WATER TYPES AND WATER MASSES - Judith...

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WATER TYPES AND WATER MASSESW. J. Emery, University of Colorado, Boulder,CO, USA

Copyright ^ 2001 Academic Press

doi:10.1006/rwos.2001.0108

Introduction

Much of what is known today about the currentsof the deep ocean has been inferred from studies ofthe water properties such as temperature, salinity,dissolved oxygen and nutrients. These are quantitiesthat can be observed with standard hydrographicmeasurement techniques which collect temperaturesand samples of water with a number of samplingbottles strung along a wire to provide the depthresolution needed. Salinity or ‘salt content’ is thenmeasured by an analysis of the water sample, whichcombined with the corresponding temperature valueat that ‘bottle’ sample yields temperature and salin-ity as a function of depth of the sample. Modernobservational methods have in part replaced thissample bottle method with electronic proRling sys-tems at least for temperature and salinity but manyof the important descriptive quantities such as oxy-gen and nutrients still require bottle samples ac-complished today with a ‘rosette’ sampler integratedwith the electronic proRling systems. These newelectronic proRling systems have been in use forover 30 years but the majority of data useful forstudying the properties of the deep and open oceanstill comes from the time before the advent of mod-ern electronic proRling systems. This knowledge isimportant in the interpretation of the data since themeasurements from sampling bottles have very dif-ferent error characteristics than those from modernelectronic proRling systems.

This article reviews the mean properties of theopen ocean, concentrating on the distributions ofthe major water masses and their relationships tothe currents of the ocean. Most of this informationis taken from published material including the fewpapers that directly address water mass structurealong with the many atlases that seek to describe thedistribution of water masses in the ocean. Coinci-dent with the shift from bottle sampling toelectronic proRling is the shift from publishing

information about water masses and ocean currentsin large atlases to the more routine research paper.In these papers the speciRc water mass character-istics are in general only a small portion of the totalpaper requiring an oceanographer interested prim-arily in the water mass distribution to review theentire paper to extract the water mass information.Although water mass characteristics often playimportant roles in today’s oceanographic researchefforts there are few studies devoted solely to a bet-ter description of the distributions and character-istics of global water masses.

What is a Water Mass?

The concept of a ‘water mass’ is borrowed frommeteorology, which classiRes different atmosphericcharacteristics as ‘air masses’. In the early part ofthe twentieth century physical oceanographers alsosought to borrow another meteorological conceptseparating the ocean waters into a ‘warm’ and ‘cold’water spheres. This designation has not survived inmodern physical oceanography but the more generalconcept of water masses persists. Some oceanogra-phers regard these as real, objective physical entities,building blocks from which the oceanic stratiRcation(vertical structure) is constructed. At the oppositeextreme, other oceanographers consider watermasses to be mainly descriptive words, summaryshorthand for pointing to prominent features inproperty distributions.

The concept adopted for this discussion is square-ly in the middle, identifying some ‘core’ water massproperties that are the building blocks. In most partsof the ocean the stratiRcation is deRned by mixing inboth vertical and horizontal orientations of thevarious water masses that advect into the location.Thus, the maps of the various water mass distribu-tions identify a ‘formation region’ where it isbelieved that the core water mass has acquired itsbasic characteristics at the surface of the ocean. Thisintroduces a fundamental concept Rrst discussed in1939 that the properties of the various subsurfacewater masses were originally formed at the surfacein the source region of that particular water mass.Since temperature and salinity are considered to be

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Figure 1 Example of TS ‘scatter plot’ for all data within a 103square with mean TS curve (centre line) and curves for onestandard deviation in salinity on either side. (1&S1 PSU.)

‘conservative properties’ (a conservative property isonly changed at the sea surface) these characteristicswould slowly erode as the water properties wereadvected at depth to various parts of the ocean.

