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

Andreas Lang,Uwe Mikolajewicz

doi:10.1007/s00382-020-05357-5

Abstract

We quantify the change in extreme high sea level (ESL) statistics in the German Bight under rising {{mathrm {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 {{mathrm {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.

Highlights

Extreme high sea levels caused by storm floods constitute a major geophysical hazard for low-lying coastal regions such as the southeastern North Sea, known as the German Bight
In this study we quantified the changes in North Sea extreme sea level statistics using a large ensemble of transient climate change simulations, downscaled with a coupled climate system model focusing on the southern North Sea
The coupled climate model approach with a global ocean allows the free propagation of signals from the North Atlantic to the North Sea as well as a consistent analysis of related drivers in the climate system

Summary

Introduction

Extreme high sea levels caused by storm floods constitute a major geophysical hazard for low-lying coastal regions such as the southeastern North Sea, known as the German Bight. Strong tidal oscillations due to the shallow shelf sea and the basin’s geometry (Fig. 1), combined with its situation along the major northern hemispheric storm-track path can lead to high storm floods. There is only little confidence on the change of upper-end extreme values (Weisse et al 2012; Wahl et al 2017), which by definition occur only rarely It is those ’high-impact-lowprobability’ extremes that are typically required for flood defense standards. While such height changes are often handled by scaling the sea level distribution with the regional or even global mean sea level rise (Buchanan et al 2017; IPCC 2019), relative changes in the upper tail of

Methods

Results

Discussion

Conclusion