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| internal waves in the atmosphere |
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| internal waves forming slicks on the ocean's surface |
Compared to surface waves, the properties and effects of internal waves in the ocean are still relatively unknown. These underwater waves have numerous causes and can affect the environment in many different ways. Recent research on the effects of internal waves in the Arctic Ocean sheds light on the profound ways that these swells can affect ocean mixing, circulation, and the planet as a whole.
The Arctic Ocean is the smallest and most shallow of the world’s oceans. It is unique in that its water is practically homogenous. This means its salty, cold, dense deep water qualities are relatively similar to those at the surface. This is due to several factors. The shallow Arctic ocean sits mainly in a basin, with only one main channel circulating water in and out of the ocean, called the Fram Strait, located near Greenland (Arctic Ocean Encyclopædia Britannica Online, 2010). This lack of major circulation potentiates a stable water column, which makes for minimal mixing of the water stratification. Tides are small, resulting in negligible tidal currents. The ocean is also covered with ice for the majority of the year. This minimizes evaporation by acting as an insulator, maintaining a cold and stable water surface temperature, and reflecting any incoming sunlight. The ice cover also prevents the wind from mixing the water (Rainville & Woodward, 2009).
Along with the Kara, Laptev, East Siberian and Barents seas, the Chuchki is one of the five seas that comprise the Arctic Ocean (Arctic Ocean Encyclopædia Britannica Online, 2010). The Chuchki is north of the Bering Strait, with Alaska on its southeastern edge and Siberia on the southwest. At its north end is the Arctic continental self.
The Arctic Ocean is the smallest and most shallow of the world’s oceans. It is unique in that its water is practically homogenous. This means its salty, cold, dense deep water qualities are relatively similar to those at the surface. This is due to several factors. The shallow Arctic ocean sits mainly in a basin, with only one main channel circulating water in and out of the ocean, called the Fram Strait, located near Greenland (Arctic Ocean Encyclopædia Britannica Online, 2010). This lack of major circulation potentiates a stable water column, which makes for minimal mixing of the water stratification. Tides are small, resulting in negligible tidal currents. The ocean is also covered with ice for the majority of the year. This minimizes evaporation by acting as an insulator, maintaining a cold and stable water surface temperature, and reflecting any incoming sunlight. The ice cover also prevents the wind from mixing the water (Rainville & Woodward, 2009).
Along with the Kara, Laptev, East Siberian and Barents seas, the Chuchki is one of the five seas that comprise the Arctic Ocean (Arctic Ocean Encyclopædia Britannica Online, 2010). The Chuchki is north of the Bering Strait, with Alaska on its southeastern edge and Siberia on the southwest. At its north end is the Arctic continental self.
Its average depth is only 77 metres, or 250 feet (Arctic Ocean Encyclopædia Britannica Online, 2010). Because the Arctic Ocean has little circulation, is usually covered with ice, and has a relatively homogeneous water density with little mixing, the water is usually quite calm with little energy output or wave action (Rainville & Woodgate, 2009). However, compared to deeper parts of the ocean, the shallow depth of the Chuchki Sea causes potential internal waves to have more significant effects when it comes to mixing and affecting water circulation (ScienceDaily, March 2010)
Internal waves occur between waters that are of different salinity/temperature/density. The differences in density cause the fresher/warmer/less dense water on the surface to behave differently than the typically saltier/colder/denser water below. The boundary between the layers, or the pycnocline, can move in a wave motion, triggered by tides, underwater topography, or wind (http://earthobservatory.nasa.gov/IOTD/view.php?id=7230). Internal waves can be visible in some circumstances due to the slicks they sometimes cause on the surface as surface currents are affected. The crest of the internal wave causes surface particles directly above to spread out, creating a dark, smooth appearance on the surface, while particles and water collect in the troughs of the wave, causing the surface water to be rough and bright in appearance (http://earthobservatory.nasa.gov/IOTD/view.php?id=3586). Otherwise the sea level remains undisturbed. Particles in the middle of the water column are moved vertically, though not horizontally, with the passing of each crest and trough of the wave.
