Salinity Patterns in the Ocean

Lynne D Talley

Volume 1, The Earth system: physical and chemical dimensions of global environmental change,pp 629–640

Edited by

Dr Michael C MacCracken and Dr John S Perry

in

Encyclopedia of Global Environmental Change(ISBN 0-471-97796-9)

Editor-in-Chief

Ted Munn

John Wiley & Sons, Ltd, Chichester, 2002

Salinity Patterns in theOcean

Lynne D Talley

Scripps Institution of Oceanography, University ofCalifornia, La Jolla, CA, USA

Ocean salinity varies geographically and with time. Freshwater input occurs at the sea surface due to precipitationand river inflow, reducing salinity. Salinity is increased byevaporation and also as a by-product of sea ice forma-tion. Because freshening and salinification occur in differentplaces, salinity at a particular location reflects the upstreamsource of the water there. In subtropical latitudes, high sur-face evaporation creates high salinity near the sea surface.In subpolar latitudes, high precipitation creates low salinitynear the sea surface. As these waters flow into the oceaninterior, they create layers of high and low salinity.

At mid-depth (i.e., around 1000 to 2000 m deep), outflowsfrom the highly evaporative Mediterranean and Red Seascreate a vertical salinity maximum in the North Atlanticand Indian Oceans, respectively. Also at mid-depth in thesubtropical and tropical regions, the relatively fresh, butdense, surface water from higher latitudes flows in andcreates a vertical salinity minimum, most prevalent in theSouthern Hemisphere and North Pacific. The North Atlanticis the most saline ocean and the North Pacific the freshest.

Salinity affects sea water density and therefore can be acontrolling factor for the depth of the ocean surface mixedlayer. This depth also depends on wind speed and cooling atthe sea surface. Feedbacks can occur as precipitation andevaporation are affected by the atmosphere, which in turn isaffected by sea surface temperature. The atmosphere in turnis affected by sea surface temperature, which is affected bymixed layer depth.

The distribution of dissolved salts (see Salinity, Volume 1)in the oceans and adjacent seas varies in space and time.Salinity is changed near the sea surface by precipitationand evaporation of fresh water and by salty water producedsea ice forms and excludes salt. Geographical variationsin inputs create regional differences in salinity at the seasurface. As seawater circulates down into the ocean awayfrom the sea surface, it carries these differences along,creating large-scale salinity patterns throughout the ocean.Changes in time in the inputs at the sea surface alsoaffect the salinity distribution. By mapping the salinitydistribution and tracking changes in salinity over time,much insight is gained into both the basic ocean circulationand the processes that change the ocean circulation andtemperature distribution.

Salinity is a seawater property related to the amount ofmatter, mainly consisting of salts, dissolved in the water(see Salinity, Volume 1). The original definition of salin-ity was in terms of grams of dissolved salts per kilogramof seawater. Salinity is now defined in terms of seawaterconductivity, and is currently calibrated using a standardsolution prepared at a laboratory in the UK. The quantity ofdissolved salts, hence the salinity, affects seawater density,which also depends on temperature and pressure. Quanti-fying these dissolved constituents as the single parameter,salinity, is possible because the source of solids is weather-ing, which occurs on geological time scales. Ocean circu-lation and mixing, on the other hand, distribute the solidsthroughout the World Ocean on a time scale of no more thanthousands of years, and so the various salt constituents areessentially well mixed. Therefore, salinity differences arecreated only by dilution or concentration as fresh water isadded or removed, or as salty water is rejected from sea iceas it freezes. Other dissolved non-salt constituents such ascarbon, calcium and silica, which are affected by biologicalcycles, have enough spatial variability to affect seawaterdensity (e.g., Millero et al., 1978); this small variation isnot usually included in salinity and density studies.

