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by: Johnathon Miller I


Johnathon Miller I
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This 12 page Class Notes was uploaded by Johnathon Miller I on Wednesday September 9, 2015. The Class Notes belongs to PATH 700 at University of Washington taught by Staff in Fall. Since its upload, it has received 13 views. For similar materials see /class/192194/path-700-university-of-washington in Pathology at University of Washington.




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Date Created: 09/09/15
0C 210 TOPIC 2 CIRCULATION IN THE DEEP SEA Fall 2008 Readings Ocean Circulation Chap 6 sections 6367 Intro to Physical Oceanography on line Chap 13 PART 1 Objectives for this Topic 1 2 5 Learn about the major water masses that input deep water to the Deep Sea Learn how to identify and quantify the presence of these deep water masses based on their seawater properties T S nutrients 02 Learn the general circulation pathway in the Deep Sea Learn how to use natural and anthropogenic chemical compounds to estimate the rate of deep water circulation Impact of deep water circulation on climate and anthropogenic C02 uptake A How does thefarmalitm of deep waters affect the earth s climate Equot The cooling of northward owing warm surface water initially Via the Gulf Stream in the far N Atlantic has two effects Fig 1 a It increases the density of the water to the point where it can sink to great depth 20003000m and enter the circulation of the Deep Sea b It releases heat from the ocean to the atmosphere i This transfer of heat from the surface ocean to the atmosphere warms the air masses over the N Atlantic Air temperatures in lands surrounding the N America are warmer than land areas outside this region at similar latitudes because of this ocean air heat transfer 0 Climatologists theorize that a slow down in this deep water formation process played a key role in the occurrence of Ice Ages during the last 1 million years Models have indicated that a reduction in the formation rate deep water in the N Atlantic would have a cooling effect on air temperature over the N Atlantic Ocean and surrounding land areas Europe Canada Fig 2 a Less heat would be released to the atmosphere and air temperatures cool One concern among climatologists and oceanographers is about future changes in deep water formation rates and the subsequent impact on the earth s heat budget a One idea is that global warming will increase ice melt on the continents surrounding the N Atlantic eg Greenland Canada Scandinavia etc b This increased ice melt will lower the salinity of the surface waters and make it more dif cult for the surface water to sink as they cool c This reduction in deep water formation rate will in turn slow the rate at which surface currents eg Gulf Stream transport heat from the tropics to the polar latitudes d This reduced poleward heat ow will likely make the tropics warmer and the polar latitudes colder i The movie Day After Tomorrow scenario but much less dramatic 5 Ocean circulation models predict a slowing of deep water formation that would e result from the predicted surface salinity decrease due to increased ice melting in polar regions and surface water warming in the N Atlantic caused by anticipated increases in atmospheric greenhouse gas concentrations Fig 3 f Currently it is difficult to determine the likelihood that this will happen and the 39 39 of the l 39 1 change ie the accuracy of the model predictions is dif cult to determine Generally oceanographers agree that the rate of deep water formation in the N Atlantic is an important factor in maintaining the present distribution of temperature on earth and human induced perturbation of this formation rate could have signi cant climatic consequences B Characteristics ofthe seawater in the Deep Sea 1 Generally the Deep Sea is the portion of the ocean at depths greater than 1500m where the water is cold and fairly homogeneous in its properties compared to surface waters Fig 4 2 Typical Water Properties in the Deep Sea Cold temperatures typically 1 to 3 C Lower salinity than most surface waters typically 34535 High potential densities Se 277 to 280 primarily due to cold temperatures High nutrient concentrations and low dissolved oxygen concentrations as a result of respiration Remember Q photosynthesis occurs in the Deep Sea because it is much too deep for sunlight to penetrate The main source of oxygen in the ocean is the dissolution of oxygen gas in air which occurs at the surface of the ocean 4 Current speeds are very slow typically 0l cmsec usually too slow to measure directly with current meters so circulation pathways are not as well known as for surface ocean circulation pathways have largely been determined