ocean circ water types masses

Water Types and Water Masses

W J Emery, University of Colorado, Boulder, CO, USA

Copyright 2003 Elsevier Science Ltd. All Rights Reserved.

Introduction

Muchofwhat is known today about the currents of thedeep ocean has been inferred from studies of the waterproperties such as temperature, salinity, dissolvedoxygen, and nutrients. These are quantities that can beobserved with standard hydrographic measurementtechniques which collect temperatures and samples ofwater with a number of sampling bottles strung alonga wire to provide the depth resolution needed. Salinityor ‘salt content’ is then measured by an analysis of thewater sample, which combined with the correspond-ing temperature value at that ‘bottle’ sample yieldstemperature and salinity as a function of depth of thesample. Modern observational methods have in partreplaced this sample bottle method with electronicprofiling systems, at least for temperature and salinity,but many of the important descriptive quantities suchas oxygen and nutrients still require bottle samplesaccomplished todaywith a ‘rosette’sampler integratedwith the electronic profiling systems. These newelectronic profiling systems have been in use for over30 years, but still the majority of data useful forstudying the properties of the deep and open oceancome from the time before the advent of modernelectronic profiling system. This knowledge is impor-tant in the interpretation of the data since themeasurements from sampling bottles have very differ-ent error characteristics than those from modernelectronic profiling systems.

This article reviews the mean properties of the openocean, concentrating on the distributions of the majorwater masses and their relationships to the currents ofthe ocean. Most of this information is taken frompublished material, including the few papers thatdirectly address water mass structure, along with themany atlases that seek to describe the distribution ofwater masses in the ocean. Coincident with the shiftfrom bottle sampling to electronic profiling is the shiftfrom publishing information about water masses andocean currents in large atlases to the more routineresearch paper. In these papers the water mass char-acteristics are generally only a small portion, requiringthe interested descriptive oceanographer to go toconsiderable trouble to extract the information he orshe may be interested in. While water mass distribu-tions play a role in many of today’s oceanographic

problems, there is very little research directed atimproving our knowledge of water mass distributionsand their changes over time.

What is a Water Mass?

The concept of a ‘water mass’ is borrowed frommeteorology, which classifies different atmosphericcharacteristics as ‘air masses’. In the early part of thetwentieth century physical oceanographers alsosought to borrow another meteorological conceptseparating the ocean waters into ‘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 stratification(vertical structure) is constructed. At the oppositeextreme, other oceanographers consider water massesto be mainly descriptive words, summary shorthandfor pointing to prominent features in property distri-butions.

The concept adopted for this discussion is squarelyin the middle, identifying some ‘core’ water massproperties that are the building blocks. Inmost parts ofthe ocean the stratification is defined bymixing in bothvertical and horizontal orientations of the variouswatermasses that advect into the location. Thus, in themaps of the various water mass distributions a‘formation region’ is identified where it is believedthat the core water mass has acquired its basiccharacteristics at the surface of the ocean. Thisintroduces a fundamental concept first discussed byIselin (1939), who suggested that the properties of thevarious subsurface water masses were originallyformed at the surface in the source region of thatparticular water mass. Since temperature and salinityare considered to be ‘conservative properties’ (prop-erty is only changed at the sea surface), these charac-teristics would slowly erode as the water propertieswere advected at depth to various parts of the ocean.

