Modeling of upwelling/relaxation events with the Navy Coastal Ocean Model

Abstract

The Navy Coastal Ocean Model (NCOM) is a free‐surface, primitive‐equation model that is under development at the Naval Research Laboratory (NRL). The NCOM‐based model of the Monterey Bay area is evaluated during a series of upwelling and relaxation wind events in August–September of 2000. The model receives open boundary conditions from a regional NCOM implementation of the California Current System and surface fluxes from the Navy Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPSTM)(COAMPS is a registered trademark of the Naval Research Laboratory). Issues investigated in this study are: NCOM‐based model simulations of upwelling and relaxation events, coupling to COAMPS, use of sigma versus hybrid (sigma‐z) vertical grids, and coupling with a larger‐scale model on the open boundaries. The NCOM simulations were able to reproduce the observed sequence of the upwelling and relaxation events, which can be attributed, in part, to the good agreement between the observed and COAMPS winds. Comparisons with the mooring observations show that COAMPS overestimates shortwave radiation values, which makes the NCOM modeled SSTs too warm in comparison with observations. The NCOM runs forced with different resolution atmospheric forcing (3 versus 9 km) do not show significant differences in the predicted SSTs and mixed‐layer depths at the mooring locations. At the same time, during the extended upwelling event, the model runs forced with 3 and 9 km resolution COAMPS fields show differences in the surface circulation patterns, which are the most distinct in the southern portion of the model domain. The model run with 9‐km forcing develops a northward flow along the coast, which is not present in the run with 3‐km forcing and in observations (for example, HF radar‐derived radials). Comparison of the wind patterns of the 3‐ and 9‐km products shows a weakening of the 9‐km wind stress along the southern coast of the NCOM model domain, which is responsible for the development of the artificial northward flow in the NCOM run with 9‐km forcing.