The role of double diffusion for the heat and salt balance in Lake Kivu

Abstract

Double diffusion in lakes and oceans can transform vertical gradients into staircases of convectively mixed layers separated by thin stable interfaces. Lake Kivu is an outstanding double‐diffusive natural laboratory with > 300 such steps over the permanently stratified deep basin. Here, we use 315 microstructure profiles (225 measured in Rwanda and 90 in the DRC) to shed light on the heat and salt balances of Lake Kivu. Comparing profiles from 2011 and 2015 reveals warming of 8.6 mK yr−1 below 80 m depth and negligible changes in salinity. The double‐diffusive layering is coherent over horizontal distances of 20–30 km and remained unchanged between 2011 and 2015, indicating little variability. The mean estimated dissipation within mixed layers is 1.5 × 10−10 W kg−1. If unshaped Batchelor microstructure spectra are interpreted as nonturbulent, the rescaled dissipation of 0.44 × 10−10 W kg−1 corresponds to a vertical heat flux of 0.10 W m−2, which agrees with the molecular heat flux through the adjacent stable interfaces. Using estimates of upwelling, temporal changes of temperature and salt, and vertical double‐diffusive fluxes, we established heat and salt balances, which require lateral heat and salt inputs. For salt, lateral input of freshwater at the main gradients balances upwelling. For temperature, however, the divergence of the vertical double‐diffusive fluxes can only be balanced by horizontal inputs supplying cool water above and warm water below the main gradients. This suggests that lateral inputs of water at various depths are the main drivers for this unique double‐diffusive phenomenon in Lake Kivu.

The entire lifetime of a distinct double-diffusive staircase in crater Lake Nyos, Cameroon

“…Powell Lake in British Columbia is the most prominent representative of this category [24,36]. With three and four density-relevant water constituents, there are examples of lakes showing triple-diffusion [23], such as the present study, and quadruple-diffusion, such as Lake Kivu [33]. More DD-type lake examples are reviewed in Wüest et al [42].…”

“…Double-diffusive stratification is generally rare in lakes, because usually T decreases and S increases with depth leading to classical diffusively stable water columns. However, prominent exceptions of DD-developing stratification were found above former salt depositions, such as in Lake Vanda [9,10] and several other lakes in the Antarctic Dry Valley [34,35], or over volcanic underground, such as in Lake Kivu [18,33]. Another set of DD-type lakes can be found on the Norwegian and Canadian West Coast, where Fig.…”

“…The mechanical energy for this process stems from extraction of potential energy from the stratification. Such staircases can develop when two agents that diffuse at different molecular rates contribute in opposing ways to the stability of the vertical density profile [14,33,38,39]. Given that temperature was destabilizing, whereas S and CO 2 were stabilizing the density profile (Fig.…”

Convection‐Diffusion Competition Within Mixed Layers of Stratified Natural Waters

“…Applying equation yields Pe = 3.4 × 10 3 , and thus, = 3.1 × 10 −10 W kg −1 . Recently, Sommer et al () reported an average dissipation within the MLs of Lake Kivu of 1.5 × 10 −10 W kg −1 . The same study considers a mixing efficiency of 0.8, and thus, a J b = 1.9 × 10 −10 W kg −1 , which is in good agreement with .…”

A multi-site, year-round turbulence microstructure atlas for the deep perialpine Lake Garda

“…Indeed, analogous loosely tethered profilers mounting fast-response thermistors are typically operated at profiling speeds between 0.1 m s −1 and 0.4 m s −1 (refs. 12 , 21 , 23 , 67 , 68 ). However, we found that the a profiling speed of ~0.75 m s −1 was a reasonable compromise between optimizing shear and fast-response thermistors performance: the ability of shear probes to be used in medium- to high-energy environments (i.e., ε > 5 × 10 −10 m 2 s −3 ) compensated the time response limitations of the FP07 sensors.…”