extremes are subject to large multidecadal variations and remain uncertain, thus potentially demanding even further safety measures.

RCP8.5-projected changes in German Bight storm surge characteristics from regionalized ensemble simulations for the end of the twenty-first century

“…Gaslikova et al (2013) compared their milder A1B scenario period 2071-2100 with the control period 1961-1990 and found the only statistically significant increase of storm surge heights (up to 13 cm in 110 years, 10% of control results) along the North Frisian and Danish coasts but insignificant and less increase (<8% of control results) along the East Frisian coast. Lang and Mikolajewicz (2020) found a similar distribution of significant 20-and 50-year ESL return level increases in their simulations. Estimated magnitudes of increases are also here not comparable, because they simulated another scenario (yearly 1% increase of atmospheric pCO2 from pre-industrial stage) and compare results being more than 100 years apart.…”

“…Climate-related investigations for the North Sea, in particular with focus on the German Bight, have been concentrating on ESL heights and return periods in recent decades, e.g., from observations (e.g., Müller-Navarra and Giese, 1999;Gönnert, 2003;Ganske et al, 2018), or their climate-induced changes from model simulations. For example, Lang and Mikolajewicz (2020) estimated from climate model projections (1% scenario) for the German Bight that potential ESL future changes would be around 15-30 cm for 1-to 30-year return periods. Their more general conclusion is an ESL increase with climate change of 50% of the expected mean sea level rise (SLR).…”

“…All these studies look at ESL in terms of absolute sea surface height (SSH) when investigating storm surges and their intensity. However, extreme sea levels are expected to change with their underlying mean (=background) sea level (Marcos and Woodworth, 2018;Lang and Mikolajewicz, 2020;Fox-Kemper et al, 2021) and consequently also with the mean tidal high water level (MHW). In the German Bight, where the wind is the most dominant factor for MSL variability (Dangendorf et al, 2013), the wind-induced component corresponds essentially to the partition of SSH above MHW (plus a threshold accounting for tidal high water variability).…”

“…Our comprehensive approach with respect to physical process representation and statistical validity thus reduces uncertainties in the characterization and quantification of potential extreme sea level events during the twenty-first century. In comparison to Lang and Mikolajewicz (2020), our study was performed with a horizontal resolution twice as high for the North Sea region, which allows in particular for a better representation of local topographic features and the orography of the coastline influencing the propagation and dissipation of tidal energy.…”

Large ensemble climate model simulations: introduction, overview, and future prospects for utilising multiple types of large ensemble

“…Kirchmeier-Young et al, 2017), and RCM SMILEs, which are increasingly being used for impact studies (e.g. Leduc et al, 2016).…”