Downwelling

Downwelling is the process of accumulation and sinking of higher density material beneath lower density material, such as cold or saline water beneath warmer or fresher water or cold air beneath warm air. It is the sinking limb of a convection cell. Upwelling is the opposite process and together these two forces are responsible in the oceans for the thermohaline circulation. The sinking of cold lithosphere at subduction zones is another example of downwelling in plate tectonics.

Oceanic downwelling

Downwelling occurs at anti-cyclonic places within the ocean where warm rings are spinning clockwise creating surface convergence. When these surface waters converge, they push the surface water downwards. Another way that downwelling can occur is by the wind driving the sea towards the coastline. Regions that have downwelling have low productivity because the nutrients in the water column are utilized but are not continuously resupplied by the cold, nutrient-rich water from below the surface.[1]

Ventilation

Downwelling also allows for deep ocean ventilation to occur because these waters are able to bring dissolved oxygen down from the surface to help facilitate aerobic respiration in organisms throughout the water column. Without this renewal, the dissolved oxygen in the sediment and within the water column would be quickly used up by biological processes. In the instance of decay, anaerobic bacteria would take over decomposition, leading to a build-up of hydrogen sulfide. In these toxic conditions, there are very few benthic animals that would survive. In the most extreme cases, a lack of downwelling could possibly lead to mass extinction. Paleontologists have suggested that 250 million years ago, deep ocean ventilation slowed nearly to a halt, and the ocean became stagnant. Sulfide and methane-rich waters low on oxygen filled the deep ocean and progressed onto the continental shelves, wiping out 95% of all marine species in the greatest extinction event in Earth history, the Permian extinction.[2]

Locations

Downwelling occurs in areas such as in the subpolar gyre of the North Atlantic where several surface currents meet. We also find downwelling along the outermost boundary of the Southern Ocean where cold Antarctic water sinks below warmer South Pacific and South Atlantic waters. There is also downwelling on a few coastlines where the wind blows in such a direction that it causes Ekman transport to move water towards the coast which then causes the water to pile up and be pushed down.[3]

See also

References

  1. ^ "Ocean Motion : Definition : Wind Driven Surface Currents - Upwelling and Downwelling". Retrieved 12 March 2016.
  2. ^ "NOAA Ocean Explorer: Education - Multimedia Discovery Missions - Lesson 8 - Ocean Currents - Activities: Currents and Marine Life". Retrieved 12 March 2016.
  3. ^ "G115 - Introduction to Oceanography". Retrieved 12 March 2016.

External links

Andaman Sea

The Andaman Sea (historically also known as the Burma Sea) is a marginal sea of the eastern Indian Ocean separated from the Bay of Bengal (to its west) by the Andaman Islands of India and Myanmar and the Nicobar Islands of India and touching Myanmar, Thailand, and the Malay Peninsula. Its southernmost end is defined by Breueh Island, an island just north of Sumatra.

Traditionally, the sea has been used for fishery and transportation of goods between the coastal countries and its coral reefs and islands are popular tourist destinations. The fishery and tourist infrastructure was severely damaged by the 2004 Indian Ocean earthquake and tsunami.

Bobtail squid

Bobtail squid (order Sepiolida) are a group of cephalopods closely related to cuttlefish. Bobtail squid tend to have a rounder mantle than cuttlefish and have no cuttlebone. They have eight suckered arms and two tentacles and are generally quite small (typical male mantle length being between 1 and 8 cm).Sepiolids live in shallow coastal waters of the Pacific Ocean and some parts of the Indian Ocean and Atlantic Ocean as well as in shallow waters on the west coast of the Cape Peninsula off South Africa.

Like cuttlefish, they can swim by either using the fins on their mantle or by jet propulsion. They are also known as dumpling squid (owing to their rounded mantle) or stubby squid.

