Further Reading

Davis RE, de Szoeke R, Halpern D and Niiler P (1981)Variability in the upper ocean during MILE. Part I:The heat and momentum balances. Deep-Sea Research28: 1427}1452.

Ekman VW (1905) On the inSuence of the earth’s rota-tion on ocean currents. Arkiv Matematik, Astronomioch Fysik 2: 1}52.

Eriksen CC, Weller RA, Rudnick DL, Pollard RT andRegier LA (1991) Ocean frontal variability in theFrontal Air}Sea Interaction Experiment. Journal ofGeophysical Research 96: 8569}8591.

Gill AE (1982) Atmosphere}Ocean Dynamics. New York:Academic Press.

Gregg MC (1989) Scaling turbulent dissipation in thethermocline. Journal of Geophysical Research 94:9686}9698.

Langmuir I (1938) Surface motion of water induced bywind. Science 87: 119}123.

Lighthill MJ and Pearce RP (eds) (1981) MonsoonDynamics. Cambridge: Cambridge University Press.

Munk W (1981) Internal waves and small-scale processes.In: Warren BA, Wunsch C (eds) Evolution of PhysicalOceanography, pp. 264}291. Cambridge, USA: MITPress.

Philander SG (1990) El NinJ o, La NinJ a, and the SouthernOscillation. San Diego: Academic Press.

Roden GI (1984) Mesoscale oceanic fronts of the NorthPaciRc. Annals of Geophysics 2: 399}410.

UPPER OCEAN VERTICAL STRUCTURE

J. Sprintall, University of California San Diego,La Jolla, CA, USAM. F. Cronin, NOAA Pacific Marine EnvironmentalLaboratory, Seattle, WA, USA

Copyright ^ 2001 Academic Press

doi:10.1006/rwos.2001.0149

Introduction

The upper ocean connects the surface forcingfrom winds, heat, and fresh water, with thequiescent deeper ocean where this heat andfresh water are sequestered and released onlonger time and global scales. Classically the surfacelayer includes both an upper mixed layer, that issubject to the direct inSuence of the atmosphere,and also the highly stratiRed zone below themixed layer, where vertical property gradientsare maximum. Although all water within thesurface layer has been exposed to the atmosphere atsome point in time, water most directly exposedlies within the mixed layer. Thus, the surface layervertical structure reSects not only immediatechanges in the forcing, but also anomaliesassociated with earlier forcing events. Further,these forcing events may have occurred eitherlocally in the region, or remotely at other locationsand transferred by ocean currents. This article RrstdeRnes the major features of the upper ocean verti-cal structure and discusses what causes and main-tains them. Then the rich variability in the verticalshapes and forms that these structures can assumethrough variation in the atmospheric forcing isdemonstrated.

Major Features of the Upper OceanVertical Structure

The vertical structure of the upper ocean is prim-arily deRned by changes in the temperature andsalinity, which together control the water column’sdensity structure. Within the upper ocean surfacelayer, a number of distinct layers can be distin-guished that are formed by different processes overdifferent timescales: the upper mixed layer, the sea-sonal pycnocline, and the permanent pycnocline(Figure 1). Right at the ocean surface in the top fewmillimeters, a cool ‘skin’ exists with lowered tem-perature caused by the combined heat losses fromlong-wave radiation, sensible and latent heat Suxes.The cool skin is always present and is the actual seasurface temperature (SST) measured by airborneinfrared radiometers. In contrast, in-situ sensors inthe top few meters measure the ‘bulk’ SST. The coolskin temperature is of the order 0.1}0.5 K coolerthan the bulk temperature. In terms of the upperocean buoyancy, the bulk SST is often a more ap-propriate characterization of the surface temper-ature. Air}sea Suxes are transported through themolecular layer almost instantaneously so that theupper mixed layer can be considered in direct con-tact with the atmosphere. For this reason, whendeRning the depth of the upper ocean structure, thechanges in water properties are generally made rela-tive to the bulk SST measurement.