Descriptive Tools: The TS Curve

Before beginning to talk about and describe theglobal distribution of water masses some of thebasic tools used in such a description are intro-duced. One of the most basic tools is the use ofproperty vs. property plots to summarize the analy-sis by making extrema easy to locate. The mostpopular of these is the temperature}salinity or TSdiagram, which relates density to the observedvalues of temperature and salinity. Originally the TScurve was constructed for a single hydrographic castand thus related the TS values collected for a singlebottle sample with the salinity computed from thatsample. In this way there was a direct relationshipbetween the TS pair and the depth of the sample. Asthe historical hydrographic record expanded it be-came possible to compute TS curves from a combi-nation of various temperature/salinity proRles. Thisapproach amounted to plotting the TS curve asa scatter diagram (Figure 1) where the salinityvalues were then averaged over a selected temper-ature interval to generate a discrete TS curve. Anaverage of all of the data in a 103 square just northeast of Hawaii in this TS curve is typical of featuresthat can be found in all TS curves. In this examplethe temperature/salinity pair remained the samewhile the depth of this pair oscillated vertically bytens of meters resulting in the absence of a preciserelationship between TS pairs and depth. As sensedeither by ‘bottle casts’ or by electronic proRlers

these vertical variations express themselves asincreased variability in the temperature or salinityproRles while the TS curve continues to retain itsshape now independent of depth. Hence compositeTS curves computed from a number of closelyspaced hydrographic stations no longer have aspeciRc relationship between temperature, salinity,and depth.

As with the more traditional ‘single station’ TScurve these area average TS curves can be used todeRne and locate water masses which is done bylocating extrema in salinity associated with particu-lar water masses. Ignoring the near-surface values,the salinity minimum in this TS curve is at about103C where there is a clear divergence of TS valuesas they move up the temperature scale from thecoldest temperatures near the bottom of Figure 1.There are two separate clusters of points at thissalinity minimum temperature with one terminatingat about 133C and the other transitioning up to thehighest temperatures. It is this termination of pointsthat results in a sharp turn in the mean TS curveand causes a very wide standard deviation. Thesetwo clusters of points represent two differentintermediate level water masses. The relatively highsalinity values that appear to terminate at 133Crepresent the Antarctic Intermediate Water (AIW)formed near the Antarctic continent, reaching itsnorthern terminus after Sowing up from the south.The coincident less salty points indicate the presenceof North PaciRc Intermediate Water moving southfrom its formation region in the northern Gulf ofAlaska.

Although there is no general practice in watermass terminology it is generally accepted thata ‘water type’ refers to a single point on a character-istic diagram such as a TS curve. As introducedabove, ‘water mass’ refers to some portion or seg-ment of the characteristic curve which described the‘core properties’ of that water mass. In the aboveexample the salinity characteristics of the two inter-mediate waters was a salinity minimum, which wasthe overall characteristic of the two intermediatewaters. The extrema associated with a particularwater mass may not remain at the same salinityvalue. Instead as one moves away from the forma-tion zone for the AIW, which is at the oceano-graphic ‘polar front’ the sharp minimum that marksthe AIW water has sunk from the surface downto about 1000 m and start to erode, broadening thesalinity minimum and slowly increasing its magni-tude. By comparing conditions of the salinity ex-treme at a location with the salinity characteristicstypical of the formation region one can estimate theamount of the source water mass still present at the

3180 WATER TYPES AND WATER MASSES


Pacific Deep

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Figure 2 Simulated three-dimensional T}S}V diagram for the cold water masses of the world ocean.

distant location. Called the ‘core-layer’ method thisprocedure was a crucial development in the earlystudy of the ocean water masses and long-termmean currents.

Many variants of the TS curve have been intro-duced over the years. One form that is particularlyinstructive is the ‘volumetric TS curve’. Here theoceanographer subjectively decides just how muchvolume is associated with a particular water mass.This becomes a three-dimensional relationship,which can be plotted, in a perspective format (Fig-ure 2). In this plot the two horizontal axes aretemperature and salinity while the elevation repres-ents the volume with those particular TS character-istics. For this presentation only the deeper watermass characteristics have been plotted which can beseen by the restriction of the temperature scale to!1.0 to 4.03C. The arrows show which parts ofthe ocean various features are from. That the Atlan-tic is the saltiest of the oceans is very clear witha branch to high salinity values at higher temper-atures. The largest volume water mass is the PaciRcdeep water that Rlls most of the PaciRc below theintermediate waters at about 1000m.

