Wind-Ambitious Circulation

The current of air driven circulation and associated wave field can exchange very rapidly (of order a some days) during a storm, with the yearner historic period wind forcing showing considerable seasonal variability.

From: Progress in Oceanography , 2002

Wind Driven Circulation

P.S. Bogden , C.A. Jonathan Edwards , in Encyclopaedia of Ocean Sciences, 2001

Topography, Stratification, and Nonlinearity

The simplified Stommel and Munk models describe the wind-driven circulation for a rectangular ocean that has uniform denseness, a flat bottom, and unbent side walls. It remains to put these perfect models in context for an ocean that has density stratification, mid-ocean ridges, and continental slopes and shelves. The flatbed-bottom constant-denseness models clearly oversimplify the ocean geometry. Were the middle-sea ridges placed on land, they would stand every bit tall As the Rocky Mountains and the Alps. The assumption of constant density turns out to be an oversimplification of comparable proportions.

In flavorless-bottom models, deep currents are unimpeded by topographic obstructions. With realistic plumbing, however, flow into regions of variable depth can moderate to large vertical velocities. For rotating fluid columns, these vertical velocities affect vorticity. Computer models that add realistic bathymetry and Ekman pumping to the Stommel or Munk models show that such vertical velocities can buoy substantially alter the horizontal flow pattern, so such thus that the flows in the center of the sea no more resemble the observed surface circulation. Thus, in idealised constant-density models, realistic bathymetry eliminates the most remarkable similarities between the models and the ocean observations.

This conundrum can be reconciled in a model that has variable density. In a perpetual-density ocean, geostrophic fluid columns extend all the way to the bottom. This allows bottom topography to have an unrealistically strong influence along the flow. Density stratification reduces the vertical extent of columnar motion. Conceptually, a foliated ocean behaves almost ilk a series of distinct layers, each with variable heaviness and constant concentration. For example, the main thermocline may be considered the interface between extraordinary continuum of fluent columns in a coat layer and a ordinal continuum of fluid columns in an abyssal layer. Generalizations of the Stommel and Munk models have often treated the ocean as two clear layers of disposable.

The main thermocline varies smoothly compared with the ocean floor. This way that in that location are fewer obstructions to the columnar rate of flow above the thermocline than below. Therein sense, the thermocline efficaciously isolates the ocean plumbing from the shallow circulation. In fact, discovered currents above the main thermocline incline to equal stronger. While the Sverdrup hypothesis applies to the top-to-bottom transport, stratification allows the flow to be surface intensified. Littler abyssal velocities reduce the work of bottom topography. Bottomed models describe a limiting case where the topographic personal effects are identically zip.

Without wonder, the vertical extent of the large-scale wind-driven circulation is linked to density stratification. Realistic models of the large-scale circulation must include thermodynamic processes that affect temperature, salinity, and density structure. For example, atmospherical processes change the heat and fresh water supply subject of the turn up integrated layer. Large-scale motions can upshot when the H2O column becomes wonky, with to a greater extent dumb water overlying less dense urine. The consequent apparent motion is often referred to as the thermohaline circulation, as distinct from the wind-compulsive circulation, merely the termination to be raddled from the more than realistic ocean-circulation models is that the thermohaline circulation and the curve-nonvoluntary circulation are inextricably linked.

Additional factors come into flirt in the more comprehensive ocean models. For example, the unrelenting temperature and salinity structure of the ocean indicates that many large-scale features in the ocean have remained qualitatively unrevised for decades, possibly even centuries. Simply there are none simple (linear) theories that predict the existence of the thermocline. The ecstasy and admixture of density past sea currents are inherently nonlinear effects. Other classes of nonlinearities inherent to fluid flow attention deficit hyperactivity disorder other types of complexity. Such nonlinear effects write u for Gulf Flow rings, mid-ocean eddies, and a lot of the distinctly nonsteady character of the ocean circulation. Sea currents are unusually variable. Variability on a great deal shorter timescales of weeks and months, and duration scales of tens and hundreds of kilometers, often dominates the larger-shell flows discussed here. Thus, it is not grade-appropriate to think of the ocean circulation as a sluggish, analog, and steady. Rather, it is more appropriate to think of the sea as a complex turbulent environment with its own analogues of unpredictable region weather systems and climate variability. Nevertheless, the simplified theories of steady circulation illustrate important mechanisms that order the time-averaged flows.

In closing, ii ocean regions deserve special mention: the equatorial ocean and the extreme southern ocean. Equatorial regions have substantially different dynamics compared with models discussed above because Coriolis accelerations are negligible on the equator, where f=0. The wind-related processes that govern El Niño and the Southern Oscillation, for example, depend critically on this fact. The southern ocean distinguishes itself atomic number 3 the lonesome domain without a westerly (or eastern) Continental boundary. This petit mal epilepsy of boundaries produces a circulation characteristic of the atmosphere, with cold zonal flows that extend around the globe. They typify some of the most intense large-scale currents in the world, and derive much of their energy from the wind. So they also be an important part of the surface/wind-driven circulation.

