On the lifecycle of Antarctic Bottom Water

269 Pages·2017·18.64 MB·English

by  Casimir De Lavergne

T
HÈSE DE DOCTORAT
D ’U P M C
E L NIVERSITÉ IERRE ET ARIE URIE
Spécialité : Océanographie physique
Écoledoctorale: “129-Sciencedel’environnement”
réaliséeau
Laboratoire d’Océanographie et du Climat
présentéepar
Casimir de LAVERGNE
pourobtenirlegradede:
DOCTEUR DE L’UNIVERSITÉ PIERRE ET MARIE CURIE
Titredelathèse:
Eléments du cycle de vie de l’Eau Antarctique de Fond
Soutenance prévue le 23/09/16
devantlejurycomposéde:
M. DavidMarshall (UniversityofOxford) Rapporteur
M. JonasNycander (MISU,Stockholm) Rapporteur
Mme PascaleBouruet-Aubertot (UPMC,Paris) Examinateur
M. GillesReverdin (LOCEAN,Paris) Examinateur
M. BrunoFerron (LPO,Brest) Examinateur
M. GurvanMadec (LOCEAN,Paris) Directeurdethèse
M. PatriceKlein (Caltech,Pasadena) Co-directeurdethèse

Eléments du cycle de vie de l’Eau Antarctique de Fond
Résumé : L’Eau Antarctique de Fond constitue la principale masse d’eau océanique par son vol-
ume, et nourrit la composante la plus profonde et la plus lente de la circulation océanique. Les
processus qui régissent son cycle de vie sont donc clé pour la capacité de stockage de l’océan en
carbone et chaleur aux échelles centennales à multi-millénaires. Cette thèse tente de caractériser
et quantifier les principaux processus responsables de la destruction (synonyme d’allègement et
de remontée) de l’Eau Antarctique de Fond dans l’océan abyssal. A partir d’une estimée issue
d’observations de la structure thermohaline de l’océan mondial et de diagnostics fondés sur le
budget de densité des eaux profondes, les rôles respectifs du chauffage géothermal, du mélange
turbulentpardéferlementd’ondesinternesetdelagéométriedesbassinssontévalués. Ilestmon-
tré que la géométrie de l’océan gouverne la structure de la circulation de l’Eau Antarctique de
Fond. La contribution du déferlement des ondes internes, bien que mal contrainte, est estimée
insuffisante pour maintenir un rythme de destruction de l’Eau Antarctique de Fond comparable à
celui de sa formation. Le chauffage géothermal a quant à lui un rôle important pour la remontée
deseauxrecouvrantunelargesurfacedulitocéanique. Lesrésultatssuggèrentuneréévaluationde
l’importancedumélangeauniveaudesdétroitsetseuilsprofonds,maisaussidurôlefondamental
delaformedesbassins,pourl’allègementetletransportdeseauxabyssales.
Mots clés : Masses d’eau, Océan abyssal, Circulation thermohaline, Chauffage géothermal,
Mélange,Ondesinternes,Turbulence,Paramétrisation,Modélisation,NEMO
On the lifecycle of Antarctic Bottom Water
Abstract : Antarctic Bottom Water is the most voluminous water mass of the World Ocean, and
it feeds the deepest and slowest component of ocean circulation. The processes that govern its
lifecyclearethereforekeytotheocean’scarbonandheatstoragecapacityoncentennialtomulti-
millennialtimescales. Thisthesisaimsatcharacterizingandquantifyingprocessesresponsiblefor
thedestruction(synonymousoflighteningandupwelling)ofAntarcticBottomWaterintheabyssal
ocean. Usinganobservationalestimateoftheglobaloceanthermohalinestructureanddiagnostics
based on the density budget of deep waters, we explore the roles of basin geometry, geothermal
heatingandmixingbybreakinginternalwavesfortheabyssalcirculation. Weshowthattheshape
of ocean basins largely controls the structure of abyssal upwelling. The contribution of mixing
powered by breaking internal waves, though poorly constrained, is estimated to be insufficient to
destroyAntarcticBottomWaterataratecomparabletothatofitsformation. Geothermalheating
plays an important role for the upwelling of waters covering large seafloor areas. The results
suggestareappraisaloftheroleofmixingindeepstraitsandsills,butalsoofthefundamentalrole
ofbasingeometry,forthelighteningandtransportofabyssalwaters.
Keywords:Watermasses,Abyssalocean,Thermohalinecirculation,Geothermalheating,Mixing,
Internalwaves,Turbulence,Parameterization,Modelling,NEMO