Carbonate compensation depth

Calcite compensation depth (CCD) is the depth in the oceans below which the rate of supply of calcite (calcium carbonate) lags behind the rate of solvation, such that no calcite is preserved. Aragonite compensation depth (hence ACD) describes the same behaviour in reference to aragonitic carbonates. Aragonite is more soluble than calcite, so the aragonite compensation depth is generally shallower than the calcite compensation depth.

Calcium carbonate is essentially insoluble in sea surface waters today. Shells of dead calcareous plankton sinking to deeper waters are practically unaltered until reaching the lysocline where the solubility increases dramatically. By the time the CCD is reached all calcium carbonate has dissolved according to this equation:

Calcareous plankton and sediment particles can be found in the water column above the CCD. If the sea bed is above the CCD, bottom sediments can consist of calcareous sediments called calcareous ooze, which is essentially a type of limestone or chalk. If the exposed sea bed is below the CCD tiny shells of CaCO3 will dissolve before reaching this level, preventing deposition of carbonate sediment. As the sea floor spreads, thermal subsidence of the plate, which has the effect of increasing depth, may bring the carbonate layer below the CCD; the carbonate layer may be prevented from chemically interacting with the sea water by overlying sediments such as a layer of siliceous ooze or abyssal clay deposited on top of the carbonate layer.

Cookiecutter shark

The cookiecutter shark (Isistius brasiliensis), also called the cigar shark, is a species of small squaliform shark in the family Dalatiidae. This shark occurs in warm, oceanic waters worldwide, particularly near islands, and has been recorded as deep as 3.7 km (2.3 mi). It migrates vertically up to 3 km (1.9 mi) every day, approaching the surface at dusk and descending with the dawn. Reaching only 42–56 cm (16.5–22 in) in length, the cookiecutter shark has a long, cylindrical body with a short, blunt snout, large eyes, two tiny spineless dorsal fins, and a large caudal fin. It is dark brown, with light-emitting photophores covering its underside except for a dark "collar" around its throat and gill slits.

The name "cookiecutter shark" refers to its feeding habit of gouging round plugs, as if cut out with a cookie cutter, out of larger animals. Marks made by cookiecutter sharks have been found on a wide variety of marine mammals and fishes, as well as on submarines, undersea cables, and even human bodies. It also consumes whole smaller prey such as squid. Cookiecutter sharks have adaptations for hovering in the water column and likely rely on stealth and subterfuge to capture more active prey. Its dark collar seems to mimic the silhouette of a small fish, while the rest of its body blends into the downwelling light via its ventral photophores. When a would-be predator approaches the lure, the shark attaches itself using its suctorial lips and specialized pharynx and neatly excises a chunk of flesh using its bandsaw-like set of lower teeth. This species has been known to travel in schools.

Though rarely encountered because of its oceanic habitat, a handful of documented attacks on humans were apparently caused by cookiecutter sharks. Nevertheless, this diminutive shark is not regarded as highly dangerous. The International Union for Conservation of Nature has listed the cookiecutter shark under least concern, as it is widely distributed, has no commercial value, and is not particularly susceptible to fisheries.

CubeSat UV Experiment

CubeSat UV Experiment (CUVE) is a space mission concept to study the atmospheric processes of the planet Venus with a small satellite. Specifically, the orbiter mission would study an enigmatic ultraviolet light absorber of unknown composition situated within the planet's uppermost cloud layer that absorbs about half the solar radiation downwelling in the planet's atmosphere.

The mission concept is still in its early formulation stage. The Principal Investigator is Valeria Cottini, at the University of Maryland in College Park.