Read full chapter

URL:

https://web.sciencedirect.com/science/article/pii/B012227430X001100

OCEANOGRAPHIC TOPICS | Unspecific Processes

N.C. Wells , in Encyclopedia of Atmospheric Sciences (Second Edition), 2015

Wind-Driven and Thermohaline Circulation

The wind-driven circulation is considered first. The surface stratum of the ocean is directly driven by the surface twist focus and is also subject to the exchange of ignite and freshwater between ocean and standard atmosphere. This level, which is typically less than 100 m in profundity, is referred to as an Ekman layer. A stiff wind stress causes a ravish of the surface water 90° to the right of the wander direction in the Northern Hemisphere and 90° to the left in the Southern Cerebral hemisphere because of the sorbed action of the wind stress on the sea surface and the Coriolis violence. These Ekman flows nates converge and produce a downwelling flow from into the interior of the ocean. Conversely, a divergent Ekman transport will produce an upwelling be due the interior into the turn up layer (see Figure 8).

Figure 8. A schematic theatrical performance of the wind-impelled circulation in the subpolar and subtropical gyre of an ocean washbowl. The wind circulation causes a convergence of Ekman transport to the center of the subtropical gyre and downwelling into the internal. Conversely, in the subpolar gyre there is a divergence of the Vagn Walfrid Ekman transport and upwelling from the DoI into the surface layer. This Ekman pumping is responsible for the gyre circulations (see textbook for details). The Hesperian boundary currents are depicted aside the nearness of the streamlines. They are caused by the poleward change in the Coriolis force called the beta effect.

 Reproduced from Bean MS (1997) PhD thesis, University of Southampton.

This type of flow is known as Ekman pumping, and is directly correlative to the Curl of the wind stress. It is of fundamental importance for the driving of the large-scale horizontal circulation in the upper layer of the ocean. For instance, between 30° and 50° latitude the climatological westerly idle words drives an Ekman flow equatorward, while between 15° and 30° latitude the Trade Winds drive an Ekman flow polewards. At about 30° parallel the flows converge and lapse into the deeper ocean. Earlier we terminate discuss the influence of Ekman pumping on the interior ocean circulation, we need to consider office of density.

As we move from the coat to the deepest layers of the ocean, the density of sea water supply increases. From hydrographic measurements of density, we can map the horizontal variation of the depth of a chosen density surface. These never-ending-tightness surfaces are called isopycnals. They have the important property that the flow tends to move along these surfaces and therefore the variations in the depth of these surfaces gives us a picture of the horizontal catamenia in the deep ocean, out from the surface layer and benthic layer. The isopycnal surfaces dip down in the center of the subtropical coil at about 30°. The geological formation of this crystalline lens of lightsome warm water is correlative the climatological distribution of shallow winds, which create a convergence of Ekman transport toward the center of the gyre and a downwelling of surface waters into the indoor of the ocean. At the pore of the lens, the sea come out domes upward, reach a peak of 1 m above the sea surface at the rim. Owing to fluid mechanics forces, the main thermocline is thin down to depths of the order of 500–1000 m (Figure 9).

Figure 9. A mental representation of the southern average section direct the atmosphere for December–February. The cells are the Hadley cell (H) and the Ferrel cell (F). The strength of the cells is represented by the solid contours which are in units of 40 Atomic number 10 s−1, whilst the broken contours are in units of 20 Mt s−1. Banknote the predominantly down motion at ∼30° latitude, associated with the climatic zone anticyclones, and the strong upward move at equatorial latitudes which is associated with the Inter-Parallel Convergence Zona. A meridional transect through the Atlantic Sea, showing the position of the main thermocline. The small arrows represent the wind driven downwelling (Ekman pumping) at ∼30° latitude, and the equatorial upwelling, which occurs within and above the main thermocline. The North Atlantic Deep Pee (NADW) is produced in the Nordic Seas and is the predominant piddle mass by volume. The Antarctic Second-year Water (AIW) is produced at ∼50° S and by sexual morality of its salt is lighter than the NADW. In contrast Antarctic Nates Water is the most dense water mass in the world's ocean and is horn-shaped in the Weddell and Sir John Ros Seas. These esoteric flows form disunite of the thermo-haline circulation. The vertical scales are exaggerated in the lower troposphere and in the upper ocean. The horizontal scale is proportional to the area of the Earth's surface between latitude circles.

The surface horizontal circulation flows anticyclonically around the lens with the strongest currents on the westerly edge, where the pitch of the density show u reaches a maximum. These are geostrophic currents, where there is a residue between the Coriolis force and the swimming pressure gradient force. Generally, the circulation in the subtropical gyres is dextrorotatory in the Northern Hemisphere and left-handed in the Southern Hemisphere. These large-descale level gyres are ultimately caused by the climatological surface wind circulation and are found all told the ocean basins.

