Downwelling

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Schematic of coastal downwelling in the Northern Hemisphere. Coastal Downwelling.png
Schematic of coastal downwelling in the Northern Hemisphere.

Downwelling is the downward movement of a fluid parcel and its properties (e.g., salinity, temperature, pH) within a larger fluid. It is closely related to upwelling, the upward movement of fluid.

Contents

While downwelling is most commonly used to describe an oceanic process, it's also used to describe a variety of Earth phenomena. This includes mantle dynamics, air movement, and movement in freshwater systems (e.g., large lakes). This article will focus on oceanic downwelling and its important implications for ocean circulation and biogeochemical cycles. Two primary mechanisms transport water downward: buoyancy forcing and wind-driven Ekman transport (i.e., Ekman pumping). [1] [2]

Downwelling has important implications for marine life. Surface water generally has a lower nutrient content compared to deep water due to primary production using nutrients in the photic zone. Surface water is, however, high in oxygen compared to the deep ocean due to photosynthesis and air-sea gas exchange. When water is moved downwards, oxygen is pumped below the surface, where it is used by decaying organisms. [3] Downwelling events are accompanied by low primary production in the surface ocean due to a lack of nutrient supply from below. [3]

Mechanisms

Buoyancy

Buoyancy-forced downwelling, often termed convection, is the deepening of a water parcel due to a change in the density of that parcel. Density changes in the surface ocean are primarily the result of evaporation, precipitation, heating, cooling, or the introduction and mixing of an alternate water or salinity source, such as river input or brine rejection. Notably, convection is the driving force behind global thermohaline circulation. For a water parcel to move downward, the density of that parcel must increase; therefore, evaporation, cooling, and brine rejection are the processes that control buoyancy-forced downwelling. [1]

Wind-driven Ekman transport

Ekman transport is the net mass transport of the ocean surface resulting from wind stress and the Coriolis force. As wind blows across the ocean surface, it causes a frictional force that drags the uppermost surface water along with it. Due to the Earth's rotation, these surface currents develop at 45° to the wind direction. However, compounding frictional forces cause the net transport across the Ekman layer to be 90° to the right of wind stress in the Northern Hemisphere and 90° to the left in the Southern Hemisphere. Ekman transport piles up water between the trade winds and westerlies in subtropical gyres, or near the shore during coastal downwelling. [4] The increased mass of surface water creates high-pressure zones that push water downward. It can also create long convergence zones during sustained winds to create Langmuir circulation.

Buoyancy-forced downwelling

Buoyancy is lost through cooling, evaporation, and brine rejection through sea ice formation. Buoyancy loss occurs on many spatial and temporal scales.

In the open ocean, there are regions where cooling and mixed layer deepening occurs at night, and the ocean re-stratifies during the day. On annual cycles, widespread cooling begins in the fall, and convective mixed layer deepening can reach hundreds of meters into the ocean interior. In comparison, the wind-driven mixed layer depth is limited to 150 m.

Large evaporation events can cause convection; however, latent heat loss associated with evaporation is usually dominant and in the winter, this process drives Mediterranean Sea deep water formation. In select locations - Greenland Sea, Labrador Sea, Weddell Sea, and Ross Sea - deep convection (>1000 m) ventilates (oxygenates) most of the deep water of the global ocean and drives the thermohaline circulation. [1]

Wind-forced downwelling

Map showing the five subtropical ocean gyres. Oceanic gyres.png
Map showing the five subtropical ocean gyres.

Subtropical gyres

Subtropical gyres act on the largest scale that we observe downwelling. Winds to the north and south of each ocean basin blow opposite each other such that Ekman transport moves water toward the basin's center. This movement piles up water, creating a high-pressure zone in the center of the gyre, low pressure on the borders, and deepens the mixed layer. The water in this zone would diffuse outward if the planet weren't spinning. However, because of the Coriolis force, the water rotates clockwise in the Northern Hemisphere and counterclockwise in the southern, creating a gyre. While it spins, the rotating high-pressure zone forces water downward, resulting in downwelling. [4] Typical downwelling rates associated with ocean gyres are on the order of 10’s of meters per year. [5]

Coastal downwelling

Coastal downwelling occurs when winds blow parallel to the shore. With such winds, Ekman transport directs water movement towards or directly away from the shore. If Ekman transport moves water towards the shore, the shoreline acts as a barrier causing surface water to pile up onshore. The piled-up water is forced downwards, pumping warm, nutrient-poor, oxygenated water below the mixed layer. [3] [4]

Langmuir circulation

Langmuir circulation develops from the wind, which, through Ekman transport, creates alternating zones of convergence and divergence at the ocean surface. In convergent zones, marked by long strips of floating debris accumulation, coherent vortices that transport surface waters to the base of the mixed layer develop. Also, direct wind stirring and current shear at the base of the mixed layer can create instabilities and turbulence that further mix properties within and at the base. [6]

Association with other ocean features

Eddies

Warm-core eddy in the Northern Hemisphere. Shown are the clockwise rotation of waters, depressed isopycnals, and low productivity at the eddy's center. Anti-cyclonic warm core eddy.png
Warm-core eddy in the Northern Hemisphere. Shown are the clockwise rotation of waters, depressed isopycnals, and low productivity at the eddy's center.