In many ocean regions, particularly in the tropics, thesubtropics and the deep ocean, temperature variations aremore important than salinity in changing density. However,at higher latitudes, the presence of freshened surface watersresulting from high precipitation and/or ice melt has astrong influence on whether the water column can be cooledenough to mix vertically with other layers. Because wintersurface cooling at high-latitudes determines water proper-ties for the global ocean at depths below about 1500 m,salinity can control the occurrence of convection and itspenetration depth. With a surface layer that is relativelyfresh compared with the underlying water that came fromanother ocean region, surface waters can cool to freezingwithout overturning. If the high-latitude surface waters arenot much freshened, as is the usual case in the northernNorth Atlantic, then overturn to great depth at the end ofwinter can occur with even modest surface cooling, leavingthe surface waters well above the freezing point. Becauseocean surface temperatures are part of the forcing for theatmosphere, changes in high-latitude salinity distributioncan be involved in climate change.

In addition to its effect on seawater density, salinity isuseful in itself as a tracer of ocean circulation and mixing.Salinity is established in the ocean’s surface layer by localprocesses, and therefore salinity variations within the oceandepend on the surface origin of a parcel of water. Mix-ing between waters of different salinities occurs within theocean, and so the original salinity changes gradually. How-ever, mixing rates are low enough that salinity (and otherchemical tracers) can provide a great deal of informationon the pathways of flow.

2 THE EARTH SYSTEM: PHYSICAL AND CHEMICAL DIMENSIONS OF GLOBAL ENVIRONMENTAL CHANGE

SEA SURFACE FORCING OF SALINITY:PRECIPITATION, EVAPORATION, RUNOFFAND ICE PROCESSES

The total amount of salt in the ocean is constant on timescales of millions of years, and hence is assumed constantfor studies at almost all time scales. However, the totalamount of water in the oceans varies daily due to exchangeat the sea surface. Rainfall, runoff from land, and ice meltall reduce salinity by increasing the amount of fresh water.Evaporation increases salinity by decreasing the amount offresh water.

Ice formation also locally increases salinity through aprocess called brine rejection. As ice forms from seawater,the dissolved salts are rejected from most of the ice and col-lect in pockets in the ice. This briny water drips through thebase of the forming ice. More brine is released during slowice formation than if the ice forms quickly, so the strengthof brine rejection depends on the local air temperature. Thebrine mixes with the seawater beneath the ice, raising theseawater’s salinity. The net salinity increase depends on thethickness of the mixing layer below the ice. In shallow loca-tions, such as on shallow continental shelves, salinity mightincrease more than over a deep bottom where mixing occursto greater depth. Because the water below sea ice is usuallyvery cold, the addition of salty brine may increase salinitywithout increasing temperature. This results in a densityincrease in the seawater. This denser water then sinks untilit reaches a depth where surrounding waters have the samedensity. Because these deeper waters may have come, atleast in part, from somewhere else (e.g., in the Arctic fromthe North Atlantic Ocean), these waters will usually not beas cold as the new ice-rejected brine. Although warmer,their density will be equal to the ice-rejected brine becausethey are even saltier. That is because the ice-rejected brinewas formed from near-freezing surface waters that initiallyhad much lower salinity (e.g., because they had a higherproportion of meltwater and river runoff), and thereforelower density, than the deeper waters. Although it seemsparadoxical, the ice formation and brine rejection processesare thus able to create the coldest and freshest waters on asurface of constant density within the ocean.

Because sea ice has lower salinity than the originalseawater from which it formed, melting of sea ice createsa fresh surface layer of low-density waters that float overthe more saline waters below.

Ocean salinity patterns at the largest scales are affectedprimarily by the annual average of evaporation and pre-cipitation, with additional major effects from ice melt andfreezing at high-latitudes. Evaporation and precipitation,usually reported in cm year�1, are primarily calculated fromobservations collected on a routine basis by most ships. Thedifference between evaporation (E) and precipitation (P)has been calculated from the complete historical data setcompiled from these ship observations (Figure 1). The

range of E–P is about �10 to 10 cm year�1. There is moreevaporation than precipitation in the subtropical regionsbetween about 15° and 40 °S and N latitude, under the atmo-sphere’s large-scale high pressure centers, with descending,dry air. At higher latitudes, there is more precipitation thanevaporation, under the atmospheric low-pressure centers.In the tropics, especially at about 10 °N over the Pacificand Atlantic, surface air rises in the intertropical conver-gence zone (ITCZ), forming enormous cumulus clouds thatproduce large amounts of rain.