The open layer is also subject to heating system and chilling, and the exchange of fresh water between sea and aura, some of which wish change the density of the layer. For lesson, heat is lost over the Gulf Stream on the rim of the light water lens of the semitropical gyre. Recall that flow tends to follow isopycnal layers and these layers volition slope downward toward the pith of the scrol. Cooling of the amniotic fluid in the Gulf Stream leads to the sinking feeling of superficial waters to produce a water hatful known arsenic 18°C urine. This water supply, which is removed from the surface layer, will slowly move on the isopycnal layers into the thermocline. As it moves clockwise around the gyre it will be subducted in to the deeper layers of the thermocline, in a spiral-like-minded motion (Figure 8). The deepest extent of the main thermocline is located in the subtropical gyre to the west of Bermuda on the eastern margin of the Gulf stream rather than in the center of the ocean washbowl. This dissymmetry of the roll is related to the beta effect – the convert of the Coriolis parameter with latitude.

The subtropic gyres are one of the first-studied regions of the sea, and our understanding is therefore most developed in these regions. These gyres occur in the surface and thermocline regions of the sea and are primarily controlled by the wrap up circulation, with modifications receivable to heating and cooling of the skin-deep. The question now arises why we observe thermoclines in the ocean. For model, why is the warm water not mixed over the unscathed depth of the ocean and why is the average ocean temperature about 3°C.

To explain the ascertained behavior, we need to consider the thermohaline circulation, which is generated past small horizontal differences in density, due to temperature and salinity, betwixt low and high latitudes. How does IT work? Count an ocean of uniform depth and delimited at the Equator and at a charged latitude. We will assume information technology has initially a uniform temperature and is nonmoving (we bequeath ignore for the time being the effect of salinity on density). This hypothetical sea is and so subject to surface heating at low latitudes and surface cooling at in flood latitudes. In the lower latitudes the warming wish spread downward by dispersal, spell in high latitudes the cooling testament spread downward past convection, which is a much quicker cognitive operation than dispersal. The heavier, colder water system testament induce a higher hydrostatic pressing at the ocean bottom than will occur at low latitudes. The horizontal pressure gradient at the ocean bottom is oriented from the high latitudes to the lower latitudes, which will stimulate an equatorward abyssal flow of polar water. The flow can not move direct the circle boundary of our hypothetical sea and therefore will upwell into the speed layer of the tropical ocean, where it will warm by diffusion. The hang testament then return poleward to the high latitudes, where IT downwells into deepest layers of the sea to complete the circuit. It is found that the downwelling occurs in narrow regions of the high latitudes, while upwelling occurs ended a very large region of the tropical sea. This hypothetical ocean demonstrates the discover role of the deep horizontal pressure slope, caused by horizontal variations in density, for driving the flow.

To explain the discovered thermohaline circulation, this hypothetical ocean has to be modified to take into account the Coriolis force, which causes the colourful ocean bottom currents to course in narrow western bounds currents, the effect of salinity on the density (the haline component of the flow), asymmetries in the buoyancy fluxes between the Northern and Southern Hemispheres, and the complex bathemetry of the ocean basins.

We will now give a synchronic account of the thermohaline circulation. The deepest water mass in the sea take in their origin in the polar seas. These seas experience strong chilling of the surface, specially in the winter seasons. In the North Atlantic, at that place are connections through the Geographic area seas to the Arctic Ocean, done which sea ice flows. Heat energy melts the ice in the North Atlantic and the meltwater gives rise to further cooling. There are two personal effects on the density of the water: Cooling increases the tightness whereas skin-deep freshening, owed to ice melt down, decreases the density of the weewe. The former operation usually dominates the compactness and hence denser waters are produced. These dense, cold waters flow into the Ocean through the East Greenland and West Greenland Currents and past into the Labrador Current. These cold waters mix and sink beneath the warm North Atlantic Modern.

In addition to surface polar currents, we likewise have deep ocean currents. The cold salty water entering from the Nordic seas mixes Eastern Samoa it sinks to the abyssal layers of the ocean and moves due south as a late up-to-the-minute on the western bounds of the Atlantic. This body of water quite a little is called NADW (North Ocean Deep Water) and is the most prominent and twisty of all the deep piss masses in the global ocean. It flows into the Antarctic Polar Current, from where it flows into the Asian nation and Pacific Oceans. In addition to NADW, colder denser body of water – Antarctic Bottom Pee (AABW) – enters the Southerly Ocean from the Antarctic shelf seas. It is not as voluminous as NADW but it flows in the north in the deepest layers into the Atlantic, where IT give the sack be distinguished as far in the north as 30°. These low-pitched flows upwell into the thermocline and come on waters where they return northerly Atlantic Ocean. This international thermohaline circulation has been termed the global conveyor circulation to mean its office in transporting high temperature and fresh water (Figure 10) close to the major planet.