Descriptive Tools: The TS Curve

Before focusing on the global distribution of watermasses, it is appropriate to introduce some of the basictools used to describe these masses. One of the mostbasic tools is the use of property versus propertyplots to summarize an analysis by making extremaeasy to locate. The most popular of these is the

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temperature–salinity or TS diagram, which relatesdensity to the observed values of temperature andsalinity. Originally the TS curve was constructed for asingle hydrographic cast and thus related the TS valuescollected for a single bottle sample with the salinitycomputed from that sample. In this way there was adirect relationship between the TS pair and the depthof the sample. As the historical hydrographic recordexpanded it became possible to compute TS curvesfrom a combination of various temperature–salinityprofiles. This approach amounted to plotting the TScurve as a scatter diagram (Figure 1) where the salinityvalues were then averaged over a selected temperatureinterval to generate a discrete TS curve. The TS curve

shown in Figure 1, which is an average of all of thedata in a 101 square just north east of Hawaii,shows features typical of those that can be foundin all TS curves. As it turned out the temperature–salinity pair remained the same while the depth of thispair oscillated vertically by tens of meters, resulting inthe absence of a precise relationship between TS pairsand depth. As sensed either by ‘bottle casts’ or byelectronic profilers, these vertical variations expressthemselves as increased variability in the temperatureor salinity profiles while the TS curve continues toretain its shape, now independent of depth. Hence acomposite TS curve computed from a number ofclosely spaced hydrographic stations no longer has a

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Tropical upper

Tropical Sal. Max.

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N. Pacific Intermediate and AAIW (mixing)

Deep

N. Pacific

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150−160°W

T/S pairs = 9428

500

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Figure 1 Example of TS ‘scatter plot’ for all data within a 101square with mean TS curve (center line) and curves for one standard

deviation in salinity on either side.

OCEAN CIRCULATION /Water Types and Water Masses 1557

specific relationship between temperature, salinityand depth.

As with the more traditional ‘single station’ TScurve, these area average TS curves can be used todefine and locate water masses. This is done bylocating extrema in salinity associated with particularwatermasses. The salinityminimum in the TS curve ofFigure 1 is at about 101C, where there is a cleardivergence of TS values as they move up the temper-ature scale from the coldest temperatures near thebottomof the diagram.There are two separate clustersof points at this salinity minimum temperature withone terminating at about 131C and the other transi-tioning on up to the warmest temperatures. It is thistermination of points that results in a sharp turn in themean TS curve and causes a very wide standarddeviation. These two clusters of points represent twodifferent intermediate level water masses. The rela-tively high salinity values that appear to terminate at131C represent the Antarctic Intermediate Water(AIW) formed near the Antarctic continent, reachingits northern terminus after flowing up from thesouth. The coincident less salty points indicate thepresence of North Pacific Intermediate Water movingsouth from its formation region in the northernGulf ofAlaska.

While there is no accepted practice in water massterminology, it is generally accepted that a ‘water type’refers to a single point on a characteristic diagram suchas aTS curve. As introduced above, ‘watermass’ refersto some portion or segment of the characteristic curve,which describes the ‘core properties’ of that watermass. In the above example the salinity characteristicsof the two intermediate waters were salinity minima,which were the overall characteristic of the twointermediate waters. We note that the extrema asso-ciated with a particular water mass may not remain atthe same salinity value. Instead, as one moves awayfrom the formation zone for the AIW, which is at theoceanographic ‘polar front’, the sharp minimum thatmarks the AIWwater which has sunk from the surfacedown to about 1000m starts to erode, broadening thesalinityminimumand slowly increasing itsmagnitude.By comparing conditions of the salinity extreme at alocation with salinity characteristics typical of theformation region one can estimate the amount of thesource water mass that is still present at the distantlocation. Called the ‘core-layer’ method, this proce-dure was a crucial development in the early study ofthe ocean water masses and long-term mean currents.

Many variants of the TS curve have been introducedover the years. One particularly instructive formwas a‘volumetric TS curve’. Here the oceanographer sub-jectively decides just how much volume is associatedwith a particular water mass. This becomes a three-

dimensional relationship, which can then be plotted ina perspective format (Figure 2). In this plot the twohorizontal axes are the usual temperature and salinity,while the elevation represents the volumes with thoseparticular TS characteristics. For this presentation,only the deeper water mass characteristics have beenplotted, which can be seen by the restriction of thetemperature scale to �1.01C to 4.01C. Arrows havebeen added to show just which parts of the oceanvarious features have come from. That the Atlantic isthe saltiest of the oceans is very clear with a branch tohigh salinity values at higher temperatures. The mostvoluminous water mass is the Pacific Deep Water thatfills most of the Pacific below the intermediate watersat about 1000m.

