Michael S. Dinniman John M. Klinck Center for Coastal Physical Oceanography

1. Modeling flow over steep topography with strong stratification: The circulation around Astoria canyon Michael S. Dinniman John M. Klinck Center for Coastal Physical Oceanography Old Dominion University

2. Acknowledgements • Much of this work was done in collaboration with Susan Allen at the University of British Columbia and Barbara Hickey at the University of Washington • Hernan Arango at Rutgers University provided us with the Beta 1.0 release of ROMS • The Office of Computing and Communications Services at ODU provided the 64-processor Sun HPC 10000 that the larger simulations were run on • Nancy Kachel at the University of Washington created the digitized version of the NOAA bathymetry charts • CCPO provided computer facilities and support • U.S. National Science Foundation provided financial support

3. Outline • Astoria Canyon: Bathymetry and Observations • Model Description • Model Results • Possible Causes of Discrepancies • Conclusions

4. Introduction • Submarine canyons along continental shelf edges are important topographical features that can have large impacts on coastal processes • These canyons make strong test cases for numerical circulation models due to the combination of steep bathymetry and strong stratification (e.g. Haidvogel and Beckmann, 1998) • Observations in Astoria canyon in summer 1983 (Hickey, 1997) provide a good opportunity for experimentation

6. Observations • Mooring array deployed along and across the canyon from May to August 1983 • Some moorings were lost and others damaged and prematurely returned: Time series range in duration from two weeks to three months • Current meters on all moorings and the uppermost float a minimum depth of 65m from the surface • Hydrographic data were obtained at the beginning and end of the deployment of the fixed instrument array and consist of several transects of CTD measurements taken across the canyon • Local wind data were not available

7. Astoria Model Description • Rutgers/UCLA Regional Ocean Model System (ROMS) • Parallel primitive equation model derived from the serial S-Coordinate Rutgers University Model (SCRUM) • Free surface and vertical s (terrain-following) coordinate • Domain: U.S. West Coast centered on Astoria and Willapa Canyons; 83km (lon) x 100km (lat) • Resolution: ~ 300m (lon) x 420m (lat) horizontal; 24 depth levels • Bathymetry: Gridded field derived from digitized NOAA bathymetry charts

10. Model Description (cont.) • Initial Stratification: Analytical approximation to a hydrographic section (T and S) obtained near Astoria canyon in May 1983. Full non-linear equation of state used • Laplacian mixing on geopotential surfaces (very weak for momentum) and KPP vertical mixing scheme • Forcing: • Wind Stress: Applied as a body force over the top three layers of the model (concentrated towards the surface). Realistic upwelling event from FNMOC wind stress • Quadratic bottom stress • Heat and fresh water flux: Set to zero

12. Model Description (cont.) • Boundary Conditions • Zero gradient condition for West open boundary; free slip for East closed boundary • North/South open boundaries: Used "Modified" Orlanski radiation scheme on all quantities (except 2D normal flow) • 2D normal flow: Separated the barotropic flow into "free" and "forced" components and used the Flather radiation scheme on the free component (e.g. Palma and Matano, 2000). • Pressure Gradient Error: Checked by running simulation with zero wind stress

14. Model Results Mean horizontal flow reproduces the observations fairly well Model upwelling does not match the observed upwelling Too weak and the isopycnals do not relax when the upwelling winds weaken

19. Possible Causes of Model Discrepancies • Wind stress problems (curl and magnitude) • Dissipation • Missing undercurrent • Incorrect initial stratification

21. Possible Causes of Model Discrepancies (cont.) • Pressure gradient errors • Tracer advection numerical truncation • S. Allen, et al.: Comparison of upwelling over submarine canyons between a numerical model (SCRUM) and a laboratory model (rotating tank) • An expression for the error in the time rate of change of the vertical tracer gradient can be calculated by using the tracer advection discretization in SCRUM and making several simplifying assumptions • For the upstream canyon edge, the calculated error was of the right sign (upward density gradient error leading to up-canyon flow error) of the observed differences • Ran Astoria model with new pressure gradient (splines density Jacobian; Shchepetkin, 2001) and tracer advection (4th-order Akima)

23. Conclusions • It is important to accurately represent the bathymetry when trying to match observations in areas of steep topography • The mean circulation in the Astoria realistic wind simulation matches the observations moderately well • The model is vertically stiff (isopycnals are relatively immobile) compared to the observations. However, the response can be improved somewhat by modifying the initial stratification and changing numerical pressure gradient and tracer advection schemes • Much work remains to be done on understanding the model / observation differences • Question for the audience: Has anyone else run a fine resolution model and seen a similar stiffness in the isopycnals and, if so, were you able to improve the response?