by theory and by using properties of seawater like 0 S oxygen nutrients to trace circulation pathways however in a few places eg along the western edge of the deep N Atlantic where North Atlantic Deep Water is owing southward current speeds can be fast enough gt1 cms to measure directly 5 The average water residence time in the Deep Sea is about 700 years thus it takes N700 years for the average water parcel in the Deep Sea depths gt 1500m to return back to the polar regions of ocean s surface where they originally sank if we divide the path length of deep water ow by the average time it takes to cover this path N700 years we nd that the mean current velocity is 0l cms or 30km per yr or 13quot per yr C Regions where surface waters sink into the Deep Sea 1 Deep water is formed when surface water gets very dense primarily cold and to a lesser degree salty and sinks to depths gt2000m This process occurs in regions where the air temperatures are very cold and water column stability AceAZ is low at polar latitudes 2 Two primary sites of deep water formation are the far north Atlantic Ocean off the coasts of Greenland Iceland and Norway and in the far S Atlantic off the coast of Antarctica primarily in the Weddell Sea Fig 5 M deep water forms in the Paci c or Indian oceans W In the far North Atlantic deep water is produced at three locations Fig 5 Greenland or Irminger Sea off the eastern coast of Greenland 80 of deep water formation in the N Atlantic Norwegian Sea between Iceland and northern Norway some of this deep water may come from the Arctic Ocean Labrador Sea between east coast of Labrador and western coast of Greenland 4 The surface water that ultimately cools to become deep water in the far N Atlantic originated as warm and salty tropical currents that owed northward via the Florida Current Gulf Stream and N Atlantic Current Fig 1 These surface water ultimately cool sufficiently by the time they reach the far N Atlantic near Greenland Labrador and Norway to reach densities that allow surface water to sink into the Deep Sea 5 The cumulative deep water formed by the sinking of surface waters in all three regions in the N Atlantic is called North Atlantic Deep Water or NADW The average ofNADW formation is about 15 Sv 15x106 m3s 6 At the other end of the Atlantic Ocean in the Weddell Sea off the coast Antarctica across from the tip of S America is the other primary site where deep waters are formed Fig 5 7 In the Weddell Sea deep waters are formed as a result of three processes Fig 6 a upwelling of deep cold water caused by winds b cooling at the surface during winter because of extremely cold air temperatures c increase in salinity due to sea ice formation in shallow ice shelves off the Antarctic coast during ice formation the ice rejects most of the salt and thus makes the remaining sea water saltier and thus denser 8 The deep water formed off of Antarctica is calledAntaretic Bottom Water or AABW o as the surface water sinks it entrains mixes with upwelling deep water in about a 5050 mix of sinking surface water and upwelling deep water Fig 6 o the estimated rate of AABW formation is 20 Sv 10Sv sinking surface water and 10Sv entrained deep water 0 AABW is dense enough to sink to the bottom of the Atlantic Ocean deeper than NADW 9 The entrainment of deep water during the formation of AABW increases its nutrient concentrations and lowers its oxygen concentration Why thus AABW has lower dissolved Oz and higher nutrients COZ P04 and N03 than NADW which is a deep water with a surface water source where typically nutrient concentrations are low and oxygen concentrations are high 10 Deep water formation rates of both NADW and AABW are sporadic both in time and space 0 more deep water is formed during years with cold winter air temperatures and less is formed during years with warm winters 0 because deep water is formed in relatively small geographic areas that are fairly inhospitable it can be dif cult to locate the exact formation region in any given year 0 it is dif cult to directly measure deep water formation rates in any one year there are better estimates of longer term formation rates over several years that are obtained from moored current meters or from the concentration of chemical tracers in the deep water 0 our best estimates of deep water formation rates are 15 SV for NADW and N 20 Sv for AABW yielding a total deep water input rate to the Deep Sea of about 35 Sv 11 There is no signi cant deep water formed in either the Indian or Paci c oceans o in the Paci c surface waters have lower salinity than in the Atlantic which makes it them less dense and thus more difficult to sink 0 the N Paci c also does not eXtend geographically as far north as the N Atlantic and thus surface waters cannot be cooled as