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1 Water Types and Water Masses / OCEAN CIRCULATION 1556 Water Types and Water Masses , University of Colorado, Boulder, CO, USA W J Emery problems, there is very little research directed at improving our knowledge of water mass distributions Copyright 2003 Elsevier Science Ltd. All Rights Reserved. and their changes over time. Introduction What is a Water Mass? Muchofwhatisknowntodayaboutthecurrentsofthe The concept of a ‘water mass’ is borrowed from deep ocean has been inferred from studies of the water meteorology, which classifies different atmospheric properties such as temperature, salinity, dissolved characteristics as ‘air masses’. In the early part of the oxygen, and nutrients. These are quantities that can be twentieth century physical oceanographers also observed with standard hydrographic measurement sought to borrow another meteorological concept techniques which collect temperatures and samples of separating the ocean waters into ‘warm’ and ‘cold’ water with a number of sampling bottles strung along water spheres. This designation has not survived in a wire to provide the depth resolution needed. Salinity modern physical oceanography but the more general or ‘salt content’ is then measured by an analysis of the concept of water masses persists. Some oceanogra- water sample, which combined with the correspond- phers regard these as real, objective physical entities, ing temperature value at that ‘bottle’ sample yields building blocks from which the oceanic stratification temperature and salinity as a function of depth of the (vertical structure) is constructed. At the opposite sample. Modern observational methods have in part extreme, other oceanographers consider water masses replaced this sample bottle method with electronic to be mainly descriptive words, summary shorthand profiling systems, at least for temperature and salinity, for pointing to prominent features in property distri- but many of the important descriptive quantities such butions. as oxygen and nutrients still require bottle samples The concept adopted for this discussion is squarely accomplishedtodaywitha‘rosette’samplerintegrated in the middle, identifying some ‘core’ water mass with the electronic profiling systems. These new propertiesthatarethebuildingblocks.Inmostpartsof electronic profiling systems have been in use for over the ocean the stratification is defined by mixing in both 30 years, but still the majority of data useful for vertical and horizontal orientations of the various studying the properties of the deep and open ocean water massesthat advect intothelocation.Thus,inthe come from the time before the advent of modern maps of the various water mass distributions a electronic profiling system. This knowledge is impor- ‘formation region’ is identified where it is believed tant in the interpretation of the data since the that the core water mass has acquired its basic measurements from sampling bottles have very differ- characteristics at the surface of the ocean. This ent error characteristics than those from modern introduces a fundamental concept first discussed by electronic profiling systems. Iselin (1939), who suggested that the properties of the This article reviews the mean properties of the open various subsurface water masses were originally ocean, concentrating on the distributions of the major formed at the surface in the source region of that water masses and their relationships to the currents of particular water mass. Since temperature and salinity the ocean. Most of this information is taken from are considered to be ‘conservative properties’ (prop- published material, including the few papers that erty is only changed at the sea surface), these charac- directly address water mass structure, along with the teristics would slowly erode as the water properties many atlases that seek to describe the distribution of were advected at depth to various parts of the ocean. water masses in the ocean. Coincident with the shift from bottle sampling to electronic profiling is the shift from publishing information about water masses and Descriptive Tools: The TS Curve ocean currents in large atlases to the more routine research paper. In these papers the water mass char- Before focusing on the global distribution of water acteristics are generally only a small portion, requiring masses, it is appropriate to introduce some of the basic the interested descriptive oceanographer to go to tools used to describe these masses. One of the most considerable trouble to extract the information he or basic tools is the use of property versus property she may be interested in. While water mass distribu- plots to summarize an analysis by making extrema tions play a role in many of today’s oceanographic easy to locate. The most popular of these is the

2 1557 Water Types and Water Masses / OCEAN CIRCULATION , which is an average of all of the Figure 1 shown in temperature–salinity or TS diagram, which relates square just north east of Hawaii, 1 data in a 10 density to the observed values of temperature and shows features typical of those that can be found salinity. Originally the TS curve was constructed for a in all TS curves. As it turned out the temperature– single hydrographic cast and thus related the TS values salinity pair remained the same while the depth of this collected for a single bottle sample with the salinity pair oscillated vertically by tens of meters, resulting in computed from that sample. In this way there was a the absence of a precise relationship between TS pairs direct relationship between the TS pair and the depth and depth. As sensed either by ‘bottle casts’ or by of the sample. As the historical hydrographic record electronic profilers, these vertical variations express expanded it became possible to compute TS curves themselves as increased variability in the temperature from a combination of various temperature–salinity or salinity profiles while the TS curve continues to profiles. This approach amounted to plotting the TS retain its shape, now independent of depth. Hence a ) where the salinity Figure 1 curve as a scatter diagram ( composite TS curve computed from a number of values were then averaged over a selected temperature closely spaced hydrographic stations no longer has a interval to generate a discrete TS curve. The TS curve Salinity, ‰ 32 37 33 34 35 36 30 ∆ m S,T 100 25 Tropical upper 600 Tropical Sal. Max. 20 150 500 N. Pacific central C) ° 15 400 200 Temperature ( N. Pacific 10 Intermediate and AAIW 300 (mixing) 500 N. Pacific 5 1000 ° 10 N 20 − Deep ° 150 − 160 W 5000 T/S pairs = 9428 0 00 200 1 0 Example of TS ‘scatter plot’ for all data within a 10 1 Figure 1 square with mean TS curve (center line) and curves for one standard deviation in salinity on either side.