Meso- (>10-100's km) and submesoscale (<1-10 km) eddies are ubiquitous features of the upper ocean. Eddies have either a cyclonic (cold-core) or anticyclonic (warm-core) rotation. Warm-core eddies are characterized by anticyclonic rotation that directs surface waters inward, creating high sea surface temperature and height. [7] The high central hydrostatic pressure maintained by this rotation causes the downwelling of water and the depression of isopycnals - surfaces of constant density (see Eddy pumping) at scales of hundreds of meters per year. [8] The typical result is a deeper surface layer of warm water often characterized by low primary production. [9] [10]

Warm-core eddies play multiple important roles in biogeochemical cycling and air-sea interactions. For example, these eddies are seen to decrease ice formation in the Southern Ocean due to their high sea surface temperatures. [11] It has also been observed that air-sea fluxes of carbon dioxide decrease at the center of these eddies and that temperature was the leading cause of this inhibited flux. [12] Warm-core eddies transport oxygen into the ocean interior (below the photic zone) which supports respiration. [13] Although compounds such as oxygen are transported into the deep ocean, there is an observed decrease in carbon export in warm-core eddies due to intensified stratification at their center. [14] Such stratification inhibits the mixing of nutrient-rich waters to the surface where they could fuel primary production. In this case, since primary production stays low, carbon export potential remains low.

Fronts and filaments

Ocean fronts are formed by the horizontal convergence of dissimilar water masses. They can develop at regions of freshwater input marked by horizontal density gradients due to salinity and temperature differences or the stretching and elongation of rotating flows. [15]

Submesoscale fronts and filaments are formed by ocean current interactions and flow instabilities. They are regions that connect the surface layer and the ocean interior. [16] These regions are characterized by horizontal buoyancy gradients < 10 km in scale, caused by sloping isopycnals. Two primary mechanisms transport surface waters to depth: the adiabatic tilting and relaxation of these isopycnals, and along-isopycnal flow or subduction. [17] These mechanisms can transport surface properties, such as heat, below the mixed layer and assist in carbon sequestration through the biological pump. [18] Numerical models predict vertical velocities at submesoscale fronts on the order of 100 m/day. [15] However, vertical velocities over 1000 m/day have been observed using ocean floats. [19] These observations are rare because ship-based sensors do not have sufficient accuracy to measure vertical velocities.

Variability

Downwelling trends differ between latitudes and can be associated with variations in wind strength and changing seasons. In some areas, coastal downwelling is a seasonal event pushing nutrient-depleted waters towards the shore. The relaxation or reversal of upwelling-favorable winds creates periods of downwelling as waters pile up along the coast. [20]

Temperature differences and wind patterns are seasonal in temperate latitudes, creating highly variable upwelling and downwelling conditions. [20] For example, in fall and winter along the Pacific Northwest coast in the United States, southerly winds in the Gulf of Alaska and California Current system create downwelling-favorable conditions, transporting offshore water from the south and west towards the coast. These downwelling events tend to last for days and can be associated with winter storms and contribute to low levels of primary production observed during fall and winter. [21] In contrast, during the "spring transition" at the end of the downwelling season and the beginning of the upwelling season is marked by the presence of cold, nutrient-rich, upwelled water at the coast, which stimulates high levels of primary production. [22] In contrast to seasonally variable temperate regions, downwelling is relatively steady at the poles as cold air decreases the temperature of salty water transported by gyres from the tropics. [23]

During the neutral and La Niña phases of the El Niño Southern Oscillation (ENSO), steady easterly trade winds in equatorial regions can cause water to pile up in the western Pacific. A weakening of these trade winds can create downwelling Kelvin waves, which propagate along the equator in the eastern Pacific. [24] Series of Kelvin waves associated with anomalously warm sea surface temperatures in the eastern Pacific can be a predecessor to an El Niño event. [25] During the El Niño phase of ENSO, the disruption of trade winds causes ocean water to pile up off the western coast of South America. This shift is associated with a decrease in upwelling and may enhance coastal downwelling. [26]