effectively 0 in the Indian ocean surface waters do not get cold enough to sink the N Indian ocean doesn t eXtend past 20 N 0 there be some deep water formed in the Ross Sea off coast of Antarctica in the S Paci c but it is less important than the AABW formed in the Weddell Sea D Formation of Intermediate Waters 1 Water masses that become dense enough at the surface to sink but are not dense enough to sink to great depths are called Intermediate Waters Intermediate Waters typically sink to depths of about 700 to 1500m that is at the bottom of the thermocline and top of the Deep Sea 2 The most important Intermediate Water is calledAntarctica Intermediate Water AAIW o AAIW is formed at around 50 S 60 S that is at the location of the Antarctic Polar Front at the northern edge of the Southern Ocean Fig 6 0 Surface waters that form AAIW get dense cool enough to sink to 8001000m o AAIW spreads northward penetrating to almost N 20 N in all ocean basins Paci c Atlantic Indian oceans Fig 7 3 The out ow from the Mediterranean Sea calledMed Water is another Intermediate Water Fig 8 0 Med Water MW sinks to about 1000m and spreads throughout the N Atlantic m not nearly as wide spread as AAIW 0 MW has very high salinity S365 which increases its density yet it is warm 6 12 C 0 Thus the location of the tongue of high salinity at 1000m caused by MW input illustrates the clockwise circulation path of water parcels at this depth in the N Atlantic Fig 8 0 MW is produced formed at a relatively slow rate NZ Sv and therefore has a much smaller distribution than AAIW 6 In the N Paci c an intermediate water is formed called North Pacific Intermediate Water or Paci c Subarctic Intermediate Water in the northwest comer of the N Paci c near the Kamchatka Peninsula and in the Sea of Okhotsk and sinks to a depth of about 500700m Fig 7 E Seawater Properties of Deep and Intermediate Waters l The various Deep and Intermediate waters that are produced in the ocean have distinct potential temperature and salinity characteristics because their surface cooling and salinity histories are different thus the pot temp and salinity characteristics are one way to identify the presence of these various water masses in water parcels located in the Deep Sea 2 In the Atlantic Ocean two deep AABW NADW and two intermediate water masses AAIW and MW are formed and potentially present in deep water parcels o the core maximum presence of each of these water mass sources are found at different depths AABW at the bottom 40005000m NADW at 20003000m AAIW at 800 1000m and MW at 10001200m o the depth at which these deep and intermediate waters are primarily found depends on the density at which these water masses reach when they sink 7 the denser the surface water when it sinks the deeper the water penetrates into the Deep Sea 3 The average 6 S and 66 properties of these Deep and Intermediate Waters are o NADW has 6 3 C S3495 66 2784 o AABW has 6 1 C S 3465 66 2787 o AAIW has 6 35 C S 342 66 272 o MW has 6 12 C S 365 66 277 4The presence of water which originally comprised AABW NADW and AAIW is found in water parcels in all three ocean basins Atlantic Indian and Paci c water that originally sank in the regions of AAB W NADW and AAIW formation is transported throughout the Deep Sea via deep currents and mixing in contrast the in uence of Med Water is found only in the Atlantic Ocean because its formation rate is small and essentially diluted by mixing beyond recognition in the S Atlantic 5 If AABW NADW and AAIW were the m sources of deep water heat and salt then the 6 and S properties of any water parcel in the Deep Sea wherever we found them would be a mixture of these three water sources only 0 This situation would occur if there are no signi cant in situ heat or salt sources or sinks in the Deep Sea and if there were no mixing with other eg thermocline waters A simple analogy is a bathtub with two faucets one of which discharges blue water and the other yellow water so the water in the tub will be either yellow close to the one faucet blue close to the other faucet or green a mixture of the two source waters Question What process that we haven t mentioned might affect temperature of deep water Hint occurs along some midocean ridges 6 A depth pro le of salinity and Pot Temp in the South Atlantic at 40 S 30 W is affected by the inputs of AAIW NADW and AABW Fig 9 there is a S minimum at 800m which results from AAIW input there is a S maximum at 2700m which results from NADW input there is a S minimum at the bottom which results from AABW input 7 The effect of AAIW NADW and AABW input on the depth pro le of Pot Temp is more dif cult to detect 8 The in uence of different sources of deep water on the 6 and S distributions can be more clearly seen ife is plotted versus S in