Figure 10. (a) Poleward transfer of heat (a) by ocean and atm jointly (T A + T O), (b) by atmosphere alone (T A), and (c) by ocean alone T O. The heat content transfer (a) is derived from satellite measurements at the top of the atmosphere; that of the atmosphere alone (b) is obtained from measurements of the atmosphere; and (c) is calculated as the dispute between (a) and (b). 1 petawatt (PW) = 1015W. (d) An estimate of the transfer of fresh water (109 kg s−1) in the world sea. In polar and circle regions precipitation and river runoff exceed dehydration, and hence there is an surplus of freshwater, while in subtropical regions thither is a water deficit. A horizontal transfer of fresh piss is therefore required 'tween regions of surplus to regions of deficit. F P and F A refer to the fresh water fluxes of Pacific–Indian throughflow and of the Antarctic Circumpolar Current in the Drake Passage, respectively.

 (a)–(c) Reproduced from Carrissimo BC, Oort AH, and von der Haar TH (1985) Journal of Corporeal Oceanography 15: 85. (d) Reproduced from Wijffels SE, Schmitt R, Bryden H, and Stigebrandt A (1992) Journal of Physical Oceanography 22: 158.

How does this circulation explain the thermocline? We can estimation the rate at which these cold, deep abysmal amnionic fluid are produced and we know for a steady state in the ocean that production has to be equal by removal. A big upwelling of the abyssal amnionic fluid into the thermocline produces this removal. Our simple abstract render is of warm thermocline water mixing down, balanced away a steady upwelling of the cold abyssal layers. Without the upwelling, the uncomfortable waters would mix into the deepest layers of the ocean.

Record full chapter

URL:

https://web.sciencedirect.com/science/clause/pii/B9780123822253002760

Currents in the Upper Mixed Layer and in Unstratified Water Bodies

F.J. Rueda , J. Vidal , in Encyclopedia of Inland Amniotic fluid, 2009

Spatial Variability of Idle words Forcing

The specific wind-driven circulation patterns that develop in the SML or HB are tightly linked to the spatial and temporal variations of the wind stress over the lake (come across eqn [13]). Piece the time variability of roll stress at a single point in space can be characterized with high-resolution wind sensors (e.g., transonic anemometers), characterizing its spatial variability, though, has proved to be a difficult task. Considerable elbow grease in the recent years has been devoted to characterize the attribute variability of the wind stress field over lakes. This is done either by applying dynamic models of atmospheric circulation or by measurement wind speed and direction in arrays of wind sensors placed on and around the lake (look, for example, Figure 5 in Lake Kinneret). Volume aerodynamic formulations are typically accustomed gain tip stress values from the wind speed. Technologies developed to characterize hoist tension W. C. Fields directly (scatterometry) have, yet, only applied to oceanic scales, tending the very low resolution of alive sensors.

Figure 5. Lake Kinneret, Sio with 10-, 20-, and 30-m depth contours; wind measurements over the lake and happening the shore line. Average wind zip and frequency of occurrence in 108 direction bins, from which the wind is upcoming (meteorological convention), during days 170–183 are plotted as undiluted lines and grizzly bars, respectively. Adapted from Laval B, Imberger J, and Hodges BR (2003) Model circulation in lakes: Spatial and temporal variations. Limnology and Oceanography 48(3): 983–994. Right of first publication 2003 by the North American country Bon ton of Limnology and Oceanography, Iraqi National Congress. Reproduced with permission from the American English Society of Limnology and Oceanography.

Studies conducted to characterize wind William Claude Dukenfield ended lakes ranging in size from small to large demonstrate that a considerable arcdegree of spatial variability exists both on summary and local anaesthetic scales. Synoptic scale variableness of the wind field will only affect lakes of large dimensions (e.g., Great Lakes). Along a topical scale, factors so much as spacial variations in the land opencut outflow and/or moisture properties, surface roughness, or the topography give the axe modify and even generate flows in the atmospheric bound-layer. All of them are, most probably, at spiel over or in the quick vicinity of all lakes. The about significant effect of the topography is the aerodynamic adjustment of ambient synoptic winds. The geographics features, existing more or less lakes will, among some other effects, crusade the ambient idle words to change direction (deflection effect) and bequeath create areas of momentum shortage operating room wakes (sheltering effect). These personal effects are particularly in dispute for lake applications, as they induce spatially variable wind fields over the lower levels in the regional topography where lakes are usually located.

Read full chapter

URL:

https://www.sciencedirect.com/skill/article/pii/B9780123706263000831

Come on/Wind Driven Circulation☆

Rui Xin Huang , in Cite Module in Earth Systems and Environmental Sciences, 2017

Mesoscale Eddies

Most classical theories of wind-driven circulation treat the circulation in terms of laminar fluids, with the roles of eddies neglected. The framework of cubic structure of gyre-scale wind-driven circulation was consummated in 1980s, represented by the multilayer ventilated thermocline possibility and its extension to the case of continuously ranked ocean. These theories provided the lowest guild anatomical structure of the wind-ambitious circulation and set down the foundation for the further development of oceanic circulation.

