Antarctic Bottom Water in CMIP5 models

226 Pages·2015·31.3 MB·English

Antarctic Bottom Water
in CMIP5 models:
characteristics, formation, evolution
A thesis submitted to the School of Environmental Sciences of the
University of East Anglia in partial fulfilment of the requirements for the
degree of Doctor of Philosophy
ByCe´lineHeuze´
March2015
(cid:13)c This copy of the thesis has been supplied on condition that anyone who consults it
is understood to recognise that its copyright rests with the author and that use of any
information derived there from must be in accordance with current UK Copyright Law.
Inaddition,anyquotationorextractmustincludefullattribution.

(cid:13)c Copyright 2015
by
Ce´line Heuze´
iii

Abstract
Observations suggest that the properties of Antarctic Bottom Water (AABW) are chang-
ing, causing significant steric sea level rise. Understanding the causes of these changes
is critical for projections of future sea level, yet previous generations of climate models
failed to represent AABW accurately. Present-day biases in AABW potential tempera-
ture,salinityanddensityareassessedformodelsfromtheCoupledModelIntercompari-
sonProjectphase5(CMIP5).
CMIP5 models either have inaccurate bottom water properties in the present-day
SouthernOceanorformAABWviathewrongprocess,openoceandeepconvectioninthe
subpolar gyres. Under climate change scenarios, open ocean deep convection is respon-
sible for bringing the warming signal to the Southern Ocean abyss. It is then advected
equatorwards by AABW transport. In turn, the decrease in density associated with the
warmingresultsinaweakeneddensity-drivenAABWtransport. Themeanof24CMIP5
models projects a mean global steric sea level rise of 3.8 mm by 2100 for the abyssal
500 m, albeit with a large uncertainty due to the cross-model disagreement on bottom
salinitychanges.
The parameterisation of overflows does not show an improvement in AABW prop-
erties. Sensitivity experiments are performed on the model HadGEM3. The trigger for
deep convection in the Weddell Sea, a positive sea ice anomaly leading to anomalies in
themixedlayerdepth,isidentified. Varyingthreeverticalmixingparametersmodifiesthe
originalmixedlayeranomaly,leadingtoarangeofresponsesfromarresteddeepconvec-
tion to deep convection over the entire Weddell Sea. In the arrested convection simula-
tions,theAntarcticCircumpolarCurrentstrengthisimprovedandtheAABWproperties
and North Atlantic Deep Water formation are unchanged. These experiments indicate a
possible way to stop Weddell Sea deep convection in models, to improve their Southern
Oceanrepresentation.
v

Acknowledgements
I dedicate this thesis to my grandfather who left us at the end of my first year of PhD. I
wouldhavenevergonethatfarwithouthim. MercipourtoutPapy!
I sincerely thank everyone who had to deal with grumpy me these last years, mostly
the3.17crew,pastandpresent. InparticularNickforprovingmethatitisnotthenumber
ofhoursspentinfrontofthecomputerthatmatters,andChris(S)forthepatienttechnical
support. Likewise,abigthankyoutoJennyandSunkefortheregularMatlabdebugging,
and to Clare and Chris (WB) for the safety at sea training. Dankjewel Karin for great
advice,Tahmeenaforexcellentfoodandmind-openingdiscussions,andAmeeandothers
for following me in the ClimateSnack adventure. Un grand merci a` la French clique,
Maud,Bastien,VivianneetAure´lien–expressingstrongfeelingsisalwaysbetterinyour
mothertongue,withsomewineandcheese. ThankyoutofriendsImadeduringcruises,
now-friends I met at conferences, and all the people who lightened these last 3+ years
butthatIwon’tnamebecauseI’dliketokeepthissectionbrief(andbyfearofforgetting
someone). Mybiggestthanksgotomymotherforhersupport,andforneverhangingup
onmeevenwhenIdeservedit,andformypartnerincrimeTomforforcingmetoalways
lookatthatsillybrightsideoflife.
Obviously, I would not have succeeded in finishing this thesis without some help.
Thanks a lot to the HPC and IT-unix teams for their relentless technical support and for
not getting too mad at me when I may have crashed the cluster (twice). I want to thank
the Met Office people for trying to make me feel part of the team at each of my visits
despite my rain-induced moodiness, and in particular my supervisor there, Jeff Ridley,
for his confusing yet helpful feedbacks. Finally, I owe most thanks to my supervisors at
UEA, Karen Heywood and David Stevens, for bringing me to Antarctica and letting me
gotoconferencesaroundtheworld,andalsoforcorrectingthesameerrorsoverandover
againandhelpingmetobecomeclearer(Ihope).
Andnow,forsomethingcompletelydifferent!
vii