what we call a T Splot Fig 9 39 to construct a TS plot the Pot Temp and Salinity values measured at each depth in the depth pro le are plotted in TS space 0 nd where the 6 and S properties for the measurements at surface 2000m and 4000m lie on T S plot 0 the S shape in a T S plot is caused by the inputs of deep and intermediate waters from AAIW NADW and AABW o the input of water parcels with the distinct 6 and S properties of AAIW NADW and AABW show up as bends in ections in the TS plot 9 On a TS plot for the Atlantic Ocean the location of the AAIW NADW AABW and MW waters sources are distinct separated from each other Fig 10 0 Recall that the mean 6 and S values for NADW is 6 3 C and S3495 for AABW is 6 l C and S3465 and for AAIW is 6 35 C and S342 o In terms of density the sequence is AAIWltMWltNADWltAABW Can you see this density trend in Fig 10 9 Let s look at where the TS values measured on a depth pro le in the Atlantic Ocean lie relative to the TS properties of AAIW NADW and AABW Fig 11 7 the numbers on the TS curve represent depth eg 6600m 404000m Below 2000m the measured TS values fall along on a straight line connecting the TS properties of NADW and AABW this linear straight line portion of the T S curve indicates that over this depth region the water is essentially a miXture of m two sources that is AABW and NADW as will be discussed below between 800m and 2000m the TS curve does not follow a straight line connecting the TS properties of AAIW and NADW this means that there is another source of T and or S in this depth interval which is most likely input from Med Water one result of this Med Water input is that water parcels with the T and S characteristics of pure AAIW 635 C and S342 are not found in this measured depth pro le although the measured T and S depth pro les are clearly in uenced by input of AAIW water F Determining the contribution that each water source eg AAIW NADW and AABW makes toward the T and S measured on a water parcel in the Deep Sea lLet s assume steadystate conditions for 6 and S in the Deep Sea that is the measured 6 and S of a water parcel isn t changing with time and that T and S are conservative neither produced or consumed in situ the steadystate assumption is probably a very good one for the Deep Sea where the residence times are very long and changes are very slow 2 We ll start with the simplest situation that is when there are only two water sources contributing to the TS properties of water parcels Fig 12 In this case miXing causes the T and S properties of the all the water parcels to fall on a straight line in TS space that connects the T and S properties of the two water sources Class Problem Plot on Fig 12 the T and S properties represented by a 5050 and 2575 mix of the two source waters I and II where source water I has 6 10 C and S 36 and water II has 6 4 C and S 34 Demonstrate that any mixture of these two water sources has a pot temp and salinity that falls along a straight line connecting the two water sources 3 In practice oceanographers measure the 6 and S of a water parcel in the Deep Sea and then calculate the proportion or fraction that a deep or intermediate water source like NADW AABW AAIW MW contribute towards the measured 6 and S values To calculate this proportion one must know assign the 6 and S properties of the water sources in addition to measuring the 6 and S properties of the water parcel 4 Determining the proportion of a deep water source present in a water parcel is very useful in testing deep ocean circulation theories and circulation models for example does the mixture of AABW NADW and AAIW predicted by a circulation model or theory for a water parcel at a speci c location in the Deep Sea agree with the mixture of these water sources calculated from measured 6 and S if not maybe the theory or model estimates of formation rates of AABW NADW and AAIW or mixing rates in the Deep Sea are incorrect 5 We can determine the fraction of each endmember water mass in any given sample by using either a graphical or mathematical approach This works because the heat and salt content of st deep waters which are measured by T and S properties are neither gained nor lost in situ ie they are conservative properties of seawater and are only affected by mixing an exception to the conservative characteristic of Pot Temp can occur near midocean ridges where input of geothermal heat and hot uids can warm deep waters This situation doesn t occur too often 6 Finding the fraction of two water sources eg NADW and AABW present in a water parcel using a graphical method Fig 12 0 on a TS gure plot the measured T and S of the two water sources I and II in this example and the measured T and S of the water parcel R in this example on a T S plot the T and S properties of