Runoff from land affects the coastal salinity distribution.On a global scale, runoff is most important at the majorriver outlets, indicated in Figure 1. The net fresh water inputis not shown in Figure 1, but the effect of these major riversis apparent in the surface salinity distribution considerednext.

SURFACE SALINITY DISTRIBUTION

Salinity at the sea surface (Figure 2) resembles the evapo-ration–precipitation pattern, with some differences due torunoff, ocean currents and ice melt. The total salinity rangeof the open ocean is about 30–40 parts per thousand or ‰(see Salinity, Volume 1). The international convention isthat salinity has no units; its value is approximately gramssalt per kilogram of sea water. Many authors continue to usethe practical salinity unit (psu) rather than leaving the quan-tity unitless. While salinity in coastal estuaries can be muchlower, it does not affect large oceanic-scale patterns. Low-est salinities occur in the Arctic and Antarctic where there isboth net precipitation and seasonal ice melt. Highest salin-ities occur in the Red Sea and Persian Gulf, both locatedin the northwestern Indian Ocean, where net evaporation ishigh. High salinity is also found in the Mediterranean Sea.In the open ocean, high salinity occurs in the subtropicalareas of net evaporation seen in Figure 1. A band of lowsalinity underlies the ITCZ at 10 °N.

The effect of continental runoff is apparent in loweredsurface salinity near the mouths of major rivers such as theAmazon and the Congo and the numerous large rivers thatempty into the Bay of Bengal, east of India, including theGanges and Brahmaputra. Runoff from many rivers aroundthe Gulf of Alaska in the northeastern Pacific and aroundthe Arctic Ocean is important in the lowered salinities ofhigh-latitude ocean regions.

VERTICAL STRUCTURE OF THE SALINITYDISTRIBUTION

Waters at the sea surface flow down into the interior.Salinities from the sea surface (Figure 2) carried withthis flow into the ocean interior are helpful in identifyingsources and pathways of the circulation. The two primary

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Figure 1 Evaporation minus precipitation (cm year�1), annual average. The major rivers of the world are also indicated. (Reproduced using data fromthe National Centers for Environmental Prediction Reanalysis Project, Kalnay et al., 1996)

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SALINITY PATTERNS IN THE OCEAN 5

mechanisms for this downward flow are: (1) flow followingconstant density surfaces that slope gently into the oceaninterior, and (2) vertical convection and brine rejection athigher latitudes.

Flow down along constant density surfaces is primarilythrough a process called subduction, occurring in subtrop-ical regions between about 15° and 40 °S and N latitude.In this process, waters in the surface mixed layer circu-late equatorward, encountering warmer, less dense watersto the south, which they are carried beneath. Surface watersare most saline in the middle of these subtropical regions.Therefore, all equatorward subtropical regions are charac-terized by a subsurface vertical maximum in salinity causedby subduction of more saline waters from the center ofthe gyre, as described later (see Ocean Circulation, Vol-ume 1).

Convection occurs in many places, but reaches mostdeeply into the ocean at high latitudes. Convection is notso much a downdraft of water from the surface as it isa deep mixing, which brings surface water to great depth,from whence the water spreads laterally along constant den-sity surfaces. Major deep convection areas can be identifiedfrom subsurface properties, such as the salinity distributionsdiscussed below. In the North Atlantic and Arctic, thesesites are in the Labrador Sea between Labrador and Green-land, in the Greenland Sea east of Greenland and north ofIceland, and in the Mediterranean Sea. In the North Pacific,convection in the Okhotsk and Japan Seas is significantin modifying water properties at mid-depth. In the South-ern Hemisphere, convection occurs all around Antarctica atabout 40° to 45 °S, which is just north of the major currentcalled the Antarctic circumpolar current. Brine rejectionis important for subsurface water properties primarily inthe Antarctic (Weddell and Ross Seas and other locationsaround the continent), the Okhotsk Sea, and the interior ofthe Arctic Ocean.

