Mechanical Energy Budgets for Regional Numerical Simulations

Kinetic Energy and Available Potential Energy (KE and APE) budgets are potentially a useful way to describe the function of fluid systems. However a difficulty of using energy is that it can be hard to form closed budgets. To address this, a method is presented for calculating nearly closed energy budgets using output from ROMS. We focus on a realistic simulation of a regional coastal-estuarine domain in the NE Pacific. Another difficulty for energy budgets in regional domains such as this is the treatment of fluxes through the open boundary. For the our domain we demonstrate that these fluxes can be handled in a reasonable way by using the definition of "local" APE from Holliday and McIntyre (1981). We are able to form meaningful volume-integrated budgets over specific sub-regions, such as the continental shelf and the Salish Sea estuary. The APE may be partitioned into that due to parcels which are displaced up or down from the flattened rest state. It is found that wind-driven upwelling has a clear seasonal cycle of up-APE. In contrast, the down-APE which dominates the estuarine system has little seasonal cycle. Dividing the size of the estuarine APE reservoir by the rate at which APE is lost to advection gives a timescale of 11 months, consistent with the small seasonal variation of APE.