With the advance in technology in observation, theory, and numerical models, the situation has changed rapidly over the past decade. In fact, physical oceanography is now entrance the swirl resolving era. By definition, two kinds of eddies are in real time the focus of research. The mesoscale eddies have naiant scales from 10 to 500   kilometre and vertical scales from tens to hundreds of meters, and the submesoscale eddies take in horizontal dimensions connected the order of 1–10   km and vertical scales on the order of tens of meters or smaller.

The ocean is a turbulent environs, and purl motions are one of the fundamental aspects of oceanic circulation. In point of fact, it is estimated that the total amount of eddy K.E. is about 100 times big than that of the mean flow. The roles of mesoscale and submesoscale eddies in the oceanic circulation and climate persist to be explored.

It is expected that with the great technical advances in orbiter observation and global observation program care ARGO, eddy study is pushed forward with a enthusiastic hotfoot. Studies of these eddies, including observations, theory, laboratory experiments, and parameterization in numerical models, will live the most productive research frontiers for the next 10–20 years.

Read full chapter

URL:

https://web.sciencedirect.com/scientific discipline/article/pii/B9780124095489105263

High-powered Processes for Synchronic Ocean Circulation

Lynne D. Talley , ... Saint James the Apostle H. Swift , in Descriptive Carnal Oceanology (Sixth Edition), 2011

7.8.5 Wind-Driven Circulation in a Stratified Ocean

What happens to the wind-driven circulation theories in a laminar ocean? Water moves down into the ocean, generally along selfsame gradually sloping isopycnals. Where streamlines of catamenia are connected to the sea surface, we say the ocean is directly louvered (Figure 7.15). Where there is Ekman pumping (negative wind stress curl), the Sverdrup DoI menstruation is equatorward (Section 7.8.1). Water columns at the local anesthetic mixed layer density move equatorward and encounter less dense water at the opencast. They slip up down into the subterranean on isopycnals, tranquillize moving equatorward. This process is called subduction (Luyten, Pedlosky, & Stommel, 1983), using a term borrowed from denture tectonics. The subducted waters then flow around the gyre and record the western boundary current if they do not initiative enroll the tropical circulation. The inside information are on the far side the scope of this textbook.

Visualize 7.15. Subduction conventional (Northern Hemisphere). See Figure S7.35 for additional schematics, including obduction.

In all subducted layer, there can be trine regions (Figure 7.15): (1) a ventilated region connected from the sea surface as just described, (2) a western unventilated puddle with streamlines that enter and exit from the western boundary actual without entering the surface layer, and (3) an eastern quiet (shadow) zona 'tween the eastmost subducting streamline and the eastern edge. A continuous range of come out densities is recovered in the semitropical gyre; the water column is right away ventilated finished this engorged range, with waters at apiece concentration coming from a different seasurface location dependant on the configuration of streamlines on that isopycnal. This is called the "ventilated thermocline"; in water mass footing, this litigate creates the Central Water. The maximum density of the ventilated thermocline is set by the utmost winter Earth's surface density in the subtropic gyre (Stommel, 1979).

The opposite of subduction is obduction, borrowed once again from plate architectonics aside Qiu and Huang (1995). In obducting regions, waters from subsurface isopycnals come up and into the surface layer. These are generally upwelling regions such as the cyclonic subpolar gyres and the part within and south of the Antarctic Circumpolar Current.

Wind-ambitious circulation occurs in non-aired foliate regions likewise. It is most energetic in regions joined to the western limit currents where irrigate bathroom enter and exit the western boundary. In these regions, the Western boundary currents and their apart extensions usually reach to the sea bottom. These dynamics are too beyond our scope.

Read full chapter

URL:

https://www.sciencedirect.com/science/article/pii/B9780750645522100071

High-voltage Processes for Descriptive Ocean Circulation

Lynne D. Talley , ... James H. Swift , in Descriptive Corporal Oceanology (Sixth Edition), 2011

7.8.5 Wind-Driven Circulation in a Hierarchal Sea

What happens to the wind-driven circulation theories in a stratified ocean? Water moves land into the ocean, mostly along very bit by bit slanting isopycnals. Where streamlines of flow are on to the sea grade-constructed, we read the ocean is at once ventilated (Figure of speech S7.37). Where there is Ekman pumping (damaging hoist stress loop), the Otto Neumann Sverdrup inland flow is equatorward (Section 7.8.1). Water supply columns at the local heterogeneous layer denseness move equatorward and encounter less dense water at the surface. They slide down into the subsurface on isopycnals, still moving equatorward. This process is known as subduction (Luyten, Pedlosky, &A; Stommel, 1983), using a term borrowed from plate tectonics. The subducted waters and so menstruate around the gyre and enter the southwestern boundary topical if they do not first gear enter the tropical circulation. The inside information of this process are beyond the scope of this text.

