Temperature statistics above a deep-ocean sloping boundary

We present a detailed analysis of the temperature statistics in an oceanographic observational dataset. The data are collected using a moored array of thermistors, 100 m tall and starting 5 m above the bottom, deployed during four months above the slopes of a Seamount in the north-eastern Atlantic Ocean. Turbulence at this location is strongly affected by the semidiurnal tidal wave. Mean stratification is stable in the entire dataset. We compute structure functions, of order up to 10, of the distributions of temperature increments. Strong intermittency is observed, in particular, during the downslope phase of the tide, and farther from the solid bottom. In the lower half of the mooring during the upslope phase, the temperature statistics are consistent with those of a passive scalar. In the upper half of the mooring, the temperature statistics deviate from those of a passive scalar, and evidence of turbulent convective activity is found. The downslope phase is generally thought to be more shear-dominated, but our results suggest on the other hand that convective activity is present. High-order moments also show that the turbulence scaling behaviour breaks at a well-defined scale (of the order of the buoyancy length scale), which is however dependent on the flow state (tidal phase, height above the bottom). At larger scales, wave motions are dominant. We suggest that our results could provide an important reference for laboratory and numerical studies of mixing in geophysical flows.Comment: 22 pages, 10 figures, 3 tables. Accepted versio

Can we link theory to observations in natural flows?

The growth in size and quality of observations of Geophysical Flows enables more detailed comparison between theoretical predictions and the real world. In this presentation, I give two examples where a "statistical look" on a classical problem can offer new insights on the underlying dynamics. In both cases, success is enabled by the collection of large, high-quality datasets in the field

Testing turbulence theories in the ocean: insights from state-of-the-art observations

During the last decade, environmental flows have been measured with a growing set of instruments, which can now offer relatively high resolution in space and time. This growing amount of data can analysed following a "statistical approach to turbulence", rather well established in terms of theory and laboratory results. I will discuss three studies, two on the near bottom boundarly layer turbulence of the deep ocean, and one on the internal wave variability in Lake Geneva. I will try to show that this approach is particularly useful in the field, where the parameters of the flow cannot be controlled. A detailed description of the statistics of a natural flow can enable precise connections with established theories, and can even suggest new theoretical developments

Dansgaard-Oeschger events: tipping points in the climate system

Dansgaard-Oeschger events are a prominent mode of variability in the records of the last glacial cycle. Various prototype models have been proposed to explain these rapid climate fluctuations, and no agreement has emerged on which may be the more correct for describing the paleoclimatic signal. In this work, we assess the bimodality of the system reconstructing the topology of the multi--dimensional attractor over which the climate system evolves. We use high-resolution ice core isotope data to investigate the statistical properties of the climate fluctuations in the period before the onset of the abrupt change. We show that Dansgaard-Oeschger events have weak early warning signals if the ensemble of events is considered. We find that the statistics are consistent with the switches between two different climate equilibrium states in response to a changing external forcing (e.g. solar, ice sheets...), either forcing directly the transition or pacing it through stochastic resonance. These findings are most consistent with a model that associates Dansgaard-Oeschger with changing boundary conditions, and with the presence of a bifurcation point.Comment: Final typeset version freely available at: Clim. Past, 9, 323-333, 2013 www.clim-past.net/9/323/2013/ doi:10.5194/cp-9-323-201

Lake Geneva: A natural laboratory for transport processes

Dynamics in Lake Geneva are analysed in view of recent observational and numerical advances. The classical linear internal wave view of Lake Dynamics is complemented by the description of near-shore, vortex-like nonlinear motions. The development of these structures is linked to bottom friction. Preliminary results from Lagrangian particle tracking simulations suggest that their existence is important for the transport of scalars within the lake

Transport and mixing in Lac Léman

Lake Geneva (Lac Léman) is the largest freshwater body in Western Europe. It is a deep peri-alpine lake whose importance stems from being not only an essential freshwater source in the region, but also a major tourist destination, a fishery and a waterway. Its dynamics have been the subject of long-term monitoring and study, and its response patterns to wind forcing (the major forcing) are relatively well understood (e.g. Lemmin et al. 2005). On the other hand, the large-scale organisation of the water circulation is less well known; the associated transport properties are even less clear. The interest in the transport of water parcels inside the lake is linked to the inflow of sediments and pollutants from the tributaries, in particular from the Rhône River, the major one in terms of water and sediment discharged into the lake. Most of the sediments entering the lake through the Rhône are believed to sink in the eastern part of the lake (Giovanoli, 1990). More recently, Halder et al. (2013) traced, using stable isotopes, water parcels from the Rhône River in the entire lake basin. This study demonstrates that the water entering the lake from its main tributary has a complex distribution inside the whole basin. How this distribution is established and evolves in time is, however, mostly unknown. The Ecological Engineering Laboratory of EPFL is trying to shed further light on this issue, by combining observational and numerical modelling tools. In particular, up to 6 Acoustic Doppler Current Profilers (ADCP’s) have been simultaneously deployed at various locations inside the lake. This data, together with available historical data, is being used to validate a hydrodynamic model of the lake, implemented using MITgcm code. Various passive-tracer release experiments were conducted using the numerical model, investigating the relative importance of wind-forcing, depth of release, stratification, and the Rhône discharge rate, for the spreading and mixing of the tracers. The preliminary numerical results confirm that the northern and southern coastal regions are preferred initial pathways for the transport of the Rhône discharge. More interestingly, the numerical simulations unmistakeably show that the transport of the Rhône River water inside the lake is highly inhomogeneous in space, and highly intermittent in time, even ignoring the discharge variability itself. This intermittency should be taken into account, in particular when interpreting point measurements, isolated in time. From a practical point of view, this is likely to have an important effect on the nutrient and oxygen availability, as well as on the concentration of pollutants. From a more fundamental point of view, this study contributes to further understanding the mixing processes in rotating, stratified flows at length scales where rotation is an important but not the only dynamic process (Rossby number small but non-zero)

Nonlinear dynamics of the near-shore boundary layer of a large lake (Lake Geneva)

We examine near-shore and pelagic current variability in Lake Geneva, a large and deep lake in western Europe, using observations from several measurement locations and a three-dimensional numerical model for the period 2014-2016. Linear internal seiche modes excited by wind forcing clearly appear as peaks in the energy spectra for measurements in off-shore locations. In contrast, spectra from the near-shore data, where currents interact withthe lake bed, reveal a negligible contribution of internal seiches to the total kinetic energy. A similar contrast is seen in the spectra obtained from the numerical model at the same locations. Comparing the contribution of the different terms in the vertically-averaged momentum equation from the modeling results shows that the nonlinear advective term dominates in the near-shore boundary layer. Its contribution decays with distance from shore. The width of this near-shore boundary layer, which may extend for several kilometers, seems to be mainly determined by local topography. Both field measurements and modeling results indicate that nonlinear dynamics are of primary importance in the near-shore boundary layer