Chapter 6 Large-Scale Momentum Exchange in the Coupled Atmosphere-Ocean

Abstract

Abstract It is shown that the exchange of momentum between the atmosphere and the ocean is essentially a two-way process. Thus the surface shearing stress, τs =τw — τF consists of two independent parts — one due to the air (the wind stress, τw) and the other due to the water (which we will call the “under-stress”, τF). This is in distinction to the single-term representation (τw) over a solid surface. The understress which retards the surface current, has the approximate form τF ∼ ρaK|ua|uo where ρa is the density of air, and ua is the wind velocity with the associated drag coefficient K and uo is the surface current. It is suggested that the primary dissipation process for the deep stratified World ocean may occur in the air-sea boundary layer rather than say near the bottom. This is demonstrated by considering the relative importance of bottom stress and understress for an individual oceanic eddy. For a scale of order 100 km which averages over the mesoscale eddies in the ocean, but resolves the synoptic disturbances in the atmosphere, it is also shown that the understress acting on the mean current has the approximate form, τF ∼ ρaK (1 + ξ2) |ūa|ūo in which ξ = (ū'o2)1/2|ūo| is the intensity of the surface two-dimensional turbulence in the ocean, and ′ and − denote respectively a mean and a fluctuation. On using ρa ∼ 1 kg m−3, K ∼ 2x 10−3, ūa ∼ 10 ms−1, and ξ ∼ 2 we obtain τ ∼ 0.1 ūo Nm−2, and it follows that τF and τW are both of the same order of magnitude. Finally, if dissipation in the air—sea boundary layer is the dominant process, it is deduced that the ratio of the dissipation rates for water and air should be ∼ 1/30, and hence by a thermodynamic argument that the meridional energy fluxes between the two fluids should be partitioned in a similar manner.