Ekman transport

Ekman transport, part of Ekman motion theory first investigated in 1902 by Vagn Walfrid Ekman. Winds are the main source of energy for ocean circulation, and Ekman Transport is a component of wind-driven ocean current.. Ekman transport occurs when ocean surface waters are influenced by the friction force acting on them via the wind. As the wind blows it casts a friction force on the ocean surface that drags the upper 10-100m of the water column with it.. However, due to the influence of the Coriolis effect, the ocean water moves at a 90° angle from the direction of the surface wind.. The direction of transport is dependent on the hemisphere: in the northern hemisphere, transport occurs at 90° clockwise from wind direction, while in the southern hemisphere it occurs at a 90° counterclockwise.. This phenomenon was first noted by Fridtjof Nansen, who recorded that ice transport appeared to occur at an angle to the wind direction during his Arctic expedition during the 1890s.. Ekman transport has significant impacts on the biogeochemical properties of the world’s oceans. This is because they lead to upwelling (Ekman suction), downwelling (Ekman pumping) in order to obey mass conservation laws. Mass conservation, in reference to Ekman transfer, requires that any water displaced within an area must be replenished, this can be done by either Ekman suction or Ekman pumping depending on wind patterns..

Heceta Bank

Heceta Bank is a rocky bank located 55 kilometers (km) off the Oregon coast near Florence, centered on approximately 44°N, 125°W, and is roughly 29 km long and upwards of 13 km wide. Heceta Bank is an area of ecological and oceanographic importance. The unique bathymetric features and seasonal circulation within the bank provides habitat for a diversity of economically-important fish species.

Heceta Bank also plays an important role in current understanding of prehistoric human migration. During the last glacial low stand, approximately 15,000–20,000 years ago, prehistoric humans migrated from northeastern Asia into North America. Scientists hypothesize that Heceta Bank, due to its topography and morphology, is a possible area of this historical migration.

Indonesian Throughflow

The Indonesian throughflow (ITF) is an ocean current with importance for global climate since it provides a low-latitude pathway for warm, fresh water to move from the Pacific to the Indian Ocean and this serves as the upper branch of the global heat conveyor belt. Higher ocean surface topography in the western Pacific than in the Indian Ocean drives upper thermocline water from the North Pacific through the western route of the Makassar Strait to either directly exit through the Lombok Strait or flow eastward into the Banda Sea. Weaker flow of saltier and denser South Pacific water passes over the Lifamatola Passage into the Banda Sea, where these water masses are mixed due to tidal effects, Ekman pumping, and heat and fresh water flux at the ocean surface. From the Banda Sea the ITF exits Timor, Ombai, and Lombok passages.

The location and topography of the channels that make up the ITF are shown in the Figure. Lombok Strait is 300m deep and roughly 35 km wide and the currents vary between 0.286 m/s (0.6 mi/hr) eastward to 0.67 m/s westward and average 0.25 m/s westward. Currents in Ombai vary between 0.12 m/s eastward to 0.16 m/s westward, averaging 0.11 m/s westward and are funneled within the 1250m deep and 35 km wide passage. Timor passage, which is 1890 m deep by 160 km wide, is the widest of the exit pathways and averages only 0.02 m/s. From 2004–2006, 11 moorings were deployed across the entrance and exit regions of the ITF and were positioned to accurately measure each passage’s contribution as part of the International Nusantara Stratification and Transport (INSTANT) program. Flow in through Makassar (11.6 Sv, 1 Sv = 106 m³/s) and Lifamatola (1.1 Sv) sums to 12.7 Sv. Total outflow transport corresponds to 15.0 Sv (varying from 10.7 to 18.7 Sv) and is made up of Lombok (2.6 Sv), Ombai (4.9 Sv) and Timor (7.5 Sv) contributions. Heat Transport of the Indonesian Throughflow is 1.087 PW (1 PW=1015 Watt).Circulation and transport within the Indonesian Seas varies along with large-scale monsoon flow. During June to August, southeasterlies of the southwest monsoon predominate over Indonesia and drive strong Ekman divergence (southwestward flow in the Southern Hemisphere thus increasing ITF to 15 Sv) whereas from December to February, Northwest Monsoon westerlies serve to directly reduce the ITF . During monsoon transitions, strong westerly winds in the eastern Indian Ocean force equatorial downwelling Kelvin waves (eastward moving, eastward flow) that propagate through the Indonesian passages as coastally trapped Kelvin waves and serve to reduce the ITF flow with a minimum in April of 9 Sv. Another way to think about it is that downwelling on the Indian Ocean side increases sea level and so reduces the normal Pacific-to-Indian pressure head reducing the flow.

