Climate Downscaling: Local Mean Sea Level, Surge and Wave Modelling

Abstract

Coastal areas are at increased risk of flooding and erosion in the future, due to a rise in mean sea level and potential changes in tides, surges and waves. The investigation of future climate impacts at the coast thus requires sufficiently detailed projections for the offshore waves and sea levels in both the present day and under future climate scenarios, to provide boundary conditions for detailed coastal impact models. The process of model downscaling, in a climate modelling sense, is generally taken to refer to the generation of locally relevant data from the output of global circulation models of the atmosphere and ocean. The aim is to connect global-scale projections and regional dynamics to generate regionally specific projections relevant to coastal management decision-making. We discuss research conducted as part of Phase 2 of the Tyndall Coastal Simulator research project on the future changes in surge and wave climate forced by winds and sea-level atmospheric pressures from a version of the Met Office Hadley Centre Climate Model (HadCM3), assuming various greenhouse gas emission scenarios and for various climate model parameter choices. The local spatial variation in mean sea level is also taken into account, incorporating deviations from global mean sea-level caused by regional variations in ocean density and circulation. The UK is still subject to glacial isostatic adjustment after the last ice age, counteracting recent sea-level rise. Dynamical downscaling from a global coupled atmosphere–ocean model, using a regional climate model, has been used to provide more realistic and detailed simulations of wind and pressure over the NW European continental shelf. These were then used to drive storm surge and wave models. This study was used in the Tyndall Coastal Simulator to understand the effects of different uncertainties in the climate system, in terms of marine parameters for the north Norfolk coast. Hence, the dominant drivers could be identified for analysis in later chapters. The changes in wave and surge conditions were less than suggested by earlier analyses, and the wave and surge conditions were assumed to be unchanged compared to today (Appendix A). These results were also used directly in UKCP09 (Lowe et al, UK Climate Projections science report: marine and coastal projections. Met Office Hadley Centre, Exeter, 2009). In the future, larger ensembles of wave simulations would provide a better description of future conditions.