Global Water Mass Distribution

Before we turn to the TS curve description of thewater masses we need to indicate the geographicdistribution of the basic water masses. The reader iscautioned that only the major water masses, whichmost oceanographers accept and agree upon, will bediscussed. In a particular region of interest close

inspection will reveal a great variety of smallerwater mass classiRcations, which can be almostinRnite, as higher resolution is obtained in bothhorizontal and vertical coverage.

Table 1 presents the TS characteristics of theworld’s water masses. Here the area name is giventogether with the corresponding acronym, and theappropriate temperature and salinity range. Recallthat the property extreme becomes less distinctbecause of diffusion the further one goes from thesource region so it is necessary to deRne a range ofproperties. This is also consistent with the view thata water mass refers to a segment of the TS curverather than a single point.

Here the water masses have been divided, as istraditionally the case, into deep and abyssal waters,intermediate waters and upper waters. Although theupper waters have the largest property ranges theyoccupy the smallest ocean volume. The reverse istrue of the deep and bottom waters, which havea fairly restricted range but occupy a substantialportion of the ocean. Since most ocean water massproperties are established at the ocean’s surfacethose water masses which spend most of their timeisolated far from the surface will diffuse the leastand have the longest lifetime. Surface waters on theother hand are strongly inSuenced by Suctuations atthe ocean surface which rapidly erode the watermass properties. In mean average TS curves as inFigure 1 the spread of the standard deviation at thehighest temperatures reSects this inSuence from theheat and freshwater Sux exchange that occurs nearand at the ocean’s surface.



Table 1 Temperature}salinity characteristics of the world’s water masses

Layer Atlantic Ocean Indian Ocean Pacific Ocean

Upper waters (0}500 m) Atlantic Subarctic UpperWater (ASUW) (0.0}4.03C,34.0}35.0&)

Bengal Bay Water (BBW)25.0}293C, 28.0}35.0&)

Pacific Subarctic UpperWater (PSUW)(3.0}15.03C, 32.6}33.6&)

Western North AtlanticCentral Water (WNACW)(7.0}20.03C, 35.0}36.7&)

Arabian Sea Water (ASW)(24.0}30.03C, 35.5}36.8&) Western North Pacific

Central Water (WNPCW)(10.0}22.03C, 34.2}35.2&)

Eastern North AtlanticCentral Water (ENACW)(8.0}18.03C, 35.2}36.7&)

Indian Equatorial Water(IEW) (8.0}23.03C,34.6}35.0&) Eastern North Pacific Central

Water (ENPCW)(12.0}20.03C, 34.2}35.0&)

South Atlantic Central Water(SACW) (5.0}18.03C,34.3}35.8&)

Indonesian Upper Water(IUW) (8.0}23.03C,34.4}35.0&) Eastern North Pacific

Transition Water (ENPTW)(11.0}20.03C, 33.8}34.3&)

South Indian Central Water(SICW) (8.0}25.03C,34.6}35.8&) Pacific Equatorial Water

(PEW) (7.0}23.03C,34.5}36.0&)

Western South PacificCentral Water (WSPCW)(6.0}22.03C, 34.5}35.8&)

Eastern South Pacific CentralWater (ESPCW)(8.0}24.03C, 34.4}36.4&)

Eastern South PacificTransition Water (ESPTW)(14.0}20.03C, 34.6}35.2&)

Intermediate waters(500}1500m)

Western Atlantic SubarcticIntermediate Water(WASIW) (3.0}9.03C,34.0}35.1&)

Antarctic Intermediate Water(AAIW) (2}103C,33.8}34.8&)

Pacific SubarcticIntermediate Water (PSIW)(5.0}12.03C, 33.8}34.3&)

Eastern Atlantic SubarcticIntermediate Water(EASIW) (3.0}9.03C,34.4}35.3&)

Indonesian IntermediateWater (IIW) (3.5}5.53C,34.6}34.7&)

California Intermediate Water(CIW) (10.0}12.03C,33.9}34.4&)


Red Sea}Persian GulfIntermediate Water(RSPGIW) (5}143C,34.8}35.4&)

Eastern South PacificIntermediate Water(ESPIW) (10.0}12.03C,34.0}34.4&)

