Kate Hedstrom

Dynamical simulations of filament formation and evolution in the Coastal Transition Zone

Using the semispectral primitive equation model of Haidvogel et al. (1991), the evolution of a forced, surface-intensified, eastern boundary current is studied in the presence of both finite-amplitude topography and irregular coastline geometry. The model domain is 1000 km in alongshore length, and extends on average 700 km in the cross-shelf direction. A representative cape, as well as smoothed continental shelf-slope topography, are included. The model is forced by inclusion of nudging terms in the equations of motion which relax the fluid system back to a prescribed reference state on a time scale of 45 days. The reference state chosen is a broad, geostrophically balanced, equatorward flow having a maximum current at the surface of 0.45 m s−1 and a transport of approximately 10 Sv. No explicit wind forcing is included. Initialized with the smooth surface current, the model quickly approaches a turbulent, time-dependent equilibrium featuring an intense, meandering alongshore jet with local velocities of 0.8–1.0 m s−1. A deep, poleward undercurrent also forms. Subsequent interaction with the protruding cape geometry causes an offshore deflection in the steepening frontal meanders, some of which produce elongated filaments which penetrate significant distances (400–500 km) offshore. The emerging filaments are characterized by a strong downwelling signal (maximum vertical velocities of 30–40 m d−1). The simulated filaments ultimately pinch off, typically within 40–50 days, to form a corotating pair of detached eddies. The existence of the cape geometry, as well as the southward surface flow, appears to be necessary in this model to produce filament generation; removal of the irregular coastal geometry or reversal of the sense of the surface circulation is shown to inhibit filament formation. Detailed analysis of both instantaneous and time-mean momentum balances show dynamical similarity to observations taken during the Coastal Transition Zone experiment and elucidate the eddy transport mechanisms responsible for the formation of the poleward undercurrent in these experiments. Offshore transport of heat by the filaments, found to be O(0.06) PW, is substantial.

Use of simulated drifter tracks to investigate general transport patterns and residence times in the Coastal Transition Zone

General transport patterns and residence times in the upper 100 m of the coastal transition zone (CTZ) were studied using simulated Lagrangian drifter experiments. The circulation fields used for the drifter experiments were obtained from a regional primitive equation model that incorporates coastal geometry and bottom topography that is representative of the CTZ region. The simulated circulation fields show velocity patterns that are consistent with those associated with the filaments and jets observed in the CTZ region. Drifters were released at 880 points in the model domain in a pattern that bracketed, both horizontally and vertically, the region in which offshore-flowing filaments were observed to form. At each location, drifters were released at depths of 30, 60, 90 m and tracked for 30 days. The depth of these drifters varied along their trajectories in response to the vertical velocities experienced by the drifters. An additional set of drifters was released at 30 m and was constrained to remain at this depth. The composite drifter trajectories show that transport patterns and residence times in the upper 100 m are determined by the proximity of the drifter release point to the offshore-flowing filament. Drifters released inshore of the filament are transported southward with the mean flow of the model California Current. Drifters released near the region in which the filament originates are transported rapidly offshore. The point at which this difference in drifter fate occurs is approximately 100 km offshore. The time required for a drifter to be transported from the nearshore region to the outer boundary of the area covered in the CTZ model domain is approximately 20 days. Comparison of the free-floating drifters with the 30-m fixed-depth drifters shows that the deeper drifters are displaced southward as they are transported offshore in the jet. The magnitude of the southward displacement for the 90-m drifters is about 15 km, implying substantial cross-jet exchange. Comparison of the simulated drifter trajectories with the trajectory followed by a drifter released during the 1988 CTZ field sampling program support the hypothesis that the deepening of the abundance maxima observed in the distributions of Dolioletta gegenbauri, juvenile forms of Euphausia pacifica and Eucalanus californicus resulted from these populations being downwelled as they were transported offshore by the flow associated with the filament observed in 1988.

