Victor Zhurbas

Estimating horizontal diffusivity in the East Sea (Sea of Japan) and the northwest Pacific from satellite‐tracked drifter data

We use data of satellite-tracked, mixed-layer drifters collected for the World Ocean Circulation Experiment/Tropical Ocean-Global Atmosphere (WOCE/TOGA) Surface Velocity Program in the East Sea (Sea of Japan) and the northwest Pacific Ocean during 1991-1997 to obtain reliable estimates of geographically varying horizontal diffusivity, integral timescale and space scale. For the diffusivity we suggest calculation of both the minor principal component of the diffusivity tensor in a definition by Davis [1991] and the half growth rate of the minor principal component of the displacement variance tensor. Numerical simulation of particle motion is used to prove that the minor principal component estimates of diffusivity, in contrast to regular estimates, are insensitive to ensemble averaging over particles taken from a finite area in a shear mean flow. In the East Sea, typical values of the diffusivity, timescale, and space scale are (1.7-5.2) × 10 7 cm -2 s -1 , 0.7-1.7 days, and 10-25 km, respectively. In the northwest Pacific Ocean, typical estimates of the diffusivity, timescale, and space scale in mean are a factor 2-3 larger, namely, (2.3-15.1) X 10 7 cm -2 s -1 , 1.7-3.7 days, and 18-62 km, respectively. It was found that the space scale L and the first mode internal Rosshy radius Ri are related as L Ri, which suggests a parameterization of lateral diffusivity k∞ of the form k∞ = u Ri, where u is the rms current velocity fluctuation.

Environmental Hazards of Sea-Dumped Chemical Weapons

Does the post-WWII burial at sea of chemical weapons still pose a human and environmental risk?

The Structure and Driving Mechanisms of the Baltic Intrusions

Abstract Data from closely spaced CTD profiling performed in the eastern Gotland Basin after the 1993 inflow event are used to study thermohaline intrusions in the Baltic Sea. Two CTD cross sections display abundant intrusive layers in the permanent halocline. Despite the overwhelming dominance of the salinity stratification, diffusive convection is shown to work in the Baltic halocline enhancing diapycnical mixing. To understand the driving mechanisms of observed intrusions, these are divided into different types depending on their structural features. Only two types of observed intrusions are suggested to be strongly influenced by diffusive convection: 1) relatively thin (3–5 m) and long (up to 8 km) intrusions inherent to high-baroclinicity regions and 2) relatively thick (∼10 m) and short (2–5 km) intrusions inherent to low-baroclinicity regions. To verify this hypothesis the linear stability models of 3D and 2D double-diffusive interleaving in approximation of a finite-width front were used. It is sh...

Generation of subsurface cyclonic eddies in the southeast Baltic Sea: Observations and numerical experiments

[1] Closely spaced CTD transects performed in the summertime reveal simultaneous downward/upward bendings of temperature/salinity contours in the seasonal thermocline/ permanent halocline of the Stolpe Channel and the Gulf of Gdansk, which may be interpreted as geostrophically balanced cyclonic eddies in the intermediate layer. To examine processes capable of forming the observed cyclonic eddies, a numerical simulation based on the Princeton Ocean Model (POM) has been initiated. The subsurface cyclones in the Stolpe Channel were satisfactory simulated in model runs under easterly/ northerly wind conditions. Their formation was shown to result from the adjustment of the high potential vorticity (PV) outflow (from the Bornholm Basin via the Stolpe Sill) to low potential vorticity environment by vortex stretching (so-called the PV outflow hypothesis by Spall and Price [1998]). In accordance with the real wind conditions, a cyclonic eddy observed in the intermediate layer of the Gulf of Gdansk was satisfactorily reproduced in a model run with the westerly wind shutdown, which implies westward transport throughout the Stolpe Channel and thereby excludes the PV outflow hypothesis. The subsurface cyclone simulated in the Gulf of Gdansk was traced to form in the course of relaxation of the coastal downwelling baroclinic jet.

Effects of Double Diffusion and Turbulence on Interleaving at Baroclinic Oceanic Fronts

Abstract Treating the problem of interleaving in ocean fronts, a linear stability analysis is applied to a thermohaline, baroclinic front in which the vertical diffusivity for mass and momentum is determined by both the double diffusion and turbulence. If the mass and momentum diffusivity is controlled by double diffusion solely, interleaving in baroclinic fronts is possible at any value of the geostrophic Richardson number Ri. However, it is shown that turbulent mixing always works to suppress double-diffusive interleaving. Due to turbulent mixing, at some range of the input parameters there is a range of Ri where the maximum growth rate of interleaving, ω′max, vanishes. Several asymptotic criteria governing the Ri dependence of ω′max are found. These criteria fit well the results of numerical calculations of ω′max(Ri). One of the criteria has been applied to describe intrusions observed in the Azores Front of the North Atlantic.