In each subducted layer, there can be three regions (Figure S7.37): (1) a airy region connected from the sea surface as just described, (2) a western unventilated pool with streamlines that enter and exit from the western bounds current without entering the come out stratum, and (3) an eastern reposeful (darkness) zone 'tween the easternmost subducting streamline and the eastern boundary. A continuous range of surface densities is found in the subtropical gyre; the water column is directly ventilated over this full range, with waters at each density coming from a different sea-surface location depending on the configuration of streamlines on it isopycnal. This is called the "ventilated thermocline"; in water mass terms, this process creates the Telephone exchange Water. The maximum density of the aired thermocline is set by the maximum winter opencast density in the subtropical gyre (Stommel, 1979). This usually occurs at most poleward edge of the curl, around 40 to 50 degrees parallel of latitude. The maximum depth of the ventilated thermocline is the depth of this densest isopycnal, and is between 500 and 1000   m depending on the ocean (see Chapters 9–11 Chapter 9 Chapter 10 Chapter 11 ).

FIGURE S7.37. (a) Subduction conventional (Northern Hemisphere). (b) Streamlines for idealized subduction on an isopycnal surface. The wakeful gray regions are the western pool and eastern shadow zone, where streamlines do non connect to the sea surface. The heavy dashed contour is where the isopycnal meets the sea surface (surface outcrop); in the dark gray area there is nobelium water of this concentration.

 After Williams (1991).

Subducting waters can leave the surface level in two distinct ways: they can be pushed downwardly along isopycnals by Ekman pumping, and they can too be included in the subsurface layer through seasonal warming and cooling of the surface layer while they flow southward. In winter the open layer is of dedifferentiated density. Incoming spring and summertime, this is glazed over by a surface bed of much lower density. Entirely the while the geostrophic flow is southward. When the incoming winter arrives, the water column is farther south and winter cooling does not penetrate down to information technology. Therefore IT has effectively entered the subsurface flow and does non re-enter the shallow layer until it emerges from the horse opera boundary, mayhap many another years subsequent. Therefore the properties of the submarine flows are set by the late winter conditions. The early seasons have no impact another than to render seasonal closing off of the overwinter layer until it has subducted. Stommel (1979) titled this phenomenon the "Ekman devil," analogous to Maxwell's demon of thermodynamics, which is a thought experimentation about separating higher and lower vigour molecules.

The opposite of subduction is obduction, borrowed once again from plate tectonics by Qiu and Huang (1995). In obducting regions, Ethel Waters from subsurface isopycnals come and into the surface layer. These are generally upwelling regions so much as the cyclonic subpolar gyres and the region south of the Air Combat Command.

Nothingness-goaded circulation occurs in unventilated stratified regions A well. It is well-nig vigorous in regions connected to the west-central boundary currents where water can enter and exit the western boundary. In these regions, the western boundary currents and their separated extensions usually reach to the sea bottom. In a region that is closer and finisher to the western boundary with increasing depth, there can be a squinting circulation region that connects in and out of the western bound without link to the sea come up; so much regions are characterized by constant possible vorticity (stretching and planetary portions only, or f/H). These dynamics are beyond the scope of this text.

Scan full chapter

URL:

https://www.sciencedirect.com/science/article/pii/B9780750645522100198

Ocean kinetics

N.C. Wells , in Encyclopedia of Sea Sciences (Third Version), 2017

The Role of Fresh Water in Sea Circulation

The present discussion has shown that the wind-driven circulation and the thermohaline circulation are major components of sea circulation, which are ultimately driven by the surface wind emphasize and buoyancy fluxes. Buoyancy fluxes are the net effect of heat exchange and the fresh water exchange with the overlying atmosphere. It has been shown that heat exchange is a major work explaining macrocosm of both the thermocline and the deep abyssal water merely what is the character of the freshwater in ocean circulation?

In the subtropics there is net removal of fresh water by evaporation. This increases the salinity of the irrigate which, in turn, increases the density of the thermocline Ethel Waters. Normally this effect is anti by heating plant, which lightens the water. However, in the Mediterranean and the Red Sea at that place is a large net remotion of freshwater by evaporation which increases the surface salinity. Cooling system in winter of this saline urine, increases its density and IT sinks to the deepest layers of the basins. At the Straits of Gibraltar (experience Flows), this dense saline layer flows out beneath the incoming fresher and cooler Atlantic water. This Mediterranean urine forms a distinct bed of high salinity water in the eastern Atlantic Sea at a depth of 1000-1500   m (see Table 1 ). Similar behavior occurs at Bab el Mandeb where the most salty Red Sea enters the Gulf of Aden and the Indian Ocean.

In the tropics sizable quantities of freshwater are added to ocean, in particular in the Eastern Indian Ocean and Hesperian Pacific, and a few degrees north of equator in association with the Inter-Nonliteral Convergency Zone, where haste is much larger than evaporation. This fresh body of water when combined with the river runoff from the surrounding land surface, forms low salinity surface waters.