The upper mixed layer is the site of active air}seaexchanges. Energy for the upper mixed layer tochange its vertical structure comes from wind mix-ing or through a surface buoyancy Sux. Wind mix-ing causes the vertical turbulence and convective

3120 UPPER OCEAN VERTICAL STRUCTURE

WindEvaporation

Solarradiation

Rainfall

Depth

Mixed layer

Seasonalpycnocline

Mainpycnocline

Turbulence

Entrainment

Figure 1 Conceptual diagram of the vertical structure in thesurface layer, and the forcing and physics that govern its exist-ence. The depth of the mixed layer, the seasonal pycnocline,and the main pycnocline are indicated.

overturning of water in the mixed layer by waves,and also by the entrainment of cooler water throughthe bottom of the mixed layer. Wind forcing alsoresults in advection by upper ocean currents thatcan change the properties and thus the verticalstructure of the mixed layer. Surface buoyancy forc-ing is due to heat and fresh water Suxed across theair}sea interface. Cooling and evaporation induceconvective mixing and overturning, whereas heatingand rainfall cause the mixed layer to restratify indepth and display alternate levels of greater andlesser vertical property gradients. Thus, if strongenough, the wind and buoyancy Suxes can generatesufRcient turbulence so that the upper portion of thesurface layer has a thick, homogeneous (low verticalgradient or stratiRcation), well-mixed layer intemperature, salinity, and density.

Variations in the strength and relative contribu-tions of the atmospheric forcing can cause substan-tial variability in the water properties and thicknessof the upper mixed layer. Large temporal variationcan occur on daily and seasonal timescales due tochanges in the solar radiation. For example, duringthe daily cycle the sun heats the ocean causing theupper surface to become increasingly warm andweakly stratiRed. The ‘classic’ vertically uniformmixed layer may not be present in the upper oceansurface layer as depicted in Figure 1. As the sun sets,the surface waters are cooled and sink, generating

turbulent convection that causes entrainment ofwater from below, and mixing that produces thevertically well-mixed layer. Temporal variabilityand the physics that govern this variability arecovered elsewhere (see Upper Ocean Time andSpace Variability and Wind and Buoyancy-forcedUpper Ocean).

Furthermore, the mixed layer structure can ex-hibit large horizontal variations. For example, thelarge meridional differences in solar radiation resultin mixed layers that generally increase in depth fromthe equator to the poles. Even in the east}westdirection, boundary currents and differential surfaceforcing can result in mixed layers of different struc-tures, although differences in the annual variationsof temperature along a given latitude will generallybe small. Spatial variability in the structure ofthe mixed layer is discussed elsewhere (see UpperOcean Mean Horizontal Structure).

Separating the upper mixed layer from the deeperocean is a region typically characterized by substan-tial vertical gradients in water properties. In temper-ature, this highly stratiRed vertical zone is referredto as the thermocline, and in salinity it is the halo-cline. Although the thermocline and the haloclinemay not always exactly coincide in their depthrange, one or the other property will control thedensity structure to form the ‘main pycnocline’.Because more often than not in the tropics andmidlatitudes temperature is the controlling factor ofdensity, the main pycnocline is also referred to asthe ‘permanent thermocline’. In midlatitudes duringsummer, surface heating from the sun can causea shallow seasonal thermocline (pycnocline), thatconnects the upper mixed layer to the permanentthermocline (see Figure 1). Similarly, in the subpolarregions the seasonal summer input of fresh water atthe surface through rainfall, rivers or ice melt, canresult in a seasonal halocline (pycnocline) separatingthe fresh surface from the deeper saltier waters.Whereas the seasonal pycnocline disappears everywinter, the permanent pycnocline is always presentin these areas.

To maintain stability in the water column, lighter(less dense) water must lie above heavier (denser)water. It follows then, that the pycnocline is a re-gion where density increases rapidly with depth. Ifthe vertical density gradient is very strong, the tur-bulence within the upper mixed layer induced by theair}sea exchanges of wind and heat, cannot over-come the great stability of the pycnocline to pen-etrate into the deeper ocean. The base of the mainpycnocline marks the depth limit of the upper oceansurface layer. Beneath this depth, the water has notseen the surface for a very long time, and above it,

UPPER OCEAN VERTICAL STRUCTURE 3121

the stability of the main pycnocline acts as a barrieragainst turbulent mixing processes.

In some polar regions, particularly in the farNorth and South Atlantic, no permanent thermo-cline exists. The isothermal water column suggeststhat the cold, dense waters are virtually continuous-ly sinking to great depths. No stable permanentpycnocline or thermocline exists as a barrier to thevertical passage of the surface water properties thatextend to the bottom. In some cases, such as alongthe shelf in Antarctica’s Weddell Sea in the SouthAtlantic, salinity can also play a role in dense waterformation. When ice forms from the sea water inthis region, it consists primarily of fresh water, andleaves behind a more saline and thus denser surfacewater that must also sink. The vertical Sow of thedense waters in the polar regions is the source of theworld’s deep and bottom waters that then slowlymix and spread horizontally via the large-scalethermohaline ocean circulation to Rll the deep oceanbasins.

