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Deep Water Properties, Velocities, and Dynamics Over Ocean Trenches

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Deep Water Properties, Velocities, and Dynamics Over Ocean Trenches Journalof MarineResearch, 56, 329–347, 1998 Deep water properties, velocities, anddynamics over ocean trenches byGregory C. Johnson 1 ABSTRACT Observationsof water properties and deep currents over several trenches in the Paci c Ocean centralbasins give consistent evidence for recent ventilation of water below the trench sills and cyclonicsense of circulationover the trenches. A dynamicalargument for this pattern is advanced. First,a reviewof previousanalyses of hydrographicdata shows that the trenches are well ventilated bydensebottom water, that within the trenches this bottom water generally spreads away from its source,and that a cyclonicsense of circulationis suggested over some trenches. Then, this cyclonic senseof circulationover the trenches is further documented using deep current meter and oatdata. Finally,bathymetry is used to motivatea simpledynamical framework for owover trenches. If the trenchsides are sufficiently steep and the trench is sufficiently removed from the equator to ensure a regionof closed geostrophic contours, then any upwelling over that region will drive a strongdeep cyclonicrecirculation in theweakly-strati ed abyssthrough vortex stretching. The magnitude of this recirculationis limitedby bottom drag. Ageostrophic owin a bottomEkman layer into the trench balancesthe water upwelled over the trench. The cyclonic recirculation is muchstronger than the upwelling-driven owpredicted across blocked geostrophic contours by the linear planetary geostrophicbalance. 1.Introduction Theclassical theoretical framework for thethermohaline circulation of the abyssal oceanassumes localized sinking at high latitudes forced by buoyancy loss from air-sea exchange.This sinking is balancedby widespreadupwelling at lowerlatitudes that in turn countersthe effects of downwarddiffusion of buoyancy from thesurface (Stommel and Arons,1960). If upwellingincreases with distance from thesea oorand bathymetry is sufficientlygentle to avoidregions of closedgeostrophic contours, the planetary geostro- phicbalance dictates overall poleward interior ows.With a boundarycondition of nonet owinto the eastern edge of each basin, the interior owis fed from deepwestern- boundarycurrents. These dynamics result in recirculation regions near the poles where interior owfeeds deep westward- owing boundary currents at poleward walls. The deep western-boundarycurrents near these recirculations sometimes owbacktoward the high latitudesources. Predictions of locations and strengths of deep boundary currents have 1.Paci c MarineEnvironmental Laboratory, NOAA, 7600Sand Point Way, N.E. (Bldg. 3), Seattle, Washington,98115-0070, U.S.A. 329 330 Journalof MarineResearch [56, 2 beencon rmed by observationsin locations throughout the W orldOcean (W arren,1981). Manyvariations on thisframework havebeen investigated, and a smallsample are given below.Interior zonal jets have been observed and modeled owingaway from aneastern deep-watersource (W arren,1982) and toward an eastern deep-water sink (W arrenand Speer,1991; Speer et al., 1995).The distortion of thecirculation by gentlebathymetry has beendiscussed (W arrenand Owens, 1985). Abyssal spin-up and nonuniform interior upwellingin uences have been modeled (Kawase, 1987).Effects of a fullynonlinear continuityequation (Speer andMcCartney, 1992) have also been investigated. Thefocus of thiswork is abyssalcirculation over deep trenches, speci cally along the westernand northern margins of thePaci c Oceancentral basins. The simple Stommel- Aronsframework mustbe modied substantially here because steep bathymetry results in regionsof closed geostrophic contours over the trenches, a situationpredisposed to strong recirculations(W elander,1969). Theoretically, a densebottom water source introduced intoa stratied rotating deep basin shaped more like a bowlthan a boxcan drive an anticyclonicinterior circulation. This tendency, coined the hypsometric effect, results from decreasingvertical velocity with decreasing depth owing to the increasing area of the basin 1 1 withdecreasing depth (Rhines and MacCready, 1989). However, 1- 2 and 2- 2 layer numericalmodels as well as analytical results demonstrate that in situations where stratication is weak, upwelling over closed geostrophic contours is associated with a strongcyclonic recirculation tangent to the contours. The results are similar for elevatedor depressedtopography in sink-driven experiments with prescribed uniform upwelling (Strauband Rhines, 1990) and in source-driven basin- lling experiments (Kawase and Straub,1991; Kawase, 1993a,b).The upwelling within the region of closed geostrophic contoursresults directly in vortex stretching that drives