NSFGEO-NERC Stirring at the Walls - A dynamical boundary model for the ocean

点击翻译按钮获取中文摘要

Climate change, because of its enormous social, economic and political consequences, reigns as the leading scientific problem of our times. The primary scientific tool in the study of future climate is the coupled numerical model, in which the various components of the climate system interact, producing an estimate of a future climate state. The resulting projections receive global exposure and impact global policy. Of primary importance in the prediction of climate on interannual to centennial time scales is the ocean. Dynamically sound ocean models are integral to reliable climate forecasting, yet due to gaps in our scientific understanding of ocean function, they suffer from a fundamental weakness. Most of the kinetic energy in the ocean resides in the so-called `mesoscale', a term referring to ocean phenomenon with time scales of days to months and length scales of 10's to 100's of km. The mesoscale through its large scale feedbacks has been shown to be a major factor determining intrinsic ocean variability on interannual to decadal timescales, which covers a significant fraction of the temporal spectrum over which the ocean contributes importantly to climate. It has been estimated that up to 80% of ocean variability is due to such intrinsic processes.This presents climate forecasting with a practical problem: the mesoscale consists of features that are small (50 km)compared to the basin scale (6000 km) and their direct numerical resolution over the entire globe for times required in climate simulations is far beyond current computer resources. Present computational resources for climate projection allow for modest but incomplete mesoscale resolution (25 km), necessitating the parametrisation of the remaining sub-grid scale dynamics. Reliable ocean models will employ parametrisations based on dynamics. This is not current modelling practice. Current practice models sub-grid scale dynamics using viscous and mixing representations and tunes the related parameters to match output to present observations, justifying this modelling by arguing that large scale low frequency winds drive the basin scale circulation that subsequently develops the mesoscale by instabilities. The amplitude of the large scale circulation is then set by balancing the energy flow into the large scale with the energy flow out of the large scale into the mesoscale. The mechanisms by which the mesoscale loses energy are not addressed directly and are not as well understood. Models are tuned to representative mesoscale energy levels so they exhibit reasonable decadal scale variability and to reproduce essential elements of the ocean circulation, such as accurate separation of the Gulf Stream from the east coast of the US. These parametrisations have, however, no basis in the dynamics of the flow. The nonlinearity of the climate system means that there is no assurance that a parametrisation tuned to present conditions will perform well when modelling a changing climate. The same difficulty arises in the modelling of palaeoclimates where the underlying flow structure is far from present day observations. A dynamically based parametrisation, especially as it addresses mesoscale dissipation, is needed to address this issue. We argue that there is a gap between the very high spatial and temporal resolution required in global ocean models to accurately resolve the flow near ocean boundaries and the lower resolution required to resolve the motion of the ocean interior. A dynamical boundary model of the form proposed here can exploit this gap allowing more accurate simulations at lower computational cost while simultaneously increasing our knowledge of boundary mixing processes. This addresses directly the NERC priority of ``studies of water circulation in seas and oceans on a variety of temporal and spatial scales based on modelling''. This project will test a key hypothesis that, if true, will change the modelling of ocean circulation.