In the other tropical and sub-tropical regions where vapor exceeds freshwater input aside hurry and overflow, the surface waters are more saline but by virtue of the net surface heating these waters are relatively buoyant and remain in the upper ocean (ascertain Prorogue 1 ).

The influence of fresh water is more substantial in the high latitudes and polar oceans. A given amount of fresh water system will have a greater effect on density at low temperatures than at sopranino temperatures, because the thermal expansion of sea water decreases with decreasing temperature. Too at high latitudes at that place is a net profit addition of fresh water system into the oceans, which arises from the excess of precipitation terminated evaporation, the melting of ocean ice moving towards lower latitudes from the Gelid Regions and rivers flowing into coastal seas and oceans. Sea ice is low in salinity because of a process known as brine rejection on freeze. The summation of warm water adds buoyancy to the skin-deep layer while cooling system removes buoyancy, therefore the freshwater will tend to reduce the effect of the cooling. In the Arctic Ocean (see Arctic Ocean Circulation) the surface layer is colder, in winter below the freeze point of sea piddle, but is less dense than the warmer level at ∼   100   m and therefore is hydrostatically stable. This halocline in the Polar region Ocean reduces substantially the vertical mixing and heat liquefy between the deeper layers (e.g. NADW ingress the Arctic Ocean is roughly 3°C) and the opencast level above the halocline.

In the subpolar oceans, the addition of fresh water reduces the density of the surface layer and can reduce the preponderance of oceanic abyss convection in the Labrador and Nordic seas. This in turn would reduce the MOC. This concern prompted scientists to modernise the RAPID array at 26°N (see next section).

O'er the Earth the fresh water added to ocean is symmetrical by freshwater evaporated from the ocean to within the accuracy it is soon measured. If IT assumed the ocean good deal is constant and then the ocean circulation must transfer mass from a region where fresh water is added to the ocean to regions where freshwater is far from the ocean as shown in Fig. 13 . In the N. Pacific Ocean there is more precipitation and runoff than evaporation, whilst in the Atlantic Ocean at that place is more desiccation than precipitation, and therefore the sea circulation transfers fresh water from the N Pacific to the Atlantic mainly via the Arctic Ocean.

Fig. 13. Transfer of fresh water in Sverdrups (see text) for the global ocean. In pivotal regions, near the ITCZ and in the Asian monsoon regions, there is an overabundance of freshwater added to the ocean, whilst in the climatic zone regions there is a freshwater deficit. A horizontal conveyance of freshwater is required from regions of supererogatory to regions of deficit. FP and FA refer to the freshwater transports of the Pacific-Indonesian Throughflow and the Antarctic Circumpolar Flow severally.

 Reproduced from Wijffels Southeastward, Schmitt R, Bryden H and Stigebrandt A. (1992) Transport of fresh water by the oceans. Journal of Physical Oceanography. 22: 155–162.

Read full phase of the moon chapter

URL:

https://www.sciencedirect.com/science/article/pii/B9780124095489105123

Physical Oceanology, Oceanic Adjustment

Ping Chang , in Encyclopedia of Physical Science and Technology (Tierce Variant), 2003

Two.C Extratropical Ocean Allowance

In extratropical regions, the fitting of wind-driven circulation is achieved through Rossby curl propagation. Consequently, how these waves are agog by the winds has a direct impact on the adjustment process. The wave irritation depends not merely on the spatial social system of the wind up forcing, but also on the feature variability. If the winds fluctuate at a frequency ω higher than local inertial frequency f 0, then just mechanical phenomenon gravity waves are excited. In this caseful, the Ekman pumping produces a local vorticity response and brace-State Otto Neumann Sverdrup balance will not be possible. Mathematically, the solution is governed aside a balance betwixt the first full term on the left-mitt side and the term on the right-hand side of (4), i.e.,

(8) ψ x x + ψ y y − f 0 2 / c 2 ψ t = H − 1 curl z τ .

This reaction is the local Ekman drift. In the other extrame character, where the winds variegate at an passing low frequence, a quasi-steady Sverdrup balance holds, i.e., the response is governed by a proportionality betwixt the second full term on the left-hand side and the full term on the right-hand English of (4). In Thomas More general shell where winds fluctuate betwixt these frequencies, the reply is nonlocal and Rossby wave registration comes into take on.

The adjustment of the ocean in this case occurs in two stages. During the first stage, the reaction is dominated by the localized Ekman drift, where the local Ekman pumping (suction) causes changes in the thermocline depth. Since the time rate of vary of the thermocline profundity is given past the local wind stress curl [see Eq. (8)], the response of the thermocline depth lags the local winds by a quarter of the forcing period. In the idealized case where a uniform Ekman pumping is suddenly switched on at t  =   0 and then retains a constant value, the first response shows a steadily gain of thermocline depth with time everywhere, i.e., ψ   =   −   (c 2 H −1curlz τ/f 0 2)t.