Contents
Introduction 1
Physicalproblem 7
I OntheconsumptionofAntarcticBottomWaterintheabyssalocean 37
II Theimpactofavariablemixingefficiencyontheabyssaloverturning 67
III Geometriccontrolsofthemeridionaloverturning 91
IV Observingandmodellingthebottomboundarylayer: challenges 109
V Towardsaparameterizationofinternalwave-drivenmixing 119
V.1 Two-dimensionalmappingofinternaltideenergysinks . . . . . . . . . . . . 121
V.2 Parameterizinginternalwave-drivenmixing . . . . . . . . . . . . . . . . . . 127
Conclusionsandperspectives 145
Appendix 159

Introduction
Earth’s climate system is composed of different interconnected media: the atmosphere,
theocean,seaice,landice,theseabed(bothsedimentsandbedrock)andcontinentalsurfaces
(soils,vegetation,riversandlakes). Climatedynamicsaredrivenbyexchangesofenergy,mo-
mentumandmatterbetweenthesemediaandwiththeouterspace. Describing,understanding
andmodellingclimatethereforehingesuponcharacterizingandquantifyingtheseexchanges.
Centraltotheclimatesystemistheatmosphere,whichregulatesradiativeexchangeswith
the outer space, shuffles heat and freshwater around the globe and across media, and largely
sets surface properties of continents and oceans. But the atmosphere’s ability to exchange
and redistribute properties over large spatial and short temporal scales goes at the expense of
its storage capacity and memory, both of which are limited relative to other media (Fig.1).
Instead, storage and memory functions are largely attributes of the other main fluid of the
climatesystem: theocean.
Twodefiningpropertiesoftheoceandistinguishitfromtheoverlyingatmosphere: itsther-
malinertiaanditsopacity. Becausetheoceanisnearly300timesheavierthantheatmosphere,
and because it costs four times more energy to warm by one Celsius degree a given mass of
waterrelativetothesamemassofair, thetotalheatcapacityoftheoceanisover1,000times
that of the atmosphere. Its total heat capacity is also much larger than that of Earth’s frozen
water,whosevolumeisonlyabout2%thatoftheocean. Hence,theoceanistheprimaryheat
reservoiroftheclimatesystem.
1
INTRODUCTION
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Figure1: Typicalequilibrationtimeandtotalheatcapacityofvariouscomponentsoftheclimatesys-
tem. The total heat capacity [J/K] is the amount of energy required to raise by one Celsius degree
the mean temperature of each component. The equilibration timescale is the typical duration for a
component to adjust to modified boundary conditions. Values from McGuffie and Henderson-Sellers
(2014).
Yet most of this reservoir is not readily accessible. Indeed, whereas solar radiation is
abletopenetratethroughouttheatmosphere,theopacityofseawaterpreventsitfromreaching
ocean depths greater than 200 meters. As a result, the incoming solar energy warms pref-
erentially the land and ocean surface. This surface warming is a destabilizing force for the
atmosphere but a stabilizing force for the ocean, because gravity needs warmer, lighter fluid
torestabovecolder,denserfluid. Solarheatingthusstimulatesarapidverticalrearrangement
of atmospheric properties, but inhibits this vertical redistribution and the downward pene-
tration of atmospheric forcing in the ocean. The ocean’s opacity thus contributes to isolate
the large deep ocean reservoir from the remainder of the climate system. Together with the
ocean’stremendousthermalinertia,theinefficientoceanicredistributionexplainswhyitmay
2