Global-scale, ocean waves such as equatorial/coastal Kelvin and Rossby waves drive interannual variation of the ITF with an amplitude of roughly +/-3 Sv. Western-central Pacific westerly winds from El Nino force westward moving-equatorial Rossby waves and eastward currents that hit eastern New Guinea and propagate around the west coast as coastal Kelvin waves and down through the ITF along the west Australia Shelf coast serving to reduce the ITF. Upwelling (i.e. reduced sea level) associated with Rossby waves on the Pacific side reduces the Pacific-to-Indian pressure gradient and reduces the ITF. Interannual variability of Indian Ocean westerlies act in the same manner as the seasonal equatorial Kelvin waves to reduce the normal westward ITF flow as well.

An important feature of the Indonesian Throughflow is that because the water in the western equatorial Pacific Ocean has a higher temperature and lower salinity than the water in the Indian Ocean, the Throughflow transports large amounts of relatively warm and fresh water to the Indian Ocean. When the Indonesian Throughflow (through Lombok Strait, Ombai and the Timor Passages) enters the Indian Ocean it is advected towards Africa within the Indian South Equatorial Current. There it eventually exits the Indian Ocean with the Agulhas Current around South Africa into the Atlantic Ocean. So the Indonesian Throughflow transports a significant amount of Pacific Ocean heat into the southwest Indian Ocean, which is approximately 10,000 km (6,200 mi) away from the Lombok Strait.

Meroplankton

Meroplankton is a wide variety of planktonic organisms, which spend a portion of their lives in the benthic region of the ocean. These organisms do not remain as plankton permanently, rather, they are planktonic components in transition, which eventually become larger organisms. Meroplankton can be contrasted with holoplankton, which are planktonic organisms that stay in the pelagic zone as plankton throughout their entire life cycle.

After a period of time in the plankton, meroplankton either graduate to the nekton or adopt a benthic (often sessile) lifestyle on the seafloor. Meroplankton consists of larval stages of organisms such as sea urchins, starfish, and crustaceans. Success of meroplankton populations depends on many factors, such as adult fecundity, fertilization success, growth and larval stage duration, behaviour, dispersal, and settlement. Mortality depends on many factors, such as predation, competition, disease, parasites, and physiological stresses. Survival and mortality of meroplankton has a direct effect on adult population numbers of many species.

Many of the common, well-known animals found on the Great Barrier Reef spend time as free-swimming meroplankton, bearing little or no resemblance to the adult they will become. The differences between the appearance of larval and adult stages led to much confusion in the past when larval forms were often believed to be completely different species from the adults. Larvae spend varying amounts of time in the plankton, from minutes to over a year. However, just how long these tiny animals can be considered truly planktonic is under some debate.

Mesopause

The mesopause is the point of minimum temperature at the boundary between the mesosphere and the thermosphere atmospheric regions. Due to the lack of solar heating and very strong radiative cooling from carbon dioxide, the mesosphere is the coldest region on Earth with temperatures as low as -100 °C (-148 °F or 173 K). The altitude of the mesopause for many years was assumed to be at around 85 km (53 mi.), but observations to higher altitudes and modeling studies in the last 10 years have shown that in fact the mesopause consists of two minima - one at about 85 km and a stronger minimum at about 100 km (62 mi).Another feature is that the summer mesopause is cooler than the winter (sometimes referred to as the mesopause anomaly). It is due to a summer-to-winter circulation giving rise to upwelling at the summer pole and downwelling at the winter pole. Air rising will expand and cool resulting in a cold summer mesopause and conversely downwelling air results in compression and associated increase in temperature at the winter mesopause. In the mesosphere the summer-to-winter circulation is due to gravity wave dissipation, which deposits momentum against the mean east-west flow, resulting in a small north-south circulation.In recent years the mesopause has also been the focus of studies on global climate change associated with increases in CO2. Unlike the troposphere, where greenhouse gases result in the atmosphere heating up, increased CO2 in the mesosphere acts to cool the atmosphere due to increased radiative emission. This results in a measurable effect - the mesopause should become cooler with increased CO2. Observations do show a decrease of temperature of the mesopause, though the magnitude of this decrease varies and is subject to further study. Modeling studies of this phenomenon have also been carried out.