3 Water Types and Water Masses / OCEAN CIRCULATION 1558 dimensional relationship, which can then be plotted in specific relationship between temperature, salinity ). In this plot the two Figure 2 a perspective format ( and depth. horizontal axes are the usual temperature and salinity, As with the more traditional ‘single station’ TS while the elevation represents the volumes with those curve, these area average TS curves can be used to particular TS characteristics. For this presentation, define and locate water masses. This is done by only the deeper water mass characteristics have been locating extrema in salinity associated with particular plotted, which can be seen by the restriction of the water masses. The salinity minimum in the TS curve of C to 4.0 1 1.0 temperature scale to  C. Arrows have 1 1 C, where there is a clear Figure 1 is at about 10 been added to show just which parts of the ocean divergence of TS values as they move up the temper- various features have come from. That the Atlantic is ature scale from the coldest temperatures near the the saltiest of the oceans is very clear with a branch to bottomofthediagram.Therearetwoseparateclusters high salinity values at higher temperatures. The most of points at this salinity minimum temperature with voluminous water mass is the Pacific Deep Water that one terminating at about 13 C and the other transi- 1 fills most of the Pacific below the intermediate waters tioning on up to the warmest temperatures. It is this at about 1000 m. termination of points that results in a sharp turn in the mean TS curve and causes a very wide standard deviation. These two clusters of points represent two Global Water Mass Distribution different intermediate level water masses. The rela- tively high salinity values that appear to terminate at Before turning to the TS curve description of the water 13 1 C represent the Antarctic Intermediate Water masses, it is necessary to indicate the geographic (AIW) formed near the Antarctic continent, reaching distribution of the basic water masses. The reader is its northern terminus after flowing up from the cautioned that this article only treats the major water south. The coincident less salty points indicate the masses, which most oceanographers accept and agree presence of North Pacific Intermediate Water moving upon. If a particular region is of interest close south from its formationregion in the northernGulfof inspection will reveal a great variety of smaller water Alaska. mass classifications; these can be almost infinite, as While there is no accepted practice in water mass higher resolution is obtained in both horizontal and terminology, it is generally accepted that a ‘water type’ vertical coverage. refers toasingle pointona characteristic diagramsuch Table 1 presentsthe TS characteristics of the world’s as a TS curve. As introduced above, ‘water mass’refers water masses. In the table are listed the area name, the to some portion or segment of the characteristic curve, corresponding acronym, and the appropriate temper- which describes the ‘core properties’ of that water ature and salinity range. Recall that the property mass. In the above example the salinity characteristics extreme erodes moving away from the source region, of the two intermediate waters were salinity minima, so it is necessary to define a range of properties. This is which were the overall characteristic of the two also consistent with the view that a water mass refers intermediate waters. We note that the extrema asso- to a segment of the TS curve rather than a single point. ciated with a particular water mass may not remain at As is traditionally the case, the water masses have the same salinity value. Instead, as one moves away been divided into deep and abyssal waters, interme- from the formation zone for the AIW, which is at the diate waters, and upper waters. While the upper oceanographic ‘polar front’, the sharp minimum that waters have the largest property ranges, physically marks the AIW water which has sunk from the surface they occupy the least amount of ocean volume. The down to about 1000 m starts to erode, broadening the reverse is true of the deep and bottom waters, which salinityminimumandslowlyincreasingitsmagnitude. have a fairly restricted range but occupy a substantial By comparing conditions of the salinity extreme at a portion of the ocean. Since most ocean water mass location with salinity characteristics typical of the properties are established at the ocean’s surface, those formation region one can estimate the amount of the water masses which spend most of their time isolated source water mass that is still present at the distant far from the surface will erode the least and have the location. Called the ‘core-layer’ method, this proce- longest lifetime. Surface waters, on the other hand, are dure was a crucial development in the early study of strongly influenced by fluctuations at the ocean the ocean water masses and long-term mean currents. surface, which rapidly erodes the water mass proper- Many variants of the TS curve have been introduced ties. In mean TS curves, as in Figure 1 , the spread over the years. One particularly instructive form was a of the standard deviation at the highest temperatures ‘volumetric TS curve’. Here the oceanographer sub- reflect this influence from the heat and fresh water jectively decides just how much volume is associated flux exchange that occurs near and at the ocean’s with a particular water mass. This becomes a three- surface.