Effects on ocean biogeochemistry

Biogeochemical cycling related to downwelling is constrained by the location and frequency at which this process occurs. The majority of downwelling, as described above, occurs in polar regions as deep and bottom water formation or in the center of subtropical gyres. Bottom and deep water formation in the Southern Ocean (Weddell Sea) and North Atlantic Ocean (Greenland, Labrador, Norwegian, and Mediterranean Seas) is a major contributor towards the removal and sequestration of anthropogenic carbon dioxide, dissolved organic carbon (DOC), and dissolved oxygen. [27] [28] [29] Dissolved gas solubility is greater in cold water allowing for increased gas concentrations. [29]

The Southern Ocean alone has been shown to be the most important high-latitude region controlling pre-industrial atmospheric carbon dioxide by general circulation model simulations. Circulation of water into the Antarctic deep-water formation region is one of the main factors drawing carbon dioxide into the surface oceans. The other is the biological pump, which is typically limited by iron in the Southern Ocean in areas with high nutrients and low chlorophyll (HNLC). DOC can become entrained during bottom and deep water formation which is a large portion of biogenic carbon export. It is thought that the export of DOC is up to 30% of the biogenic carbon that makes it into the deep ocean. The intensity of the DOC flux to depth relies on the strength of winter convection, which also affects the microbial food web, causing variations in the DOC exported to depth. Dissolved oxygen is also downwelled at bottom and deep water formation sites, contributing to elevated dissolved oxygen concentrations below 1000 meters.

Subtropical gyres are typically limited in macro and micro nutrients such as nitrogen, phosphorus, and iron; resulting in picophytoplankton communities that have low nutrient requirements. This is in part due to consistent downwelling, which transports nutrients away from the photic zone. These oligotrophic areas are thought to be sustained by rapid nutrient cycling which could leave little carbon remaining that could be sequestered. The dynamics of picophytoplankton's role in carbon cycling in subtropical gyres is poorly understood and is being actively researched.

Areas with the highest primary productivity play significant roles in biogeochemical cycling of carbon and nitrogen. Downwelling can either alleviate or induce anoxic conditions, depending on the initial conditions and location. Sustained periods of upwelling can cause deoxygenation which is relieved by a downwelling event transporting dissolved oxygen back down to depths. Anoxic conditions can also result from persistent downwelling after an algal bloom of high-biomass dinoflagellates. The accumulation of dinoflagellates and other forms of biomass nearshore due to downwelling will eventually cause nutrient depletion and mortality of organisms. As the biomass decays, oxygen becomes depleted by heterotrophic bacteria, inducing anoxic conditions.

Related Research Articles

<span class="mw-page-title-main">North Atlantic Deep Water</span> Deep water mass formed in the North Atlantic Ocean

North Atlantic Deep Water (NADW) is a deep water mass formed in the North Atlantic Ocean. Thermohaline circulation of the world's oceans involves the flow of warm surface waters from the southern hemisphere into the North Atlantic. Water flowing northward becomes modified through evaporation and mixing with other water masses, leading to increased salinity. When this water reaches the North Atlantic, it cools and sinks through convection, due to its decreased temperature and increased salinity resulting in increased density. NADW is the outflow of this thick deep layer, which can be detected by its high salinity, high oxygen content, nutrient minima, high 14C/12C, and chlorofluorocarbons (CFCs).

<span class="mw-page-title-main">Upwelling</span> Replacement by deep water moving upwards of surface water driven offshore by wind

Upwelling is an oceanographic phenomenon that involves wind-driven motion of dense, cooler, and usually nutrient-rich water from deep water towards the ocean surface. It replaces the warmer and usually nutrient-depleted surface water. The nutrient-rich upwelled water stimulates the growth and reproduction of primary producers such as phytoplankton. The biomass of phytoplankton and the presence of cool water in those regions allow upwelling zones to be identified by cool sea surface temperatures (SST) and high concentrations of chlorophyll a.

<span class="mw-page-title-main">Ocean gyre</span> Any large system of circulating ocean surface currents

In oceanography, a gyre is any large system of circulating ocean surface currents, particularly those involved with large wind movements. Gyres are caused by the Coriolis effect; planetary vorticity, horizontal friction and vertical friction determine the circulatory patterns from the wind stress curl (torque).