Ovda Regio

Ovda Regio is a Venusian crustal plateau located near the equator in the western highland region of Aphrodite Terra that stretches from 10°N to 15°S and 50°E to 110°E. Known as the largest crustal plateau in Venus, the regio covers an area of approximately 15,000,000 square kilometres (5,800,000 sq mi) and is bounded by regional plains to the north, Salus Tessera to the west, Thetis Regio to the east, and Kuanja as well as Ix Chel chasmata to the south. The crustal plateau serves as a place to hold the localized tessera terrains in the planet, which makes up roughly 8% of Venus' surface area. The kinematic evolution of crustal plateaus on Venus has been a debated topic in the planetary science community. Understanding its complex evolution is expected to contribute to a better knowledge of the geodynamic history of Venus. It is named after a Marijian forest spirit that can appear as both male and female.

Somali Current

The Somali Current is a cold ocean boundary current that runs along the coast of Somalia and Oman in the Western Indian Ocean and is analogous to the Gulf Stream in the Atlantic Ocean. This current is heavily influenced by the monsoons and is the only major upwelling system that occurs on a western boundary of an ocean. The water that is upwelled by the current merges with another upwelling system, creating one of the most productive ecosystems in the ocean.The Somali current is characterized by seasonal changes influenced by the Southwest monsoon and the Northeast Monsoon. During the months of June to September, the warm Southwest monsoon moves the coastal waters northeastward, creating coastal upwelling. The upwelled water is carried offshore by Ekman transport and merges with water that was brought to the surface by open-ocean upwelling. The Findlater jet, a narrow low-level, atmospheric jet, also develops during the Southwest monsoon, and blows diagonally across the Indian Ocean, parallel to the coasts of Somalia and Oman. As a result, an Ekman transport is created to the right of the wind. At the center of the jet, the transport is maximum and decreases to the right and left with increasing distance. To the left of the jet center, there is less water movement toward the center than is leaving, creating a divergence in the upper layer and resulting in an upwelling event (Ekman suction). In contrast, to the right of the center of the jet, more water is coming from the center than is leaving, creating a downwelling event (Ekman pumping). This open-ocean upwelling in combination with the coastal upwelling cause a massive upwelling. The Northeast monsoon, which occurs from December to February, causes a reversal of the Somalia current, moving the coastal waters southwest. Cooler air causes the surface water to cool and creates deep mixing, bringing abundant nutrients to the surface.

Squaliformes

The Squaliformes are an order of sharks that includes about 126 species in seven families.

Members of the order have two dorsal fins, which usually possess spines,they usually have a sharp head, no anal fin or nictitating membrane, and five to seven gill slits. In most other respects, however, they are quite variable in form and size. Most species of the squaliform order live in a saltwater or brackish waters. They are found worldwide, from northern to tropical waters, and from shallow coastal seas to the open ocean.All members of the family Eptomeridae and Dalatiidae and Zameus squamulosus possess photophores, luminous organs, and exhibit intrinsic bioluminescence . Bioluminescence evolved once in Squaliformes, approximately 111-153 million years ago, and helped the Squaliformes radiate and adapt to the deep sea. The common ancestor of Dalatiidae, Etmopteridae, Somniosidae, and Oxynotidae possessed a luminous organ and used bioluminescence for camouflage by counterillumination . Counterillumination is an active form of camouflage in which an organism emits light to match the intensity of downwelling light to hide from predators below. Currently, bioluminescence provides different functions for Squaliformes based on the family. Dalatiidae and Zameus squamulosus possess simple photophores and use bioluminescence for ventral counter-illumination. Etmopteridae possess more complex photophores and utilize bioluminescence for ventral counter illumination as well as species recognition .

