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Register a new userChallenger Deep internal wave turbulence events
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Authors
Hans van Haren
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Marine life has been detected in the oceans trenches at great depths down to nearly 11 km. Such life is subject to particular environmental conditions of large static pressure exceeding 1000 atmosphere. While current flows are expected to be slow, waters cannot be stagnant with limited exchange of fresh nutrients needed to support life. For sufficient nutrient supply, the physics process of turbulent exchange is required. However, the environmental conditions hamper research in such waters. To study potential turbulent water motions, a string equipped with specially designed high-resolution temperature sensors was moored near the deepest point on Earth in the Challenger Deep, Mariana Trench for nearly three years. A preliminary analysis of a six-day period when the mooring was still demonstrates hundreds of meters slanted convection due to internal waves breaking from above. The associated turbulence dissipation rate with peak values hundred times above the background value is considered sufficient to maintain deep-trench life. Turbulence associates with one-ten thousandth of a degree temperature anomalies of about one hour duration.
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Knowledge about the characteristics of the atmospheric boundary layer are vital for the redistribution of air and suspended contents that are particularly driven by turbulent motions. Despite many modelling studies, detailed observations are still demanded of the development of turbulent exchange under stable and unstable conditions. In this paper we present an attempt to observationally detail atmospheric internal waves, under stable conditions, and associated turbulent overturning, under quasi-stable and unstable conditions. Therefore, we mounted 198 high-resolution temperature T-sensors on a cable. The instrumented cable was attached along the 213 m tall mast of Cabauw, the Netherlands, during late-summer 2017. The mast has standard and special meteorological equipment at extendable booms every 20 m in height. A sonic turbulence anemometer is at 60 m above ground. The extra, originally underwater-, T-sensor cable was suspended down from the 206-m level, temporarily for about 3 months. While in water the sensors have a response time of tw=0.4 s and drift of 0.001 degC per month, in air the response time ta=3 s is relatively slow and the apparent drift of about 0.1 degC per month relatively large. Least performance is during daytime. These T-sensor characteristics hamper quantitative atmospheric turbulence research, as it results in a relatively narrow inertial subrange of only one order of magnitude. Nevertheless, height-time images from two contrasting days show common nocturnal marginally stable density stratification supporting internal waves up to the buoyancy period of about 300 s, shear and convective deformation of the stratification over the entire 197 m range of observations.
A parabolic equation for the propagation of periodic internal waves over varying bottom topography is derived using the multiple-scale perturbation method. Some computational aspects of the numerical implementation are discussed. The results of numerical experiments on propagation of an incident plane wave over a circular-type shoal are presented in comparison with the analytical result, based on Born approximation.
The impact of large atmospheric disturbances on deep benthic communities is not well known quantitatively. Observations are scarce but may reveal specific processes leading to turbulent disturbances. Here, we present high-resolution deep-ocean observations to study potential turbulent mixing by large atmospheric disturbances. We deployed an array of 100-Hz sampling-rate geophysical broadband Ocean Bottom Seismometers (OBSs) on the seafloor. Within the footprint of this array we also deployed an oceanographic 1-Hz sampling-rate vertical temperature sensor string covering the water phase between 7 and 207 m above the seafloor at about 3000 m depth off eastern Taiwan between June 2017 and April 2018. All instruments registered Category 4 cyclone Typhoon Talims passage northeast of the array one day ahead of the cyclones closest approach when the cyclones eye was more than 1,000 km away. For 10 days, a group of near-inertial motions appeared most clearly in temperature. The registration reflects the importance of barotropic response to cyclones and the propagation of inertio-gravity waves in weak density stratification. In addition to internal tides, these waves drove turbulent overturns larger than 200 m that were concurrently registered by OBSs. The turbulent signals were neither due to seismic activity nor to ocean-surface wave action. Cyclones can generate not only microseisms and earth hums, as well as turbulence in the water column, producing additional ground motions. Quantified turbulence processes may help constrain models on sediment resuspension and its effect on deep-sea benthic life.
By performing two parallel numerical experiments -- solving the dynamical Hamiltonian equations and solving the Hasselmann kinetic equation -- we examined the applicability of the theory of weak turbulence to the description of the time evolution of an ensemble of free surface waves (a swell) on deep water. We observed qualitative coincidence of the results. To achieve quantitative coincidence, we augmented the kinetic equation by an empirical dissipation term modelling the strongly nonlinear process of white-capping. Fitting the two experiments, we determined the dissipation function due to wave breaking and found that it depends very sharply on the parameter of nonlinearity (the surface steepness). The onset of white-capping can be compared to a second-order phase transition. This result corroborates with experimental observations by Banner, Babanin, Young.
Deep water circulation and mixing processes in deep lakes are largely unknown, although they are responsible for the transport of matter, nutrients and pollutants. Such a lack of knowledge cannot be reliably provided by numerical hydrodynamic modelling studies because detailed observations are typically not available to validate them. To overcome some of these deficiencies, a dedicated yearlong mooring comprising 100 high-resolution temperature sensors and a single current meter were located in the deeper half of the 344 m deepest point of the subalpine Lake Garda (Italy). The observations show peaks and calms of turbulent exchange, besides ubiquitous internal wave activity. In late winter, northerly winds activate episodic deep convective overturning, the dense water being subsequently advected along the lake-floor. Besides deep convection, such winds also set-up seiches and inertial waves that are associated with about 100 times larger turbulence dissipation rates than that by semidiurnal internal wave breaking observed in summer. In the lower 60 m above the lake-floor however, the average turbulence dissipation rate is approximately constant in value year-around, being about 10 times larger than open-ocean values, except during deep convection episodes.
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