On the structure and dynamical features of intrusive layering in the Eurasian Basin in the Arctic Ocean

Abstract

Numerous conductivity‐temperature‐depth data obtained in the Arctic basins are analyzed to describe structural features of intrusive layering. Special attention is paid to large intrusions (vertical scale of 40–100 m) observed in upper, intermediate and deep layers (depth ranges of 150–200, 200–600 and 600–1000 m, respectively). The analysis of the intrusions is accompanied by descriptions of the frontal zones where the layering was observed. Based on observations detailed estimates of frontal zone parameters are presented. Vertical profiles of temperature and salinity are found to have a well‐defined “sawtooth” or “cog” shape, displaying a sequence of relatively thick, weak gradient layers, where temperature and salinity are decreasing with depth, interleaved with relatively thin, high‐gradient sublayers, where temperature and salinity are increasing with depth. Some hypotheses about causes responsible for cog structure existence are discussed. Intrusions with high‐amplitude anomalies in the vertical profiles of temperature and salinity are shown to be present at baroclinic fronts. Based on models of interleaving and data analysis the apparent vertical and lateral diffusivity in the frontal zones of the upper and deep ocean layers are estimated, and the slope of unstable modes relative to the isopycnals is examined at the baroclinic front in the situation when both temperature and salinity are stably stratified.

Generation of large-scale intrusions at baroclinic fronts: an analytical consideration with a reference to the Arctic Ocean

“…From the point of view of the author of this paper, a simple quasi-stationary (geostrophic) system of equations accurately describes the large-scale movement especially in the Arctic Ocean, where the influence of the β effect is not significant, the baroclinic fronts of large width in the ocean interior are often not intense (Kuzmina et al, 2011), and the N. Kuzmina: Generation of large-scale intrusions at baroclinic fronts baroclinic radius of deformation, H N/f , at H ∼ 100 m does not exceed 2-5 km (see also Sect. 3).…”

“…According to Kuzmina et al (2011), in the upper layer of the PDW where the large-scale intrusions are observed in the Eurasian Basin at stable-stable stratification, the following estimates of N, f , and β are typical: N ≈ 2 × 10 −3 s −1 , f = 1.4×10 −4 s −1 , and β < 0.3×10 −11 m −1 s −1 (at latitude of 83 • N and higher). Therefore, for disturbances, for example, with the vertical scale of H = 100 m, the Rossby radius of deformation is only H N/f ≈1 km.…”

“…(7), the value of the linear shear s is limited by the inequality of |s| f L N 2 0 . Given that the horizontal scale of the baroclinic fronts (along the cross-front axis y) in the upper layer of the PDW is approximately L ≈ 50-100 km (see examples of transection across the fronts of different types observed in the PDW; Kuzmina et al, 2011), the maximal linear shear can be estimated as |s| ≈ (1 − 2) × 10 −7 m −1 s −1 . Such value of the linear shear is large enough to neglect the β-effect term relative to the linear shear term in Eq.…”

“…One of the major mechanisms responsible for the instability of both thermohaline and baroclinic fronts is related to the double diffusion (Stern, 1967;Ruddick and Turner, 1979;Toole and Georgi, 1981;McDougall, 1985a, b;Niino, 1986;Yoshida et al, 1989;Richards, 1991;Kuzmina and Rodionov, 1992;May and Kelley, 1997;Kuzmina, 2000). However, in the Eurasian Basin of the Arctic Ocean there are baroclinic and thermohaline fronts within the upper layer of the Polar Deep Water (PDW) populated with intrusive layers of vertical length scale as large as 30 m and with horizontal scale reaching up to more than 100 km (Rudels et al, 1999(Rudels et al, , 2009Kuzmina et al, 2011) observed at the stable-stable stratification (i.e., increasing for the mean salinity while decreasing for the mean temperature with depth). It can be suggested that the thermohaline intrusions within the upper layer of PDW are driven by differential mixing.…”

“…The study of intrusions in oceanic frontal zones is required to understand the mechanism of ventilation and mixing in the ocean interior (see, e.g., Zhurbas et al, 1993Zhurbas et al, , 1987Rudels et al, 1999Rudels et al, , 2009Kuzmina and Zhurbas, 2000;Walsh and Ruddick, 2000;Merryfield, 2000;Radko, 2003;Richards and Edwards, 2003;Kuzmina et al, , 2011Smyth and Ruddick, 2010). Intrusive layering, as a rule, results from the instability of oceanic fronts.…”