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Open-Ocean Convection' Observations, Theory, and Models OPEN-OCEAN CONVECTION' OBSERVATIONS, THEORY, AND MODELS John Marshall Friedrich Schott Departmentof Earth,Atmospheric, and Planetary Institut f•ir Meereskunde an der UniversitJt Kiel Sciences Kiel, Germany MassachusettsInstitute of Technology,Cambridge Abstract. We review what is known about the convec- processis localized in space so that vertical buoyancy tive processin the open ocean,in whichthe propertiesof transfer by upright convectioncan give way to slantwise large volumes of water are changed by intermittent, transfer by baroclinic instability. Moreover, the convec- deep-reachingconvection, u lggczcu '• by w•.tc,'-'•- storms. tive dllU •C;UbLIUI. JIII•, bCi;tlCb •lC; 11UL VC;iy Ulbl. Jill ate 11UIII Observational,laboratory, and modelingstudies reveal a one another.Detailed observationsof the processin the fascinating and complex interplay of convective and Labrador, Greenland, and Mediterranean Seas are de- geostrophicscales, the large-scale circulation of the scribed,which were made possibleby new observing ocean, and the prevailing meteorology. Two aspects technology.When interpreted in terms of underlying make ocean convectioninteresting from a theoretical dynamicsand theory and the contextprovided by labo- point of view. First, the timescalesof the convective ratory and numerical experimentsof rotating convec- processin the ocean are sufficientlylong that it may be tion, great progressin our descriptionand understand- modified by the Earth's rotation; second,the convective ing of the processesat work is being made. CONTENTS 5. Parameterization of water mass transformation in models ................................................... 52 Introduction ............................................... 1 5.1. One-dimensionalrepresentation of 1.1. Backgroundand scope........................... 1 plumes ...................................................... 53 1.2. Some preliminaries................................ 3 5.2. Geostrophiceddies and the spreading . Observationalbackground ............................ 5 phase....................................................... 54 2.1. Phasesand scalesof deep convection...... 5 5.3. Putting it all together ............................ 58 2.2. Major ocean convectionsites .................. 6 6. Conclusions and outlook .............................. 58 2.3. Meteorologicalforcing ........................... 11 . Convective scale .......................................... 16 3.1. Gravitational instability;"upright" 1. INTRODUCTION convection ................................................. 17 3.2. Convectionlayer ................................... 18 1.1. Backgroundand Scope 3.3. Plume dynamics.................................... 22 The strongvertical density gradientsof the thermo- 3.4. Observationsof plumes in the ocean....... 26 cline of the ocean inhibit the vertical exchangeof fluid 3.5. Numerical and laboratory studiesof and fluid propertiesbetween the surfaceand the abyss, oceanic convection ..................................... 31 insulating the deep ocean from variations in surface 3.6. Role of lateral inhomogeneities.............. 33 meteorology.However, in a few special regions (see Figure 1) characterizedby weak stratificationand, in 3.7. Complicationsarising from the equation of state of seawater .................................... 36 winter, exposedto intense buoyancyloss to the atmo- sphere,violent and deep-reachingconvection mixes sur- . Dynamicsof mixed patches.......................... 38 face waters to great depth, settingand maintainingthe 4.1. Observed volumes and water mass properties of the abyss.This paper reviews observa- transformation rates ................................... 38 tional, modeling,laboratory, and theoreticalstudies that 4.2. Mixed patchesin numerical and have elucidatedthe physicsof the convectiveprocess laboratory experiments............................... 42 and its effect on its larger-scaleenvironment. 4.3. Theoretical considerations ...................... 44 In the present climate, open-oceandeep convection 4.4. Restratificationand geostrophic occursonly in the Atlantic Ocean: the Labrador, Green- eddy effects............................................... 49 land, and MediterraneanSeas (Figure 1), and occasion- Copyright1999 by the AmericanGeophysical Union. Reviewsof Geophysics,37, 1 / February1999 pages 1-64 8755-12 09/99/98 RG-02 73 9 $15.00 Papernumber 98RG02739 el ß 2 ß Marshall and Schott: OPEN-OCEAN CONVECTION 37, 1 / REVIEWS OF GEOPHYSICS 0 •o 26.8 •o 26.6 •O o •6.