the water parcel must fall on a straight line connecting the TS points of the two water sources assuming there are only two water mass sources and that T and S are conservative on a TS plot measure using a ruler the length of the line between the observed T S point measured on the water parcel R and the T S point of the water source which is closest source II in this case the ratio of this line length to the entire line length between water source I and II is the fractional contribution of I Fl where Fl bba to the measured T and S of water parcel R for example if the measured 6 and S are 60 C and 347 respectively then the water parcel contains 33 water source I and 67 water source II 7 Finding the fraction of m water sources eg AAIW NADW and AABW present in a water parcel using a graphical method Fig 13 0 When there are three water sources contributing to the T and S properties of water parcels then the TS of all water parcels st fall within the triangle de ned by the TS points for the three water sources This situation is analogous to the T and S properties falling on a straight line for mixing of two water sources To determine the each fraction draw lines from the measured TS points of each of the three water sources I II and III in the gure through the measured TS point of the water parcel R Measure the ratio of the length of the line between the measured TS point and the point where the line intersects the opposite side of the triangle to the entire line length between the TS point of the water source and the intersection with the triangle side This ratio equals the fraction of the measured TS property of the water parcel that was derived from that water source For example the fraction of water source 111 present in R is the length of line segment f divided by the length of line segment fe eg fraction ofI bba and fraction of II dcd 8 NADW is actually made up of three separate water sources forming in the Labrador Norwegian and Greenland Seas Fig 14 9 NADW can be separated into a shallow 2000m and deep 3500m component with the shallower component thought to be composed mainly of Labrador Sea water the deeper component called Lower NADW seen as the circle inside the triangle in Fig 14 is composed mainly of Greenland Sea Water p A O For other examples of using TS plots to determine water mass contributions look at Questions 69 and 610 in the Open University textbook Class problem Graphically estimate the fraction of each of the three water sources LSW GSW and NSW that make up Lower NADW which is represented by the circle in the middle of the cluster of points in Fig 14 In this gure LSW represents Labrador Sea Water GSW represent Greenland Sea Water and NSW represents Norwegian Sea Water Use the centers of the circles to represent the TS points of the sources waters and water parcel 11 A mathematical approach can be used to calculate the fraction of each water source in which you set up enough equations to calculate the fraction of each source water 2 equations for 2 sources 3 equations for 3 sources etc For three sources the three equations describe the temperature salinity and mass balances By solving them simultaneously each fractional contribution F1 F2 and F3 representing the fraction of each water source is determined equation 1 F1 F2 F3 1 Mass balance equation 2 F1T1 F2T2 F3T3 Tm Heat balance equation 3 FlS1 F2S2 F3S3 Sm Salt balance where F is fraction of the water source present in the water parcel T1 and S1 are the Pot Temperature and Salinity water source 1 same for water source 2 and Tm and Sm are the Pot Temp and salinity measured on the water parcel of interest It is much easier to solve for F1 F2 and F3 using the graphical approach Note You only need to know the graphical approach not the mathematical approach to determining F1 F2 and F3 from T and S measurements F How does mixing affect the depth pro le ofT and S 1 Water parcels mix with each other as a result of turbulence in the ocean mixing occurs most easily and thus fastest along surfaces of equal density isopycnals which are oriented sort of horizontally in most places because there is no buoyancy resistance to overcome by mixing mixing occurs most slowly across isopycnals sort of vertically because buoyancy resistance has to be overcome by mixing mixing occurs at several spatial scales that is via large scale organized motions like currents via large scale features like eddies at smaller scales via turbulent random motions of water parcels and at the smallest scales via molecular motion the length scale being considered often determines which process dominates generally turbulent mixing dominates on short length scales lt1m to 10km whereas currents and eddies dominates over longer length scales 1001000kms turbulence is an effective