Stratification (water)

Water stratification is when water masses with different properties - salinity (halocline), oxygenation (chemocline), density (pycnocline), temperature (thermocline) - form layers that act as barriers to water mixing which could lead to anoxia or euxinia. These layers are normally arranged according to density, with the least dense water masses sitting above the more dense layers.

Water stratification also creates barriers to nutrient mixing between layers. This can affect the primary production in an area by limiting photosynthetic processes. When nutrients from the benthos cannot travel up into the photic zone, phytoplankton may be limited by nutrient availability. Lower primary production also leads to lower net productivity in waters.

Stratosphere

The stratosphere () is the second major layer of Earth's atmosphere, just above the troposphere, and below the mesosphere. The stratosphere is stratified (layered) in temperature, with warmer layers higher and cooler layers closer to the Earth; this increase of temperature with altitude is a result of the absorption of the Sun's ultraviolet radiation by the ozone layer. This is in contrast to the troposphere, near the Earth's surface, where temperature decreases with altitude. The border between the troposphere and stratosphere, the tropopause, marks where this temperature inversion begins. Near the equator, the stratosphere starts at as high as 20 km (66,000 ft; 12 mi), around 10 km (33,000 ft; 6.2 mi) at midlatitudes, and at about 7 km (23,000 ft; 4.3 mi) at the poles. Temperatures range from an average of −51 °C (−60 °F; 220 K) near the tropopause to an average of −15 °C (5.0 °F; 260 K) near the mesosphere. Stratospheric temperatures also vary within the stratosphere as the seasons change, reaching particularly low temperatures in the polar night (winter). Winds in the stratosphere can far exceed those in the troposphere, reaching near 60 m/s (220 km/h; 130 mph) in the Southern polar vortex.

Tessera (Venus)

Tesserae are regions of heavily deformed terrain on Venus, characterized by two or more intersecting tectonic elements, high topography, and subsequent high radar backscatter. Tesserae often represent the oldest material at any given location and are among the most tectonically deformed terrains on Venus's surface. Diverse types of tessera terrain exist. It is not currently clear if this is due to a variety in the interactions of Venus's mantle with regional crustal or lithospheric stresses, or if these diverse terrains represent different locations in the timeline of crustal plateau formation and fall. Multiple models of tessera formation exist and further extensive studies of Venus's surface are necessary to fully understand this complex terrain.

Venus In situ Composition Investigations

Venus In situ Composition Investigations (VICI) is a concept lander mission to Venus in order to answer long-standing questions about its origins and evolution, and provide new insights needed to understand terrestrial planet formation, evolution, and habitability.

VICI was one of 12 considerations for New Frontiers 4, but was not one of the two missions selected to be finalists in late 2017.

Wave base

The wave base, in physical oceanography, is the maximum depth at which a water wave's passage causes significant water motion. For water depths deeper than the wave base, bottom sediments and the seafloor are no longer stirred by the wave motion above.

Zooplankton

Zooplankton (, ) are heterotrophic (sometimes detritivorous) plankton (cf. phytoplankton). Plankton are organisms drifting in oceans, seas, and bodies of fresh water. The word zooplankton is derived from the Greek zoon (ζῴον), meaning "animal", and planktos (πλαγκτός), meaning "wanderer" or "drifter". Individual zooplankton are usually microscopic, but some (such as jellyfish) are larger and visible to the naked eye.

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