In fact, the thermohaline circulation also plays animportant role in maintaining the permanent ther-mocline at a relatively constant depth in the low andmiddle latitudes. Despite the fact that the pycnoclineis extremely stable, it might be assumed that onsome timescale it could be eroded away throughmixing of water above and below it. Humboldtrecognized early in the nineteenth century thatocean circulation must help maintain the low tem-peratures of the deeper oceans; the equatorwardmovement of the cold deep and bottom watermasses are continually renewed through sinking (or‘convection’) in the polar region. However, it wasnot until the mid-twentieth century, that Stommelsuggested that there was also a slow but continualupward movement of this cool water to balance thedownward diffusion of heat from the surface. It isthis balance, that occurs over very small space andtimescales, that sustains the permanent thermoclineobserved at middle and low latitudes. Thus, thevertical structure of the upper ocean helps us tounderstand not only the wind- and thermohaline-forced ocean circulation, but also the responsebetween the coupled air}sea system and the deeperocean on a global scale.

De\nitions

Surface Layer Depth

Conceptually the surface layer includes the mixedlayer, where active air}sea exchanges are occurring,plus those waters below the mixed layer and abovethe base of the permanent thermocline that have

been exposed to the atmosphere in their recenthistory. Note the important detail that the surfacelayer includes the mixed layer, a fact that has oftenbeen blurred in the criteria used to determine theirrespective depth levels. A satisfactory depth cri-terion for the surface layer should thus include allthe major features of the upper ocean surface layerdescribed above and illustrated in Figure 1. Further,the surface layer depth criterion should be applic-able to all geographic regimes, and include thosewaters that have recently been in contact with theatmosphere, at least on timescales of up to a year.Finally, the deRnition should preferably be based onreadily measurable properties such as temperature,salinity, or density.

Ideally then, we could specify the surface layer tobe the depth where, for instance, the temperature isequal to the previous winter’s minimum SST. How-ever in practice, this surface layer deRnition wouldvary temporally, making it difRcult to decipheryear-to-year variability. Oceanographers thereforegenerally prefer a static criterion, and thus modifythe deRnition to be the depth where the temperatureis equal to the coldest SST ever observed using thehistorical data available at a particular geographiclocation. This deRnition is analogous to a local‘ventilation’ depth: the deepest surface to whichrecent atmospheric inSuence has been felt at leastover the timescale of the available historical data.

Note that the deRnition suggested for the surfacelayer is primarily one-dimensional involving onlythe temperature and salinity information froma given location. Lateral advective effects have notbeen included. The roles of velocity and shear, andthree-dimensional processes in the surface layerstructure may be important on occasions. However,their roles are harder to quantify and have not, asyet, been adequately incorporated into a workingdeRnition for the depth of the surface layer.

Mixed-layer Depth

The mixed layer is the upper portion of the surfacelayer where active air}sea exchanges generate sur-face turbulence which causes the water to mix andbecome vertically uniform in temperature and salin-ity, and thus density. Obviously direct measurementof the upper layer turbulence would provide themost accurate data for determining the mixed layeror ‘mixing’ depth, as dissipation levels typicallydecrease by an order of magnitude below the activeconvective depth. However, turbulence measure-ments can be problematic and are not widespread atpresent. For this reason, as in the surface-layerdepth criterion, deRnitions of the mixed-layer depth

3122 UPPER OCEAN VERTICAL STRUCTURE

are most commonly based on temperature, salinity,or density. The mixed-layer depth must deRne thedepth of the transition from a homogeneous upperlayer to the stratiRed layer of the pycnocline.

