Effect of coastal boundary representation on basin-scale internal waves

Abstract. Lake Iseo is undergoing a dramatic de-oxygenation of the hypolimnion, representing an emblematic example among the deep lakes of the prealpine area that are, to a different extent, suffering from reduced deep water mixing. In the anoxic deep waters, the release and accumulation of reduced substances and phosphorus from the sediments is a major concern. Since the hydrodynamics of this lake was shown to be dominated by internal waves, in this study we investigate for the first time the role of these oscillatory motions on the vertical fluctuations of the oxycline, currently situated at a depth of around 95 m, where a permanent chemocline inhibits deep mixing by convection. Temperature and dissolved oxygen data measured at moored stations show large and periodic oscillations of the oxycline, with amplitude up to 20 m and periods ranging from 1 to 4 days. A deep dynamics characterized by larger amplitudes at lower frequencies is shown to be favoured by the excitation of second vertical modes in strongly thermally stratified periods and of first vertical modes in weakly thermally stratified periods, when the deep chemical gradient can support baroclinicity anyhow. These basin-scale internal waves cause in the water layer between 85 and 105 m depth a fluctuation of the oxygen concentration between 0 and 3 mg L−1 that, due to the bathymetry of the lake, changes the redox condition at the sediment surface. This forcing, involving about 3 % of the lake's sediment area, can have major implications for the biogeochemical processes at the sediment water interface and for the internal matter cycle.

Download Full-text

The Effect of a Bottom Shelf Front upon the Generation and Propagation of Near-Inertial Internal Waves in the Coastal Ocean

Journal of Physical Oceanography ◽

10.1175/jpo2732.1 ◽

2005 ◽

Vol 35 (6) ◽

pp. 976-990 ◽

Cited By ~ 17

Author(s):

Alan M. Davies ◽

Jiuxing Xing

Keyword(s):

Internal Waves ◽

Vertical Wall ◽

Coastal Region ◽

Initial Response ◽

Ekman Pumping ◽

Infinite Domain ◽

Cross Sectional ◽

Positive Vorticity ◽

Coastal Boundary ◽

Inertial Energy

Abstract A three-dimensional nonlinear baroclinic model in cross-sectional form is used to study the generation and propagation of wind-forced near-inertial internal waves in a coastal region in the presence of a bottom front. Initially calculations are performed with the front in an infinite domain region. By this means coastal effects are removed. The initial response is in terms of inertial oscillations in the surface layer. However, in the frontal area these are modified by interaction through the nonlinear momentum terms with regions of positive and negative vorticity associated with the alongfront flow. This leads to a change in amplitude, phase, and frequency of the inertial current, and a resulting Ekman pumping that drives near-inertial internal waves in the frontal region. On the positive vorticity side of the front these waves are at the superinertial frequency and rapidly propagate away. On the negative side they are at the subinertial frequency and are trapped and inertial energy leaks to depth. Calculations with a coastal boundary and no front show that the offshore propagation of near-inertial waves is similar to that found on the positive vorticity side of the front. This shows that in terms of superinertial internal wave generation and propagation the front acts in a similar manner to a coastal boundary. However, with a coastal boundary, inertial currents below the thermocline are phase shifted by 180° from those above. This phase shift is only found in the frontal case when the front is adjacent to a coast and is due to the no-normal-flow condition at the coast. For the case of a stratified region between the coast and the front, near-inertial energy is trapped in this region and is dissipated at a rate depending upon the local value of vertical eddy viscosity. The offshore spatial distribution of near-inertial internal waves is found to be independent of whether the coastal region is represented by a vertical wall or a more realistic sloping seabed. However, in the case of a weak coastal front, the near-frontal distribution of near-inertial energy is influenced by the horizontal spatial variability of stratification within the front.

