Mixed layer | EPFL Graph Search

The oceanic or limnological mixed layer is a layer in which active turbulence has homogenized some range of depths. The surface mixed layer is a layer where this turbulence is generated by winds, surface heat fluxes, or processes such as evaporation or sea ice formation which result in an increase in salinity. The atmospheric mixed layer is a zone having nearly constant potential temperature and specific humidity with height. The depth of the atmospheric mixed layer is known as the mixing height. Turbulence typically plays a role in the formation of fluid mixed layers. Oceanic mixed layer Importance of the mixed layer The mixed layer plays an important role in the physical climate. Because the specific heat of ocean water is much larger than that of air, the top 2.5 m of the ocean holds as much heat as the entire atmosphere above it. Thus the heat required to change a mixed layer of 2.5 m by 1 °C would be sufficient to raise the temperature of the atmo

Density currents driven by differential cooling in lakes: occurrence, dynamics and implications for littoral-pelagic exchange

Tomy Doda

Cross-shore flows exchange water laterally in lakes, with ecological implications for the ecosystem. One example is the convective circulation induced by differential cooling, also known as the thermal siphon. This lateral flow forms when the sloping sides of lakes experience surface cooling. Shallow areas cool faster, which generates lateral density gradients and drives a cold downslope gravity current and an onshore surface flow. The role of this two-layer-circulation in lateral exchange in lakes is poorly understood, due to the lack of long-term and high-resolution measurements of thermal siphons. This thesis aims at filling this gap by investigating the occurrence and dynamics of thermal siphons from extensive in situ observations in a wind-sheltered Swiss lake (Rotsee).We first quantified the seasonality of thermal siphons from one-year-long observations in Rotsee. Thermal siphons occurred on a daily basis from late summer to winter and flushed the littoral region in ~ 10 h. Their duration increased but their intensity decreased over the same period. We linked this seasonality to the change in forcing conditions by testing scaling relationships from theoretical and laboratory-based studies.We then focused on the short-term temporal variability of the transport by quantifying the dynamics of thermal siphons over a diurnal cycle and their interaction with convective plumes. Our results reveal that convective plumes penetrated into the gravity current at night and eroded its upper interface. This vertical mixing generated vertical interface fluctuations and reduced the lateral transport at night. The maximal transport was delayed to daylight conditions when radiative heating weakened penetrative convection.After having quantified the physical water transport, we assessed the role of thermal siphons in the lateral exchange of dissolved gases. We found that both branches of the circulation were capable of transporting gases laterally. The downslope current brought littoral gases to the base of the mixed layer in the stratified region whereas the surface flow transported gases towards the shore. We quantified this exchange for oxygen and methane.Finally, we generalized our observations to other lakes with different bathymetry and forcing conditions. The frequent occurrence of thermal siphons observed in six lakes confirmed that this lateral transport process is ubiquitous in lakes with shallow littoral regions. We showed that our results from Rotsee were applicable to other systems, where the same scaling relationships predicted the formation and intensity of thermal siphons. Based on these results, we proposed a procedure to predict the contribution of thermal siphons for lateral transport in any lake.This thesis provides a comprehensive understanding of the formation and dynamics of thermal siphons and paves the route for integrating this lateral transport process in lake ecosystem research.

EPFL2022

Flushing the Lake Littoral Region: The Interaction of Differential Cooling and Mild Winds

Damien Bouffard, Tomy Doda, Cintia Luz Ramon Casanas, Hugo Nicolás Ulloa Sánchez

The interaction of a uniform cooling rate at the lake surface with sloping bathymetry efficiently drives cross-shore water exchanges between the shallow littoral and deep interior regions. The faster cooling rate of the shallows results in the formation of density-driven currents, known as thermal siphons, that flow downslope until they intrude horizontally at the base of the surface mixed layer. Existing parameterizations of the resulting buoyancy-driven cross-shore transport assume calm wind conditions, which are rarely observed in lakes and thereby restrict their applicability. Here, we examine how moderate winds (less than or similar to 5 m s(-1)) affect this convective cross-shore transport. We derive simple analytical solutions that we further test against realistic three-dimensional numerical hydrodynamic simulations of an enclosed stratified basin subject to uniform and steady surface cooling rate and cross-shore winds. We show cross-shore winds modify the convective circulation, stopping or even reversing it in the upwind littoral region and enhancing the cross-shore exchange in the downwind region. The analytical parameterization satisfactorily predicted the magnitude of the simulated offshore unit-width discharges in the upwind and downwind littoral regions. Our scaling expands the previous formulation to a regime where both wind and buoyancy forces drive cross-shore discharges of similar magnitude. This range is defined by the non-dimensional Monin-Obukhov length scale, chi(MO): 0.1 less than or similar to chi(MO) less than or similar to 0.5. The information needed to evaluate the scaling formula can be readily obtained from a traditional set of in situ observations.

AMER GEOPHYSICAL UNION2022

Persistence of bioconvection-induced mixed layers in a stratified lake

Damien Bouffard, Oscar Rodrigo Sepúlveda Steiner, Alfred Johny Wüest

In situ observations of biophysical interactions in natural waters typically focus on physical mechanisms influencing biological activity. Yet, biological activity can also drive physical processes in aquatic environments. A community of photoautotrophic, motile and heavy bacteria—Chromatium okenii, which requires light, sulfide, and anoxic conditions to perform anoxygenic photosynthesis, accumulates below the chemocline of the meromictic Lake Cadagno (Switzerland). Upward vertical migration drives bioconvection, which modifies the physical environment of the bacteria-populated water to create a deep and homogeneous mixed layer of up to 1 m thickness. Continuous convection within the mixed layer and diapycnal diffusivity from its adjacent stratified surroundings determine ecologically relevant gradients. The daytime vertical migration that induce convective instabilities is well-established. It consists in bacteria swimming upward towards light and accumulating at the upper part of the anoxic layer, leading to a locally-unstable density excess. However, nocturnal activity has not yet been analyzed. An intensive 48-h survey was conducted in August 2018 using standard and microstructure profilers, as well as a moored high-resolution current meter coupled with temperature and turbidity sensors deployed across the mixed layer depth. This survey revealed a persistent mixed layer also during nighttime hours. Using a mixed layer shape model, vertical velocity observations and turbulent dissipation estimates, we conclude that photoautotrophic bacteria continue their vertical migration at night. This nocturnal activity thereby drives “dark bioconvection” and maintains the subsurface mixed bacterial layer in Lake Cadagno throughout the diel cycle.

2021

Density currents driven by differential cooling in lakes: occurrence, dynamics and implications for littoral-pelagic exchange

Tomy Doda

EPFL2022