AbstractIn global ocean models, the representation of small scale, high frequency processes considerably influences the large scale oceanic circulation and its low frequency variability. In this study we investigate the impact of stochastic perturbation schemes based on three different sub-grid scale parametrizations in multidecadal ocean-only simulations with the NEMO ocean model at 1° resolution. The three parametrizations are an enhanced vertical diffusion scheme for unstable stratification, the Gent-McWilliams (GM) scheme and a turbulent kinetic energy mixing scheme, all commonly used in state-of-the-art ocean models. We focus on changes in interannual variability caused by the comparatively high frequency stochastic perturbations with sub-seasonal decorrelation timescales. These perturbations lead to significant improvements in the representation of low frequency variability in the ocean, with the stochastic GM scheme showing the strongest impact. Interannual variability of the Southern Ocean eddy and Eulerian streamfunctions are increased by an order of magnitude and 20%, respectively. Interannual sea surface height variability is increased by about 20-25% as well, especially in the Southern Ocean and in the Kuroshio region, consistent with a strong underestimation of interannual variability in the model when compared to reanalysis and altimetry observations. These results suggest that enhancing sub-grid scale variability in ocean models can improve model variability and potentially its response to forcing on much longer timescales, while also providing an estimate of model uncertainty.
Journal of Climate – American Meteorological Society
Published: Mar 20, 2017
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