Adjoint sensitivity of sea-level and inter-basin transports to surface forcing and circulation anomalies in present and future climates

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Sea level rise is one of the few undisputed elements of future climate change to which we are already committed regardless of future emissions scenario. Projections for regional sea level change over the next century are similar in magnitude to those for global sea level rise. Yet there are wide discrepancies between model projections of its pattern, amplitude and the underlying causes. The broad aim of this project is to determine the sensitivities of regional sea level changes, and associated changes in ocean circulation, to surface fluxes of heat, freshwater and momentum, to remote circulation anomalies, and to uncertain model parameters. These sensitivities will be determined for both the present climate and various future climate scenarios. We will use a state of the art ocean model and its so-called 'adjoint' to efficiently determine these sensitivities. In contrast to traditional modelling studies, in which a large number of experiments must be performed to determine the sensitivity of a model output to model inputs, the adjoint model provides each of these sensitivities from just a single model run. The project is timely in that it addresses key issues identified in the Intergovernmental Panel for Climate Change report, released last month, concerning uncertainties in projections of sea level change and circulation. It also draws on the expertise and success that the team has built up in using the ocean model and its adjoint through a current NERC-funded project to attribute changes observed in the overturning circulation in the Atlantic to local and remote surface forcing.