mixing process in both the Deep Sea and surface ocean turbulence can mix both vertically and horizontally however the vertical cross isopycnal mixing rate is much slower 106x slower than the horizontal along isopycnal mixing rate because cross isopycnal mixing has to overcome stability AceAZ in most regions of the ocean in certain high latitude regions of the ocean where the isopycnal orientation is close to vertical along isopycnal mixing does correspond to vertical mixing 2 We ll use a hypothetical example to demonstrate how mixing affects the vertical shape of depth pro les of T and S and the shape of the TS plot when three water sources are present Fig 15 a Start out with three distinct water sources in three separate layers top row in Fig 15 Notice the shapes T and S depth pro les and the TS plot b Look at the effect of turbulent mixing on the T and S depth pro le and TS plot middle row of gure Explain why the TS plot has a sharp angle c After continued mixing the T and S values of the middle layer no longer equal the initial T and S of the middle layer Explain why this occurs with continued mixing The effect of continued mixing is to round the sharpness of the angle d Generally the further away from the formation region of a deep or intermediate water source the longer the waters parcels have had to mix the smoother less sharp the TS curvature and the fainter the original T and S properties of the source water appear Question What would the TS plot look like in Fig 15 after an in nite amount of time was allowed for mixing to occur 3 TS plots with rounded curves rather than sharp angles are typically seen in the deep ocean Fig 16 4 The TS plot also provides information about the stability of the water colunm if the depths along the TS curve are indicated Density increases as you decrease temp and increase salinity Isopycnals constant 66 appear as curved lines on TS plot Remember why stab111ty is maximized where the TS plot crosses isopycnals over the shortest depth interval that is highest AceAZ in contrast if the TS plot follows along an isopycnal surface then there is little stability Question Over what depth interval is the water column stability the greatest based on the TS plot in Fig 16 Over what depth interval is the water column stability the least Consider depths gt100m only 5 An unexpected result occurs when water masses with two different temperatures and salinities but with the same density are mixed together The resulting mixture has a density that is greater M the starting density 0 you can show this graphically on a TS plot where lines of constant density are shown see Fig 10 if you make a mixture of two water masses with different T and S properties but which lie on the same isopycnal then the T and S property of the resulting mixture follows the straight line connecting the two water mass sources This mixture however is denser than the density of either source that is the density of the mixture lies on a denser isopycnal than the isopycnal of the two sources this occurs because the relationship between temperature and density is not linear especially at cold temperatures lt2 C 0 this phenomenon is called cabelling o For example make a 5050 mixture oftwo water masses with T1 2 and S1 3504 09 28 and T2 85 and S360 6928 You might predict that the mixture would have a Ge 28 however the mixture T525 and S3552 has a 66 of 2804 slightly but signi cantly denser than the source waters 6 NOTE Although TS properties can tell us whatprapartian of each deep water mass is present in a water parcel the TS properties cannot tell us is the rate at which the deep water masses have formed or circulated We have to use other methods which provide ages of water parcels to determine how long it takes for water parcels to circulate and mix in the deep sea G The Conveyor Belt circulation pathway in the Deep Sea 1 The T and S properties of the deep water in the Atlantic Ocean indicate that deep water sinking in the both the far north near Greenland NADW and in the far south near Antarctica AABW ow toward the equator equatorward 0 Henry Stommel predicted this observation based on theory in the early 1950s Fig 17 o Stommel s theory indicated that the deep equatorward ow should occur in intensi ed western boundary currents while the poleward return ow in the Deep Sea should occur via weaker currents in the middle and eastern portions of the deep ocean basins o In the late 1950s Stommel s postulated southward intensi ed ow of deep water along the western edge of the deep N Atlantic was veri ed using neutrally buoyant oats this deep current was called Deep Western Boundary Current 0 Because deep ows generally are very slow except in the intensi ed western boundary currents it has been difficult to verify whether the Stommel