Two deRnitions of mixed-layer depth are wide-spread in the literature. The Rrst determines thedepth where a critical temperature or density gradi-ent corresponding to the top of the maximum prop-erty gradient (i.e., the thermocline or pycnocline) isexceeded, and the second determines a net temper-ature or density change from the surface isothermor isopycnal. The critical gradient criteria rangebetween 0.02 and 0.053C m~1 in temperature, and0.005 and 0.015 p

5units m~1 in density. This cri-

terion may be sensitive to the vertical depth intervalover which the gradient is calculated. In the seconddeRnition, common values used for the net changecriterion are 0.5}13C in temperature from the sur-face isotherm, or 0.125 pt units from the surfaceisopycnal. Because of the different dynamical pro-cesses associated with the molecular skin SST,oceanographers generally prefer the readily deter-mined bulk SST estimate as the surface referencetemperature. Ranges used in the temperature anddensity values used in both mixed-layer depth deRni-tions will distinguish weakly stratiRed regions fromunstratiRed. Another form of the net change cri-terion used to deRne the mixed-layer depth (mld),determines the depth at which,

o(z"mld)"o(z"0)!dT do/dT [1]

where o(z"0) is the surface density, dT the netchange in temperature from the surface (e.g.0.53}13C), and do/dT is the thermal expansion coef-Rcient calculated from the equation of state for seawater using surface temperature and salinity values.This criterion thus determines the depth at whichdensity is greater than the surface density by anamount equivalent to the dT temperature change.Through the use of density and the thermal expan-sion coefRcient, this deRnition has the advantage ofrevealing mixed layers where salinity stratiRcationmay be important. Criteria based on salinitychanges, although inherent in the density criterion,are not evident in the literature as typically heatSuxes are large compared to freshwater Suxes, andthe gravitational stability of the water column istypically controlled by the temperature stratiRca-tion. In addition, subsurface salinity observationsare not as regularly available as temperature.

To illustrate the differences between the mixed-layer depth criterion, Figure 2(A) shows the mixed-layer depth from an expendable conductivity}temperature}depth (XCTD: see Expendable Sensors)

proRle, using the net temperature (0.53C) anddensity (0.125) pt change criteria, the gradientdensity criterion (0.01 m~1), and eqn [1]. In thisparticular case, there is little difference between themixed-layer depth determined from any method orproperty. However, Figure 2(B) shows an XCTDcast from the western PaciRc Ocean, and the strongsalinity halocline that deRnes the bottom of theupper mixed layer is only correctly identiRed usingthe density-deRned criteria.

Finally, to illustrate the distinction between thesurface layer and the upper mixed layer, Figure 3(A)shows a temperature section of the upper 300 mfrom Auckland to Seattle during April 1996. Thecorresponding temperature stratiRcation (i.e., thevertical temperature gradient) is shown in Figure3(B). The surface layer, determined as the depth ofthe climatological minimum SST isotherm, and alsothe mixed-layer depth from a 13C net temperaturechange from the surface (i.e., SST!13C) areindicated on both panels. This cross-equatorialnorth}south section also serves to illustrate the sea-sonal differences expected in the mixed layer. In theSouthern Hemisphere in early fall the net temper-ature mixed-layer depth criterion picks out the topof the remaining seasonal thermocline, as depictedby the increase in temperature stratiRcation in Fig-ure 3(B). The mixed-layer depth criterion thereforeexcludes information about the depth of the priorwinter local wind stirring or heat exchange at theair}sea surface, that has been successfully capturedin the surface layer using the historical minimumSST criterion. In the Northern Hemisphere tropicalregions where there is little seasonal cycle, the sur-face layer and the mixed-layer criteria are nearlycoincident. The depth of the mixed-layer and thesurface layer extend down to the main thermocline.Finally in the early-spring northern latitudes, themixed-layer criterion again mainly picks out theupper layer of increased stratiRcation that was likelycaused through early seasonal surface heating. Thesurface layer deRnition lies deeper in the water col-umn within the main thermocline, and below a sec-ond layer of relatively low stratiRcation. The deeper,weakly stratiRed region indicates the presence offossil layers, which are deRned in the next section.

Variability in Upper Ocean VerticalStructures

Fossil Layers

Fossil layers are nearly isothermal layers that occurbelow the upper well-mixed layer, and are separatedby a well-stratiRed layer (see Figure 3(B), 313}373N).

UPPER OCEAN VERTICAL STRUCTURE 3123

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(A) (B)rt rt

Figure 2 Temperature (black line), salinity (blue line) and density (green line) during March 1995 from expendable conductivity}temperature}depth profiles in the Pacific Ocean at (A) 6.93N, 173.23W and (B) 63S, 1663W. In temperature, mixed-layer depth iscalculated using criteria of a net temperature change of 0.53C (crossed-box) and 13C (circle) from the sea surface; anda temperature gradient criteria of 0.013C m~1 (small cross). In density, mixed-layer depth is determined using criteria of a netdensity change of 0.125 pt units from the surface isopycnal (crossed box); a density gradient of 0.01 pt units m~1 (circle) and thethermal expansion method of eqn [1] (cross). Note the barrier layer defined as the difference between the deeper isothermal layerand the shallow density-defined mixed layer in (B).