The following definitions are useful for discussing thesalinity distribution in the vertical: (1) halocline – wherethe salinity value changes rapidly in the vertical in com-parison with changes above and below the halocline;(2) halostad – where salinity changes very slowly in thevertical; (3) intermediate water – usually referring to aminimum or maximum in salinity in the vertical whichis found over a large horizontal area at mid-depth in theocean. Equivalent terms for vertical variations in temper-ature and density are thermocline, thermostat, pycnoclineand pycnostad.

SUBTROPICAL SALINITY STRUCTURE

In the subtropical regions of every ocean, salinity (Figures 3and 4a) is high close to the sea surface due to strongevaporation. At the location of the highest surface salinity(Figure 2), the highest salinity in vertical profiles is at the

sea surface. This high salinity water is carried downwardand equatorward through subduction. Therefore, equator-ward of the center of the highest surface salinity, thereis a vertical maximum in salinity. This salinity maximumis at about 50 m depth, which is very shallow comparedwith the ocean depth. These subsurface salinity maximaare sometimes called Subtropical Underwater (Worthing-ton, 1976).

At all locations in the subtropical gyres, and below thenear-surface salinity maximum, salinity decreases smoothlywith depth to a salinity minimum. Temperature alsodecreases smoothly and rapidly (Figure 4b). This layer ofchanging properties is called the thermocline (region ofhigh temperature change in the vertical), and is also oftencalled central water. Central water is the subducted water ofthe subtropical gyre; the colder waters come from farthernorth within the subtropical circulation, with little inputfrom the subpolar region. It is clear from Figure 4(b) thatthe North Atlantic central waters are the most saline andthe North Pacific the freshest.

Below the central waters, at depths between about500 and 1500 m, each subtropical gyre (except in theNorth Atlantic) has a vertical minimum in salinity. Thesalinity minimum in the North Pacific is called NorthPacific intermediate water (Reid, 1965). It arises fromcold, low salinity surface waters in the subpolar regionof the North Pacific, specifically in the Okhotsk Seaand near the Kuril Islands (Talley, 1991, 1993). Thesewaters flow southward along the western boundary in theOyashio current and enter the subtropical circulation eastof Japan. The existence of the low salinity layer through-out the subtropical circulation reflects the presence ofthis subpolar water underlying the waters from subtropicalsources.

The salinity minimum in the Southern Hemisphere iscalled Antarctic intermediate water (Reid, 1965). It comesfrom low salinity surface waters found just west of Chileand just east of Argentina (McCartney, 1977). Antarcticintermediate water in the Pacific Ocean originates primar-ily from the Chilean source. The Atlantic and Indian OceanAntarctic intermediate waters originate from the Argen-tinean source. The Argentinean waters arise from Chileansurface waters, which flow eastward through the northernside of Drake Passage between South America and Antarc-tica. The Chilean waters are modified somewhat duringtheir passage into the Atlantic. Therefore, the properties ofthe Atlantic and Indian Antarctic intermediate waters dif-fer slightly from those of the Pacific Antarctic intermediatewater.

The salinity structure in the subtropical North Atlanticis somewhat different. While there is a salinity minimumat the base of the central water, this results primarilyfrom the northernmost subducted waters of the subtropicalregion, and is not the major fresh intermediate water of

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Figure 3 (a) Vertical cross-section of the Atlantic Ocean, along about 20 °W. (b) Vertical cross-section of the Pacific Ocean, along about 150 °W. (c) Verticalcross-section of the Indian Ocean, along about 90 °E. (Reproduced using data from the World Ocean Circulation Experiment)

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SALINITY PATTERNS IN THE OCEAN 9

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Figure 4 (a) Salinity distribution with depth from eachsubtropical gyre. All profiles are chosen from the centersof the subtropical circulations, at about 25 °N (NorthernHemisphere) and 30 °S (Southern Hemisphere), and fromthe center of the basins rather than near the boundaries.(b) Potential temperature as a function of salinity for eachsubtropical ocean basin

the North Atlantic, which dominates the subpolar region(described later). The main fresh intermediate water isdisplaced in the subtropical gyre by the high salinity outflowat mid-depth from the Mediterranean Sea. This outflowcreates an intermediate depth salinity maximum in thevertical.

