biological structure and exchange across the thermohaline

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May 20, 1983 - time of the conductivity signal is increased to match that of the temperature. ... using a Turner Model 10 fluorometer enclosed in a water-tight housing ...... Cullen and Horrigan [1981] have supplied laboratory evidence for these ...



VOL. 88, NO. C7, PAGES 4467-4481, MAY 20, 1983

Physical/BiologicalStructureand ExchangeAcrossthe Thermohaline

Shelf/Slope Front in the New York Bight ROBERT W. HOUGHTON AND JOHN MARRA

Lamont-DohertyGeolooicalObservatoryof ColumbiaUniversity,Palisades,New York 10964 During the summer,the shelf/slopewater from in the New York Bight is thermohalinewith strong temperatureand salinitygradientsoccuringon isopycnalsconnectingshelfand slopewater.The dominant lateral exchangeis via slopewater intrusionsaboveand belowthe cold pool, a remnantof winter-mixed shelf water. These intrusions create environments favorable for double-diffusiveprocesses.However,

measured values ofthedensity ratioRpovertheshelf implyweakvertical mixing(Kv,-, 10-• cm-•),which is consistentwith heat and salt fluxes into the persistentcold pool. Convergentcirculation driven by cabbelingand vertical,mixingenhancedby double-diffusionat the outer edgeof the cold pool may be responsible for the abruptdeepeningof the chlorophylla maximumlayer.



The structureof the frontal boundary separatingshelfand slopewater in the Middle Atlantic Bight(MAB) from Hatteras to Georges Bank has been previouslyreported by Bigelow [1933], Ketchurnand Corwin[1964], Cresswell[1967], Boicourt andHacker [1976], Beardsleyet al. [1976], and Posrnentier and Houghton[1981]. Exchangeof heat and salt acrossthis frontal boundaryis a largelyunknownfactorin boxmodelsof theshelf

compensating,and there exist large temperatureand salinity gradientson isopycnalsurfaces. We will investigatethe potential role of thermohaline processes,cabbeling and doublediffusionin this regime. Subsequentsectionsof this paper will presentthe field work and sampling procedure(section 2), the large-scalefrontal

boundary(section3), the fii•e structure(section4), frontal mixingandexchange (section 5),thermohaline processes (sec-

water[BrownandBeards!ey, 1978].Forcingby wind,Gulf tion 6), and chlorophylldistributions(section7), followedby a Streammeanders,and warm core ringsoften producea highly convolutedfrontal boundary[Movers et al. 1979;Halliwell and Movers, 1979], whoselarge cross-shelf excursionscan resultin a substantialnet exchangeof heat and salt betweenthe slope and shelf waters [MoriTan and Bishop, 1977]. However, the


acrossthefrontalboundaryis reduced. The focusof our atten-

array of current meters deployed by Woods Hole



The studyarea (Figure 1)was situatedat the shelfbreak of the NYB betweenHudsonCanyonand the NantucketShoals

presence of a frontimplies thaton a smaller scale, exchangenear the Nantucket Shoals Flux experiment (NSFE79), an tion will be on this smaller-scalecross-shelf exchange.

Frontalregio,ns havebeenfoundto beregions of higher biologicalproductivity[Pingree,1979; Fournieret al., 1979]. Enhancedvertical mixing or upwellingof nutrients into the euphoticzone is usuallyinvokedas the dominant mechanism. Alternatively, the high biomass associatedwith this pro-

OceanographicInstitution, (WHOI), U.S. Geological Survey

(USGS) and National Marine FisheriesService (NMFS) [Beardsleyet al., 1983]. Following a large-scalesurvey(dashed line in Figure 1) 9 dayswerespentin the regionnear 40øN and

71ø30'Wwith intensivephysicaland biologicalsamplingto resolve thetemporalandspatialvariabilityof thefrontalstruc-

ture. For clarity, only selectedcruise tracks from which data convergentcirculation at the front. Here, we investigatethe will be presentedare shownin Figure 1. Theseincludea grid of distribution of phytoplankton in relation to a thermohaline alongshelfsections,MS2, MS3, MS4 and cross-shelfsections front, a type of front, to our knowledge,not heretoforeinvesti- MS5, MS6, MS7 (not shown); an array of cross-shelfsections (labeledi, II, III) that were sampledrepeatedlyfor 90 hours; a gatedbiologically. We present results of a cruise• SWIGIII, in theNewYork sectionof 1-km spacedstations("microline")extendingalong I Bight(NYB)July25to August 4, 1979,whose objective wasto to the 100-m isobath; larger-scalealongshelfsections,MS8, Study theroleof finestructure intrusions in cross-frontal ex- MS9; and finally cross-shelfsectionsMS 10, MS 11, MS 12 ("machange andbiological processes attheshelf/slope front. Repea-croline")of widelyspacedstationsextendinginto the midshelf. Vertical distributionsof temperature,salinity,and dissolved ted sampling of closely spacedstations allows us to study

ductivitycouldbemerelytheaccumulation of planktondueto

oxygenwereobtainedfroma Neil BrownMark III CTD/O2

small-scalefeaturesthat have not been resolvedin much of the

previouswork on the frontal regime.During this periodthe system,with a fast responsethermistor, attached to a rosette shelfand slopewatersare fully stratifiedso that the surface containingtwelve 1.7-1Niskin-type water samplers.Comparithermal expressionof the front is obliterated by the strong seasonalthermoclineoverlyingthe cold pool. Likewisethereis no Surfaceexpression in the chlorophylla distribution.Vertically,chlorophylla is characterizedby a maximumat approximately 30-m depth. Down to 50 m the front is thermohaline, i.e., horizontal temperatureand salinity gradientsare density Copyright1983by theAmericanGeophysical Union. Papernumber3C0159. 0148-0227/83/003C-0159505.00

son with reversingthermometers(_0.01øC) and with salinities

(___0.005%0) analyzed on a Model 840 Guildline autosalinometer resultedin no correctionto the CTD temperature and salinityvalues.Dissolvedoxygen(_0.01 ml 1-•) profiles werecalibrated,usingbottle samplesanalyzedby the Carpenter modification

of the Winklet


In spite of the fast responsethermistor, high temperature gradients produced spurious salinity values which were not eliminated by lagging the conductivityrecord with respectto

the temperatu. re record[Horne and Toole,1980].An alter4467




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