Global Water Mass Distribution

Before turning to the TS curve description of the watermasses, it is necessary to indicate the geographicdistribution of the basic water masses. The reader iscautioned that this article only treats the major watermasses, which most oceanographers accept and agreeupon. If a particular region is of interest closeinspection will reveal a great variety of smaller watermass classifications; these can be almost infinite, ashigher resolution is obtained in both horizontal andvertical coverage.

Table 1presents the TS characteristics of theworld’swater masses. In the table are listed the area name, thecorresponding acronym, and the appropriate temper-ature and salinity range. Recall that the propertyextreme erodes moving away from the source region,so it is necessary to define a range of properties. This isalso consistent with the view that a water mass refersto a segment of the TS curve rather than a single point.

As is traditionally the case, the water masses havebeen divided into deep and abyssal waters, interme-diate waters, and upper waters. While the upperwaters have the largest property ranges, physicallythey occupy the least amount of ocean volume. Thereverse is true of the deep and bottom waters, whichhave a fairly restricted range but occupy a substantialportion of the ocean. Since most ocean water massproperties are established at the ocean’s surface, thosewater masses which spend most of their time isolatedfar from the surface will erode the least and have thelongest lifetime. Surfacewaters, on the other hand, arestrongly influenced by fluctuations at the oceansurface, which rapidly erodes the water mass proper-ties. In mean TS curves, as in Figure 1, the spreadof the standard deviation at the highest temperaturesreflect this influence from the heat and fresh waterflux exchange that occurs near and at the ocean’ssurface.


Accompanying the table are global maps of watermasses at all three of these levels. The upper waters inFigure 3 have the most complex distribution withsignificant meridional and zonal changes. A ‘bestguess’ at the formation regions for the correspondingwater mass is indicated by the hatched regions. For itsrelatively small size, the Indian Ocean has a verycomplex upper water mass structure. This is caused bysome unique geographic conditions. First is the mon-soon, which completely changes the wind patternstwice a year. This causes reversals in ocean currents,which also influence the water masses by altering thecontributions of the very saline Arabian Gulf and thefresh Bay of Bengal into the main body of the IndianOcean. All of the major rivers in India flow to the eastand discharge into the Bay of Bengal, making it a veryfresh body of ocean water. To the west of the Indiansubcontinent is the Arabian Sea with its connection tothe Persian Gulf and the Red Sea, both locations ofextremely salty water, making the west side of Indiavery salty and the east side very fresh. The other upperocean water masses in the Indian Ocean are thoseassociated with the Antarctic Circumpolar Current(ACC), which are found at all of the longitudes in theSouthern Ocean.

As the largest ocean basin, the Pacific has thestrongest east–west variations in upper water masses,with east andwest central waters in both the north andsouth hemispheres. Unique to the Pacific is the fairlywide band of the Pacific Equatorial Water, which isstrongly linked to the equatorial upwelling, whichmay not exist in El Nino years. None of the other twoocean basins have this equatorial water mass in theupper ocean. The Atlantic has northern hemisphereupper water masses that can be separated east–westwhile the South Atlantic upper water mass cannot beseparated east–west into two parts. Note the interac-tion between the North Atlantic and the Arctic Oceanthrough the Norwegian Sea and Fram Strait. Also inthese locations are found the source regions for anumber of Atlantic water masses. Compared with theother twooceans, theAtlantic has themostwatermasssource regions, which produce a large part of the deepand bottom waters of the world ocean.

