A nested grid model of the Oregon Coastal Transition Zone: Simulations and comparisons with observations during the 2001 upwelling season

Abstract

[1] The Oregon Coastal Transition Zone (OCTZ) extends several hundred kilometers offshore where shelf flows interact with the northern California Current. A primitive-equation numerical ocean model is used to study the upwelling circulation in this region from 1 May to 1 November 2001. This OCTZ model obtains initial and boundary conditions from a larger-scale model of the California Current System and forcing from a regional atmospheric model product. The model results are compared with extensive in situ and remotely sensed data, and the model is found to provide a realistic representation of flows both over the shelf and in the broader OCTZ. Simulation of coastal sea level and shelf currents over the complex topography of the central Oregon coast is improved quantitatively relative to previous regional models. A particularly significant qualitative improvement is realistic representation of coastal jet separation and eddy formation offshore of Cape Blanco. Three-dimensional Lagrangian analysis of water parcel displacement shows that the surface waters inshore of the separated jet are upwelled from near the bottom along the shelf as far north as 45.5°N. A large eddy, which incorporates some of this upwelled water and carries it farther westward, forms offshore in the late summer. Ensemble simulations show a distinction between the strongly deterministic response to wind forcing over the shelf and the more unstable, less predictable jet separation and offshore eddy formation processes in the region near Cape Blanco.

Toward an Uncertainty Budget for a Coastal Ocean Model

“…For these simulations, we adopt the nested grid model configuration for the Oregon Coastal Transition Zone (OCTZ) implemented by Springer et al (2009), who studied the upwelling circulation in the OCTZ region during summer 2001, and showed that this model reproduces relatively realistic OCTZ circulation features including shelf flow over the Heceta Bank, coastal jet separation and eddy formation offshore of Cape Blanco, and jet and eddy evolution in the offshore region. Simulations with this model configuration represent the OCTZ more faithfully than the periodic-channel domain used in related studies by Oke et al (2002a,b,c), Kurapov et al (2005a,b), and Kim et al (2009).…”

“…For the basic-case simulation, the model configuration and surface forcing is the same as that used by Springer et al (2009). The ocean initial and boundary conditions were obtained from spatial interpolation of the NCOM-CCS output, with the initial conditions taken from day 120 and the boundary information linearly interpolated in time between twice-daily NCOM-CCS output files.…”

“…The openboundary data are provided every 12 h by the model output of 2001 in the Navy Coastal Ocean Model (NCOM) and the California Current System (CCS; Shulman et al 2004). Following the best nesting strategy found by Springer et al (2009), radiation and nudging conditions are applied at all open boundaries for both the three-dimensional velocities and tracers of temperature and salinity, and sponge layers are adopted at the western and southern boundaries, using diffusivity and viscosity equal to 100 m 2 s 21 and tapered over 150 km to their interior values. The model includes neither freshwater nor salinity flux through the ocean's surface and no tidal forcing, and the heat fluxes are calculated from the bulk flux formulation of Fairall et al (1996).…”

“…The Springer et al (2009) OCTZ implementation uses a three-dimensional free-surface and hydrostatic primitive equation model discretized over abruptly varying topography with terrain-following s coordinates. The numerical code used is based on the Regional Ocean Modeling System (ROMS) described by Shchepetkin and McWilliams (2005).…”

Variational assimilation of satellite observations in a coastal ocean model off Oregon

“…Subgrid turbulence is parameterized using the level 2.5 Mellor-Yamada scheme [Mellor and Yamada, 1982;Wijesekera et al, 2003]. Process-oriented studies and extensive model-data comparisons of summer circulation have been performed in a similar domain using ROMS at the 3 km horizontal resolution [Springer et al, 2009;Kim et al, 2009;Koch et al, 2010]. Since variational DA requires repeated runs of the TL and ADJ codes, its computational cost can be on the order of 10-100 times as large as that of the forward nonlinear model run.…”