Mesoscale heterogeneity of the wind-driven mixed layer: Influence of a quasigeostrophic flow

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

The mesoscale variability of the sea surface temperature: An analytical and numerical mode

This study examines the emergence and evolution of a mesoscale sea surface temperature (SST) variability induced by a uniform and impulsive wind stress when an embedding quasigeostrophic (QG) flow is present. The SST variability which is triggered by the mixed-layer deepening closely resembles some characteristic properties of the QG flow, namely either the subsurface temperature or relative vorticity, depending on the amplitude of the deepening. The SST variance can have the same order of magnitude as the subsurface temperature variance. Within 10 days, the SST field, which is stirred only by the horizontal QG flow, displays a rapid spectral evolution characterized by the emergence of small-scale structures and the appearance of thermal fronts located in the QG jet areas. This evolution depends only on the deformation of the large-scale structures of the SST field, initially resulting from the mixed-layer deepening, by the QG strain field. In contrast with SST, later evolution of the mixed-layer depth is characterized by the emergence of large-scale structures. From these dynamical results, it is speculated that, when nonuniform initial conditions are considered, the resulting SST spatial variability should be more closely related to the subsurface temperature and the SST variance could be significantly increased

Equatorial zonal jets formation

After a brief review of oceanic mid -latitude zonal jets observations and of their currently proposed formation mechanisms, the characteristics of oceanic equatorial zonal jets are presented. A new mechanism for zonal jets formation is proposed based on the destabilization of short Mixed Rossby Gravity waves, providing a rationale for the difference of scales observed in the Pacific and in the Atlantic deep equatorial jets. Results of numerical simulations performed on the Earth Simulator are presented to validate the analytical theory

The equatorial thermostad and subsurface countercurrents in the light of the dynamics of atmospheric Hadley cells

The simple Held and Hou (1980) nearly inviscid model of the axisymmetric atmospheric circulation, rationalizing the existence of Hadley cells, Jet Streams, and tropical homogenization of potential temperature and vorticity, is adapted to the oceanic subthermocline region. The meridional profile of radiative equilibrium temperature, which provides the driving in the atmospheric case, is replaced, in the oceanic case, by the large-scale equatorial doming of the thermocline. The meridional structure of the equatorial thermostad and the existence, at its poleward flanks, of eastward subsurface countercurrents coincident with sharp potential vorticity gradients are thus explained via angular momentum redistribution by secondary ageostrophic over-turning cells in the meridional plane. Taking into account the existence of the equatorial undercurrent within the thermocline has little effect on the overall structure of the subthermocline solution at a given longitude, even though homogenization of properties is less efficient

Three-dimensional stirring of thermohaline fronts

This study investigates the stirring of the thermohaline anomalies in a fully turbulent quasigeostrophic stratified flow. Temperature and salinity fields are permanently forced at large scales and are related to density by a linear equation of state. We show, using some inherent properties of quasi-geostrophic turbulence, that the 3-D ageostrophic circulation is the key dynamical characteristic that governs the strength and the spatial distribution of small-scale thermohaline fronts that are strongly density compensated. The numerical simulations well illustrate the formation by the mesoscale eddy field of sharp thermohaline fronts that are mainly located in the saddle regions and around the eddy cores and have a weak signature on the density field. One important aspect revealed by the numerical results is that the thermohaline anomalies experience not only a direct horizontal cascade but also a significant vertical cascade. One consequence of this 3-D cascade is that the ultimate mixing of the thermohaline anomalies will not be necessarily maximum at the depth where the large-scale temperature and salinity anomalies are maximum. Some analytical arguments allow us to identify some of the mechanisms that drive this 3-D cascade