Mediterranean Water (MW)(2.6}11.03C, 35.0}36.2&)


Arctic Intermediate Water(AIW) (!1.5}3.03C,34.7}34.9&)

Deep and abyssal waters(1500m}bottom)

North Atlantic Deep Water(NADW) (1.5}4.03C,34.8}35.0&)

Circumpolar Deep Water(CDW) (1.0}2.03C,34.62}34.73&)

Circumpolar Deep Water(CDW) (0.1}2.03C,34.62}34.73&)

Antarctic Bottom Water(AABW) (!0.9}1.73C,34.64}34.72&)

Arctic Bottom Water (ABW)(!1.8}!10.53C,34.88}34.94&)

Circumpolar Surface Waters Subantarctic Surface Water(SASW) (3.2}15.03C,34.0}35.5&)

Antarctic Surface Water(AASW) (!1.0}1.03C,34.0}34.6&)

To accompany Table 1 global maps of watermass at all three of these levels are presented. Theupper waters in Figure 3 have the most complexdistributions with signiRcant meridional and zonal

changes. We have also indicated a best guess at theformation regions for the corresponding water massas indicated by the hatched regions. For its relativelysmall size the Indian Ocean has a very complex



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IEW

Figure 3 Global distribution of upper waters (0}500 m). Water masses are labeled in abbreviated form with their boundariesindicated by solid lines. Formation regions for these water masses are marked by cross-hatching and labeled with the correspondingacronym title.

upper water mass structure. This is caused by someunique geographic conditions. First is the monsoon,which completely changes the wind patterns twicea year. This causes reversals in ocean currents,which also inSuence the water masses by alteringthe contributions of the very saline Arabian Gulfand the fresh Bay of Bengal into the main body ofthe Indian Ocean. All of the major rivers in IndiaSow to the east and discharge into the Bay of Bengalmaking it a very fresh body of ocean water. To thewest of the Indian subcontinent is the Arabian Seawith its connection to the Persian Gulf and the RedSea, both locations of extremely salty water makingthe west side of India very salty and the east sidevery fresh. The other upper ocean water masses inthe Indian Ocean are those associated with the Ant-arctic Circumpolar Current (ACC) and are found atall of the longitudes in the Southern Ocean.

As the largest ocean basin the PaciRc has thestrongest east}west variations in upper water masseswith east and west central waters in both the northand south hemispheres. Unique to the PaciRc is thefairly wide band of the PaciRc Equatorial Water,which is strongly linked to the equatorial upwelling,which may not exist in El Nin8 o years. Neither of theother two ocean basins has this equatorial watermass in the upper ocean. The Atlantic has northernhemisphere upper water masses that can be separ-ated east}west and the South Atlantic upper watermass cannot be separated east}west into two parts.Note the interaction between the North Atlanticand the Arctic Ocean through the Norwegian Sea

and Fram Strait. Also in these locations there aresource regions for a number of Atlantic watermasses. Compared with the other two oceans theAtlantic has the most water mass source regionswhich produce a large part of the deep and bottomwaters of the world ocean.

The chart of intermediate water masses in Figure4 is much simpler than that of the upper oceanwater masses (Figure 3). This reSects the fact thatthere are far fewer intermediate waters and thosethat are present Rll large volumes of the intermedi-ate depth ocean. The North Atlantic has the mostcomplex horizontal structure of the three oceans.Here intermediate waters form at the source regionsin the northern North Atlantic. One exception is theMediterranean Intermediate Water, which is a con-sequence of climatic conditions in the Mediterra-nean Sea. This salty water Sows out through theStraits of Gibraltar at about 320 m depth where itthen descends to 1000 m where it sinks below thevertical range of the less saline Antarctic Intermedi-ate Water (AIW) and joins with the higher salinityof the deeper North Atlantic Deep Water (NADW),which maintains the salinity maximum indicative ofthe NADW.