4 1559 OCEAN CIRCULATION Water Types and Water Masses / Pacific deep World ocean Pacific 34.40 Atlantic 34.50 4° 34.70 34.80 ° C) 3 ° 34.90 35.00 ° 2 4° ° 1 ° 3 Potential temperature ( ° 0 ° 2 ° 1 34.40 34.50 34.60 ° 0 34.70 34.80 34.90 Salinity ( ‰) 35.00 Southern Indian Simulated three-dimensional T–S–V diagram for the cold water masses of the World Ocean. Figure 2 As the largest ocean basin, the Pacific has the Accompanying the table are global maps of water strongest east–west variations in upper water masses, masses at all three of these levels. The upper waters in with east andwest central waters in both the north and have the most complex distribution with Figure 3 south hemispheres. Unique to the Pacific is the fairly significant meridional and zonal changes. A ‘best wide band of the Pacific Equatorial Water, which is guess’ at the formation regions for the corresponding strongly linked to the equatorial upwelling, which water mass is indicated by the hatched regions. For its may not exist in El Nin o years. None of the other two relatively small size, the Indian Ocean has a very ̃ ocean basins have this equatorial water mass in the complex upper water mass structure. This is caused by upper ocean. The Atlantic has northern hemisphere some unique geographic conditions. First is the mon- upper water masses that can be separated east–west soon, which completely changes the wind patterns while the South Atlantic upper water mass cannot be twice a year. This causes reversals in ocean currents, separated east–west into two parts. Note the interac- which also influence the water masses by altering the tion between the North Atlantic and the Arctic Ocean contributions of the very saline Arabian Gulf and the through the Norwegian Sea and Fram Strait. Also in fresh Bay of Bengal into the main body of the Indian these locations are found the source regions for a Ocean. All of the major rivers in India flow to the east number of Atlantic water masses. Compared with the and discharge into the Bay of Bengal, making it a very other two oceans, theAtlantic hasthemost water mass fresh body of ocean water. To the west of the Indian source regions, which produce a large part of the deep subcontinent is the Arabian Sea with its connection to and bottom waters of the world ocean. the Persian Gulf and the Red Sea, both locations of Figure 4 The chartofintermediatewatermassesin is extremely salty water, making the west side of India much simpler than was the upper ocean water masses very salty and the east side very fresh. The other upper Figure3 .Thisreflectsthefactthattherearefarfewer in ocean water masses in the Indian Ocean are those intermediate waters andthose thatarepresentfill large associated with the Antarctic Circumpolar Current volumes of the intermediate depth ocean. The North (ACC), which are found at all of the longitudes in the Atlantic has the most complex horizontal structure of Southern Ocean.

