On open boundary conditions for a limited-area coastal model off Oregon. Part 1: Response to idealized wind forcing

Abstract

A two-part study focusing on the development and application of open boundary conditions (OBCs) for a limited-area high-resolution coastal ocean model off Oregon is pursued. In Part 1, the OBCs are formulated and evaluated in numerical experiments utilizing idealized wind forcing. In Part 2, the OBCs are applied in an experiment with forcing by space and time variable winds obtained from a regional mesoscale atmospheric model. The initial experiments in Part 1 involve forcing by winds that are constant in time and are spatially uniform. The response to both upwelling favorable (southward) and downwelling favorable (northward) winds are examined and compared to results obtained with cyclic alongshore boundary conditions (CBCs). Additional experiments are conducted with idealized spatial variability in the wind forcing to include assessment of the OBC performance in situations when coastal trapped wave (CTW) dynamics play an important role in the shelf flow response. Some aspects of OBC performance with time-dependent wind forcing are investigated in these latter experiments by examining the response after wind forcing is turned off. The OBCs separate the total baroclinic solution at the open boundary into local and global components. In these applications, the local solution is obtained from an across-shore, depth and time-dependent two-dimensional submodel. The existence of inflow or outflow is determined from the global solutions using radiation conditions. During inflow, the local solution is smoothly imposed. During outflow, the global part of the solution is radiated out of the domain. Favorable, dynamically consistent performance of these OBCs is found in all experiments.