Spatial variability of the Arctic Ocean's double-diffusive staircase

Abstract

The Arctic Ocean thermohaline stratification frequently exhibits a staircase structure overlying the Atlantic Water Layer that can be attributed to the diffusive form of double‐diffusive convection. The staircase consists of multiple layers of O(1) m in thickness separated by sharp interfaces, across which temperature and salinity change abruptly. Through a detailed analysis of Ice‐Tethered Profiler measurements from 2004 to 2013, the double‐diffusive staircase structure is characterized across the entire Arctic Ocean. We demonstrate how the large‐scale Arctic Ocean circulation influences the small‐scale staircase properties. These staircase properties (layer thicknesses and temperature and salinity jumps across interfaces) are examined in relation to a bulk vertical density ratio spanning the staircase stratification. We show that the Lomonosov Ridge serves as an approximate boundary between regions of low density ratio (approximately 3–4) on the Eurasian side and higher density ratio (approximately 6–7) on the Canadian side. We find that the Eurasian Basin staircase is characterized by fewer, thinner layers than that in the Canadian Basin, although the margins of all basins are characterized by relatively thin layers and the absence of a well‐defined staircase. A double‐diffusive 4/3 flux law parametrization is used to estimate vertical heat fluxes in the Canadian Basin to be O(0.1) W m−2. It is shown that the 4/3 flux law may not be an appropriate representation of heat fluxes through the Eurasian Basin staircase. Here molecular heat fluxes are estimated to be between O(0.01) and O(0.1) W m−2. However, many uncertainties remain about the exact nature of these fluxes.

Variability of Internal Wave‐Driven Mixing and Stratification in Canadian Arctic Shelf and Shelf‐Slope Waters

“…The distribution of the AW thermocline-averaged internal wave-driven turbulent heat flux ranges from ∼0.1 to ∼90 W/m 2 , with a geometric mean of ∼1 W/m 2 (Figure 13a). This peak value is 1 to 2 orders of magnitude larger than typical Arctic double-diffusive heat fluxes (Shibley et al, 2017) and is comparable to many other observed values of Arctic Ocean turbulent heat fluxes (see Table 1 and references therein). While the average value of F H appears typical, it is also important to consider the implications of anomalously high values evident in the skew symmetric shape of its distribution.…”

“…While the distribution of F H only depicts heat fluxes in turbulent regimes, it is also relevant and worthwhile to consider a hypothetical distribution of heat fluxes that also includes the nonturbulent regimes, and is thus representative of the study domain as a whole. For this purpose, we assume that F H is 0.1 W/m 2 everywhere Re b < 20, consistent with the order of magnitude of double-diffusive heat fluxes that Shibley et al (2017) observed in the Canada Basin. When the contributions from these nonturbulent environments are taken into account, we find that despite the fact that only ∼60% of our domain can be characterized as turbulent according to the Re b > 20 criterion, turbulent heat fluxes in these regions account for 96% of the net integrated heat flux over the whole study region.…”

“…It has thus predominantly been viewed as a quiescent, weakly mixed ocean. This description of the Arctic Ocean is reinforced by observations of low internal wave activity (e.g., Guthrie et al, ; Halle & Pinkel, ; Levine et al, ) and pervasive double‐diffusive staircases that dominate much of the thermocline (e.g., Shibley et al, ; Timmermans et al, ), the existence of which implies weak shear instabilities and turbulence. However, the scarcity of existing measurements in this remote, harsh environment poses significant challenges in our ability to accurately quantify typical Arctic Ocean mixing rates and understand their variability in space and time.…”

Tidal Conversion and Mixing Poleward of the Critical Latitude (an Arctic Case Study)

“…Recent observations have shown continental shelf break regions of the Arctic Ocean host significant barotropic tidal energy conversion (Rippeth et al, 2015), with evidence of greatly enhanced , and by implication mixing, over sloping topography in the presence of significant tides (e.g., D'Asaro & Morison, 1992;Fer et al, 2010;Padman & Dillon, 1991;Rainville & Winsor, 2008;Rippeth et al, 2015). This is in sharp contrast to much of the central Arctic Ocean that is found to be remarkably quiescent (e.g., Lincoln et al, 2016;Shibley et al, 2017). However, much of the Arctic Ocean lies poleward of the critical latitude and so the conversion of barotropic tidal energy to a freely propagating linear internal tide, over sloping topography, is suppressed by rotation.…”

Double‐Diffusive Layering in the Canada Basin: An Explanation of Along‐Layer Temperature and Salinity Gradients

“…Estimates of heat and salt fluxes can be made by considering the steady state energy balance for a layer (e.g., Hieronymus & Carpenter, ): which relates the dissipation of turbulent kinetic energy ϵ within a layer and the net buoyancy flux for large Rayleigh numbers (Rayleigh numbers are for the Canada Basin layers; Shibley et al, ). Note that assumes that there is no large‐scale background shear which is a reasonable assumption given that the lateral density gradient in the Canada Basin at these depths is effectively 0.…”