5 Water Types and Water Masses / 1560 OCEAN CIRCULATION Table 1 Temperature–salinity characteristics of the world’s water masses Atlantic Ocean Indian Ocean Pacific Ocean Layer Atlantic Subarctic Upper Water Upper waters Pacific Subarctic Upper Water Bengal Bay Water (BBW) C, (ASUW) (0.0–4.0 1 C, 1 (PSUW) (3.0–15.0 ) (0–500m) (25.0–29 1 C, 28.0–35.0 % ) % 34.0–35.0 32.6–33.6 % ) Arabian Sea Water (ASW) Western North Pacific Central ) Western North Atlantic Central % C, 35.5–36.8 1 (24.0–30.0 C, 1 Water (WNACW) (7.0–20.0 Indian Equatorial Water (IEW) Water(WNPCW)(10.0–22.0 1 C, 1 (8.0–23.0 ) C, 34.6–35.0 % ) % % ) 35.0–36.7 34.2–35.2 Eastern North Pacific Central Indonesian Upper Water (IUW) Eastern North Atlantic Central ) Water (ENPCW) (12.0–20.0 C, 1 1 (8.0–23.0 1 C, 34.4–35.0 % C, Water (ENACW) (8.0–18.0 South Indian Central Water ) % 35.2–36.7 34.2–35.0 % ) 1 C, 34.6– Eastern North Pacific Transition (SICW) (8.0–25.0 South Atlantic Central Water 1 % 35.8 Water (ENPTW) (11.0–20.0 C, ) C, 1 (SACW) (5.0–18.0 33.8–34.3 % ) ) % 34.3–35.8 Pacific Equatorial Water (PEW) 1 C, 34.5–36.0 % ) (7.0–23.0 Western South Pacific Central Water (WSPCW) (6.0–22.0 1 C, 34.5–35.8 ) % Eastern South Pacific Central 1 C, Water (ESPCW) (8.0–24.0 34.4–36.4 % ) Eastern South Pacific Transition Water (ESPTW) (14.0–20.0 1 C, 34.6–35.2 % ) Pacific Subarctic Intermediate Intermediate waters Western Atlantic Subarctic Antarctic Intermediate Water ) Water (PSIW) (5.0–12.0 (500–1500m) 1 Intermediate Water (WASIW) % C, (AAIW) (2–10 1 C, 33.8–34.8 % 1 C, 34.0–35.1 % ) 33.8–34.3 Indonesian Intermediate Water (3.0–9.0 ) Eastern Atlantic Subarctic California Intermediate Water (IIW) (3.5–5.5 1 C, 34.6– ) C, 1 (CIW) (10.0–12.0 34.7 Intermediate Water (EASIW) % (3.0–9.0 Red Sea–Persian Gulf ) % C, 34.4–35.3 1 33.9–34.4 % ) Antarctic Intermediate Water EasternSouth Pacific Intermediate Intermediate Water (RSPGIW) (AAIW) (2–6 % ) C, C, 33.8–34.8 (5–14 1 C, 34.8–35.4 % ) 1 Water (ESPIW) (10.0–12.0 1 Mediterranean Water (MW) 34.0–34.4 % ) (2.6–11.0 1 C, 35.0–36.2 % ) Antarctic Intermediate Water 1 Arctic Intermediate Water (AIW) ) (AAIW) (2–10 % C, 33.8–34.5 C, 34.7–34.9 1  ( 1.5–3.0 ) % Circumpolar Deep Water (CDW) Deep and abyssal North Atlantic Deep Water Circumpolar Deep Water (CDW) (0.1–2.0 C, 34.62–34.73 ) waters ) % (NADW) (1.5–4.0 1 C, % C, 34.62–34.73 1 1 (1.0–2.0 ) 34.8–35.0 (1500m-bottom) % Antarctic Bottom Water (AABW) % C, 34.64–34.72 1 0.9–1.7  ) ( Arctic Bottom Water (ABW)  C, 34.88– 1 ( 10.5 1.8 to  ) 34.94 % Circumpolar Surface Waters Subantarctic Surface Water (SASW) (3.2–15.0 C, 1 ) 34.0–35.5 % Antarctic Surface Water (AASW) % C, 34.0–34.6 1 1.0–1.0  ( ) the three oceans. Here intermediate waters form at the more. It now sinks below the vertical range of the less source regions in the northern North Atlantic. One saline Antarctic Intermediate Water (AAIW), instead exception is the Mediterranean Intermediate Water, joining with the higher salinity of the deeper North which is a consequence of climatic conditions in the Atlantic Deep Water (NADW), which maintains the Mediterranean Sea. This salty water flows out through salinity maximum indicative of the NADW. the Straits of Gibraltar at about 320 m depth, where it In the Southern Ocean the formation region for the then descends to at least 1000 m, and maybe a bit AAIW is marked as the location of the oceanic Polar

6 1561 OCEAN CIRCULATION Water Types and Water Masses / ° ° ° ° ° ° ° 0 N S 20 60 40 20 40 60 . t a r W e t e a c W a f r e u c E E s for these water masses are S a f ° r ° c i u t . c S 30 . l r t 30 t ° W ASUW c a a ° i A t t 0 . . t n 0 W c Centr. Water South Atlantic ° a NAC r . ° N A c i r E t a . t W t c t r n 30 n s a 30 e ° a b a r ° b u C e E u S 60 p S ° . p SACW l t W U A W 60 C A W 90 N W Centr. Water West. N. Atl. 500m) − Transition Wat. East. S. Pacific Upper Waters (0 Transition Water East. N. Pacific W ter ° a W 60 Centr. Water East. S. Pacific ° e c r a e 90 t ° a urf S W l c i a t 120 i W W . c r t ° r o C a ° t a t ESPCW P a W n u N r a 150 q e E b ° 120 p E u ° p c S i U f Ice East. N. Pacific Centr. Water ° i 180 c c i ° 150 a t ° c W Antarctic Surface Water P r 180 a U b S u 150 P S ° E 150 . c W a PEW C West. S. Pacific Centr. Water P W P C S E 120 P W N W Centr. Water West. N. Pacific IUW E ° r r e t e t a a BBW 150 W ° W e e c c a SICW f a r f 120 r u ° Wat. u Indo. Upper Wat. E S S c ° 90 i c t Bay Bengal ° i t c . r r c 90 t r e a ° a t 60 t a t ° a n W n A l 60 W a a . ° i b r r t u o n IEW t E 30 S e a ASW C u E 30 q Water n Arab. Sea E a i n d a n i I d . n I S Global distribution of upper waters (0–500m). Water masses are in abbreviated form with their boundaries indicated by solid lines. Formation region ° ° ° ° ° ° ° 0 N S 20 40 20 60 40 60 marked by cross-hatching and labelled with the corresponding acronym title. Figure 3

7 Water Types and Water Masses OCEAN CIRCULATION 1562 / ° ° ° ° ° ° ° N S 20 20 40 40 60 0 60 MW MW E E ° . t Arctic Int. Water ° a 30 AIW r W ° e 30 t . . t a d ° 0 n I e ° 0 AAIW c W i t ° M c East. Atl. r EASIW 30 a ° b 30 u ° S 60 c i t ° c r . a WA SIW W 60 b Antarctic Int. Water u r S W 90 e . l t t a A . W t Figure 3 . s t e n I W W I P S E 1500 m) − Int. Wat. East S. Pacific (500 Intermediate Waters W W ° I C 60 ° 90 ° W 120 ° ° Calif. Int. Water 150 120 ° ° r e t ° a 180 150 AAIW W ° ° Antarctic Int. Water . t 180 n I c 150 i t PSIW ° c E 150 r a b u S E 120 c i f i c a P IIW E r e ° t a W . 150 t ° n I . o d 120 n ° E I ° AAIW 90 ° 90 ° 60 ° 60 ° − E 30 E 30 Pers. Gulf Int. Water Red Sea Antarctic Int. Water Global distribution of intermediate water (550–1500 m). Lines, labels and hatching follow the same format as described for ° ° ° ° ° ° ° 0 N S 20 40 20 40 60 Figure 4 60

8 1563 Water Types and Water Masses / OCEAN CIRCULATION water flowing off the continental shelf. It then sinks Front, which isknown tovary considerably instrength and encounters the upwelling NADW, which adds a and location, moving the formation region north and bit of salinity to the cold, fresh water, making it even south. That this AAIW fills a large part of the ocean denser. This very dense product of Weddell Sea shelf can be clearly seen in all of the ocean basins. In the water and NADW becomes the AABW, which then Pacific the AAIWextends north to about 20 1 N, where sinks to the very bottom and flows out of the Weddell . The it meets the NPIWas already noted from Figure 1 Sea tofill mostof thebottom layersoftheworld ocean. AAIW reaches about the same latitude in the North It is probable that a similar process works in the Ross Atlantic but it only reaches to about 5 1 S in the Indian Sea and some other areas of the continental shelf to Ocean. In the Pacific the northern intermediate waters form additional AABW,but theWeddell Seais thought are mostly from the North Pacific where the NPIW is to be the primary formation region of AABW. formed. There is, however, another smaller volume intermediate water that is formed in the transition region west of California, mostly as a consequence of coastal upwelling. A similar intermediate water for- Summary TS Relationships mation zone can be found in the south Pacific mainly off the coast of South America, which generates a As pointed out earlier, one of the best ways to detect minor intermediate water mass. specific water masses is with the TS relationship, are Figure 5 The deep and bottom waters mapped in whether computed for single hydrographic casts or restricted in their movements to the deeper reaches of from an historical accumulation of such hydro casts. the ocean. For this reason the 4000 m depth contour Here traditional practice is followed and the summary Figure 5 has been plotted in and a good correspond- TS curves are divided into the major ocean basins ence can be seen between the distribution of bottom ). Once again, the Figure 6 starting with the Atlantic ( water and the deepest bottom topography. Some higher salinities typical of the Atlantic can be clearly interesting aspects of this bottom water can be seen seen. The highest salinities are introduced by the in the eastern South Atlantic. As the dense bottom . Figure 6 Mediterranean outflow marked as MW in water makes its way north from the