Quasigeostrophic energetics of open ocean regions

Abstract

We present a method for local energy and vorticity analysis (EVA) of open regions of oceanic flow governed by quasigeostrophic dynamics. The purpose is to infer from real and simulated data sets the physics of synoptic/mesoscale processes, and to identify general signatures of such processes. We first derive, via a Rossby number expansion, the form of the local conservation law for quasigeostrophic energy density in terms of the geostrophic pressure field. We relate the quasigeostrophic terms to their more general form and also identify the different local ageostrophic contributions to the pressure work flux divergences. Analysis methods include time series of maps of terms, space-time integral time series, and schematic open region diagrams. Rossby wave and normal mode barotropic and baroclinic instability processes are studied in open regions, and local conversion/transport properties are defined. It is found that the instability process is indicated by both Reynolds-stress-like terms ( ΔF ϰ , ΔF A ) and ageostrophic pressure work divergence ( ΔF π a, δƒ π a ). The process of local growth of energy is indicated by the local growth of asymmetries in the divergence terms. The application of EVA to real data situations which are made self-consistent by quasigeostrophic filtering is introduced. Real data initialization of a quasigeostrophic dynamical model provides the required dynamical interpolation procedure. Finally an eddy merger event captured during a successful dynamical forecast in the California Current region (Robinson et al.) is described and interpreted via EVA.