Natalia Kuzmina

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...

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.

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...

Driving Forces of Interleaving in the Baroclinic Front at the Equator

Abstract The different types of instability in the equatorial β-plane approximation are analyzed by means of a 2D linear stability problem. The double-diffusive (DD) and diffusive/baroclinic (2D baroclinic and McIntyre) instabilities are shown not to develop if contours of the mean salinity/density have a parabolic, symmetrical-relative-to-the-equator shape. Using modeling results, an illustrative scheme of Equatorial Undercurrent (EUC) regions where different types of instability can develop is presented and subsequently applied to understand the driving forces of the intrusions observed in a closed spaced CTD section, located between the equator and 1°N. Long coherence intrusions are situated within two isopycnal layers, aligned to 25 (layer 1) and 26.3 (layer 2) σT, where the vertical shear is low. It was shown from the model that the layer-1 intrusions being observed in the midlayer of the EUC where the mean horizontal gradient of salinity is approximately constant are likely generated by a combined e...