In the Southern Ocean the formation region forthe AIW is marked as the location of the oceanicpolar front, which is known to vary considerably instrength and location moving the formation regionnorth and south. It can clearly be seen in all of theocean basins that this AIW Rlls a large part of theocean. In the PaciRc the AIW extends north to



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Figure 4 Global distribution of intermediate water (550}1500m). Lines, labels and hatching follow the same format as describedfor Figure 3.

about 203N where it meets the North PaciRc Inter-mediate Water (NPIW) as shown in Figure 1. TheAIW reaches about the same latitude in the NorthAtlantic but it only reaches to about 53S in theIndian Ocean. In the PaciRc the northern intermedi-ate waters are mostly from the North PaciRc wherethe NPIW is formed. There is, however, anothersmaller volume intermediate water that is formed inthe transition region west of California mostly asa consequence of coastal upwelling. A similarintermediate water formation zone can be found inthe south PaciRc mainly off the coast of SouthAmerica, which generates a minor intermediatewater mass.

The deep and bottom waters mapped in Figure 5are restricted in their movements to the deeperreaches of the ocean. For this reason the 4000mdepth contour is plotted in Figure 5 and a goodcorrespondence can be seen between the distributionof bottom water and the deepest bottom topogra-phy. Some interesting aspects of this bottom watercan be seen in the eastern South Atlantic. As thedense bottom water makes its way north from theSouthern Ocean in the east it runs into the WalvisRidge which blocks it from further northward ex-tension. Instead the bottom water Sows north alongthe west of the mid-Atlantic ridge and Rndinga deep passage in the Romanche Gap Sows east-ward and then south to Rll the basin north of theWalvis Ridge. A similar complex pattern of distribu-tion can be seen in the Indian Ocean where the eastand west portions of the basin Rll from the southseparately because of the central ridge in the bottom

topography. In spite of the requisite depth of theNorth PaciRc the Antarctic Bottom Water (AABW)does not extend as far northward in the NorthPaciRc meaning that some variant of the AABW,created by mixing with other deep and intermediatewaters, occupies the most northern reaches of thedeep North PaciRc. Because the North PaciRc isessentially ‘cut-off’ from the Arctic there is noformation region of deep and bottom water in theNorth PaciRc.

The three-dimensional TS curve of Figure 2showed that the most abundant water massmarked by the highest peak in this TS curve corre-sponded to PaciRc Deep Water. Table 1 shows thereis something called ‘Circumpolar Deep Water’ inthe deeper reaches of both the PaciRc and IndianOceans. This water mass is not formed at the sur-face but is instead a mixture of North Atlantic DeepWater (NADW), AABW and the two intermediatewaters present in the PaciRc. The AABW forms inthe Weddell Sea as the product of very cold, dense,fresh water Sowing off the continental shelf whichthen sinks and encounters the upwelling NADWwhich adds a little salinity to the cold, fresh water,making it even denser. This very dense product ofWeddell Sea shelf water and NADW becomes theAABW which then sinks to the very bottom andSows out of the Weddell Sea to Rll most of thebottom layers of the world ocean. It is probable thata similar process works in the Ross Sea and someother areas of the continental shelf to form addi-tional AABW but the Weddell Sea is thought to bethe primary formation region of AABW.



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Figure 5 Global distribution of deep and abyssal waters (1500}bottom). Contour lines describe the spreading of abyssal water(primarily AABW). The formation of NADW is indicated by hatching and its spreading terminus, near the Antarctic, by a dashed linewhich also suggests the global communication of this deep water around the Antarctic. The formation and distribution of CDW is notshown since it overlies the abyssal water in both the Pacific and Indian Oceans.

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Figure 6 Characteristic temperature}salinity (TS) curves forthe main water masses of the Atlantic Ocean. Water massesare labeled by the appropriate acronym (see Table 1) and corewater properties are indicated by a dark square with an arrow tosuggest their spread. The cross-isopycnal nature of some ofthese arrows is not intended to suggest a mixing processbut merely to connect source waters with their correspondingcharacteristic extrema.