<span class="mw-page-title-main">Kuroshio Current</span> North flowing ocean current on the west side of the North Pacific Ocean

The Kuroshio Current, also known as the Black Current or Japan Current is a north-flowing, warm ocean current on the west side of the North Pacific Ocean basin. It was named for the deep blue appearance of its waters. Similar to the Gulf Stream in the North Atlantic, the Kuroshio is a powerful western boundary current that transports warm equatorial water poleward and forms the western limb of the North Pacific Subtropical Gyre. Off the East Coast of Japan, it merges with the Oyashio Current to form the North Pacific Current.

<span class="mw-page-title-main">California Current</span> Pacific Ocean current

The California Current is a cold water Pacific Ocean current that moves southward along the western coast of North America, beginning off southern British Columbia and ending off southern Baja California Sur. It is considered an Eastern boundary current due to the influence of the North American coastline on its course. It is also one of six major coastal currents affiliated with strong upwelling zones, the others being the Humboldt Current, the Canary Current, the Benguela Current, the Oyashio Current, and the Somali Current. The California Current is part of the North Pacific Gyre, a large swirling current that occupies the northern basin of the Pacific.

<span class="mw-page-title-main">Alaska Current</span> Warm-water current flowing nortwards along the coast of British Columbia and the Alaska Panhandle

The Alaska Current is a southwestern shallow warm-water current alongside the west coast of the North American continent beginning at about 48-50°N. The Alaska Current produces large clockwise eddies at two sites: west of the Haida Gwaii and west of Sitka, Alaska.

<span class="mw-page-title-main">Loop Current</span> Ocean current between Cuba and Yucatán Peninsula

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<span class="mw-page-title-main">Ekman transport</span> Net transport of surface water perpendicular to wind direction

Ekman transport is 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 90° anticlockwise. 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 of the 1890s. Ekman transport has significant impacts on the biogeochemical properties of the world's oceans. This is because it leads to upwelling and downwelling 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.

<span class="mw-page-title-main">Ecosystem of the North Pacific Subtropical Gyre</span> Major circulating ecosystem of ocean currents

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<span class="mw-page-title-main">Somali Current</span> Ocean boundary current that flows along the coast of Somalia and Oman in the Western Indian Ocean

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<span class="mw-page-title-main">Haida Eddies</span>

Haida Eddies are episodic, clockwise rotating ocean eddies that form during the winter off the west coast of British Columbia's Haida Gwaii and Alaska's Alexander Archipelago. These eddies are notable for their large size, persistence, and frequent recurrence. Rivers flowing off the North American continent supply the continental shelf in the Hecate Strait with warmer, fresher, and nutrient-enriched water. Haida eddies are formed every winter when this rapid outflow of water through the strait wraps around Cape St. James at the southern tip of Haida Gwaii, and meets with the cooler waters of the Alaska Current. This forms a series of plumes which can merge into large eddies that are shed into the northeast Pacific Ocean by late winter, and may persist for up to two years.

<span class="mw-page-title-main">Stratification (water)</span> Layering of a body of water due to density variations

Stratification in water is the formation in a body of water of relatively distinct and stable layers by density. It occurs in all water bodies where there is stable density variation with depth. Stratification is a barrier to the vertical mixing of water, which affects the exchange of heat, carbon, oxygen and nutrients. Wind-driven upwelling and downwelling of open water can induce mixing of different layers through the stratification, and force the rise of denser cold, nutrient-rich, or saline water and the sinking of lighter warm or fresher water, respectively. Layers are based on water density: denser water remains below less dense water in stable stratification in the absence of forced mixing.

A Wind generated current is a flow in a body of water that is generated by wind friction on its surface. Wind can generate surface currents on water bodies of any size. The depth and strength of the current depend on the wind strength and duration, and on friction and viscosity losses, but are limited to about 400 m depth by the mechanism, and to lesser depths where the water is shallower. The direction of flow is influenced by the Coriolis effect, and is offset to the right of the wind direction in the Northern Hemisphere, and to the left in the Southern Hemisphere. A wind current can induce secondary water flow in the form of upwelling and downwelling, geostrophic flow, and western boundary currents.

Open ocean convection is a process in which the mesoscale ocean circulation and large, strong winds mix layers of water at different depths. Fresher water lying over the saltier or warmer over the colder leads to the stratification of water, or its separation into layers. Strong winds cause evaporation, so the ocean surface cools, weakening the stratification. As a result, the surface waters are overturned and sink while the "warmer" waters rise to the surface, starting the process of convection. This process has a crucial role in the formation of both bottom and intermediate water and in the large-scale thermohaline circulation, which largely determines global climate. It is also an important phenomena that controls the intensity of the Atlantic Meridional Overturning Circulation (AMOC).