The chart of intermediatewatermasses inFigure 4 ismuch simpler than was the upper ocean water massesinFigure 3. This reflects the fact that there are far fewerintermediatewaters and those that are present fill largevolumes of the intermediate depth ocean. The NorthAtlantic has the most complex horizontal structure of

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Figure 2 Simulated three-dimensional T–S–V diagram for the cold water masses of the World Ocean.


the three oceans. Here intermediate waters form at thesource regions in the northern North Atlantic. Oneexception is the Mediterranean Intermediate Water,which is a consequence of climatic conditions in theMediterranean Sea. This saltywater flows out throughthe Straits of Gibraltar at about 320m depth, where itthen descends to at least 1000m, and maybe a bit

more. It now sinks below the vertical range of the lesssaline Antarctic Intermediate Water (AAIW), insteadjoining with the higher salinity of the deeper NorthAtlantic Deep Water (NADW), which maintains thesalinity maximum indicative of the NADW.

In the Southern Ocean the formation region for theAAIW is marked as the location of the oceanic Polar

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

Layer Atlantic Ocean Indian Ocean Pacific Ocean

Upper waters

(0–500m)

Atlantic Subarctic Upper Water

(ASUW) (0.0–4.01C,34.0–35.0%)

Bengal Bay Water (BBW)

(25.0–291C, 28.0–35.0%)

Arabian Sea Water (ASW)

Pacific Subarctic Upper Water

(PSUW) (3.0–15.01C,32.6–33.6%)

Western North Atlantic Central

Water (WNACW) (7.0–20.01C,35.0–36.7%)

(24.0–30.01C, 35.5–36.8%)

Indian Equatorial Water (IEW)

(8.0–23.01C, 34.6–35.0%)

Western North Pacific Central

Water (WNPCW) (10.0–22.01C,34.2–35.2%)

Eastern North Atlantic Central

Water (ENACW) (8.0–18.01C,35.2–36.7%)

Indonesian Upper Water (IUW)

(8.0–23.01C, 34.4–35.0%)

South Indian Central Water

Eastern North Pacific Central

Water (ENPCW) (12.0–20.01C,34.2–35.0%)

South Atlantic Central Water

(SACW) (5.0–18.01C,34.3–35.8%)

(SICW) (8.0–25.01C, 34.6–35.8%)

Eastern North Pacific Transition

Water (ENPTW) (11.0–20.01C,33.8–34.3%)

Pacific Equatorial Water (PEW)

(7.0–23.01C, 34.5–36.0%)

Western South Pacific Central

Water (WSPCW) (6.0–22.01C,34.5–35.8%)

Eastern South Pacific Central

Water (ESPCW) (8.0–24.01C,34.4–36.4%)

Eastern South Pacific Transition

Water (ESPTW) (14.0–20.01C,34.6–35.2%)

Intermediate waters

(500–1500m)

Western Atlantic Subarctic

Intermediate Water (WASIW)

(3.0–9.01C, 34.0–35.1%)

Antarctic Intermediate Water

(AAIW) (2–101C, 33.8–34.8%)

Indonesian Intermediate Water

Pacific Subarctic Intermediate

Water (PSIW) (5.0–12.01C,33.8–34.3%)

Eastern Atlantic Subarctic

Intermediate Water (EASIW)

(3.0–9.01C, 34.4–35.3%)

(IIW) (3.5–5.51C, 34.6–34.7%)

Red Sea–Persian Gulf

California Intermediate Water

(CIW) (10.0–12.01C,33.9–34.4%)


(AAIW) (2–61C, 33.8–34.8%)

Mediterranean Water (MW)

Intermediate Water (RSPGIW)

(5–141C, 34.8–35.4%)

EasternSouthPacific Intermediate

Water (ESPIW) (10.0–12.01C,34.0–34.4%)

(2.6–11.01C, 35.0–36.2%)

Arctic Intermediate Water (AIW)


(AAIW) (2–101C, 33.8–34.5%)

(�1.5–3.01C, 34.7–34.9%)

