Effects of nonlinear physical processes on optimal error growth in predictability experiments of the Kuroshio Large Meander

Abstract

Within a 1.5-layer shallow-water model, the effects of nonlinear physical processes on the optimal error growth in the predictability experiments of the Kuroshio Large Meander (LM) are investigated. To clarify these effects, we use both the conditional nonlinear optimal perturbation (CNOP) and the first singular vector (FSV) methods. By examining the nonlinear evolution of the CNOPs and the FSVs, we find that the nonlinear physical processes play an important role in the error growth, in particular for the initial errors with large amplitudes. The specific roles of nonlinear processes in error growth are identified by determining the error development in modified nonlinear equations in which particular terms are neglected. The results demonstrate that advection of momentum perturbations, associated with the nonlinear development of barotropic instabilities, tend to enhance the evolution of the errors and cause the forecasted Kuroshio Large Meander to be strengthened. The nonlinear process related to the divergence of velocity perturbations caused by upper-layer thickness perturbations enhances the optimal error growth, whereas the advection of upper-layer thickness perturbations by velocity perturbations suppresses it. Key Points Nonlinearities have important effects on prediction of Kuroshio large meander Role of nonlinearity in prediction of Kuroshio large meander is revealed Nonlinear method is more suitable to study predictability of Kuroshio meander © 2013. American Geophysical Union. All Rights Reserved.