In the Indian Ocean, a dense, high salinity source in theRed Sea creates a high-salinity intermediate water in thenorth, similar to the Mediterranean water of the Atlantic.Because there are no northern high-latitude sources of waterin the Indian Ocean, there is no fresh intermediate layerarising from the north. The fresh water at the surface in theBay of Bengal arising from major river runoff is much toowarm to sink to mid-depth. Thus, the mid-depth northernIndian Ocean is dominated by the highly saline outflowfrom the Red Sea.

Beneath the low salinity intermediate waters of the NorthPacific and Southern Hemisphere, salinity increases towardsthe bottom. The higher salinity of the deep waters of theSouthern Hemisphere and North Pacific results from inputof saline North Atlantic waters. Beneath the saline inter-mediate water of the North Atlantic and tropical IndianOceans, salinity decreases to the bottom. In the SouthAtlantic Ocean, low-salinity intermediate water (Antarcticintermediate water) lies above higher salinity deep watersfrom the North Atlantic. Beneath the high-salinity NorthAtlantic deep water, salinity decreases to the bottom. Thelower salinity of the bottom waters in the Atlantic andIndian Oceans results from deepwater sources in the Antarc-tic that are of lower salinity.

SUBPOLAR SALINITY STRUCTURE

In high latitude regions dominated by precipitation and icemelt, salinity is lowest at the sea surface and increasesdownward through a halocline (Figure 5, North and South

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N.Pacific

S.Pacific

1500

Dep

th

1000

500

0

35.535.034.534.033.5

Salinity33.032.532.0

Figure 5 Salinity distribution with depth from the subpo-lar regions. The Northern Hemisphere profiles are fromabout 50 °N. The Southern Hemisphere profile is from thePacific Ocean at about 65 °S

10 THE EARTH SYSTEM: PHYSICAL AND CHEMICAL DIMENSIONS OF GLOBAL ENVIRONMENTAL CHANGE

Pacific profiles). In the North Pacific and Southern Hemi-sphere, and also in most of the Arctic Ocean, the haloclineis very sharp, with a large change in salinity over severaltens of meters. Because of the salinity structure, the sur-face layer can be colder than the water below the halocline.This pre-conditions the region for sea ice formation. Thecooled surface waters can circulate away from their winteroutcrop locations and appear as a subsurface temperatureminimum between warmer water below from inflow fromother regions and warmer water above from local seasonalwarming.

The North Atlantic’s subpolar gyre is affected by inflowof a thick layer of saline, warm surface water from thesubtropical gyre. Therefore, the halocline typical of the sub-polar Pacific and subpolar Southern Hemisphere is absentin most of the subpolar North Atlantic (Figure 5, NorthAtlantic profile). The saline flow into the central and easternsubpolar gyre is affected by precipitation, and the surfacewaters become gradually fresher as they circulate counter-clockwise. When the surface waters enter the Labrador Sea,they are much fresher than when they first enter the sub-polar region. Winter convection to about 1500 m depth inthe Labrador Sea then mixes the relatively fresh water tointermediate depth. This spreads away from the LabradorSea and forms a salinity minimum throughout the subpolargyre, beneath the inflowing warm, saline waters. This dif-fers from subpolar regions in the other oceans where thesurface waters are strongly dominated by local fresh watersources. Therefore, most of the North Atlantic’s subpolargyre has a vertical salinity structure similar to a subtropicalstructure, with saline waters at the surface and an interme-diate depth salinity minimum.

TROPICAL SALINITY STRUCTURE

The upper ocean salinity distribution depends on localprecipitation patterns. Thus, a band of low salinity is foundunder the high precipitation of the ITCZ at 5–10 °N in thePacific. This band is separated from the saline waters belowit by a halocline. Excess precipitation is also found in thewestern Pacific at the equator, creating a halocline thereas well. The eastern equatorial Pacific on the other hand isdominated by evaporation and so there is no halocline there.The Bay of Bengal in the Indian Ocean is the site of majorrunoff from Asia; this relatively fresh, warm water createsa shallow halocline throughout the northeast Indian Ocean.The intermediate and deepwater salinity distributions ineach ocean within 15° of the equator are similar to thoseof the adjacent subtropical regions.

