Patrice Klein

Impact of sub-mesoscale physics on production and subduction of phytoplankton in an oligotrophic regime

Using a protocol of numerical experiments where horizontal resolution is progressively increased, we show that small-scale (or sub-mesoscale) physics has a strong impact on both mesoscale physics and phytoplankton production/subduction. Mesoscale and sub-mesoscale physics result from the nonlinear equilibration of an unstable baroclinic jet. The biogeochemical context is oligotrophy. The explicitly resolved sub-mesoscales, at least smaller than one e fth of the internal Rossby radius of deformation, reinforce the mesoscale eddy e eld and contribute to double the primary production and phytoplankton subduction budgets. This enhancement is due to the reinforced mesoscale physics and is also achieved by the small-scale frontal dynamics. This sub-mesoscale physics is associated with density and vorticity gradients around and between the eddies. It triggers a signie cant small-scale nutrient injection in the surface layers, leading to a phytoplankton e eld mostly dominated by e ne spatial structures. It is believed that, depending on wind forcings, this scenario should work alternately with that of Abraham (1998) which invokes horizontal stirring of nutrient injected at large scales. Results also reveal a strong relationship between new production and negative vorticity, in the absence of wind forcing and during the period of formation of the eddies.

Upper Ocean Turbulence from High-Resolution 3D Simulations

Abstract The authors examine the turbulent properties of a baroclinically unstable oceanic flow using primitive equation (PE) simulations with high resolution (in both horizontal and vertical directions). Resulting dynamics in the surface layers involve large Rossby numbers and significant vortical asymmetries. Furthermore, the ageostrophic divergent motions associated with small-scale surface frontogenesis are shown to significantly alter the nonlinear transfers of kinetic energy and consequently the time evolution of the surface dynamics. Such impact of the ageostrophic motions explains the emergence of the significant cyclone–anticyclone asymmetry and of a strong restratification in the upper layers, which are not allowed by the quasigeostrophic (QG) or surface quasigeostrophic (SQG) theory. However, despite this strong ageostrophic character, some of the main surface properties are surprisingly still close to the surface quasigeostrophic equilibrium. They include a noticeable shallow (≈k−2) velocity s...

Dynamics of the Upper Oceanic Layers in Terms of Surface Quasigeostrophy Theory

In this study, the relation between the interior and the surface dynamics for nonlinear baroclinically unstable flows is examined using the concepts of potential vorticity. First, it is demonstrated that baroclinic unstable flows present the property that the potential vorticity mesoscale and submesoscale anomalies in the ocean interior are strongly correlated to the surface density anomalies. Then, using the invertibility of potential vorticity, the dynamics are decomposed in terms of a solution forced by the three-dimensional (3D) potential vorticity and a solution forced by the surface boundary condition in density. It is found that, in the upper oceanic layers, the balanced flow induced only by potential vorticity is strongly anticorrelated with that induced only by the surface density with a dominance of the latter. The major consequence is that the 3D balanced motions can be determined from only the surface density and the characteristics of the basin-scale stratification by solving an elliptic equation. These properties allow for the possibility to reconstruct the 3D balanced velocity field of the upper layers from just the knowledge of the surface density by using a simpler model, that is, an “effective” surface quasigeostrophic model. All these results are validated through the examination of a primitive equation simulation reproducing the dynamics of the Antarctic Circumpolar Current.

Do Altimeter Wavenumber Spectra Agree with the Interior or Surface Quasigeostrophic Theory

In high-eddy-energy regions, it is generally assumed that sea level wavenumber spectra compare well with quasigeostrophic (QG) turbulence models and that spectral slopes are close to the expected k−5 law. This issue is revisited here. Sea level wavenumber spectra in the Gulf Stream, Kuroshio, and Agulhas regions are estimated using the most recent altimeter datasets [the Ocean Topography Experiment (TOPEX)/Poseidon, Jason-1, the Environmental Satellite (Envisat), and the Geosat Follow-On]. The authors show that spectral slopes in the mesoscale band are significantly different from a k−5 law, in disagreement with the QG turbulence theory. However, they very closely follow a k−11/3 slope, which indicates that the surface quasigeostrophic theory (SQG) is a much better dynamical framework than the QG turbulence theory to describe the ocean surface dynamics. Because of the specific properties of the SQG theory, these results offer new perspectives for the analysis and interpretation of satellite data.

An exact criterion for the stirring properties of nearly two-dimensional turbulence

Abstract An exact criterion can be found for partitioning the fluid into regions with different dynamical properties, from both the points of view of particle dispersion and tracer gradient evolution. This criterion differs markedly, both in its magnitude and spatial scales, from the Okubo-Weiss criterion which depends upon the differential geometry of the streamfunction field and coincides with the eigenvalues of the velocity gradient tensor. The new criterion corresponds to the eigenvalues of the acceleration gradient tensor, whose spatial distribution depends instead upon the topology of the pressure field. This result holds for all flows for which a continuous momentum equation can be prescribed. We provide numerical evidence for the quantitative importance of the time change of the strain-rate components in the dispersion problem in freely decaying two-dimensional turbulence.

