Helmuth Thomas 1 ,

Helmuth Thomas1, Friederike Prowe1,4, Ivan D. Lima2, Scott C. Doney2, Rik Wanninkhof3,

Richard Greatbach1,4, Antoine Corbière5 & Ute Schuster6

1: Dalhousie University, Halifax, Canada. 2: Woods Hole Oceanographic Institution, Woods Hole MA, USA. 3: NOAA, Miami, USA. 4: now at IfM-Geomar, Kiel Germany. 6: Laboratoire

d'Océanographie et du Climat: Expérimentations et Approches

Numériques, Pairs France. 6:University of East Anglia, Norwich, UK

Temporal variability of the CO2 system in the North Atlantic Ocean

Observations in the North Atlanticduring 1995-2002/4

BATS, BermudaΔpCO2 constantBates, 2001/7

North SeaΔpCO2 declining

VOS line UK –CaribbeanΔpCO2 decliningSchuster and Watson2007Sub Polar North AtlanticΔpCO2 decliningOmar and Olsen, 2006

South of GreenlandΔpCO2 decliningCorbiere et al., 2007

North Atlantic Drift RegionΔpCO2 decliningLefèvre et al., 2004

NW AtlanticΔpCO2 increasingLueger et al., 2006

Thomas et al., 2008, GBC

•Simulations from 1958-2004 •Community Climate System Model, POP

coarse resolution•Ecosystem model coupled to modified

OCMIP-II biogeochemistry•NCEP/NCAR reanalysis forcing

•Further reading: Doney et al., 2004, Moore et al. 2004, Yeager et al., 2006

•pCO2 decomposition (DICnorm, ATnorm, T, S) •Trend regression analysis

•Anthropogenic and preindustrial runs

Simulations for the North Atlantic Ocean

What is the driver for the observed CO2 flux variability in the North Atlantic Ocean?

situin

situinnorm Sal

DICDIC

35*

.Thomas et al., 2008, GBC

Simulations vs. Observations for the North Atlantic Ocean

Corbière et al.

Model

water +2.5

ppm yr-1

+3.0

ppm yr-1

Atmos. +1.6

ppm yr-1

+1.6

ppm yr-1

temp.-

pCO2

+ +1.2

ppm yr-1

DIC +0.6

μmol (kg yr)-1

AT -0.3

μmol (kg yr)-1 Thomas et al., 2008, GBC

Period 1993-2003 2001-2008

Atm 1.9 µatm/yr 2.1 µatm/yr

Ocean 3.6 µatm/yr 5-7 µatm/yr

Why ? Warming +DIC/-TA

Full story presented by

Corbière et al. (Poster, CT1)

Ocean CO2 trends in the North Atlantic Subpolar Gyre :

winter 1993-2003/2001-2008 (based on DIC/TA Suratlant data)

Skògafoss

Nuka Arctica

1994-1995

2002-2005Schuster and Watson, 2007

JGR

mean

CO2 uptake in 1994/5 and 2002/4

[mo

l C

O2

m-2 y

r-1]

0

1.4

The overall sink (14-65 degrees N):1995: 0.40 PgC (Takahashi climatology)2005: 0.24 Pg C for 2005

40% decline in CO2 uptake?

UK-Caribbean route


One conclusion:We need to maintain an observing system.

Tool:Trend regression analysisinstead of differences.


Tool:Trend regression analysisinstead ofdifferences


neutr./neg. NAO

Most of the observationscover the 95-04 period

ΔpCO2 trends

long term

pos. NAO

Note changein scale!

Thomas et al.2008, GBC

Outline

1: Fundamentalcontrol

2: Adjustments

3: Overlayingperturbance

Real World Ocean

Building the house

You move in

Your in-laws move in

NAO & the subpolar and subtropical gyres

Positive NAO:Strong westerliesStrong NAC

Negative NAO:weaker westerlies

weaker NAC

http://www.noc.soton.ac.uk

↑ http://www.ldeo.columbia.edu/res/pi/NAO/http://www.ldeo.columbia.edu/res/pi/NAO/

↑

1: Fundamental control

North Atlantic Oscillation during the past two decades

15

20

MO

C[S

v]

-5

0

5

Win

ter

NA

O

1,980 1,985 1,990 1,995 2,000 2,005



15

20

MO

C[S

v]

-5

0

5

Win

terN

AO

1,980 1,985 1,990 1,995 2,000 2,005

positive NAO(1989-1995 average)

Response of surface ocean to NAO forcing

Surface velocity

(each 2nd grid point shown)

x

x


Subpolar gyre

Subtropical gyre

Labrador Current


Neutral - positive NAO(1996/2004av.) - (1989/1995av.)