Deep and abyssal

waters

(1500m-bottom)

North Atlantic Deep Water

(NADW) (1.5–4.01C,34.8–35.0%)

Circumpolar Deep Water (CDW)

(1.0–2.01C, 34.62–34.73%)

Circumpolar Deep Water (CDW)

(0.1–2.01C, 34.62–34.73%)

Antarctic Bottom Water (AABW)

(�0.9–1.71C, 34.64–34.72%)

Arctic Bottom Water (ABW)

(�1.8 to �10.51C, 34.88–34.94%)

Circumpolar Surface Waters Subantarctic Surface Water

(SASW) (3.2–15.01C,34.0–35.5%)

Antarctic Surface Water (AASW)

(�1.0–1.01C, 34.0–34.6%)


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IEW

Bengal Bay

Wat.

Arab. SeaWater

Indian Equatorial Wat. Indo. Upper Wat.

SICW

PEW

ESPCW

IUW

BBW

S. Indian Centr. Water

Subantarctic Surface Water

Antarctic Surface Water

Ice

P S U W

Pac. Subarctic Upper Wat.

WNPCW ENPCW

West. N. Pacific Centr. Water

West. S. Pacific Centr. Water

East. S. Pacific Centr. Water

East. N. Pacific Centr. Water

East. N. Pacific Transition Water

East. S. Pacific Transition Wat.

Pacific Equatorial Water

WSPCW


Antarctic Surface Water

ASUW

WNACW

ENACW

West. N. Atl.Centr. Water East. N. Atl .

Centr. Wat.

South AtlanticCentr. Water

SACW

Antarctic Surface Wat.

Upper Waters (0−500m)

180°


U pperW

at.

A tl. Subarctic

Figure 3 Global distribution of upper waters (0–500m). Water masses are in abbreviated form with their boundaries indicated by solid lines. Formation regions for these water masses are

marked by cross-hatching and labelled with the corresponding acronym title.

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Red Sea−Pers. GulfInt. Water

Antarctic Int. Water

AAIW

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PSIW

Pacific Subarctic Int. Water

Calif. Int. Water


AAIW

Arctic Int. Water

AIW

WA SIW EASIW

East. Atl.

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East S. PacificInt. Wat.

ESP

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AAIW

Intermediate Waters (500 − 1500 m)

MWMWWest. Atl. Subarctic

Int. Water

Indo. Int. Water

CIW

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

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Front,which is known to vary considerably in strengthand location, moving the formation region north andsouth. That this AAIW fills a large part of the oceancan be clearly seen in all of the ocean basins. In thePacific the AAIWextends north to about 201N,whereit meets the NPIWas already noted from Figure 1. TheAAIW reaches about the same latitude in the NorthAtlantic but it only reaches to about 51 S in the IndianOcean. In the Pacific the northern intermediate watersare mostly from the North Pacific where the NPIW isformed. There is, however, another smaller volumeintermediate water that is formed in the transitionregion west of California, mostly as a consequence ofcoastal upwelling. A similar intermediate water for-mation zone can be found in the south Pacific mainlyoff the coast of South America, which generates aminor intermediate water mass.

The deep and bottomwatersmapped in Figure 5 arerestricted in their movements to the deeper reaches ofthe ocean. For this reason the 4000m depth contourhas been plotted in Figure 5 and a good correspond-ence can be seen between the distribution of bottomwater and the deepest bottom topography. Someinteresting aspects of this bottom water can be seenin the eastern South Atlantic. As the dense bottomwater makes its way north from the Southern Ocean,in the east it runs into theWalvisRidge,whichblocks itfrom further northward extension. Instead the bottomwater flows north along the west of the mid-Atlanticridge and, finding a deep passage in the RomancheGap, flows eastward and then south to fill the basinnorth of the Walvis Ridge. A similar complex patternof distribution can be seen in the Indian Ocean, wherethe east and west portions of the basin fill from thesouth separately because of the central ridge in thebottom topography. In spite of the requisite depth ofthe North Pacific, the Antarctic Bottom Water(AABW) does not extend as far northward in theNorth Pacific. This means that some variant of theAABW, created by mixing with other deep andintermediate waters, occupies the most northernreaches of the deep North Pacific. Because the NorthPacific is essentially ‘cut-off’ from the Arctic, there isno formation region of deep and bottom water in theNorth Pacific.