Internal waves occur between waters that are of different salinity/temperature/density. The differences in density cause the fresher/warmer/less dense water on the surface to behave differently than the typically saltier/colder/denser water below. The boundary between the layers, or the pycnocline, can move in a wave motion, triggered by tides, underwater topography, or wind (http://earthobservatory.nasa.gov/IOTD/view.php?id=7230). Internal waves can be visible in some circumstances due to the slicks they sometimes cause on the surface as surface currents are affected. The crest of the internal wave causes surface particles directly above to spread out, creating a dark, smooth appearance on the surface, while particles and water collect in the troughs of the wave, causing the surface water to be rough and bright in appearance (http://earthobservatory.nasa.gov/IOTD/view.php?id=3586). Otherwise the sea level remains undisturbed. Particles in the middle of the water column are moved vertically, though not horizontally, with the passing of each crest and trough of the wave.
http://www.es.flinders.edu.au/~mattom/IntroOc/lecture10.html
“An internal wave propagating on the interface between two layers. The undisturbed sea level is indicated by the yellow line. Water particles are shown as yellow and magenta dots. Yellow dots sit in the middle of the water column and move only up and down. Magenta dots sit at the top and bottom of the water column and move only in the horizontal.” - Tomczak, M.(2005) Introduction to Physical Oceanography [HTML].
Rainville & Woodgate (2009) conducted a study in the Chuchki Sea to determine the effects of receding summer ice coverage in the Arctic on internal wave activity and the consequential ocean mixing. The researchers used two subsurface moorings, one placed at 70 metres below the surface and another, 30 kilometres from the first, at 110 metres below the surface. Attached to the moorings were devices measuring wind speed, and acoustic devices measuring inertial oscillations in the water as well as ice coverage. This data was combined with satellite data and ice records from previous years to provide the researchers with as complete a picture as possible on which to compare their experimental observations.
Mooring data recorded storms occurring at all times during the year. The researchers observed that during the winter, when most of the sea was covered by sea ice, there were minimal inertial oscillations under the ice and therefore minimal ocean mixing. These oscillations were not affected by winds from any of the storms that occurred. When the ice receded in the summer, inertial oscillations increased. The melting ice increased water stratification by lowering the density and salinity of surface water as ice water is fresher than ocean water because of the brine rejection that occurs during its formation. The newly exposed water is now susceptible to internal waves caused by wind mixing.
At first Rainville and Woodgate speculated that the stronger inertial oscillations and internal wave formation were a result of previously restricted tidal currents now having room to move more freely across underwater topography and manifesting as stronger inertial oscillations and internal waves. However, data showed that the oscillations and internal wave movements did not correspond with tidal movements and instead were correlated with the recorded storms; As a result, researchers concluded that wind mixing was causing the internal waves.
Mooring data showed buoyancy frequency (oscillations) beneath the mixed surface layer were positive throughout most the year, meaning that the water column and stratification were stable. However, during storms, wind caused internal waves. Inertial oscillations increased when exposed to the wind, and the increased oscillations flowing over the underwater topography create internal waves (Zhang et al., 2008). These underwater waves create a sort of friction in the boundary between fresher/less dense surface water and the dense deep water, called shear. The shear caused by the internal waves appears to cause the pycnocline to dissipate, and the mixed surface layer to become deeper (Rainville & Woodgate, 2009). The eroded pycnocline becomes less of a boundary to cross for the (previously separated) deep water that the internal wave naturally brings to the surface. Along with the shallow depths of the Chuchki Sea, this potentiates conditions for mixing.
The Chuchki Sea’s underwater topography consists of Mendeleyev Ridge to the northwest, the Chuchki Plain at the southern base of this ridge, the Chuchki Plateau to the northeast and the Arctic continental slope to the north (Arctic Ocean Encyclopædia Britannica Online, 2010). According to research by Zhang et al. (2008), if the internal waves occurring in the Chuchki Sea had the same slope as the that of the Arctic continental slope, the strong shear and angle at which they encountered the slope would cause what are called boundary flows. Boundary flows are intense currents caused by internal waves travelling up a continental slope (Zhang et al., 2008). The internal wave rushes up the slope, carrying the unstable isocline until it reaches a critical point. Here the current billows and then breaks on the slope, resulting in active mixing in what is usually a relatively stagnant ocean. The diminishing ice coverage in the Arctic results in increased internal wave formation which in turn causes ocean mixing. The Arctic Ocean water stratification levels are usually stable, so this increased mixing could affect delicate ecosystems, such as the conditions photoplankton blooms require in order to continue to survive.
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| arctic phytoplankton bloom larger than the size of greece |
Reduced ice levels and the increased mixing could also change the circulation in the Arctic; for example, impeding or potentiating the one major circulation channel: the Fram Strait. This could in turn affect global water circulation, or thermohaline circulation. Thermohaline circulation keeps the Earth’s overall temperature balanced.
If this heating/cooling pump were to be altered or brought to a halt, the Earth’s delicate ecological, atmospheric, and geographic balances (among any other number of things) would be in disrupted, and the survival of life on the planet would be thrown into question. Adding urgency to the need for more study is recent research suggesting cloud cover in the Arctic is decreasing, meaning increasing ice-melt and warming of surface waters (The American Geophysical Union & American Meteorological Society, 2008).
