WHOI PO

The mechanical energy budget can be a compact, integrative way to understand the dynamics of fluid systems. This is particularly true when looking at system adjustment times and the exchange of work and energy among different reservoirs and forcing terms. Here we use the "local" Available Potential Energy (APE) formulation of Holliday and McIntyre (1981, JFM) in which APE is defined relative to a state in which all isopycnals in a domain are allowed to flatten adiabatically, as per Lorenz (1955) and Winters et al. (1995). We develop a numerical method (MacCready and Giddings 2016, JPO) to calculate complete, closed energy budgets for ROMS model simulations, and apply it to a realistic simulation of the NE Pacific and Salish Sea, and to an idealized estuary simulation. One interesting feature of the local APE is that it may be decomposed into portions for water parcels that are displaced up- or down- relative to the flattened state, clearly distinguishing types of eddies, wind-driven upwelling, and the stored APE of the estuarine salt intrusion. There is a large annual cycle of up-APE on the shelf, driven by wind and dissipated by vertical mixing. In contrast, in estuarine waters there is a large reservoir of down-APE modulated by tidal mixing and conversion to KE by the exchange flow. The ratio of stored APE to the major fluxes that influence it gives an estimate of the system adjustment time.