a strongcyclonic recirculation, limitedonly by bottom drag. A bottomEkman layer associated with this drag transports mass intothe region normal to the closed contours. If thecirculation is in steady-state, the bottomEkman transport balances the mass lostto upwellingwithin the region. Inthe following, rst aliteraturereview shows several inferences about this deep ow from hydrographicdata. W aterproperties below the trench sills reveal that bottom water thereis not stagnant. That is to say,each series of trenchesis activelyventilated by bottom water.In addition, water propertiesin some locations suggest owpatterns.In the Paci c, theinferences from thewater properties are that the bottom water inthese trenches is spreadrelatively rapidly away from itssouthern source and often that a cyclonicsense of circulationis evident.These two inferences are not contradictorysince the southernbottom watersignature can be carriedrapidly away from itssource on one branch of thecyclonic circulationaround the trench. Next, velocity measurements are used to document a cyclonicsense of circulation over several trenches. Finally, the trench bathymetry is shown toresult in closedgeostrophic contours, and this fact is usedto placethe observations in a theoreticalframework. 1998] Johnson:Deep circulation over ocean trenches 331 Figure1. Deep trenches at the western and northern edges of the Paci c Oceancentral basins revealedby theGEBCO 7000-misobath (IOC, IHO, andBODC, 1994).The labeled boxed areas correspondto individual panels of Figure5, and contain the trenches extensively discussed, (a) TheKermadec and T ongaTrenches, (b) the southern end of the Mariana Trench, (c) the Izu-Ogasawara,Japan, and Kuril Kamchatka Trenches, and (d)the Aleutian Trench. 2.Deep waterproperties ThePaci c Oceanis ringed around much of its margins by deep trenches (Fig. 1). Signicant trenches are found elsewhere in the W orldOcean. However, readily available long-termdeep current measurements over trenches are all found at the western and northernedges of the Paci c Oceancentral basins. For thesake of brevity, only water propertiesin this region will be discussedin detail. No bottom water forms intheNorth Pacic Ocean;instead it entersthe Pacic Oceancentral basins in a deepwestern-boundary currentfrom thesouth. The following literature review starts closest to thisbottom water source,with water propertiesof theKermadec Trench, moves clockwise around the Paci c Ocean,and ends with the water propertiesof theAleutianTrench, farthest from thesource. Zonaltranspaci c hydrographicsections at 32.5 and 28S (Whitworth et al., 1996; Warren,1973) cross the Kermadec Trench. The sections show that the water belowthe sill isabout as cold (in potential temperature), fresh, dense, oxygen-rich, and silica-poor as bottomwater justto the east. These properties are characteristics that distinguish the more recentlyventilated modi ed Antarctic Bottom W aterfrom theless recently ventilated 332 Journalof MarineResearch [56, 2 modied NorthAtlantic Deep W ater.These water masses are the two main components of theCircumpolar Deep W aterthat ventilates the deep Paci c Ocean,with the former being dominantin the bottom water ofthesouthern hemisphere, and the latter being dominant in thebottom water ofthenorthern hemisphere. Properties below the trench sill distinguish water thereas strongly southern in origin. This conclusion is not surprising, since the trenchis coincident with the northward- owing deep western-boundary current banked againstthe Kermadec Island Arc (Whitworth et al., 1996;W arren,1973). The oxygen-rich natureof thewater belowthe sill clearly shows that water thereis not stagnant. This water mustbe sufficiently ventilated to maintain this characteristic in the face of biological consumption.In addition, a troughin theisotherms immediately over the trench is visible tovarying degrees in thefourzonal sections near 32.5S and the single zonal section at 28S. Assuminggeostrophy and a referencevelocity of zero somewhat above the trough, this persistentfeature is consistentwith a bottom-intensied cyclonic sense of circulation over thetrench, as notedby Whitworth et al. (1996). Farthernorth, a zonaltranspaci c sectionacross the north end of theMariana Trench at 24N(Roemmich et al., 1991)shows that the water belowthe trench still is relativelycold, salty,dense, oxygen-rich, and silica-poor .Asectionnear the saddle point of the Izu- Ogasawaraand Japan Trenches at 35N (Kenyon, 1983) also shows these tendencies. In the NorthPaci c Ocean,these tendencies distinguish more newly arrived Circumpolar Deep Waterbelow from theolder Paci c DeepW aterabove. By this latitude the modi ed North AtlanticDeep W atercomponent of Circumpolar Deep W ateris dominant in the bottom water,which is why the distinguishing salinity and silica criteria for southernorigin bottom waterchange from
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