Southern Ocean, This joins with water from the North Atlantic to intheeastitrunsintotheWalvisRidge,whichblocks it become part of the NADW, which is marked by a from further northward extension. Instead the bottom salinity maximum in these TS curves. The AAIW is water flows north along the west of the mid-Atlantic indicated by the sharp salinity minimum at lower ridge and, finding a deep passage in the Romanche temperatures. The source water for the AAIW is Gap, flows eastward and then south to fill the basin marked by a dark square in the figure. The AABW is a north of the Walvis Ridge. A similar complex pattern single point, which now does not represent a ‘water of distribution can be seen in the Indian Ocean, where type’ but rather a water mass. The difference is that the east and west portions of the basin fill from the this water mass has very constant TS properties south separately because of the central ridge in the represented by a single point in the TS curves. Note bottom topography. In spite of the requisite depth of that this is the densest water on this TS diagram (the the North Pacific, the Antarctic Bottom Water density lines are shown as the dashed curves in the TS (AABW) does not extend as far northward in the s diagram marked with the value of ). North Pacific. This means that some variant of the The rather long segments stretching to the upper AABW, created by mixing with other deep and temperature and salinity values represent the upper intermediate waters, occupies the most northern waters in the Atlantic. While this occupies a large reaches of the deep North Pacific. Because the North portionoftheTSspace,it onlycoversarelativelysmall Pacific is essentially ‘cut-off’ from the Arctic, there is part of the upper ocean when compared to the large no formation region of deep and bottom water in the volumes occupied by the deep and bottom water North Pacific. masses. From this TS diagram it can be seen that the indicated that the most The 3D TS curve of Figure 2 upper waters are slightly different in the South abundant water, mass marked by the highest peak in Atlantic, the East North Atlantic and the West North this TS curve, corresponded to Pacific Deep Water. In Atlantic.OfthesedifferencestheSouthAtlanticdiffers Table 1 there is listed something called ‘Circumpolar more strongly from the other two than they do from Deep Water’ in the deeper reaches of both the Pacific each other. and Indian Oceans. This water mass is not formed at By comparison with Figure 6 , the Pacific TS curves the surface but is instead a mixture of NADW, AABW, Figure 7 ) are very fresh, with all but the of the Pacific ( and the two intermediate waters present in the Pacific. highest upper water mass having salinities below The Antarctic Bottom Water (AABW) forms in the 35 % . The bottom property anchoring this curve is the Weddell Sea as the product of very cold, dense, fresh Circumpolar Deep Water (CDW), which is used to

9 Water Types and Water Masses 1564 / OCEAN CIRCULATION ° ° ° ° ° ° ° N S 40 20 20 40 60 60 0 E Arctic Deep Water ADW E ° ° n of NADW is indicated again by 30 ° 30 ° 0 ° 0 ° tarctic. 30 NADW ° 30 Antarctic Bottom Water ° AABW 60 ° W 60 W 90 bottom) − Deep and Abyssal (1500 m Waters W ° 60 ° 90 ° Arctic Deep Water W 120 ° ° 150 120 ° ° Antarctic Bottom Water ° 180 150 ° ° 180 150 ° E 150 E 120 ADW 4000 m Depth contour E ° r e t 150 a ° W m o 120 t t ° E ADW o ° B 90 c ° i 90 t ° c r 60 a ° t 60 n ° A E 30 E 30 Arctic Deep Water Global distribution of deep and abyssal waters (1500–bottom). Contour lines describe the spreading of abyssal water (primarily AABW). The formatio ° ° ° ° ° ° ° 0 N S 60 20 40 40 20 60 Figure5 hatching and its spreading terminus, near the Antarctic, by a dashed line which also suggests the global communication of this deep water around the An

