IBI-CCS: a regional high-resolution model to simulate sea level in western Europe

Abstract

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 (tides, atmospheric surface pressure forcing, waves). 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 (Iberian–Biscay–Ireland Climate Change Scenarios) regional ocean model based on a 1/12∘ northeastern Atlantic Nucleus for European Modelling of the Ocean (NEMO) model configuration to dynamically downscale CNRM-CM6-1-HR, a GCM with a 1/4∘ resolution ocean model component participating in the sixth phase of the Coupled Model Intercomparison Project (CMIP6) by the Centre National de Recherches Météorologiques (CNRM). For a more complete representation of the 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 performance of IBI-CCS is better than that 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 shows 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, except in the Mediterranean Sea, where GCM biases were substantial.

Impact of wave-water level non-linear interactions for the projections of mean and extreme wave conditions along the coasts of western Europe

“…2.3), the decrease in the wave setup in the deep ocean can reach between -2.8 cm and -4.9 cm. These changes are thus rather small (but not negligible) compared to the projected sea level rise of about +80 cm over the northeastern Atlantic domain (Chaigneau et al, 2022) but are of opposite sign. These changes in wave setup are mainly related to moderate changes in the peak period which is squared in formulation (1): 63 % of the wave setup changes are due to those in peak period and 37 % are due to changes in significant wave height.…”

“…12 at the end of the century (2081-2100) under the SSP5-8.5 scenario. Sea level projected changes in the IBI-CCS ocean model during the 21 st century are mainly dominated by the mean sea level rise, with rather small changes in tides and storm surges (Chaigneau et al 2022). Therefore, as IBI-CCS-WAV and IBI-CCS-WAV_ssh are forced by the same winds, Figure 12 mostly shows the impact of mean sea level rise on waves, even if not completely linear.…”

“…Regional wave projections are also forced by IBI-CCS, a regional ocean model at a 1/12 ° horizontal resolution. IBI-CCS was implemented in Chaigneau et al, 2022 to refine sea level projections of CNRM-CM6-1-HR over the northeastern Atlantic region using dynamical downscaling. For a more complete representation of processes driving coastal water level changes, tides and atmospheric surface pressure forcing are explicitly resolved in IBI-CCS in addition to the ocean general circulation (dynamic sea level).…”

“…A minimum time-mean water depth of 6 m was chosen to be consistent with that applied in the ocean simulation from IBI-CCS (Chaigneau et al, 2022). This value avoids the occurrence of uncovered banks in macro-tidal areas, especially around Mont Saint Michel in France.…”

“…The simulations are produced over the northeastern Atlantic, called the IBI domain (Iberian-Biscay-Ireland). To assess the sensitivity of wave projections to the wave-water level non-linear interactions, the regional wave model is adapted to consider hourly variations of water level from a regional ocean model described in Chaigneau et al, 2022 for the same IBI domain.…”