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

Some physical factors affecting ecosystems

The response of a simple pelagic ecosystem is investigated in a domain with zero, one- and two-dimensional descriptions of the physical processes. Assuming complete mixing, internal recycling and external exchange of nutrients and their biological products are not additive in terms of the rate of primary production. In a one-dimensional system, advection without diffusion leads to low values of primary production. With two horizontal dimensions, cross diffusion at the boundary gives higher values of production. The consequences for higher trophic levels are described

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

Interannual to Decadal Variations of Submesoscale Motions around the North Pacific Subtropical Countercurrent

The outputs from a submesoscale permitting hindcast simulation from 1990 to 2016 are used to investigate the interannual to decadal variations of submesoscale motions. The region we focus on is the subtropical Northwestern Pacific including the subtropical countercurrent. The submesoscale kinetic energy (KE) is characterized by strong interannual and decadal variability, displaying larger magnitudes in 1996, 2003, and 2015, and smaller magnitudes in 1999, 2009, 2010, and 2016. These variations are partially explained by those of the available potential energy (APE) release at submesoscale driven by mixed layer instability in winter. Indeed, this APE release depends on the mixed layer depth and horizontal buoyancy gradient, both of them modulated with the Pacific Decadal Oscillation (PDO). As a result of the inverse KE cascade, the submesoscale KE variability possibly leads to interannual to decadal variations of the mesoscale KE (eddy KE (EKE)). These results show that submesoscale motions are a possible pathway to explain the impact associated with the PDO on the decadal EKE variability. The winter APE release estimated from the Argo float observations varies synchronously with that in the simulation on the interannual time scales, which suggests the observation capability to diagnose the submesoscale KE variability

Dissipation of the energy imparted by mid-latitude storms in the Southern Ocean

The aim of this study is to clarify the role of the Southern Ocean storms on interior mixing and meridional overturning circulation. A periodic and idealized numerical model has been designed to represent the key physical processes of a zonal portion of the Southern Ocean located between 70 and 40°?S. It incorporates physical ingredients deemed essential for Southern Ocean functioning: rough topography, seasonally varying air–sea fluxes, and high-latitude storms with analytical form. The forcing strategy ensures that the time mean wind stress is the same between the different simulations, so the effect of the storms on the mean wind stress and resulting impacts on the Southern Ocean dynamics are not considered in this study. Level and distribution of mixing attributable to high-frequency winds are quantified and compared to those generated by eddy–topography interactions and dissipation of the balanced flow. Results suggest that (1) the synoptic atmospheric variability alone can generate the levels of mid-depth dissipation frequently observed in the Southern Ocean (10?10–10?9?W?kg?1) and (2) the storms strengthen the overturning, primarily through enhanced mixing in the upper 300?m, whereas deeper mixing has a minor effect. The sensitivity of the results to horizontal resolution (20, 5, 2 and 1?km), vertical resolution and numerical choices is evaluated. Challenging issues concerning how numerical models are able to represent interior mixing forced by high-frequency winds are exposed and discussed, particularly in the context of the overturning circulation. Overall, submesoscale-permitting ocean modeling exhibits important delicacies owing to a lack of convergence of key components of its energetics even when reaching ?x?=??1?km