The 3D TS curve of Figure 2 indicated that the mostabundant water, mass marked by the highest peak inthis TS curve, corresponded to Pacific Deep Water. InTable 1 there is listed something called ‘CircumpolarDeep Water’ in the deeper reaches of both the Pacificand Indian Oceans. This water mass is not formed atthe surface but is instead amixture of NADW,AABW,and the two intermediate waters present in the Pacific.The Antarctic Bottom Water (AABW) forms in theWeddell Sea as the product of very cold, dense, fresh

water flowing off the continental shelf. It then sinksand encounters the upwelling NADW, which adds abit of salinity to the cold, fresh water, making it evendenser. This very dense product of Weddell Sea shelfwater and NADW becomes the AABW, which thensinks to the very bottom and flows out of the WeddellSea to fillmost of the bottom layers of theworld ocean.It is probable that a similar process works in the RossSea and some other areas of the continental shelf toformadditional AABW,but theWeddell Sea is thoughtto be the primary formation region of AABW.

Summary TS Relationships

As pointed out earlier, one of the best ways to detectspecific water masses is with the TS relationship,whether computed for single hydrographic casts orfrom an historical accumulation of such hydro casts.Here traditional practice is followed and the summaryTS curves are divided into the major ocean basinsstarting with the Atlantic (Figure 6). Once again, thehigher salinities typical of the Atlantic can be clearlyseen. The highest salinities are introduced by theMediterranean outflow marked as MW in Figure 6.This joins with water from the North Atlantic tobecome part of the NADW, which is marked by asalinity maximum in these TS curves. The AAIW isindicated by the sharp salinity minimum at lowertemperatures. The source water for the AAIW ismarked by a dark square in the figure. The AABW is asingle point, which now does not represent a ‘watertype’ but rather a water mass. The difference is thatthis water mass has very constant TS propertiesrepresented by a single point in the TS curves. Notethat this is the densest water on this TS diagram (thedensity lines are shown as the dashed curves in the TSdiagram marked with the value of s).

The rather long segments stretching to the uppertemperature and salinity values represent the upperwaters in the Atlantic. While this occupies a largeportion of theTS space, it only covers a relatively smallpart of the upper ocean when compared to the largevolumes occupied by the deep and bottom watermasses. From this TS diagram it can be seen that theupper waters are slightly different in the SouthAtlantic, the East North Atlantic and the West NorthAtlantic.Of these differences the SouthAtlantic differsmore strongly from the other two than they do fromeach other.

By comparison with Figure 6, the Pacific TS curvesof the Pacific (Figure 7) are very fresh, with all but thehighest upper water mass having salinities below35%. The bottom property anchoring this curve is theCircumpolar Deep Water (CDW), which is used to


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ADW ArcticDeep Water

Antarctic Bottom Water

4000 m Depth contour



ADW Arctic Deep Water

Deep and Abyssal Waters(1500 m − bottom)

AABW

Arctic Deep WaterADW

NADW

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 again by

hatching and its spreading terminus, near the Antarctic, by a dashed line which also suggests the global communication of this deep water around the Antarctic.