Summary TS Relationships

As pointed out earlier one of the best ways to detectspeciRc water masses is with the TS relationshipwhether computed for single hydrographic casts orfrom a historical accumulation of such hydro casts.Here traditional practice is followed and the sum-mary TS curves are divided into the major oceanbasins starting with the Atlantic (Figure 6). Onceagain the higher salinities typical of the Atlantic canclearly be seen. The highest salinities are introducedby the Mediterranean outSow marked as MW inFigure 6. This joins with water from the NorthAtlantic to become part of the NADW, which ismarked by a salinity maximum in these TS curves.The AAIW is indicated by the sharp salinity min-imum at lower temperatures. The source water forthe AAIW is marked by a dark square in the Rgure.The AABW is a single point, which now does notrepresent a ‘water type’ but rather a water mass.The difference is that this water mass has veryconstant TS properties represented by a single pointin the TS curves. Note that this is the densest wateron this TS diagram (the density lines are shown asthe dashed curves in the TS diagram marked aspt").

The rather long segments stretching to the highesttemperature and salinity values represent the upperwater in the Atlantic. Although this occupies a largeportion of the TS space it only covers a relativelysmall part of the upper ocean when compared to thelarge volumes occupied by the deep and bottom

water masses. From this TS diagram it can be seenthat the upper waters are slightly different in theSouth Atlantic, the East North Atlantic and theWest North Atlantic. Of these differences the SouthAtlantic differs more strongly from the other twothan they do from each other.

By comparison with Figure 6 the PaciRc TS curvesof the PaciRc (Figure 7) are very fresh with all but



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Figure 7 Characteristic temperature}salinity (TS) curves forthe main water masses of the Pacific Ocean. All labels as inFigure 6.

BBW

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Salinity (‰)Figure 8 Characteristic temperature}salinity (TS) curves forthe main water masses of the Indian Ocean. All labels as inFigure 6.

the highest upper water mass having salinities below35&. The bottom property anchoring this curve isthe Circumpolar Deep Water (CDW) which is usedto identify a wide range of TS properties that areknown to be deep and bottom water but have notbeen identiRed in terms of a speciRc formation re-gion and TS properties. As with the AABW a singlepoint at the bottom of the curves represents theCDW. The relationship between the AAIW and thePaciRc Subarctic Intermediate Waters (PSIW) can beclearly seen in this diagram. The AAIW is colderand saltier than the PSIW, which is generally a bithigher in the water column indicated by the lowerdensity of this feature. There are no external sourcesof deep salinity as for the Mediterranean Water inthe Atlantic. Instead there is a confusing plethoraof upper water masses that clearly separate theeast}west, and north}south portions of the basin.There is therefore Eastern North PaciRc CentralWater (ENPCW) and Western North PaciRc CentralWater (WNPCW), as well as Eastern South PaciRcCentral Water (ESPCW) and Western South PaciRcCentral Water (WSPCW).

The central waters all refer to open ocean upperwater masses. The more coastal water masses suchas the Eastern North PaciRc Transition Water(ENPTW) are typical of the change in upper watermass properties that occurs near the coastal regions.The same is also true of the South PaciRc. In generalthe fresher upper-layer water masses of the PaciRcare located in the east where river runoff introducesa lot of fresh water into the upper ocean. To thewest the upper water masses are saltier as shown bythe quasi-linear portions of the TS curves corre-sponding to the western upper water masses. The

PaciRc Equatorial Water (PEW) is unique in thePaciRc probably due to the well-developed equato-rial circulation system. As seen in Figure 7 the PEWTS properties lie between the east and west centralwaters.

The Indian Ocean TS curves in Figure 8 are quitedifferent from either the Atlantic or the PaciRc.Overall the Indian Ocean is saltier than the PaciRcbut not quite as salty as the Atlantic. Also like theAtlantic the Indian Ocean receives salinity inputfrom a marginal sea as the Red Sea deposits itssalt-laden water into the Arabian Sea. Its presenceis noted in Figure 8 as the black square markedRSPGIW (Red Sea}Persian Gulf IntermediateWater). Added at the sill depth of the Red Sea thisintermediate water contributes to a salinity max-imum that is seasonally dependent.

The bottom water is the same CDW as seen in thePaciRc. Unlike the PaciRc the Indian Ocean equa-torial water masses are nearly isohaline above thepoint representing the CDW. In fact the line thatrepresents the Indian Ocean Equatorial Water (IEW)runs almost straight up from the CDW at about 03Cto the maximum temperature at 203C. There is anexpression of the AAIW in the curve that corres-ponds to the South Indian Ocean Central Water(SICW). A competing Indonesian IntermediateWater (IIW) has higher temperature and highersalinity characteristics which result in it having anonly slightly lower density creating the weak salinityminimum in the curve transitioning to the IndianOcean Upper Water (IUW). The warmest and salti-est part of these TS curves represents the ArabianSea Water (ASW) on the western side of the Indiansubcontinent.