10 1565 / OCEAN CIRCULATION Water Types and Water Masses 20 = 26  t 15 WNACW MW 27 SACW ENACW C) ° 10 28 Temperature ( 5 AAIW NADW EASIW WASIW 0 Atlantic Ocean AABW 2 − 36 35 34 Salinity (‰) Characteristic temperature–salinity (TS) curves for the main water masses of the Atlantic Ocean. Water masses are labelled Figure 6 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. identify a wide range of TS properties that are known relationship between the AAIW and the PSIW can be to be deep and bottom water but which have not been clearly seen in this diagram. The AAIW is colder and saltier than is the PSIW, which is generally a bit higher identified in terms of a specific formation region and TS properties. As with the AABW, a single point at the in the water column, indicated by the lower density of bottom of the curves represents the CDW. The this feature. There are no external sources of deep 20 = 26  ENPCW ENPTW WNACW 15 27 ESPTW PEW C) ° WSPCW ESPCW 28 10 Temperature ( PSIW 5 AAIW 0 Pacific Ocean CDW 2 − 36 35 34 Salinity (‰) Figure 7 Characteristic TS curves for the main water masses of the Pacific Ocean.

11 Water Types and Water Masses / OCEAN CIRCULATION 1566 astheblack boxmarkedRSPGIW for Figure8 notedin salinity like with the Mediterranean Water in the the Red Sea–Persian Gulf Intermediate Water. Added Atlantic. Instead there is a confusing plethora of upper at the sill depth of the Red Sea, this intermediate water water masses that clearly separate the east–west and contributes to a salinity maximum that is seasonally north–south portions of the basin. So we have Eastern dependent. North Pacific Central Water (ENPCW) and Western The bottom water is the same CDW that we saw in North Pacific Central Water (WNPCW), as well as the Pacific. Unlike the Pacific, the Indian Ocean Eastern South Pacific Central Water (ESPCW) and equatorial water masses are nearly isohaline above Western South Pacific Central Water (WSPCW). the point representing the CDW. In fact the line that The central waters all refer to open ocean upper represents the Indian Ocean Equatorial Water (IEW) water masses. The more coastal water masses such as 1 C runs almost straight up from the CDWat about 0.0 the Eastern North Pacific Transition Water (ENPTW) to the maximum temperature at 20 1 C. There is are typical of the change in upper water mass expression of the AAIW in the curve that corresponds properties that occurs near the coastal regions. The to the South Indian Ocean Central Water (SICW). A same is true of the South Pacific as well. In general the competing Indonesian Intermediate Water (IIW) has fresher upper-layer water masses of the Pacific are higher temperature and higher salinity characteristics located in the east where river runoff introduces a lot which result in it having an only slightly lower density, of fresh water into the upper ocean. To the west the creating the weak salinity minimum in the curve upper water masses are saltier as shown by the transitioning to the Indian Ocean Upper Water (IUW). quasilinear portions of the TS curves corresponding The warmest and saltiest part of these TS curves to the western upper water masses. The Pacific represents the Arabian Sea Water (ASW) on the Equatorial Water (PEW) is unique in the Pacific western side of the Indian subcontinent. probably due to well-developed equatorial circulation Figure 7 , the PEW TS properties lie system. As seen in between the east and west central waters. Discussion and Conclusion Figure 8 The Indian Ocean TS curves in are quite different from either the Atlantic or the Pacific. The descriptions provided in this article cover only the Overall the Indian Ocean is quite a bit saltier than mostgeneralofwatermasses,theircorepropertiesand the Pacific but not quite as salty as the Atlantic. Also their geographic distribution. In most regions of the like the Atlantic, the Indian Ocean receives salinity ocean it is possible to resolve the water mass structure input from a marginal sea as the Red Sea deposits its into even finer elements describing more precisely the salt-laden water into the Arabian Sea. Its presence is differences in temperature and salinity. In addition, BBW 20 = 26  IEW ASW 15 27 SICW C) RSPGIW ° IUW 10 28 IIW Temperature ( 5 AAIW 0 Indian Ocean CDW − 2 35 34 36 Salinity (‰) Figure 8 Characteristic TS curves for the main water masses of the Indian Ocean. All lables as in Figure 6 .