GLOBAL SCALE SALINITY PATTERNS

As indicated above, there are significant differences insalinity between oceans. Considering the total ocean basin,

the North Atlantic is the most saline and the North Pacificthe freshest of the major oceans. Because of significantriver inflows, the surface waters of the Arctic are the fresh-est. The three Southern Hemisphere oceans are connectedaround Antarctica and hence their salinities are similar,midway between the North Atlantic and North Pacific. Themain reason for the differences between the North Pacificand North Atlantic salinities is the difference in net evapo-ration per unit area. There is relatively more evaporation inthe North Atlantic’s subtropical regions than in the NorthPacific’s subtropics. The evaporative regions occur wherethere are easterly trade winds. The North Atlantic’s tradewinds originate over the deserts of Africa and the Middle-East. The North Pacific’s trade winds originate over themuch smaller continental areas of Central America andMexico. Therefore, the North Atlantic’s trade winds aredrier than the North Pacific’s and cause more evaporationper unit area. High salinity in the North Atlantic is presentat intermediate depth because of the Mediterranean Sea.The North Atlantic exchanges water with the MediterraneanSea through the Strait of Gibraltar. Evaporation and cool-ing in the Mediterranean cause the outflowing water to bemuch denser than the inflow. This dense outflow sinks tointermediate depth and creates the mid-depth high salin-ity that marks North Atlantic waters. This input of highsalinity at mid-depth in the North Atlantic is one rea-son for the overall higher salinity of the North Atlanticcompared with the North Pacific. Similarly, in the IndianOcean, high evaporation in the Arabian region creates verysaline and dense water in the Red Sea, which enters theIndian Ocean and raises the salinity at mid-depths comparedwith the inflowing fresher deep waters from the Antarc-tic.

ROLE OF SALINITY IN CLIMATE CHANGE

Salinity is a major factor in climate change because itcan affect the depth of mixing at the sea surface. If forinstance there is a shallow, fresh layer atop a denser, moresaline layer, even large surface cooling can act only onthe surface layer, creating a large temperature drop withoutdeep convection. In the absence of a halocline, heat can beremoved from a much thicker layer, which then results ina smaller temperature change. The processes that set thesalinity of the waters beneath the surface layer, and hencethe strength of the halocline, are complex. This salinitycontrol of surface-layer processes in the present climate hasbeen studied, e.g., in the tropical Pacific, as part of the ElNino and La Nina cycle. In the normal state in the northernAtlantic, up to and including the region between Greenlandand Norway, surface salinities are not much lower thanthose of the underlying waters, permitting overturn to morethan several hundred meters depth. This overturn is part ofthe global thermohaline circulation in which deep waters

SALINITY PATTERNS IN THE OCEAN 11

are produced from surface waters at high-latitudes, mainlythrough heat loss to the atmosphere. (The presence ofa permanent and strong halocline in the northern NorthPacific and in the Arctic Ocean prevents deep overturning ofwaters there.) During observed decadal climate changes inthe North Atlantic, a halocline sometimes forms as a resultof excessive ice melt and runoff and is associated withshutdown of deep wintertime convection. This then affectsthe sea surface temperature as described previously, and canthen affect the atmosphere. From ocean computer modelingexperiments it is hypothesized that similar controls bysalinity on deep overturn have occurred on much longerclimate timescales.

See also: Broecker, Wallace S, Volume 1; Ocean Cir-culation, Volume 1.

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Levitus, S, Burgett, R, and Boyer, T (1994) World Ocean Atlas1994, Vol. 3, Salinity, NOAA Atlas NESDIS 3, 99, US Gov-ernment Printing Office, Washington, DC.

McCartney, M S (1977) Subantarctic Mode Water, in A Voy-age of Discovery, ed M Angel, Pergamon Press, New York,103–120.

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Talley, L D (1993) Distribution and Formation of North PacificIntermediate Water, J. Phys. Oceanogr., 23, 517–537.

Worthington, L V (1976) On the North Atlantic Circulation, Num-ber 6, Johns Hopkins Oceanographic Studies, Baltimore, MD.