Discussion and Conclusion

The descriptions provided in this review cover onlythe most general of water masses, their core proper-ties and their geographic distribution. In most re-gions of the ocean it is possible to resolve the watermass structure into even Rner elements describingmore precisely the differences in temperature andsalinity. In addition other important properties canbe used to specify water masses not obvious in theTS curves. Although dissolved oxygen is often usedto deRne water mass boundaries care must be takenas this nonconservative property is inSuenced bybiological activity and the chemical dissolution ofdead organic material falling through the water col-umn. Nutrients also suffer from modiRcation withinthe water column making their interpretation aswater mass boundaries more difRcult. Characteristicdiagrams that plot oxygen against salinity or nutri-ents can be used to seek extrema that mark theboundaries of various water masses.

The higher vertical resolution property proRlespossible with electronic proRling instruments alsomake it possible to resolve water mass structure thatwas not even visible with the lower vertical resolu-tion of earlier bottle sampling. Again this complex-ity is only merited in local water mass descriptionsand cannot be used on the global scale description.

At this global scale the descriptive data availablefrom the accumulation of historical hydrographicdata are adequate to map the large-scale water massdistribution as in this review article.

See also

California and Alaska Currents. Kuroshio andOyashio Currents. Ocean Circulation. Ocean Sub-duction. Paci\c Ocean Equatorial Currents.Thermohaline Circulation. Wind Driven Circula-tion.

Further ReadingEmery WJ and Meincke J (1986) Global water masses:

summary and review. Oceanologica Acta 9:383}391.

Iselin CO’D (1939) The inSuence of vertical and lateralturbulence on the characteristics of the waters at mid-depths. Transactions of the American GeophysicalUnion 20: 414}417.

Pickard GL and Emery WJ (1992) Descriptive PhysicalOceanography, 5th edn. Oxford: Pergamon Press.

Worthington LV (1981) The water masses of the worldocean: some results of a Rne-scale census. In: WarrenBA and Wunsch C (eds) Evolution of PhysicalOceanography, ch. 2, pp. 42}69. Cambridge, MA:MIT Press.

WAVE ENERGY

M. E. McCormick and D. R. B. Kraemer,The Johns Hopkins University, Baltimore,MD, USA

Copyright ^ 2001 Academic Press

doi:10.1006/rwos.2001.0034

Introduction

In the last half of the twentieth century, humankindRnally realized that fossil fuel resources are Rniteand that use of those fuels has environmental conse-quences. These realizations have prompted thesearch for other energy resources that are bothrenewable and environmentally ‘friendly’. One suchresource is the ocean wind wave. This is a form ofsolar energy in that the sun is partly responsible forthe winds that generate water waves.

The exploitation of water waves has been a goalfor thousands of years. Until recent times, however,only sporadic efforts were made, and these weregenerally directed at a speciRc function. In the

1960s, Yoshio Masuda, the ‘renaissance man’ ofwave energy conversion, came up with a scheme toconvert the energy of water waves into electricity byusing a Soating pneumatic device. Originally, theMasuda system was used to power remote navi-gation aides, such as buoys. One such buoy systemwas purchased by the US Coast Guard which, inturn, requested an analysis of the performance ofthe system. The results of that analysis were re-ported by McCormick (1974). This was the Rrst ofa long list of theoretical and experimental studies ofthe pneumatic and other wave energy conversionsystems. (For summaries of some of the works, seethe Further Reading section.) The most recent col-lective type of publication is that edited by Nicholls(1999), written under the joint sponsorship of theEngineering Committee on Oceanic Resources(ECOR) and the Japan Marine Science and Techno-logy Center (JAMSTEC). In the late 1970s and early1980s, JAMSTEC co-sponsored a full-scale trialof a Soating, offshore pneumatic system calledthe Kaimei. The 80 m long, 10 m wide Kaimei

WAVE ENERGY 3187


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