FVCOM validation experiments : comparisons with ROMS for three idealized barotropic test problems

[1] The unstructured-grid Finite-Volume Coastal Ocean Model (FVCOM) is evaluated using three idealized benchmark test problems: the Rossby equatorial soliton, the hydraulic jump, and the three-dimensional barotropic wind-driven basin. These test cases examine the properties of numerical dispersion and damping, the performance of the nonlinear advection scheme for supercritical flow conditions, and the accuracy of the implicit vertical viscosity scheme in barotropic settings, respectively. It is demonstrated that FVCOM provides overall a second-order spatial accuracy for the vertically averaged equations (i.e., external mode), and with increasing grid resolution the model-computed solutions show a fast convergence toward the analytic solutions regardless of the particular triangulation method. Examples are provided to illustrate the ability of FVCOM to facilitate local grid refinement and speed up computation. Comparisons are also made between FVCOM and the structured-grid Regional Ocean Modeling System (ROMS) for these test cases. For the linear problem in a simple rectangular domain, i.e., the winddriven basin case, the performance of the two models is quite similar. For the nonlinear case, such as the Rossby equatorial soliton, the second-order advection scheme used in FVCOM is almost as accurate as the fourth-order advection scheme implemented in ROMS if the horizontal resolution is relatively high. FVCOM has taken advantage of the new development in computational fluid dynamics in resolving flow problems containing discontinuities. One salient feature illustrated by the three-dimensional barotropic winddriven basin case is that FVCOM and ROMS simulations show different responses to the refinement of grid size in the horizontal and in the vertical.

The Benguela Upwelling System: Quantifying the Sensitivity to Resolution and Coastal Wind Representation in a Global Climate Model*

AbstractOf all the major coastal upwelling systems in the world’s oceans, the Benguela, located off southwest Africa, is the one that climate models find hardest to simulate well. This paper investigates the sensitivity of upwelling processes, and of sea surface temperature (SST), in this region to resolution of the climate model and to the offshore wind structure. The Community Climate System Model (version 4) is used here, together with the Regional Ocean Modeling System. The main result is that a realistic wind stress curl at the eastern boundary, and a high-resolution ocean model, are required to well simulate the Benguela upwelling system. When the wind stress curl is too broad (as with a 1° atmosphere model or coarser), a Sverdrup balance prevails at the eastern boundary, implying southward ocean transport extending as far as 30°S and warm advection. Higher atmosphere resolution, up to 0.5°, does bring the atmospheric jet closer to the coast, but there can be too strong a wind stress curl. The most ...

Interdecadal changes in mesoscale eddy variance in the Gulf of Alaska circulation : Possible implications for the steller sea lion decline

Abstract A distinct change in the ocean circulation of the Gulf of Alaska after the 1976–77 climate shift is studied in an eddy‐permitting primitive equation model forced by observed wind stresses from 1951–99. When the Aleutian Low strengthens after 1976–77, strong changes occur in the mean velocity of the Alaskan Stream and in its associated mesoscale eddy field. In contrast, the Alaska Current and the eddy flows in the eastern Gulf remain relatively unchanged after the shift. Since mesoscale eddies provide a possible mechanism for transporting nutrient‐ rich open‐ocean waters to the productive shelf region, the flow of energy through the food web may have been altered by this physical oceanographic change. This climate‐driven mechanism, which has a characteristic eastwest spatial asymmetry, may potentially help to explain changes in forage fish quality in diet diversity of Steller sea lions whose populations have declined precipitously since the mid‐1970s in the western Gulf while remaining stable in the eastern Gulf.

Environmental conditions impacting juvenile Chinook salmon growth off central California: An ecosystem model analysis

A fully coupled ecosystem model is used to identify the effects of environmental conditions and upwelling variability on growth of juvenile Chinook salmon in central California coastal waters. The ecosystem model framework consists of an ocean circulation submodel, a biogeochemical submodel, and an individual-based submodel for salmon. Simulation results indicate that years favorable for juvenile salmon growth off central California are characterized by particularly intense early season upwelling (i.e., March through May), leading to enhanced krill concentrations during summer near the location of ocean entry (i.e., Gulf of the Farallones). Seasonally averaged growth rates in the model are generally consistent with observed values and suggest that juvenile salmon emigrating later in the season (i.e., late May and June) achieve higher weight gains during their first 90 days of ocean residency.