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

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

Low -wind condition

Strong winds

Strong winds

•Turbulence closure model in POM

•Forcing by constant wind stress

25-day simulations

•red - original boundary condition

•black - Craig-Banner boundary condition

q2 - turbulent kinetic energy

0,2 32

zuz

qkq

Km

2

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.

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

)2exp(2 kca kUs

dfdfEeft kcS ),(4),( 2kxU

10003.02

10 )(0186.0 WU gLWs

Representation of the Stokes drift

(1) Single wave (Stokes, 1847)

(2) Wave field (Jenkins, 1989)

(3) Parameterization by wind speed (Wu, 1983)

Model currents with the Stokes drift

dkeSfdf

zA

ztkz

dsSE

VEE 222 kU

uu

uff .

Momentum equation for the Ekman layer and boundary condition

0c

EV cA

u

o

T

a dSfdf ink2

nl ds in gS S S f Et

f E

), (), (

C

dfdfEeft kcS ),(4),( 2kxU

Equation for wave energy spectrum

00dz dztSEEUufu o

o

T

wo aw a oTTTT

o aw2 Tdf dSfink

owo2TdfdSfdsk

Vertically integrated Ekman equation

•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.

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 .