MESoscale Ocean eddies and Climate Predictions (MESO-CLIP)

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Mesoscale ocean eddies (MOEs) are swirls of water (typically a few hundred km in diameter) that are ubiquitous in the World Ocean. MOEs are the oceanic equivalent of weather systems in the atmosphere. In analogy to weather systems MOEs cannot be predicted a long time in advance. In computer models of the ocean MOEs can only develop if the spatial scale that the model can resolve is small enough. Typically a model needs to be able to resolve scales of about 30km (at mid-latitudes) to start generating MOEs. An ocean model is then said to be eddy-permitting. For a good representation of MOEs the resolved spatial scales need to be at least 10 km. Ocean models with that resolution are often referred to as eddy-resolving. Until recently, the grid resolution in climate models used for climate prediction was too coarse (100 km and more) for MOEs to be simulated. This is now changing and the latest generation of climate models under development use ocean components that are eddy-permitting (and soon eddy-resolving). When and where MOEs occur in high resolution models depends on initial conditions (the temperature, salinity and velocities at the beginning of the model simulation). Even small changes in initial conditions will eventually lead to different MOE fields. This is analogous to weather patterns typically adopting different patterns in a matter of days when the initial conditions are perturbed at the beginning of a forecast. How MOEs feed back on climate variability and predictability is still largely unknown. However, some recent studies suggest that MOEs could affect ocean and atmosphere variability on interannual to decadal timescales. Cutting edge climate models currently under development use eddy-permitting (e.g. HadGEM3-H in the UK) and eddy-resolving (e.g. CM2.6 in the US) oceans and therefore there is a need to get a better understanding of how MOEs affect forecasts based on such models. The main goal of MESO-CLIP will be to determine how initial conditions (temperatures, salinities, velocities) have to be perturbed in eddy-permitting/resolving ocean models to assess the uncertainty in forecasts. We will use a hierarchy of numerical models: (i) an uncoupled global ocean model run at horizontal grid resolutions of 1/4degree (25km at Equator) and 1/12degree (9 km at Equator), (ii) the latest coupled ocean-atmosphere model currently under development at the UK MetOffice (HadGEM3-H) which uses a 1/4degree ocean component, and (iii) an eddy-resolving (1/20degree) resolution idealised coupled ocean-atmosphere model. With this set of models we will be able to address how the presence of MOEs in the ocean affect the predictability and variability of ocean and atmosphere and how important coupled processes (interactions between the ocean and the atmosphere) are likely to be. MESO-CLIP will therefore provide valuable knowledge about forecast uncertainties in present and future high resolution coupled models that will be used for climate predictions.