Abstract

Weller (1982) and Kunze (1985) have shown that the presence of a geostrophic flow may be responsible for a large part of the observed mesoscale heterogeneity and intermittency of inertial oscillations. In this paper, the effects of this influence on the dynamics of the wind-driven mixed-layer (ML), in particular on entrainment and ML depth evolution, are analytically derived. A simple ML model including these effects is coupled with the quasigeostrophic numerical model of Hua and Haidvogel (1986) in order to investigate and characterize the specific effects of a quasigeostrophic flow on the ML spatial heterogeneity. Numerical results clearly show that the presence of a quasigeostrophic shear is capable of producing a non-negligible mesoscale heterogeneity of the ML as a response to uniform and constant strong wind. This mesoscale heterogeneity, which is mainly induced by the quasigeostrophic deformation and strain field, is characterized by the presence of significant spatial intermittency. It is shown that the ML heterogeneity is a wind-biased vorticity mirror during the first two days when the entrainment is dominant. On a longer time scale other processes such as the inertial Ekman pumping affect the ML mesoscale heterogeneity characteristics. Nonuniform initial conditions and nonstationary atmospheric forcings should also affect the ML mesoscale heterogeneity in reality. However the physical processes considered in this study could explain a non-negligible part of the ML mesoscale variability observed on satellite imagery and during in-situ experiments.

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