The fact that these layers are warmer than the localminimum SST, deRning the surface layer depth, in-dicates that they have at some time been subject tolocal surface forcing. The solar heating and reducedwind stirring of spring can cause the upper layer tobecome thermally restratiRed. The newly formedupper mixed layer of light, warm water is separatedfrom the older, deeper winter mixed layer bya well-stratiRed thermocline. The fairly stablewaters in this seasonal thermocline may isolate thelower isothermal layer and prevent further modiRca-tion of its properties, so that this layer retains thewater characteristics of its winter formation periodand becomes ‘fossilized’. Hence, fossil layers tend toform in regions with signiRcant seasonal heating,a large annual range in wind stress, and deep wintermixed layers. These conditions can be found at thepoleward edges of the subtropical gyres.

In the north-east PaciRc Ocean off California andin the south-west PaciRc Ocean near New Zealand,particularly deep and thick fossil layers have beenassociated with the formation of Subtropical ModeWaters. As with the fossil layers, the mode waters

are characterized by low vertical gradients intemperature and density. The isothermal layer orthermostad of winter water trapped in the fossilizedlayers may be subducted into the permanent ther-mocline through the action of Ekman pumping, inresponse to a convergence in the wind Reld. Themode waters are then transported, retaining theircharacteristic thermostad, with Sow in the subtropi-cal gyre.

However, not all fossil layers are associated withthe formation regions of mode waters. Shallowfossil layers have been observed where there arestrong diurnal cycles, such as in the western equa-torial PaciRc Ocean. Here, the same alternatingprocesses of heating/cooling and wind mixingas found in the mode water formation regions,cause the fossil layers to form. Fossil layers havealso been observed around areas of abrupt topogra-phy, such as along island chains where strong cur-rents are found. In this case, the fossil layers areprobably formed by the advection of water withproperties different from those found in the uppermixed layer.

3124 UPPER OCEAN VERTICAL STRUCTURE

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Figure 3 (A) The temperature section from expendable bathythermograph data collected along a transect from Auckland, NewZealand to Seattle in April 1996, and (B) the corresponding temperature gradient with depth. The heavy line indicates the depth ofthe surface layer, according to the depth of the coldest sea surface temperature measured at each location. The light line indicatesthe depth of the mixed layer according to the (SST!13C) criterion.

Barrier Layers

In some regions the freshwater Sux can dominatethe mixed-layer thermodynamics. This is evident inthe tropics where heavy precipitation can causea surface-trapped freshwater pool that forms a shal-lower mixed layer within a deeper nearly isothermallayer. The region between the shallower densitydeRned well-mixed layer and the deeper isothermallayer (Figure 2B), is referred to as a salinity-stratiRed barrier layer.

The barrier layer may have important implica-tions on the heat balance within the surface layerbecause, as the name suggests, it effectively limitscommunication between the ocean mixed layer andthe deeper permanent thermocline. Even if under

light wind conditions water is entrained from belowinto the mixed layer, it will have the same temper-ature as the water in this upper layer. Thus, there isno heat Sux through the bottom of the mixedlayer and other sinks must come into play to bal-ance the solar warming that is conRned to the sur-face, or more likely, the barrier layer is transientin nature.

Inversions

Occasionally temperature stratiRcation within thesurface layer can be inverted (i.e., cool water liesabove warmer water). The temperature inversioncan be maintained in a stable water column since itis density-compensated by a corresponding salinity

UPPER OCEAN VERTICAL STRUCTURE 3125

increase with depth throughout the inversion layer.Inversions are a ubiquitous feature in the verticalstructure of the surface layer from the equator tosubpolar latitudes, although their shape and forma-tion mechanisms may differ.

