THREE-DIMENSIONAL LANGMUIR CIRCULATIONS AND ENHANCED TURBULENCE IN UPPER MIXED OCEAN LAYERS

Abstract

FIELD AND LABORATORY DATA CON¯RM THE PRESENCE OF LONGITUDINAL BILLOWS IN FLUID FLOW UNDER WIND-WAVY SURFACES. IN THE OCEAN THESE VORTICES (CALLED LANGMUIR CELLS) ACT BY MIXING NUTRIENTS AND OTHER BIOLOGICAL MATERIAL, AND THUS THEIR ROLE CANNOT BE NEGLECTED IN VERTICAL TRANSFER MODELLING. IN THIS WORK, NON-DIMENSIONAL MEAN VELOCITY FIELD EQUATIONS ARE FORMULATED WITH CRAIK AND LEIBOVICH THEORY, INCLUDING INTERACTION TERMS BETWEEN SURFACE WAVE STOKES DRIFT AND MEAN CURRENT. A FIRST ORDER TURBULENCE CLOSURE MODEL (K,") IS USED TO MODEL THE REYNOLDS STRESS TENSOR. THE MODEL IS FORMULATED IN NON-DIMENSIONAL GROUNDS, AND NUMERICAL EXPERIMENTS ARE PERFORMED USING A ¯NIT-VOLUME TECHNIQUE. IN THE ¯RST SET OF SIMULATIONS, MODEL OUTPUTS ARE COMPARED TO MEASUREMENTS OBTAINED AT THREE DIFFERENT LABORATORY WIND-WATER FACILITIES (CHEUNG AND STREET, 1988; THAIS AND MAGNAUDET, 1996). RESULTS SUGGEST THAT THE PRESENCE OF SECONDARY MOTIONS IS NECESSARY FOR EXPLAINING THE OBSERVED CHANNEL FLOWS. A SECOND GROUP OF SIMULATIONS INVOLVES ¯ELD SITUATIONS, WHEN NUMERICAL RESULTS ARE COMPARED TO SOME TYPICAL ENVIRONMENTAL CASES (KITAIGORODSKII ET AL., 1983). MODEL RESULTS FOR THIS SECOND GROUP OF EXPERIMENTS SHOW (K/U^2 = O(10^2)) (K IS THE SURFACE TURBULENT KINETIC ENERGY, AND U¤ IS THE WATER FRICTION VELOCITY), REPRESENTING THE SAME ORDER OF MAGNITUDE CURRENTLY FOUND IN SITU FOR TURBULENCE.