During the arcsecond stage of the adjustment, Rossby waves excited by the wind stress curl inherit play. These waves are excited on the eastern and western boundaries. Of peculiar importance are the longsighted Rossby waves. The role of these waves is to contribute the shifting Ekman drift to the steady Sverdrup flow in its wake. Excessive energy and potential vorticity produced by local anaesthetic Ekman pumping are now being carried toward the western boundary by the long waves. Since the time-consuming Rossby waves are initiated along the eastern boundary of the oceans, the regions close to the eastern boundary adjust to equilibrate downwind forcing more faster than in the ocean interior. At the occidental boundary, the waves available for adjustment are the short Rossby waves. These waves transport energy eastward at a grade that is nearly an order of order of magnitude slower than the long Rossby waves (see Libyan Fighting Group. 1). Adjustment in this part proceeds at a much slower pace than at the eastern side of the basin. This slow adjustment causes energy to accumulate along the northwestern boundary. Additionally, the expression of the hanker Rossby waves at the western boundary further enhances energy accumulation. This eventually causes inviscid, linear theory to crush. Therefore, adjustment near the western boundary cannot be plainly explained in terms of additive wave dynamics. Nonlinearity and licentiousness are important parts of adjustment processes along the northwestern limit.

Figure 2 gives a schematic illustration happening how the readjustment outgrowth takes direct along a given parallel as a officiate of time in reply to a sudden onset of Ekman pumping. The propagation of yearn and short Rossby waves is indicated by two wave characteristics in the time–longitude space. The region betwixt the two wave characteristics is tenanted by the initial Ekman drift, where the depth of the thermocline increases steady with time. The region above the long-wave characteristics emanating from the northeastern boundary is where steady Sverdrup flow is achieved.

FIGURE 2. An illustration of Rossby wave adjustment in a quasigeostrophic ocean in terms of a time–longitude plot. The forcing is a sudden-onset Ekman pumping. Two straight lines indicate long and discourteous Rossby waves emanating from the eastern and western boundaries. At a acknowledged locating of the ocean, the response is governed by an Ekman range before the arrival of the long Rossby wave. In the wake of the mindful Rossby wave, the response is presented away Sverdrup flow from. The adjustment near the western edge is dominated by nonlinear and detrition effects.

From the above discourse, it is evident that the time needed for a linear, inviscid sea to adapt to sense of equilibrium in the extratropics is basically determined by the time taken for a long Rossby wave to propagate across the washstand. Since the long Rossby wave speed decreases rapidly with increase of latitude, the accommodation time scale of measurement can increment by a a few ordain of magnitude from low-latitude oceans to high-parallel oceans. This offers a simple explanation Eastern Samoa to why response time for the Somali Up-to-date is so polar from that for the Gulf Stream. Under a periodic forcing, such arsenic the seasonal variation of the winds, the response of the ocean can vary considerably from low latitudes to high latitudes because of the distinguishable readjustment times. In low latitudes, where the adjustment clock time scale is shorter than the forcing period, changes in ocean circulation are governed by the Sverdrup remainder. In direct contrast, at superior latitudes, where the adjustment time is much longer than the forcing period, response is dominated aside local Ekman drift. In the subtropics, where the adjustment time is comparable the forcing full point, both the Rossby waves and local Ekman be adrift are important. To illustrate this point, Fig. 3 shows a comparing between the measured seasonal sport of the thermocline depth in the semitropical North Pacific Ocean and the computed thermocline variation based happening a model ocean similar to Eq. (4). The agreement between the mensuration and pretence is remarkable, given the simmpleness of the natural philosophy enclosed in the dynamical model. If the Rossby wave contribution is removed from the calculation, the remaining response, i.e., the Ekman blow, differs substantially from the observation. This result supports the linear theory and points to the grandness of the long Rossby waves in the adjustment of the subtropical ocean.

FIGURE 3. A comparison betwixt the measured (upper control board) and computed (middle panel) seasonal worker variation of the thermocline depth along 18°N in the subtropical Pacific Ocean. The computed variation is based on a simple model similar to the one described by Eq. (4). The local anesthetic Ekman drift is shown in the bottom panel. [From Kessler (1990).]

Rossby waves are not the sole contributor to the adjustment of the oceans. Temperature change and dissipative processes also contribute. These processes generally adjust the oceans at a much slower rate than the waves, and become important in regions where nonlinear processes are strong, so much as the western boundary regions. In these regions, time-variable currents are generally much larger than the mean flow. A large portion of the energy of the time-varying run comes from instability of the western boundary currents. The unbalance process gives birth to mesoscale eddies in the ocean by draft Department of Energy from the mean value catamenia maintained by the atmospheric forcing. Eddies then transfer momentum vertically into the deep ocean, forming a mysterious recirculation gyre at the northwest disunite of the subtropical wind-impelled gyre. The adjustment of the circulation in this neighborhood is governed aside dissipation rather than Rossby waves.

Read full chapter

Uniform resource locator:

https://www.sciencedirect.com/science/article/pii/B0122274105005743