The reduced overturning state of the North Atlantic observed

by the RAPID-MOCHA-WBTS array

DavidSmeedBillJohns,MollyBaringer,BenMoat,DarrenRayner,

ChrisMeinen,DenisVolkovGerardMcCarthy,

EleanorFrajka-Williams,andHarryBrydenJuly2018

Outline

•  Introduction•  TheRAPID26°NarraytimeseriesApril2004–February2017•  ThereducedstateofAMOC2009-2017compared

with2004-2008•  ChangesintheupperandlowerlimbsoftheAMOC•  ConcurrentchangesintheNorthAtlantic•  Conclusions

Smeed,D.A.,etal.(2018).TheNorthAtlanticOceanIsinaStateofReducedOverturning.GeophysicalResearchLetters,370(1962),1228–1533.

•  Timeseriesfrom2ndApril2004to28thFebruary2017•  Thecurrentarrayhas29mooringsandlandersand226instruments•  Todatetherehavebeen28cruises

andmorethan600daysatsea•  Almost300technicians,scientists,

students,officersandcrewhavetakenpartinUKRAPIDcruises

•  NextcruiseAutumn2018

TheRAPID26°Narray

TheRAPID26°Narray

Mean

13.3Sv

3.7Sv

Change

-2.7Sv

+0.3Sv

Decadalchange

Upper-limboftheAMOCTMOC-TEKM=TFS+TUMO=TWBC+TGYR

0 5 10 15 20 25 30 35 40 45Distance (km)

-5000

-4500

-4000

-3500

-3000

-2500

-2000

-1500

-1000

-500

0

Dep

th (m

)

WBADP WB0

WB1 WB2 WBH2 WB3

-0.15

-0.1

-0.05

0

0.05

0.1

0.15

0.2

0.25

0.3

TWBC=TFS+TANTTGYR=TUMO-TANT

Mean33.1Sv

-19.7Sv

13.3Sv

3.7Sv

Change-  0.7Sv

-2.0Sv

-2.7Sv

+0.3Sv

Decadalchange–upperlimb

Total17Sv

Thelower-limboftheAMOC

12Sv6Sv-1Sv

Mean33.1Sv

-19.7Sv

13.3Sv

3.7Sv

-5.8Sv

-12.0Sv

Change-0.7Sv

-2.0Sv

-2.7Sv

+0.3Sv

+2.0Sv

+0.1Sv

Decadalchange–lowerlimb

Baroclinictransportchange

-10000 -8000 -6000 -4000 -2000 0 2000Transport per depth (m 2s-1)

-6000

-5000

-4000

-3000

-2000

-1000

0

Dep

th (m

)

Transport change

TotalFl StEkmanWestEast

Totaltransportchange

-8000 -7000 -6000 -5000 -4000 -3000 -2000 -1000 0 1000 2000Transport per depth (m 2s-1)

-6000

-5000

-4000

-3000

-2000

-1000

0

Dep

th (m

)

Compensated transport change

TotalFl StEkmanWestEast

Streamfunctionchange

-3 -2.5 -2 -1.5 -1 -0.5 0 0.5Transport (Sv)

-6000

-5000

-4000

-3000

-2000

-1000

0

Dep

th (m

)

Streamfunction change

AllUpperLower

Deep(>2000db)densitieschangesaccountforabout20%changeatdepthofstreamfunctionmaximum

WatermasschangesconcurrentwithAMOCchange

2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018-10

-5

0

5

10

Tran

spor

t ano

mal

ly (S

v)

AMOC

2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018-0.05

0

0.05

Tem

pera

ture

ano

mal

ly (C

)

Temperture anomally on density surface

1800m3000m

ConcurrentchangesinthenorthAtlantic

Modelpredictedresponse

toincreasingAMOC

Zhang(2008)

ConcurrentchangesinthenorthAtlantic

Indicesofchange

2004 2006 2008 2010 2012 2014 2016

-2

-1

0

1

2

AMO

C-EK

MAN

(Sv)

-0.1

-0.05

0

0.05

0.1

SSH

inde

x (m

)

-20

-10

0

10

20

LHF

inde

x (W

m-2

)

-1-0.8-0.6-0.4-0.200.20.40.60.81

Tem

p in

dex

(°C)

1985 1990 1995 2000 2005 2010 2015

-0.5

0

0.5

Tem

p in

dex

(°C)

-20

-10

0

10

20

LHF

inde

x (W

m-2

)

-1

-0.5

0

0.5

1

SST

inde

x (°

C)

-0.05

0

0.05

SSH

inde

x (m

)

Two-yearfiltered

Indicesofchangesince1985

Historicalvaluesofindicessuggest:•  Variabilitywith8-10yearstimescale•  RAPIDobservationbeganatatimewhenAMOCwasatamaximum•  Inrecentyearstheindicesarethelowestsince1985

2004 2006 2008 2010 2012 2014 2016

-2

-1

0

1

2

AMO

C-EK

MAN

(Sv)

-0.1

-0.05

0

0.05

0.1

SSH

inde

x (m

)

-20

-10

0

10

20

LHF

inde

x (W

m-2

)

-1-0.8-0.6-0.4-0.200.20.40.60.81

Tem

p in

dex

(°C)

1985 1990 1995 2000 2005 2010 2015

-0.5

0

0.5

Tem

p in

dex

(°C)

-20

-10

0

10

20

LHF

inde

x (W

m-2

)

-1

-0.5

0

0.5

1

SST

inde

x (°

C)

-0.05

0

0.05

SSH

inde

x (m

)

3-yearrunningmeans

Summary

www.rapid.ac.uk/rapidmocdas@noc.ac.uk

•  TheRAPID26°NtimeseriesnowspansApril2004–February2017

•  Since2009theAMOChasbeen2.5Svlessthanfrom2004to2008

•  Changesinthelowerlimbarealmostallinthelower-NADW

•  TherehavebeenconcurrentchangesintheNorthAtlanticthataresimilartothosepredictedbyclimatemodelsinresponsetoareducedAMOC