12 1567 OPERATIONAL METEOROLOGY General Processes; Sur- Ocean Circulation: . Models other important properties can be used to specify face–Wind Driven Circulation; Thermohaline Circulation. water masses not obvious in TS space. While dissolved oxygen is often used to define water mass boundaries, care must be taken as this nonconservative property is Further Reading influenced by biological activity and the chemical dissolution of dead organic material falling through Emery WJ and Meincke J (1986) Global water masses: summary and review. Oceanologica Acta 9: the water column. Nutrients also suffer from modifi- 383–391. cation within the water column, making their inter- Mamayev OI (1975) Temperature–Salinity Analysis of pretation as water mass boundaries more difficult. World Ocean Waters . Elsevier Oceanography Series, Characteristic diagrams that plot oxygen against #11, Elsevier Scientific Pub. Co., Amsterdam, 374 pp. salinity or nutrients can be used to seek extrema that Iselin CO’D (1939) The influence of vertical and lateral mark the boundaries of various water masses. turbulence on the characteristics of the waters at mid- The higher vertical resolution property profiles Transactions of the American Geophysical Union depths. possible with electronic profiling instruments also 20: 414–417. make it possible to resolve water mass structure that Pickard GL and Emery WJ (1992) Descriptive Physical was not even visible with the lower vertical resolution Oceanography , 5th edn. Oxford, England: Pergamon Press. of earlier bottle sampling. Again, this complexity is Reid JL (1973) . Northwest Pacific Ocean Water in Winter only merited in local water mass descriptions and The Johns Hopkins Oceanographic Studies #5, Johns cannot be used on the global-scale description. At this Hopkins Press, 96 pp. global scale the descriptive data available from the The Sverdrup HU, Johnson MW and Fleming RH (1941) accumulation of historical hydrographic data are Oceans . Prentice-Hall Inc., 1087 pp. adequate to map the large-scale water mass distribu- . Worthington LV (1976) On the North Atlantic Circulation tion, as has been done in this article. The Johns Hopkins Oceanographic Studies #6, Johns Hopkins Press, 110 pp. Worthington LV (1981) The water masses of the world See also ocean: some results of a fine-scale census. In: Warren BA and Wunsch C (eds) Evolution of Physical Oceanogra- Convection: Ocean Mixed Layer. Boundary Layers: phy , Ch. 2, pp. 42–69. Cambridge, MA: MIT Press. Coupled Ocean–Atmosphere Convection in the Ocean. OPERATIONAL METEOROLOGY , University of Oklahoma/Cooperative J V Cortinas Jr process typically involves the acquisition, examina- Institute for Mesoscale Meteorological Studies, Norman, tion, and interpretation of vast quantities of both OK, USA observational meteorological data and numerical forecast model output, and thus tends to be extremely W Blier , National Weather Service WASC, Monterey, computer intensive. The content of the disseminated CA, USA weather products varies according to the issuing Copyright 2003 Elsevier Science Ltd. All Rights Reserved. organization and the type of weather information required by its customers; typically, they include periodic reports and forecasts of weather conditions Introduction over particular geographic regions and notification of any anticipated or observed hazardous weather con- Operational meteorology involves the generation and ditions. These products are disseminated in many widespread dissemination of weather information. ways, including dedicated electronic communications Although the types of weather-related information systems maintained by national and local govern- generated and the methods of distribution have ments, the World Wide Web, newspapers, radio, and evolved throughout the history of operational fore- television. casting, the basic focus of this subdiscipline of the Prior to the 1990s, countries with significant atmospheric sciences has remained unchanged: to use weather service programs generally relied on their scientific knowledge about how the atmosphere respective national governments to provide weather behaves to develop and distribute useful forecasts of information. While this practice is still common in weather conditions. In its modern application, this

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