Surface velocity difference

Subpolar gyre

(each 2nd grid point shown)

Subtropical gyre


Labrador Current





Annual salinity anomalies:

1: Fundamental control (one more detail)

Expansion of subtropical gyre during positive NAO

Relevance western subpolar gyre:

Labrador Current cannot move south, it is diverted into the subpolar gyre



Pos. NAO

Neutr./neg. NAO


DICnorm and Salinity in the North Atlantic1: Fundamental control

Pos. NAONeutr./neg. NAO

North Atlantic Current is fed by low DICnorm / high salinity water:Northward transport of aCO2 deficiency


Main driver of variability:DICnorm

(AT,norm exerts minor control)

Example:2 stations atthe easternand westernsubpolar gyre

WE

ST


EA

ST

Note the phase lag between west and east


Large scale control:NAC and its low DICnorm

Pos. NAO:•Fast NAC•High supply of low DICnorm

•High CO2 uptake

Neut. / Neg NAO:•Slow NAC•Low supply of low DICnorm

•low CO2 uptake

Long term:no significant trend

Corrected for anthropogenic CO2!!

NAO+

NAO-/


Longterm


Conclusions 1• Key process:

– NAC exports CO2 sink from the tropics northward

• Pos. NAO:– Fast NAC– High supply of low DICnorm

– Higher CO2 uptake

• Neut. / Neg NAO:– Slow NAC– Low supply of low DICnorm

– lower CO2 uptake

• Long term:– no significant trend

• Temporal aspect:– Observed response depends on distance to NAC source region


however:

2: Adjustments

Effects of atmospheric temperature

Pos. NAO

neutr./neg. NAO

Long termAtm. Temperature effects:•Fast response•Spatially not uniform across the basin•Particularly dominant in the NW Atlantic

Note changein scale!


Perturbing process:Great Salinity anomalies

Annual salinity anomalies:

Annual DICnorm anomalies:

3. Overlaying perturbance

Recall:1. Fundamental control:Pos. NAO = enhanced northward flow of low DICnorm Thomas et al., 2008, GBC

Do we still expect correlations?(??)

-1.0

-0.5

0.0

0.5

1.0

R

-2 0 2 4

Lag [yr]

Northern North Atlantic

Western Subpolar Gyre

CorrelationCO2 air-sea flux vs. NAO

>60ºN

15

20

MO

C[S

v]

-5

0

5W

inte

rN

AO

1,980 1,985 1,990 1,995 2,000 2,005

Annual SST anomalies:

+ NAO

neutr./neg. NAO

p<0.05


Outline

1: Fundamentalcontrol

2: Adjustments

3: Overlayingperturbance

Real World Ocean

Building the house

Response of surface

circulation to NAO

You move in Atmospheric Temperature

Your in-laws move in

Great Salinity Anomaly

Conclusions• Key process:

– NAC exports CO2 sink from the tropics northward

• Pos. NAO:– Fast NAC– High supply of low DICnorm

– Higher CO2 uptake– Strong cooling in NW Atlantic

• Neut. / Neg NAO:– Slow NAC– Low supply of low DICnorm

– lower CO2 uptake– Strong warming in NW Atlantic

• Alternative view:– Partial redistribution of North Atlantic CO2 sink between

subpolar and subtropical gyres

• Long term:– no significant trend other than (global) warming and rising

atmos. CO2.

• Perturbation:– Great Salinity Anomalies

Consequences are timevariant across the basinbecause of water masstravel time.

High correlations unlikely.

Reference:

• Thomas, H., F. Prowe, I.D. Lima, S.C. Doney, R. Wanninkhof, R.J. Greatbatch, A. Corbière and U. Schuster (2008). Changes in the North Atlantic Oscillation influence CO2 uptake in the North Atlantic over the past two decades Global Biogeochemical Cycles, in press.

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Helmuth Thomas 1 ,