½ I 40ø 20ø 0ø convectionobserved sections 'Bravo' Figure 1. The major deep convectionsites of the North Atlantic sector:the Labrador Sea (box a), the GreenlandSea (box b), and the westernMediterranean (box c). Detailed descriptionsand discussionsof the water mass transformationprocess occurring in the three "boxes" are reviewed here. To indicate the preconditionedstate of early winter, the potential densityat a depth of 100 m is shownfor November from the climatologicaldata of Levituset al. [1994b] and Levitusand Boyer[ 1994].Deep-reaching convection has been observedin the shadedregions. ally also in the Weddell Sea [see Gordon, 1982]. Con- Geologists speculate about possible North Pacific vection in these regions feeds the thermohaline Deep Water formationin pastclimates (for example,see circulation, the global meridional-overturningcircula- Mammerickx [1985]). There is some evidencefor en- tion of the ocean responsiblefor roughly half of the hancedconvection in the North Pacificat the last glacial polewardheat transportdemanded of the atmosphere- maximum(the •4C age reduction observed by Duplessy et oceansystem [see Macdonald and Wunsch,1996]. Warm, al. [1989], for example). However, the patterns of evi- salty water is drawn poleward, becomesdense in polar dence are contradictory,and as yet, there is no consen- seas,and then sinks to depth and flows equatorward. sus [see Keigwin, 1987; Curry et al., 1988; Boyle, 1992; Water massesmodified by deep convection in these Adkins and Boyle, 1997]. small regionsare taggedwith temperature and salinity In this review we discussthe dynamicsof the water valuescharacteristic of them (togetherwith other tracers masstransformation process itself, and its effect on the suchas tritium from the atomicweapon tests and freons stratification and circulation of its immediate environ- from industrialand householduse), allowingthem to be ment. Some of the relevant fluid mechanics, that of tracked far from their formation region. convectionin "open" domains,is reviewedby Maxworthy 37, 1 / REVIEWS OF GEOPHYSICS Marshall and Schott: OPEN-OCEAN CONVECTION ß 3 [1997]. Our scope here is more specificallyoceano- traordinary measuresare taken, only Rayleigh numbers graphic and similar to that of Killworth [1983]. Since in therange 109-10 •6 are attainable in thelaboratory or Killworth's review, however, there has been much in the computer,compared with 1026 in the ocean[see progressin our understandingof the kinematics and Whiteheadet al., 1996]. However, when laboratory and dynamicsof ocean convectionthrough new resultsfrom numerical experimentshave been used in concert and field experiments,through focused laboratory experi- scaledfor comparisonwith the observations,they have ments, and through numerical simulation. We bring led to great insight. things up to date and draw together threads from new It is interestingto note how little the developments observations,theory, and models. that will be describedhere have been influenced by Observationsof the processesinvolved in open-ocean "classicalconvection studies" that trace their lineage deep convectionbegan with the now classicalMediter- back to "Rayleigh-Benard"convection [Rayleigh, 1916; raneanOcean Convection (MEDOC) experimentin the Benard, 1900]. In the ocean the Rayleigh number in Gulf of Lions, northwesternMediterranean [MEDOC convectingregions is many orders of magnitudegreater Group,1970]. Rapid (in a day or so) mixingof the water than the critical value, and the convectionis fully turbu- column down to 2000 rn was observed.Strong vertical lent with transfer properties that do not depend, we currents,of the order of 10 cm s-1 associatedwith believe, on molecular viscositiesand diffusivities(see convective elements were observed for the first time section3.3). Even more importantly,the convectivepro- [Stommelet al., 1971]. Observationsof convectionprior cessin the oceanis localizedin space,making it distinct to MEDOC were limited to descriptionsof hydrostatic from the myriad classicalstudies of convectionrooted in changesand timescalesestimated from changesin the the Rayleigh problem (convectionbetween two plates inventoryof water massproperties. Since MEDOC, and extending laterally to _+•). As one might anticipate, particularly in the past decade, new technologieshave edge effectsand baroclinicinstability come to dominate led to different kindsof observationsand deeperinsights the evolvingflow fieldsand, as describedin section4, are into the processesat work. Moored acousticDoppler a distinctiveand controllingfactor in ocean convection. current profilers(ADCPs) were deployedin a convec- Finally, one of the goalsof the researchreviewed here tion regime over a winter period to documentthe three- is to improve the parametric representationof convec- dimensional(3-D) currents occurringin conjunction tion in large-scalemodels used in climate research,in with deepmixing. From a first ADCP experiment,Schott which one cannot,and doesnot wishto, explicitlyresolve and Leaman [1991]
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