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Frontogenesis, Subduction, and Cross-Front Exchange at Upper Ocean Fronts

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Frontogenesis, Subduction, and Cross-Front Exchange at Upper Ocean Fronts JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 100, NO. C2, PAGES 2543-2557, FEBRUARY 15, 1995 Frontogenesis, subduction, and cross-front exchange at upper ocean fronts Michael A. Spall Department of PhysicalOceanography, Woods Hole OceanographicInstitution, Woods Hole, Massachusetts Abstract. The subduction of water at upper ocean fronts driven by internal instabilities is investigated.A simpleanalytic model which assumesconservation of potential densityand potential vorticity is used to characterizethe expectedstructure of subductedparcels as a function of the surface layer depth and the length scale of the parcel. Parcels subducted from a deep surfacelayer will be characterizedby anomalouslylow potential vorticity and anticycloniccirculation, while shallowsurface layers result in subductedparcels with high potential vorticity and cycloniccirculation. A nonlinear isopycnalprimitive equation model is used to demonstratehow baroclinicinstability and the resulting frontogenesisforce the subductionof parcelsbelow and acrossthe front. Frontogenesisgenerates vertical velocities of 0(30 m d-•) and ageostrophiccross front flowsof 3 to 5 cm s-•. The subductionis achievedby the deep ageostrophicflow which carries water from the surface layer below and acrossthe front. The subductedparcels quickly developa circulation through balanced adjustment, anticyclonicfor deep surfacelayers and cyclonicfor shallow surface layers. The characteristics of parcels subducted in winter conditions closely resemble a class of eddies called submesoscale coherent vortices. The horizontal and vertical scales of the vortices are determined by the frontogenetic mechanism and the depth of the surface layer. The total permanentsubduction rate is estimatedto be 20 m yr-• for frontal subductiondriven by frontogenesis,although local (and temporary)subduction can be muchlarger. The subducted anticyclonic vortices are often coupled to upper ocean cyclonic vortices, thus forminga baroclinicdipole that propagatesat approximately3.5 cm s-• and is capableof transporting the anomalouswater propertiesfar from the region of formation. Similarities with observationsin the North Atlantic Subtropical ConvergenceZone and near the Gulf Stream are discussed. 1. Introduction ter through subductioncan significantlyinfluence the biologicalcycle [Strass, 1992]. The processby which water is transferred from the The physicsof oceanicsubduction are not well known. mixed layer into the stratified interior of the ocean is It is believedto be at leastpartially controlledby the generally referred to as subduction. This processis be- large-scalevariations in atmosphericwind stresscurl lieved to be important to the physicsof both the oceans and net buoyancyflux (see, for example,Marshall et and the atmosphere. The properties of the main ther- al. [1993,and references therein]). The generalpattern mocline in the ocean are to a large degree set when the of westerlies at midlatitudes and trade winds at low water parcels were last in contact with the atmosphere. latitudes sets up a convergentEkman transport with Dynamical properties, such as potential vorticity, den- implied downwellingat the regionof the subtropical sity, and temperature, play an important role in de- convergence zone where the wind stress curl is a maxi- termining the large-scalecirculation and stratification mum. This vertical velocitycontributes at leastpart of within the main thermocline. The absorption of chemi- the massflux from the mixed layer into the permanent cals from the atmosphereby the ocean dependsstrongly thermocline. The surfaceof the permanentthermocline on the subductionprocess and may play a key role in is not horizontal, however, and a horizontal flow from a the globalclimate system.In addition, biologicalactiv- regionof a deepmixed layer into a regionof a shallow ity is particularly large in the near-surfacelayers, and mixed layer also transfersmass from the mixed layer the exchangeof nutrients and biologicallyactive mat- into the permanent thermocline. Simple calculations indicate that, on large scales,this mechanismcan be of the sameorder of magnitudeas Ekmanpumping [Mar- Copyright 1995 by the American GeophysicalUnion. shall et al., 1993]. Paper number 94JC02860. Upper ocean fronts are also thought to be places 0148-0227/95/94JC-02860505.00 where subduction might occur becausethey are re- 2543 2544 SPALL: FRONTAL SUBDUCTION gions where deep isopycnalshave large vertical excur- This paper is outlined as follows. A simple analytical sions and often outcrop into the surface mixed layer. solution that providessome guidelines for the structure Surface-intensifiedfrontal regions are found in