Ice levels during the Arctic summer are diminishing, partly due to increased sun exposure and polar warming. As a result, water is exposed and internal waves are more likely to form. This potentiates conditions for ocean mixing, which in turn could affect ecosystems and thermohaline circulation as the receding ice worsens year by year. The potentially profound and detrimental effects of the receding ice on the Earth’s environmental systems make the causes and effects of internal waves, specifically at the poles, an area of study requiring further research and attention.
References
Arctic Ocean. (2010). In Encyclopædia Britannica. Retrieved November 17, 2010, from Encyclopædia Britannica Online: http://www.britannica.com/EBchecked/topic/33188/Arctic-Ocean
Chukchi Sea. (2010). In Encyclopædia Britannica. Retrieved November 17, 2010, from Encyclopædia Britannica Online: http://www.britannica.com/EBchecked/topic/116851/Chukchi-Sea
Internal Waves in the Tsushima Strait (2006). Retrieved from http://earthobservatory.nasa.gov/IOTD/view.php?id=7230
Internal Waves, Sulu Sea (2003). Retrieved from http://earthobservatory.nasa.gov/IOTD/view.php?id=3586
Rainville et al. Observations of internal wave generation in the seasonally ice-free Arctic. Geophysical Research Letters, 2009; 36 (23): L23604. doi:10.1029/2009GL041291
The American Geophysical Union and the American Meteorological Society (2008, April 1). Inside The Clouds: Meteorologists Gather Important Information With 5-satellite 'A-Train' Group. Science Daily. Retrieved November 17, 2010, from http://www.sciencedaily.com/videos/2008/0402-inside_the_clouds.htm.
Tomczak, M. (2005) Introduction to Physical Oceanography [HTML]. Retrieved from http://www.es.flinders.edu.au/~mattom/IntroOc/lecture10.html.
Zhang et al. Resonant Generation of Internal Waves on a Model Continental Slope. Physical Review Letters, 2008; PRL 100, 244504. doi:10.1103/PhysRevLett.100.244504
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| the earth's oceanic "heating/cooling pump" |
If this heating/cooling pump were to be altered or brought to a halt, the Earth’s delicate ecological, atmospheric, and geographic balances (among any other number of things) would be in disrupted, and the survival of life on the planet would be thrown into question. Adding urgency to the need for more study is recent research suggesting cloud cover in the Arctic is decreasing, meaning increasing ice-melt and warming of surface waters (The American Geophysical Union & American Meteorological Society, 2008).
Ice levels during the Arctic summer are diminishing, partly due to increased sun exposure and polar warming. As a result, water is exposed and internal waves are more likely to form. This potentiates conditions for ocean mixing, which in turn could affect ecosystems and thermohaline circulation as the receding ice worsens year by year. The potentially profound and detrimental effects of the receding ice on the Earth’s environmental systems make the causes and effects of internal waves, specifically at the poles, an area of study requiring further research and attention.
References
Arctic Ocean. (2010). In Encyclopædia Britannica. Retrieved November 17, 2010, from Encyclopædia Britannica Online: http://www.britannica.com/EBchecked/topic/33188/Arctic-Ocean
Chukchi Sea. (2010). In Encyclopædia Britannica. Retrieved November 17, 2010, from Encyclopædia Britannica Online: http://www.britannica.com/EBchecked/topic/116851/Chukchi-Sea
Internal Waves in the Tsushima Strait (2006). Retrieved from http://earthobservatory.nasa.gov/IOTD/view.php?id=7230
Internal Waves, Sulu Sea (2003). Retrieved from http://earthobservatory.nasa.gov/IOTD/view.php?id=3586
Rainville et al. Observations of internal wave generation in the seasonally ice-free Arctic. Geophysical Research Letters, 2009; 36 (23): L23604. doi:10.1029/2009GL041291
The American Geophysical Union and the American Meteorological Society (2008, April 1). Inside The Clouds: Meteorologists Gather Important Information With 5-satellite 'A-Train' Group. Science Daily. Retrieved November 17, 2010, from http://www.sciencedaily.com/videos/2008/0402-inside_the_clouds.htm.
Tomczak, M. (2005) Introduction to Physical Oceanography [HTML]. Retrieved from http://www.es.flinders.edu.au/~mattom/IntroOc/lecture10.html.
Zhang et al. Resonant Generation of Internal Waves on a Model Continental Slope. Physical Review Letters, 2008; PRL 100, 244504. doi:10.1103/PhysRevLett.100.244504
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