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identify a wide range of TS properties that are knownto be deep and bottom water but which have not beenidentified in terms of a specific formation region andTS properties. As with the AABW, a single point at thebottom of the curves represents the CDW. The

relationship between the AAIW and the PSIW can beclearly seen in this diagram. The AAIW is colder andsaltier than is the PSIW, which is generally a bit higherin the water column, indicated by the lower density ofthis feature. There are no external sources of deep

Atlantic Ocean

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AAIW

EASIWWASIW

AABW

NADW

MW

28

27ENACW

WNACW

SACW

� t = 26

Figure 6 Characteristic temperature–salinity (TS) curves for the main water masses of the Atlantic Ocean.Water masses are labelled

by the appropriate acronym and core water properties are indicated by a dark square with an arrow to suggest their spread. The cross

isopycnal nature of some of these arrows is not intended to suggest a mixing process but merely to connect source waters with their

corresponding characteristic extrema.

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Salinity (‰)

CDW

AAIW

PSIW

ENPT

W

ENPCW

WNACW

ESPTW

ESPCW

PEW

WSPCW

27

28

� = 26

Figure 7 Characteristic TS curves for the main water masses of the Pacific Ocean.


salinity like with the Mediterranean Water in theAtlantic. Instead there is a confusing plethora of upperwater masses that clearly separate the east–west andnorth–south portions of the basin. So we have EasternNorth Pacific Central Water (ENPCW) and WesternNorth Pacific Central Water (WNPCW), as well asEastern South Pacific Central Water (ESPCW) andWestern South Pacific Central Water (WSPCW).

The central waters all refer to open ocean upperwater masses. The more coastal water masses such asthe Eastern North Pacific Transition Water (ENPTW)are typical of the change in upper water massproperties that occurs near the coastal regions. Thesame is true of the South Pacific as well. In general thefresher upper-layer water masses of the Pacific arelocated in the east where river runoff introduces a lotof fresh water into the upper ocean. To the west theupper water masses are saltier as shown by thequasilinear portions of the TS curves correspondingto the western upper water masses. The PacificEquatorial Water (PEW) is unique in the Pacificprobably due to well-developed equatorial circulationsystem. As seen in Figure 7, the PEW TS properties liebetween the east and west central waters.

The Indian Ocean TS curves in Figure 8 are quitedifferent from either the Atlantic or the Pacific.Overall the Indian Ocean is quite a bit saltier thanthe Pacific but not quite as salty as the Atlantic. Alsolike the Atlantic, the Indian Ocean receives salinityinput from a marginal sea as the Red Sea deposits itssalt-laden water into the Arabian Sea. Its presence is

noted inFigure 8 as the black boxmarkedRSPGIW forthe Red Sea–Persian Gulf Intermediate Water. Addedat the sill depth of the Red Sea, this intermediate watercontributes to a salinity maximum that is seasonallydependent.

The bottom water is the same CDW that we saw inthe Pacific. Unlike the Pacific, the Indian Oceanequatorial water masses are nearly isohaline abovethe point representing the CDW. In fact the line thatrepresents the Indian Ocean Equatorial Water (IEW)runs almost straight up from the CDWat about 0.01Cto the maximum temperature at 201C. There isexpression of the AAIW in the curve that correspondsto the South Indian Ocean Central Water (SICW). Acompeting Indonesian Intermediate Water (IIW) hashigher temperature and higher salinity characteristicswhich result in it having an only slightly lower density,creating the weak salinity minimum in the curvetransitioning to the IndianOceanUpperWater (IUW).The warmest and saltiest part of these TS curvesrepresents the Arabian Sea Water (ASW) on thewestern side of the Indian subcontinent.