<span class="mw-page-title-main">Kuroshio Current Intrusion</span> Movement of water from the Pacific to the West Philippine/South China Sea

The Kuroshio Current is a northward flowing Western Boundary Current (WBC) in the Pacific Ocean. It is a bifurcation arm of the North Equatorial Current and consists of northwestern Pacific Ocean water. The other arm is the southward flowing Mindanao Current. The Kuroshio Current flows along the eastern Philippine coast, up to 13.7 Sv... of it leaking into the Luzon Strait - the gap between the Philippines and Taiwan - before continuing along the Japanese coast. Some of the leaked water manages to intrude into the South China Sea (SCS). This affects the heat and salt budgets and circulation and eddy generation mechanisms in the SCS. There are various theories about possible intrusion paths and what mechanisms initiate them.

Eddy saturation and eddy compensation are phenomena found in the Southern Ocean. Both are limiting processes where eddy activity increases due to the momentum of strong westerlies, and hence do not enhance their respective mean currents. Where eddy saturations impacts the Antarctic Circumpolar Current (ACC), eddy compensation influences the associated Meridional Overturning Circulation (MOC).

Low-nutrient, low-chlorophyll (LNLC)regions are aquatic zones that are low in nutrients and consequently have low rate of primary production, as indicated by low chlorophyll concentrations. These regions can be described as oligotrophic, and about 75% of the world's oceans encompass LNLC regions. A majority of LNLC regions are associated with subtropical gyres but are also present in areas of the Mediterranean Sea, and some inland lakes. Physical processes limit nutrient availability in LNLC regions, which favors nutrient recycling in the photic zone and selects for smaller phytoplankton species. LNLC regions are generally not found near coasts, owing to the fact that coastal areas receive more nutrients from terrestrial sources and upwelling. In marine systems, seasonal and decadal variability of primary productivity in LNLC regions is driven in part by large-scale climatic regimes leading to important effects on the global carbon cycle and the oceanic carbon cycle.

Eddy pumping is a component of mesoscale eddy-induced vertical motion in the ocean. It is a physical mechanism through which vertical motion is created from variations in an eddy's rotational strength. Cyclonic (Anticyclonic) eddies lead primarily to upwelling (downwelling). It is a key mechanism driving biological and biogeochemical processes in the ocean such as algal blooms and the carbon cycle.

<span class="mw-page-title-main">Southern Ocean overturning circulation</span> Southern half of the global ocean current system

Southern Ocean overturning circulation is the southern half of a global thermohaline circulation, which connects different water basins across the global ocean. Its better-known northern counterpart is the Atlantic meridional overturning circulation (AMOC). This circulation operates when certain currents send warm, oxygenated, nutrient-poor water into the deep ocean (downwelling), while the cold, oxygen-limited, nutrient-rich water travels upwards at specific points. Thermohaline circulation transports not only massive volumes of warm and cold water across the planet, but also dissolved oxygen, dissolved organic carbon and other nutrients such as iron. Thus, both halves of the circulation have a great effect on Earth's energy budget and oceanic carbon cycle, and so play an essential role in the Earth's climate system.

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  23. ↑ "Downwelling". oceanexplorer.noaa.gov. Retrieved 2023-11-27.
  24. ↑ "Kelvin Wave - an overview | ScienceDirect Topics". www.sciencedirect.com. Retrieved 2023-11-27.
  25. ↑ NASA. "International Sea Level Satellite Spots Early Signs of El Niño". NASA Jet Propulsion Laboratory (JPL). Retrieved 2023-11-27.
  26. ↑ "ENSO". www.weather.gov. Retrieved 2023-12-06.
  27. ↑ Marinov, I.; Gnanadesikan, A.; Toggweiler, J. R.; Sarmiento, J. L. (June 2006). "The Southern Ocean biogeochemical divide". Nature. 441 (7096): 964–967. Bibcode:2006Natur.441..964M. doi:10.1038/nature04883. ISSN   1476-4687. PMID   16791191. S2CID   4428683.
  28. ↑ Tian, Ru Cheng; Vézina, Alain F.; Deibel, Don; Rivkin, Richard B. (2003-10-03). "Sensitivity of biogenic carbon export to ocean climate in the Labrador Sea, a deep‐water formation region". Global Biogeochemical Cycles. 17 (4): 1090. Bibcode:2003GBioC..17.1090T. doi: 10.1029/2002gb001969 . ISSN   0886-6236.
  29. 1 2 Emerson, Steven R.; Hedges, John (2008). Chemical Oceanography and the Marine Carbon Cycle (4th ed.). Cambridge. pp. 205–206. ISBN   978-0-521-83313-4.

Wind-Driven Surface Currents: Upwelling and Downwelling Background

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