Download Full-text

The structure of basin-scale internal waves in a stratified lake in response to lake bathymetry and wind spatial and temporal distribution: Lake Iseo, Italy

Limnology and Oceanography ◽

10.4319/lo.2012.57.3.0772 ◽

2012 ◽

Vol 57 (3) ◽

pp. 772-786 ◽

Cited By ~ 22

Author(s):

Giulia Valerio ◽

Marco Pilotti ◽

Clelia Luisa Marti ◽

Jö rg Imberger

Keyword(s):

Internal Waves ◽

Temporal Distribution ◽

Spatial And Temporal Distribution ◽

Stratified Lake ◽

Basin Scale

Download Full-text

Energy Flux Paths in Lakes and Reservoirs

Water ◽

10.3390/w13223270 ◽

2021 ◽

Vol 13 (22) ◽

pp. 3270

Author(s):

Sofya Guseva ◽

Peter Casper ◽

Torsten Sachs ◽

Uwe Spank ◽

Andreas Lorke

Keyword(s):

Wind Energy ◽

Turbulent Flows ◽

Internal Waves ◽

Energy Flux ◽

Energy Content ◽

Mechanical Energy ◽

Energy Fluxes ◽

Ecological Processes ◽

Basin Scale ◽

Lakes And Reservoirs

Mechanical energy in lakes is present in various types of water motion, including turbulent flows, surface and internal waves. The major source of kinetic energy is wind forcing at the water surface. Although a small portion of the vertical wind energy flux in the atmosphere is transferred to water, it is crucial for physical, biogeochemical and ecological processes in lentic ecosystems. To examine energy fluxes and energy content in surface and internal waves, we analyze extensive datasets of air- and water-side measurements collected at two small water bodies (<10 km2). For the first time we use directly measured atmospheric momentum fluxes. The estimated energy fluxes and content agree well with results reported for larger lakes, suggesting that the energetics governing water motions in enclosed basins is similar, independent of basin size. The largest fraction of wind energy flux is transferred to surface waves and increases strongly nonlinearly for wind speeds exceeding 3 m s−1. The energy content is largest in basin-scale and high-frequency internal waves but shows seasonal variability and varies among aquatic systems. At one of the study sites, energy dissipation rates varied diurnally, suggesting biogenic turbulence, which appears to be a widespread phenomenon in lakes and reservoirs.

Download Full-text

Basin-scale internal waves in the bottom boundary layer of ice-covered Lake Müggelsee, Germany

Aquatic Ecology ◽

10.1007/s10452-009-9274-3 ◽

2009 ◽

Vol 43 (3) ◽

pp. 641-651 ◽

Cited By ~ 18

Author(s):

Georgiy Kirillin ◽

Christof Engelhardt ◽

Sergey Golosov ◽

Thomas Hintze

Keyword(s):

Boundary Layer ◽

Internal Waves ◽

Bottom Boundary Layer ◽

Bottom Boundary ◽

Basin Scale

Download Full-text

Oxycline oscillations induced by internal waves in deep Lake Iseo

Hydrology and Earth System Sciences ◽

10.5194/hess-23-1763-2019 ◽

2019 ◽

Vol 23 (3) ◽

pp. 1763-1777 ◽

Cited By ~ 2

Author(s):

Giulia Valerio ◽

Marco Pilotti ◽

Maximilian Peter Lau ◽

Michael Hupfer

Keyword(s):

Internal Waves ◽

Water Layer ◽

Sediment Surface ◽

Chemical Gradient ◽

Deep Lakes ◽

Deep Lake ◽

Deep Waters ◽

Basin Scale ◽

First Time

Abstract. Lake Iseo is undergoing a dramatic deoxygenation of the hypolimnion, representing an emblematic example among the deep lakes of the pre-alpine area that are, to a different extent, undergoing reduced deep-water mixing. In the anoxic deep waters, the release and accumulation of reduced substances and phosphorus from the sediments are a major concern. Because the hydrodynamics of this lake was shown to be dominated by internal waves, in this study we investigated, for the first time, the role of these oscillatory motions on the vertical fluctuations of the oxycline, currently situated at a depth of approximately 95 m, where a permanent chemocline inhibits deep mixing via convection. Temperature and dissolved oxygen data measured at moored stations show large and periodic oscillations of the oxycline, with an amplitude of up to 20 m and periods ranging from 1 to 4 days. Deep motions characterized by larger amplitudes at lower frequencies are favored by the excitation of second vertical modes in strongly thermally stratified periods and of first vertical modes in weakly thermally stratified periods, when the deep chemical gradient can support baroclinicity regardless. These basin-scale internal waves cause a fluctuation in the oxygen concentration between 0 and 3 mg L−1 in the water layer between 85 and 105 m in depth, changing the redox condition at the sediment surface. This forcing, involving approximately 3 % of the lake's sediment area, can have major implications for the biogeochemical processes at the sediment–water interface and for the internal matter cycle.

Download Full-text