s theorized ow pattern really exists 2 The reasons why deep western boundary currents are present are dynamical and related to the earth s spin 0 Stommel theorized that there must be equatorward ow of deep water formed in the polar regions to balance the loss of deep water due to upwelling This upwelling of deep water is needed to balance the input of deep waters like NADW and AABW to the Deep Sea and offset the downward diffusion of heat resulting from turbulent mixing 0 Stommel hypothesized that the equatorward ow would be concentrated in intensi ed deep western boundary currents 0 Note Our theoretical understanding of abyssal circulation is still evolving As our observations of deep circulation pathways improve our ability to verify the theory of deep circulation will improve 3 Stommel s theorized ow path indicates that deep water that forms in the far N Atlantic NADW should ow southward all the way to the Antarctic Circumpolar Current ACC at 50 60 S Fig 17 In contrast most of the northward equatorward ow of the AABW gets entrained into the eastward ow of the ACC but some passes underneath the ACC and ows northward at the bottom along the western boundary of the S Atlantic 4 The NADW and AABW that enter into the eastward ow of the ACC are transported into the deep basins of the Indian and Paci c Oceans 5 In the Indian and Paci c oceans the deep water enters in the southwestern corner of the basin and spreads northward along the western boundary Some of this deep water eventually ows southward out of each basin back to the ACC in the central and eastern portions of the Indian and Paci c basins Fig 17 6 Deep water circulation pathways simulated by stateoftheart General Circulation Models GCM of the Deep Sea yield Stommel s theorized circulation Fig 18 In Fig 18 the contours are in units of years and represent the time it takes for 1 of an idealized tracer eg dye injected in the southern ocean to get to a particular location in the deep Paci c Generally the shortest times are found along the western boundary of the deep Paci c which agrees with Stommel s theory 7 How does the deep water produced in the Atlantic and canied to the Indian and Paci c oceans return to the surface in the N and S Atlantic to complete the round trip 0 deep water warms during its trip from the Atlantic Indian Paci c via downward miXing of heat from the warm upper ocean 0 this warming increases the buoyancy decreases the density of the deep water and causes it to rise Eventually this density decrease is sufficient to allow the deep water to rise through the thermocline and ultimately become part of the surface ocean circulation 8 Note At steadystate the rate of deep water returning to the surface ocean has to equal the total deep water formation rates in the N Atlantic NADW and Weddell Sea AABW that is there is a mass ow balance between deep water input NADWAABW and loss upwelling in the deep sea at steadystate However we don t know well the pathway that returns deep water to the surface ocean oceanographers think a substantial amount of the upwelling from the Deep Sea occurs in the Southern Ocean south of 50 S in the region of the Antarctic Circumpolar Current see Fig 6 9 Ultimately the upwelled deep water must return to the site of deep water formation to complete the circulation circuit 0 this general path of deep water ow gave rise to a Conveyor Belt analogy Fig 19 although the actual ow path is not this simple the analogy is convenient in the Paci c the major surface return path is through the Indonesian archipelago return ow of surface currents to the Atlantic from the Indian and Paci c occurs via the Agulhas Current around the tip of S Africa once in the Atlantic 15Sv gets incorporated into the Gulf Stream ow and is ultimately advected to the far N Atlantic to cool sink and start all over again as NADW 10 Some deep water returns to the Weddell Sea without ever contacting the surface this is the water that is entrained during the sinking of AABW 11 Our best estimates are that 25Sv returns via in the surface ow back to the Atlantic Ocean about 15 Sv ow into the North Atlantic to form NADW and about 10 Sv ow to Antarctica where it sinks and entrains another 10 Sv of deep water to form AABW thus our best estimate is that there is a total of about 35 Sv of deep water forming in today s ocean 15 Sv via NADW and 20 Sv via AABW Class Problem Calculate the upwelling velocity ms of deep water needed to balance 35Sv of deep water forming in the polar regions if this upwelling occurred uniformly over the entire ocean ocean surface area 360XlO12 m2 How easy would it be to measure this rate directly How does this upwelling velocity compare to the average current speeds in the Deep Sea


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