Generation of cyclonic eddies in the Eastern Gotland Basin of the Baltic Sea following dense water inflows: numerical experiments

Abstract A sigma ( σ )-coordinate ocean model by Blumberg and Mellor (POM) is applied to study the formation processes of mesoscale cyclones observed in the Eastern Gotland Basin following the dense water inflows. The initial conditions simulate a situation when the Arkona and Bornholm basins and partially the Slupsk Furrow are already filled with the inflow water of the North Sea origin, while the Eastern Gotland and Gdansk basins still contain the old water of pre-inflow stratification. Model runs with constant and time-dependent winds, changing the buoyancy forcing, grid geometry and bottom topography display the following. Entering the Eastern Gotland Basin from the Slupsk Furrow, the bottom intrusion of saline inflow water splits in two: one goes northeast towards the Gotland Deep, and second moves southeast towards the Gulf of Gdansk. An intensive mesoscale cyclonic eddy carrying the inflow water is generated just east of the Slupsk Furrow with the inflow pulse. A number of smaller cyclones with boluses of the inflow water are formed in the permanent halocline along the saline intrusion pathway to the Gotland Deep. Following Spall and Price [J. Phys. Oceanogr. 28 (1998) 1598], the cyclones are suggested to form by the adjustment of the high potential vorticity inflow water column to a low potential vorticity environment.

What drives thermohaline intrusions in the Baltic Sea

Abstract Using data from closely spaced CTD profiles taken after the 1993 major inflow of North Sea water, we investigate the characteristics and generation mechanisms of finestructure intrusions in the Baltic halocline. Due to the strong prevalence of salinity over temperature in density stratification, the salt fingering is not possible, and diffusive convection has a limited effect on thermohaline intrusions in the halocline. Some evidence for inertial oscillations as a driving mechanism for the intrusions is found. In several occasions, we observed odd intrusion-like structures crossing isopycnals in such a way that the along-layer density ratio is approximately equal to the vertical density ratio. We show that these ‘odd intrusions’ are not likely to be generated from the interleaving process, and several alternate scenarios for their generation are suggested.

Can Turbulence Suppress Double-Diffusively Driven Interleaving Completely?

Abstract A linear stability problem is formulated to investigate the effect of turbulence on double-diffusively driven thermohaline interleaving in rotating media. Three cases are considered: (a) intrusions with an alongfront slope in rotating media, (b) intrusions with zero alongfront slope in nonrotating media, (c) intrusions with zero alongfront slope, where the Coriolis force is retained. The physical reason for case c is that the large-scale vertical geostrophic shear in baroclinic fronts will rotate any intrusion with nonzero alongfront slope as long as the alongfront slope vanishes. In all three cases, turbulence works to suppress interleaving so that the growth rate of the fastest growing intrusion decreases with the increase of turbulent diffusivity k*. However, in cases a and b the growing intrusions exist for any finite value of k*, while in case c there is a marginal (maximum) value of k* beyond which growing intrusions do not exist.

Study of spatial spectra of horizontal turbulence in the ocean using drifter data

The statistics of a pair of Lagrangian particles offer, in principle, a possibility to estimate the structure functions of velocity, then the spatial autocorrelations, and finally the spatial spectra. On the basis of this strategy, the authors have developed an approach to estimate spatial spectra of mesoscale horizontal turbulence in the ocean using data of satellite-tracked drifters. The approach was applied to the data of 19 drifters deployed in the California Current System in 1993. It was found that the shape of both those spectra and this spectra calculated using drifterborne longitudinal and transverse correlations estimated by other authors are qualitatively in good accordance with theoretical predictions for 2D isotropic nondivergent turbulent flow. To relate obtained spectra to some physical parameters, kinematic stochastic models were developed that consisted of a population of randomly spaced, 2D axisymmetric eddies of a given shape. Numerical experiments with different eddy shapes showed that the model spectra obey a self-similarity; that is, at a given eddy shape they depend on the variance of stochastic process and a length scale of the eddy only. A model with the exponential eddy shape was found to fit drifterborne spectra better than other models. The best agreement between the drifterborne and model spectra was achieved when the radius of an exponentially shaped model eddy was taken equal to the internal Rossby radius.

Effects of Turbulent Mixing and Horizontal Shear on Double-Diffusive Interleaving in the Central and Western Equatorial Pacific

Abstract The effects of small-scale turbulence and the horizontal shear on double-diffusive interleaving are treated in the framework of a linear stability problem in the equatorial β-plane approximation. The model predicts the decrease of the vertical wavenumber (mmax) and growth rate (λmax) for the fastest-growing intrusion with the increase in the turbulent diffusivity and predicts increased λmax and decreased mmax with the horizontal shear of the mean zonal flow. The effects of turbulent mixing are analyzed at different values of the Prandtl number, and the criteria of instability are obtained analytically for some simple cases. Special attention is focused on the influence of the horizontal shear on instability, as with the f-plane models of interleaving. The CTD profiling data obtained from Korean and Russian research vessels are used to prove the model. A closely spaced CTD section across the equator at 176°16′W shows several intrusive layers with high lateral coherence throughout the section, whic...