Inversions that form in response to a change inthe seasonal heating at the surface are most com-monly found in the subpolar regions. They can formwhen the relatively warmer surface water of sum-mer is trapped by the cooler, fresher conditions thatexist during winter. The vertical structure of thesurface layer is described by a well-mixed upperlayer in temperature, salinity, and density, lyingabove the inversion layer that contains the haloclineand subsequent pycnocline (Figure 4A). Conversely,during summer the weak subpolar solar heating cantrap the very cold surface waters of winter, sand-wiching them between the warmer surface anddeeper layers. In this case, the vertical structure ofthe surface layer consists of a temperature minimumlayer below the warm stratiRed surface layer,and above the relatively warmer deeper layer(Figure 4B). The density-deRned mixed layeroccurs above the temperature minimum. With con-tinual but slow summer heating, the cold waterfound in this inversion layer slowly mixes withthe warmer water masses above and below, anderodes away.

Inversions can also form through horizontaladvection of water with different properties orwater-mass interleaving. For example, in the tropicswhere there may be velocity shear between opposingcurrents, inversions are typically characterized assmall abrupt features, often only 10 m thick, foundat the base of a well-mixed upper layer, and withinthe top of the halocline and pycnocline (Figure 4C).Just west of San Francisco (1303}1403W), the lowtemperature and salinity properties of the Subantar-ctic Water Mass found in the California Currenttransition toward the higher-salinity water massesformed in the evaporative-regime of the mid-sub-tropical gyre. The interleaving of the various watermasses results in inversions that are quite differentin structure from those observed in the tropicalPaciRc or the subpolar regions (Figure 4D). Thesurface layer vertical structure is further complic-ated by frequent energetic eddies and meanders thatperturb the Sow and have their own distinctivewater properties. In the transition zone, the inver-sions can be thick, and occur well within thepycnocline and not at the base of the mixed layer(Figure 4D). Typically within the broad regionof the inversion, there are sharp gradients in tem-perature and salinity, both horizontally and verti-cally, that are characteristic of water-mass interleav-

ing from the advective penetrations of the currentsand eddies.

Other Properties of the Upper OceanVertical Structure

Other water properties, such as dissolved oxygenand nutrients (e.g., phosphates, silica, and nitrates),can also vary in structure in the upper ocean surfacelayer. These properties are considered to be noncon-servative, that is, their distribution in the watercolumn may change as they are produced or con-sumed by marine organisms. Thus, although theyare of great importance to the marine biology,their value in deRning the physical structure of theupper ocean surface layer must be viewed withcaution. In addition, until recently these propertieswere not routinely measured on hydrographiccruises. Nonetheless, the dissolved oxygen satura-tion of the upper ocean in particular, has beena useful property for determining the depth ofpenetration of air}sea exchanges, and alsofor tracing water masses. For example, in the farNorth PaciRc Ocean, it has been suggested that thedegree of saturation of the dissolved oxygen concen-tration may be a better indicator than temperatureor density for determining the surface-layer depth ofconvective events. During summer, the upper layermay be restratiRed in temperature and salinitythrough local warming or freshening at the surface,or through the horizontal advection of less densewaters. However, these upper level processes maynot erode the high-oxygen saturation signature ofthe deeper winter convection. Thus the deep high-oxygen saturation level provides a clear record ofthe depth of convective penetration from the air}sea exchange of the previous winter, and a uniquesignal for deRning the true depth of the surfacelayer.

Conclusions

The vertical structure of the upper surface layer canbe characterized at its simplest as having a near-surface mixed layer, below which there may exista seasonal thermocline, where temperature changesrelatively rapidly, connected to the permanent ther-mocline or main pycnocline. The vertical structure isprimarily deRned by stratiRcation in the water prop-erties of temperature, salinity, and density, althoughin some regions oxygen saturation and nutrient dis-tribution can play an important biochemical role.The vertical structure of the surface layer can becomplex and variable. There exist distinct variations

3126 UPPER OCEAN VERTICAL STRUCTURE

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Figure 4 Temperature (black line), salinity (blue line) and density (pt , green line) from expendable conductivity}temper-ature}density casts at (A) 58.23N, 147.33W in March 1996, (B) 613S, 63.93W in January 1997, (C) 11.93S, 176.13W in August 1998,and (D) 33.53N, 134.63W in May 1995. Note the presence of temperature inversions at the base of the mixed layer in all casts.

in the forms and thickness of the upper-layer struc-ture both in time and in space, through transientvariations in the air}sea forcing from winds, heat,and fresh water that cause the turbulent mixing of

the upper ocean. Understanding the variation in theupper ocean vertical structure is crucial for under-standing the coupled air}sea climate system, and thestorage of the heat and fresh water that is ultimately

UPPER OCEAN VERTICAL STRUCTURE 3127

redistributed throughout the world oceans by thegeneral circulation.

