Abstract

Projections of coastal sea level (SL) changes are of great interest for coastal risk assessment and decision-making. SL projections are typically produced using global climate models (GCMs) which cannot fully resolve SL changes at the coast due to their coarse resolution and lack of representation of some relevant processes. To overcome these limitations and refine projections at regional scales, GCMs can be dynamically downscaled through the implementation of a high-resolution regional climate model (RCM). In this study, we developed the IBI-CCS regional ocean model based on a 1/12 ° north-eastern Atlantic NEMO ocean model configuration to dynamically downscale CNRM-CM6-1-HR, a GCM with a ¼ ° resolution ocean model component developed for the Coupled Model Intercomparison Project 6th Phase (CMIP6) by the Centre National de Recherches Météorologiques (CNRM). For a more complete representation of processes driving coastal SL changes, tides and atmospheric surface pressure forcing are explicitly resolved in IBI-CCS in addition to the ocean general circulation. To limit the propagation of climate drifts and biases from the GCM into the regional simulations, several corrections are applied to the GCM fields used to force the RCM. The regional simulations are performed over the 1950 to 2100 period for two climate change scenarios (SSP1-2.6 and SSP5-8.5). To validate the dynamical downscaling method, the RCM and GCM simulations are compared to reanalyses and observations over the 1993–2014 period for a selection of ocean variables including SL. Results indicate that large-scale performances of IBI-CCS are better than those of the GCM thanks to the corrections applied to the RCM. Extreme SLs are also satisfactorily represented in the IBI-CCS historical simulation. Comparison of the RCM and GCM 21st century projections show a limited impact of increased resolution (1/4° to 1/12°) on SL changes. Overall, bias corrections have a moderate impact on projected coastal SL changes projections, except in the Mediterranean Sea where GCM biases were substantial.

Highlights

  • Sea level (SL) changes are a major threat for coastal and low-lying regions

  • We developed the IBI-CCS regional ocean model based on a 1/12 ° northeastern Atlantic NEMO ocean model configuration to dynamically downscale CNRM-CM6-1-HR, a global climate models (GCMs) with a 1⁄4 ° 15 resolution ocean model component developed for the Coupled Model Intercomparison Project 6th Phase (CMIP6) by the Centre National de Recherches Météorologiques (CNRM)

  • Previous dynamical downscaling studies have provided regional projections of sea level (SL) based on CMIP5 GCMs (e.g. Hermans et al, 2020; Liu et al, 2016; Zhang et al, 2017; Gomis et al, 2016, Jin et al, 2021)

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Summary

Introduction

Sea level (SL) changes are a major threat for coastal and low-lying regions. Risks associated to sea level rise (SLR) are even more important because coastal regions are subject to an increasing anthropogenic pressure with 10 % of the world’s population living in low elevation coastal zones (McGranahan et al, 2007). In Europe, the coastal population represents 50 million people (Neumann et al, 2015). The annual number of European people exposed to coastal flooding could reach 1.5 to 3.6 million by the end of the century and the associated expected annual 35 damage could reach 90 to 960 billion euros (Vousdoukas et al, 2018a). Projections of coastal SL changes are of great interest for coastal risk assessment and decision-making processes

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