Observational validation of the diffusive convection flux laws in the Amundsen Basin, Arctic Ocean

Abstract

The low levels of mechanically driven mixing in many regions of the Arctic Ocean make double diffusive convection virtually the only mechanism for moving heat up from the core of Atlantic Water towards the surface. In an attempt to quantify double diffusive heat fluxes in the Arctic Ocean, a temperature microstructure experiment was performed as a part of the North Pole Environmental Observatory (NPEO) 2013 field season from the drifting ice station Barneo located in the Amundsen Basin near the Lomonosov Ridge (89.5°N, 75°W). A diffusive convective thermohaline staircase was present between 150 and 250 m in nearly all of the profiles. Typical vertical heat fluxes across the high‐gradient interfaces were consistently small, O(10−1) W m−2. Our experiment was designed to resolve the staircase and differed from earlier Arctic studies that utilized inadequate instrumentation or sampling. Our measured fluxes from temperature microstructure agree well with the laboratory derived flux laws compared to previous studies, which could find agreement only to within a factor of two to four. Correlations between measured and parameterized heat fluxes are slightly higher when using the more recent Flanagan et al. [2013] laboratory derivation than the more commonly used derivation presented in Kelley [1990]. Nusselt versus Rayleigh number scaling reveals the convective exponent, η, to be closer to 0.29 as predicted by recent numerical simulations of single‐component convection rather than the canonical 1/3 assumed for double diffusion. However, the exponent appears to be sensitive to how convective layer height is defined.

Energy and Variance Budgets of a Diffusive Staircase with Implications for Heat Flux Scaling

“…This is likely to occur also for double diffusion at high enough Ra and may well be the case in some of the thicker diffusive layers in the Arctic Ocean. In fact, Guthrie et al (2015) found a decent agreement between observed fluxes and fluxes calculated using the 1 /3 exponent, although the agreement was even better with a 0.29 exponent. More research is needed before we can propose an accurate flux law for diffusive convection that applies to all ocean and lake environments.…”

“…Both these studies support a downward revision of the commonly used Rayleigh number exponent of 1 /3. However, the two studies support rather different revised exponents, 0.2 in the case of Sommer et al (2013) and 0.29 in the case of Guthrie et al (2015), 1 and there is a large scatter of points in both studies. The Rayleigh numbers are generally larger in the Amundsen Basin than in Lake Kivu with a mean value of 10 9 compared to 2.4 3 10 8 for the latter location, but there is a large spread within each staircase and Rayleigh numbers vary between about 10 5 and 10 10 at both locations.…”

“…This poses a difficult challenge for researchers trying to derive accurate flux laws. In this paper, we have focused on a range of Rayleigh numbers that commonly occur both in lakes (Sommer et al 2013;Scheifele et al 2014) and in the Amundsen Basin in the Arctic Ocean (Guthrie et al 2015), as can be seen in our Fig. 5.…”

“…Ra exponent Ra range Method Turner (1965) 1/3 N/A Theory Kelley (1990) 0:284*(0:27*) 10 4 -10 10 (10 7 -10 10 ) Laboratory experiments (theory) Flanagan et al (2013) 0.28 (0.25) 3 3 10 7 -7.6 3 10 9 (1.9 3 10 6 -7.6 3 10 9 ) DNS Sommer et al (2013) 1/5 10 5 -10 10 Measurements Guthrie et al (2015) 0:29**(0:25**) 10 5 -10 11 Measurements The current study 0.27 (0.09) 1.6 3 10 7 -8.2 3 10 7 (1.5 3 10 6 -1.6 3 10 7 ) DNS GL theory 1/3***, 1/4, 1/5**** N/A Theory TABLE 2. Heat flux scaling laws for the different regimes in the GL theory.…”

“…Sommer et al (2013), who took measurements in Lake Kivu on the border between the Democratic Republic of the Congo and Rwanda, and Guthrie et al (2015), who took their measurements in the Amundsen Basin in the Arctic Ocean, are two examples. Both these studies support a downward revision of the commonly used Rayleigh number exponent of 1 /3.…”