many and circulation of parcels exchangedacross a potential places throughout the world's oceans. Some are per- vorticity gradient is presentedin section2. In section3 manent, large-scalefeatures of the general circulation, a nonlinear primitive equation model is applied to the such as the Gulf Stream, Kuroshio, and the Antarctic problem of frontal subduction for two distinct initial Circumpolar Current. Other fronts are intermittent in conditions. The subduction mechanism and its rela- nature where they may exist for timescalesof weeks tionship to submesoscalecoherent vortices are discussed to months and then break down, such as those found in section 4, and final conclusionsare presentedin sec- in regions of Ekman convergencelike the Subtropical tion 5. ConvergenceZone (STCZ) in the North Atlantic and Pacific. Strong frontal regionsare often observedalong the periphery of larger-scale mesoscaleeddies in the 2. A Simple Model for Subducted North Atlantic STCZ between22øN and 32øN [Halli- Parcels well et al., 1991]. Simplescaling estimates by Follows and Marshall [1994]that are basedon the ageostrophic A simple model is developedto describe the struc- axial circulation associatedwith frontogenesissuggest ture of parcels that are transferred across a localized that subduction driven by frontal processesmay be potential vorticity gradient. The model is developed as large as that due to the large-scaleEkman conver- and discussedwithin the context of frontal subduction, gence. (Similar ideas are applied to exchangein the however, the underlying dynamics are relevant to the atmospherictropopause by Followsand Austin [1992].) more general class of problems. Frontal regionsoften Pollard [1986]found parcelsof low potential vorticity mark a transition between different water masses and water below the mixed layer on the warm side of one havean associatedchange in stratification(or potential of these fronts whose temperature and salinity charac- vorticity) acrossthe front. This is particularlytrue for teristics suggestedthat it originated upstream in the isopycnallayers that outcrop in the frontal region. The mixed layer on the cold side of the front. We focus here thicknessof the surface layer is strongly influencedby on this subclassof fronts which are largely confinedto local atmosphericforcing, however;once subducted be- the upper few hundred meters, although many of the re- low warmer surfacelayer waters, the water is no longer sults presentedhere are relevant in a generalway to the directly forced by the atmosphere and approximately broader class of surface-intensifiedfronts. The present conservesquantities such as heat and potential vorticity. focusis motivated by their abundanceand the relatively A consequenceof this is that the depth of the surface high-resolutiondata setswhich have beencollected near layer on the cold side of the front is generally different these fronts (such as the Frontal Air-Sea Interaction from the thickness of the same isopycnal layer below Experiment(FASINEX); see Weller [1991], Pollard and the surface on the warm side of the front. In the vicin- Regier[1992] (hereinafter referred to as PR92), and the ity of mixed layer fronts this transition can take place data reportedby Voorhisand Bruce[1982] (hereinafter overhorizontal scales of O(10 km). If a columnof wa- referredto as VB82)). ter is transferred from the surfacelayer to the stratified The relative influences of internal frontal processes interior, its thicknesswill be anomalousrelative to the and external atmosphericforcing on frontal subduction ambient stratification. A schematic of such a subducted are not known. The evolution of actual upper ocean water parcelis shownin Figure I for the situationwhere frontsis likelyto be influencedby surface•rcing (wind the surfacelayer is thickerthan the sameisopycnal layer and buoyancy),turbulent mixing in the mixed layer, in the stratified interior. This situation is typical of and large-scaleflows in addition to the front's own in- fronts observed in the North Atlantic STCZ in winter. trinsic time dependence.In the presentstudy the mech- If the parcel conservespotential vorticity as it is carried anisms inherent in the internaT instability of the front acrossthe front, the thermal wind relation requiresthat are isolated from these other complicating factors in the subductedparcel be squashedand spreadlaterally order to obtain a clearer understandingof this underly- as it developsa horizontal circulationand negativerel- ing component.This is justified by the scalinganalysis ative vorticity. of Followsand Marshall [1994],by the relativelyshort The model consistsof three isopycnal layers, as in- timescales of baroclinic instability and the associated dicated in Figure 1. The thicknessof the surfacelayer frontogenesisthat are O(1 week), and by the consis- on the cold side of the front is hm, the thickness
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