See also

Air+Sea Gas Exchange. Bottom Water Formation.Deep Convection. Ekman Transport and Pumping.Expendable Sensors. Heat and Momentum Fluxesat the Sea Surface. Ocean Subduction. OpenOcean Convection. Penetrating Shortwave Radi-ation. Thermohaline Circulation. Upper OceanHeat and Freshwater Budgets. Upper Ocean MeanHorizontal Structure. Upper Ocean Mixing Pro-cesses. Upper Ocean Time and Space Variability.Water Types and Water Masses. Wind Driven Cir-culation. Wind and Buoyancy-forced Upper Ocean.

Further readingKraus EB and Businger JA (1994) Atmosphere}Ocean

Interaction, Oxford Monographs on Geology andGeophysics, 2nd edn. New York: Oxford UniversityPress.

Philips OM (1977) The Dynamics of the Upper Ocean,2nd edn. London and New York: Cambridge Univer-sity press.

Reid JL (1982) On the use of dissolved oxygen concentra-tion as an indicator of winter convection. Naval Re-search Reviews 3: 28}39.

Warren B and Wunsch W (1981) Evolution of PhysicalOceanography: ScientiTc Surveys in Honor of HenryStommel. Cambridge, MA: MIT Press.

Tomczak M and Godfrey JS (1994) Regional Oceano-graphy: An Introduction. Oxford: Pergamon Press.

UPWELLING ECOSYSTEMS

R. T. Barber, Duke University Marine Laboratory,Beaufort, NC, USA

Copyright ^ 2001 Academic Press

doi:10.1006/rwos.2001.0295

Introduction

An ecosystem is a natural unit in which physical andbiological processes interact to organize the Sow ofenergy, mass, and information. The result of thisself-organizing activity is that each kind of eco-system has a characteristic trophic structure andmaterial cycle, some degree of internal homogeneity,objectively deRnable boundaries, and predictablepatterns of seasonality. Oceanic ecosystems arethose ecosystems that exist in the open ocean inde-pendently of solid substrates; for example, oceanicecosystems are fundamentally distinct from coral orintertidal ecosystems.

Upwelling ecosystems are those that occupyregions of the ocean where there is a persistentupward motion of sea water that transports subsur-face water with increased inorganic plant nutrientsinto the sunlit surface layer. The upwelling water isnot only rich in nutrients, but also frequently coolerthan the surface water it replaces; this results ina variety of atmospheric changes, such as coastaldeserts or arid zones. The increased nutrient supplyand favorable light regime of upwelling ecosystems,however, distinguish them from other oceanic eco-systems and generate characteristic food webs thatare both quantitatively and qualitatively differentfrom those of other oceanic ecosystems.

For persistent upwelling to take place it is neces-sary for the surface layer to be displaced laterally ina process physical oceanographers call divergenceand then for subsurface water to Sow upward toreplace the displaced water. The physical concept ofupwelling is simple in principle but, as with manyocean processes, it becomes surprisingly complexwhen real examples are studied. To begin with,there are two fundamental kinds of upwelling eco-systems: coastal and oceanic. They differ in thenature of their divergence. In coastal upwelling, thesurface layer diverges from the coastline and Sowsoffshore in a shallow layer; subsurface water Sowsinshore toward the coast, up to the surface layer,then offshore in the surface divergence. In contrast,oceanic upwelling, which occurs in many regions ofthe ocean, depends on the divergence of one surfacelayer of water from another. One such oceanicdivergence is created when an increasing gradientin wind strength forces one surface layer to movefaster, thereby leaving behind, or diverging from,another surface layer. Major regions of this kind ofoceanic upwelling are found in high latitudes in theSubpolar gyres of the Northern Hemisphere and theAntarctic divergence in the Southern Ocean. Thefood webs of polar upwelling ecosystems aredescribed elsewhere in the Encyclopedia and thisarticle will focus on coastal and equatorial upwell-ing ecosystems that occur in low and mid-latituderegions of the world’s oceans.

The physical boundary organizing oceanic diver-gence in equatorial upwelling is the Coriolis force,which changes sign at the equator, causing the east-erly Trade Winds to force a northward divergence

3128 UPWELLING ECOSYSTEMS