Discussion and Conclusion

The descriptions provided in this article cover only themost general ofwatermasses, their core properties andtheir geographic distribution. In most regions of theocean it is possible to resolve the water mass structureinto even finer elements describing more precisely thedifferences in temperature and salinity. In addition,

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5

10

15

20BBW

Salinity (‰)

Tem

pera

ture

(° C

)

IIW

CDW

AAIW

IUW

IEW

SICW

ASW

RSPGIW

28

27

� = 26

Indian Ocean

Figure 8 Characteristic TS curves for the main water masses of the Indian Ocean. All lables as in Figure 6.


other important properties can be used to specifywatermasses not obvious in TS space.While dissolvedoxygen is often used to define water mass boundaries,care must be taken as this nonconservative property isinfluenced by biological activity and the chemicaldissolution of dead organic material falling throughthe water column. Nutrients also suffer from modifi-cation within the water column, making their inter-pretation as water mass boundaries more difficult.Characteristic diagrams that plot oxygen againstsalinity or nutrients can be used to seek extrema thatmark the boundaries of various water masses.

The higher vertical resolution property profilespossible with electronic profiling instruments alsomake it possible to resolve water mass structure thatwas not even visible with the lower vertical resolutionof earlier bottle sampling. Again, this complexity isonly merited in local water mass descriptions andcannot be used on the global-scale description. At thisglobal scale the descriptive data available from theaccumulation of historical hydrographic data areadequate to map the large-scale water mass distribu-tion, as has been done in this article.

See also

Boundary Layers: Ocean Mixed Layer. Convection:Convection in the Ocean.CoupledOcean–Atmosphere

Models. Ocean Circulation: General Processes; Sur-face–Wind Driven Circulation; Thermohaline Circulation.

Further Reading

Emery WJ and Meincke J (1986) Global watermasses: summary and review. Oceanologica Acta 9:383–391.

Mamayev OI (1975) Temperature–Salinity Analysis ofWorld Ocean Waters. Elsevier Oceanography Series,#11, Elsevier Scientific Pub. Co., Amsterdam, 374 pp.

Iselin CO’D (1939) The influence of vertical and lateralturbulence on the characteristics of the waters at mid-depths.Transactions of theAmericanGeophysicalUnion20: 414–417.

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

Reid JL (1973) Northwest Pacific Ocean Water in Winter.The Johns Hopkins Oceanographic Studies #5, JohnsHopkins Press, 96 pp.

Sverdrup HU, Johnson MW and Fleming RH (1941) TheOceans. Prentice-Hall Inc., 1087 pp.

Worthington LV (1976)On the North Atlantic Circulation.The Johns Hopkins Oceanographic Studies #6, JohnsHopkins Press, 110 pp.

Worthington LV (1981) The water masses of the worldocean: some results of a fine-scale census. In: Warren BAand Wunsch C (eds) Evolution of Physical Oceanogra-phy, Ch. 2, pp. 42–69. Cambridge, MA: MIT Press.

OPERATIONAL METEOROLOGY

JV Cortinas Jr, University of Oklahoma/CooperativeInstitute for Mesoscale Meteorological Studies, Norman,OK, USA

WBlier, National Weather Service WASC, Monterey,CA, USA

Copyright 2003 Elsevier Science Ltd. All Rights Reserved.

Introduction

Operational meteorology involves the generation andwidespread dissemination of weather information.Although the types of weather-related informationgenerated and the methods of distribution haveevolved throughout the history of operational fore-casting, the basic focus of this subdiscipline of theatmospheric sciences has remained unchanged: to usescientific knowledge about how the atmospherebehaves to develop and distribute useful forecasts ofweather conditions. In its modern application, this

process typically involves the acquisition, examina-tion, and interpretation of vast quantities of bothobservational meteorological data and numericalforecast model output, and thus tends to be extremelycomputer intensive. The content of the disseminatedweather products varies according to the issuingorganization and the type of weather informationrequired by its customers; typically, they includeperiodic reports and forecasts of weather conditionsover particular geographic regions and notification ofany anticipated or observed hazardous weather con-ditions. These products are disseminated in manyways, including dedicated electronic communicationssystems maintained by national and local govern-ments, the World Wide Web, newspapers, radio, andtelevision.

Prior to the 1990s, countries with significantweather service programs generally relied on theirrespective national governments to provide weatherinformation. While this practice is still common in

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