Oceanic Restratification Forced by Surface Frontogenesis

Potential vorticity (PV) conservation implies a strong constraint on the time evolution of the mean density at a given depth. The authors show that, on an f plane and in the absence of sources and sinks of PV, it only depends on two terms, namely, the time evolution of the product between density anomaly and relative vorticity and the vertical PV flux. This primitive equation result, which applies at any depth, suggests that the ageostrophic dynamics induced by baroclinic eddies strongly affect the mean oceanic stratification profile. This result is illustrated for two simple initial-value simulations of a baroclinic, balanced jet. For initial situations propitious to surface frontogenesis, the simulations show a restratification over the whole water column characterized by the amplification in time of the Brunt–Vaisala frequency in the upper oceanic layers. In the absence of surface frontogenesis, such as when the jet is initialized at the middepth of the water column, the restratification is much weaker and slower. Because both simulations have similar kinetic energy and growth rate of baroclinic instability, the results clearly reveal that the restratification is driven by surface frontogenesis in the first case and by vertical PV flux in the interior in the second case. The authors also point out that the dynamics of the interior PV is tightly related to the surface dynamics because of total mass conservation.

Three‐dimensional reconstruction of oceanic mesoscale currents from surface information

The ability to reconstruct the three-dimensional (3D) dynamics of the ocean by an effective version of Surface Quasi-Geostrophy (eSQG) is examined. Using the fact that surface density plays an analogous role as interior potential vorticity (PV), the eSQG method consists in inverting the QG PV generated by sea-surface density only. We also make the extra assumption that sea-surface temperature (SST) anomalies fully represent surface density anomalies. This approach requires a single snapshot of SST and the setup of two parameters: the mean Brunt-Vaisala frequency and a parameter that determines the energy level at the ocean surface. The validity of this approach is tested using an Ocean General Circulation Model simulation representing the North Atlantic in winter. It is shown that the method is quite successful in reconstructing the velocity field at the ocean surface for mesoscales (between 30 and 300 km). The eSQG framework can also be applied to reconstruct subsurface fields using surface information. Results show that the reconstruction of velocities and vorticity from surface fields is reasonably good for the upper 500 m and that the success of the method mainly depends on the quality of the SST as a proxy of the density anomaly at the base of the mixed layer. This situation happens after a mixed-layer deepening period. Therefore the ideal situation for the application of this method would be after strong wind events.

Potential use of microwave sea surface temperatures for the estimation of ocean currents

In this paper, we examine the emerging potential offered by satellite microwave radiometer SST measurements to complement altimeter data to quantitatively derive surface ocean currents. The proposed methodology does not follow standard sequential temporal analysis but follows the application of the Surface Quasi-Geostrophic (SQG) theory. Accordingly, under favourable environmental conditions, the implementation for this methodology is simple and robust, and most importantly, solely requires a single SST image. For the present demonstration, altimetric measurements are used to infer a necessary adjustment to match the kinetic energy level for length scales smaller than 300 km. This helps to derive a regional effective Brunt-Vaisala frequency to produce SQG surface current estimates. As demonstrated, the results are very encouraging and strongly invite to consider the systematic use of satellite microwave radiometer measurements.

Does the tracer gradient vector align with the strain eigenvectors in 2D turbulence

This paper investigates the dynamics of tracer gradient for a two-dimensional flow. More precisely, the alignment of the tracer gradient vector with the eigenvectors of the strain-rate tensor is studied theoretically and numerically. We show that the basic mechanism of the gradient dynamics is the competition between the effects due to strain and an effective rotation due to both the vorticity and to the rotation of the principal axes of the strain-rate tensor. A nondimensional criterion is derived to partition the flow into different regimes: In the strain dominated regions, the tracer gradient vector aligns with a direction different from the strain axes and the gradient magnitude grows exponentially in time. In the strain-effective rotation compensated regions, the tracer gradient vector aligns with the bisector of the strain axes and its growth is only algebraic in time. In the effective rotation dominated regions, the tracer gradient vector is rotating but is often close to the bisector of the strain...

Impact of oceanic-scale interactions on the seasonal modulation of ocean dynamics by the atmosphere.

Ocean eddies (with a size of 100–300 km), ubiquitous in satellite observations, are known to represent about 80% of the total ocean kinetic energy. Recent studies have pointed out the unexpected role of smaller oceanic structures (with 1–50 km scales) in generating and sustaining these eddies. The interpretation proposed so far invokes the internal instability resulting from the large-scale interaction between upper and interior oceanic layers. Here we show, using a new high-resolution simulation of the realistic North Pacific Ocean, that ocean eddies are instead sustained by a different process that involves small-scale mixed-layer instabilities set up by large-scale atmospheric forcing in winter. This leads to a seasonal evolution of the eddy kinetic energy in a very large part of this ocean, with an amplitude varying by a factor almost equal to 2. Perspectives in terms of the impacts on climate dynamics and future satellite observational systems are briefly discussed.