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The concept of a ‘water mass’ is borrowed from meteorology, which classifies different atmospheric characteristics as ‘air masses’. In the early part of the twentieth century physical oceanographers also sought to borrow another meteorological concept separating the ocean waters into ‘warm’ and ‘cold’ water spheres. This designation has not survived in modern physical oceanography but the more general concept of water masses persists.

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  • 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 rosettesampler 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 open

    ocean, 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 todays 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 coldwater 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 squarely

    in 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 aformation 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

    1556 OCEAN CIRCULATION /Water Types and Water Masses

  • temperaturesalinity 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 temperaturesalinityprofiles. 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 temperaturesalinity 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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    Figure 1 Example of TS scatter plot for all data within a 101square with mean TS curve (center line) and curves for one standarddeviation 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 TS

    curve, 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 mass

    terminology, it is generally accepted that a water typerefers 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 introduced

    over the years. One particularly instructive formwas avolumetric 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 t

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