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Ocean surface currents and the Craig-Banner boundary condition Charles Tang Bedford Institute of Oceanography Dartmouth

Ocean surface currents and the Craig-Banner boundary condition Charles Tang Bedford Institute of Oceanography Dartmouth, Nova Scotia, Canada. POM for the Labrador Sea and eastern Canadian shelves Simulations of circulation and winter convection Wind-driven currents on the Grand Banks

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Ocean surface currents and the Craig-Banner boundary condition Charles Tang Bedford Institute of Oceanography Dartmouth

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  1. Ocean surface currents and the Craig-Banner boundary condition Charles Tang Bedford Institute of Oceanography Dartmouth, Nova Scotia, Canada • POM for the Labrador Sea and eastern Canadian shelves • Simulations of circulation and winter convection • Wind-driven currents on the Grand Banks • Testing of the Craig-Banner boundary condition • Wave-current interaction

  2. Short-term forecasts of surface currents • 2-day forecasts • wind-driven currents • no data assimilation • comparison with surface drifter trajectories • Questions: • Representation of surface currents in POM • Wave effects

  3. Low -wind condition

  4. Strong winds

  5. Strong winds

  6. Turbulence closure model in POM • Forcing by constant wind stress

  7. 25-day simulations • red - original boundary condition • black - Craig-Banner boundary condition

  8. q2 - turbulent kinetic energy Km

  9. Application of theories of wave-induced currents Weber, J., Steady wind- and wave-induced currents in the open ocean. JPO, 1983. Jenkins, A.D., A theory for steady and variable wind- and wave-induced currents. JPO, 1986. Jenkins, A.D., Wind and wave induced currents in a rotation sea with depth- varying eddy viscosity. JPO, 1987. Jenkins, A.D., The use of a wave prediction model for driving a near-surface current model. Dt.Hydrogra.Z, 1989. Weber, J. & A. Melsom, Transient ocean currents induced by wind and growing waves. JPO, 1993.

  10. Wave effects on surface currents • The Stokes drift • Modification of diffusivity due to wave breaking • Partition of wind momentum for generation of waves and currents

  11. Representation of the Stokes drift (1) Single wave (Stokes, 1847) (2) Wave field (Jenkins, 1989) (3) Parameterization by wind speed (Wu, 1983)

  12. Model currents with the Stokes drift

  13. Momentum equation for the Ekman layer and boundary condition Equation for wave energy spectrum

  14. Vertically integrated Ekman equation

  15. Summary • POM simulates seasonal-scale variables in the Labrador Sea reasonably well, but produces surface currents not always in agreement with observations in short-term forecasts . • Drifter data show surface currents in the direction of wind vectors during strong winds. • The Craig-Banner boundary condition increases the turbulent kinetic energy and mixing intensity at the surface, and reduces surface currents. • A formulation based on Jenkins’ theory of wave-current interaction is proposed to calculate surface currents.

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