Contents
Abstract v
Acknowledgements vii
1 Introduction 1
1.1 AntarcticBottomWaters . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.1.1 Definition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.1.2 FormationofAntarcticBottomWaters . . . . . . . . . . . . . . . 3
1.1.3 CirculationofAntarcticBottomWater . . . . . . . . . . . . . . . 6
1.1.4 Observations of Antarctic Bottom Water and climatologies used
inthisthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
1.1.5 ModesofvariabilityoftheSouthernOcean . . . . . . . . . . . . 11
1.1.6 Observationsofchanges . . . . . . . . . . . . . . . . . . . . . . 13
1.2 GlobalClimateModels . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
1.2.1 BasicdescriptionofGlobalClimateModels . . . . . . . . . . . . 16
1.2.2 CMIP5andwhattodowithaGCM . . . . . . . . . . . . . . . . 18
1.2.3 SomelimitationsofCMIP5modelstobearinmindwhilereading
thisthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
1.3 Thisthesis: researchinterestandaims . . . . . . . . . . . . . . . . . . . 21
2 SouthernOceanbottomwatercharacteristicsinCMIP5models 23
2.1 Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
2.2 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
2.3 Methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
2.4 Results. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
ix
2.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
2.6 SupplementarymaterialforSouthernOceanbottomwatercharacteristics
inCMIP5models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
2.6.1 Bottomsalinitymaps . . . . . . . . . . . . . . . . . . . . . . . . 37
2.6.2 Aremodelsformingtheirdensebottomwaterontheshelf? . . . . 40
2.6.3 Neither shelf export nor open ocean deep convection - how to
formbottomwater . . . . . . . . . . . . . . . . . . . . . . . . . 48
2.6.4 Definingthemixedlayerdepth . . . . . . . . . . . . . . . . . . . 51
2.7 ConcludingremarksandmotivationforChapter3 . . . . . . . . . . . . . 57
3 ChangesinglobaloceanbottompropertiesandvolumetransportsinCMIP5
modelsunderclimatechangescenarios 59
3.1 Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
3.2 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
3.3 DataandMethods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
3.3.1 CMIP5models . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
3.3.2 Oceanpropertiesandsealevel . . . . . . . . . . . . . . . . . . . 64
3.3.3 Volumetransports . . . . . . . . . . . . . . . . . . . . . . . . . 66
3.4 Results. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68
3.4.1 Bottompropertychanges . . . . . . . . . . . . . . . . . . . . . . 68
3.4.2 Meanvolumetransports: AMOC,ACCandSMOCs . . . . . . . 77
3.4.3 Relationships between the changes in bottom properties and the
transports . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
3.4.4 DeepconvectionintheNorthAtlantic . . . . . . . . . . . . . . . 88
3.5 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93
3.6 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
3.7 Appendix: Abriefcomparisonoftheclimatechangesignalandthemodel
driftinCanESM2,GFDL-ESM2GandMIROC-ESM-CHEM. . . . . . . 99
3.8 Canthepropertychangesbeinferredfromthehistoricalbiases? . . . . . 102
4 AcloserlookattwopuzzlingCMIP5models: CCSM4andinmcm4 111
4.1 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111
x