simulation the impact of shifts in southern ocean …simulation the impact of shifts in southern...
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Simulation the impact of shifts in Southern Ocean westerliesat LGM on ocean physics and atmospheric CO2
Peter Kohler, Christoph Volker, Xu Zhang, Gregor Knorr, Gerrit LohmannAlfred Wegener Institute, Helmholtz Centre for Polar and Marine Research P.O. Box 12 01 61, D-27515 Bremerhaven, Germanyemail: [email protected]
AbstractWe explore the impact of a latitudinal shift in the we-sterly wind belt over the Southern Ocean (SO) on theAtlantic meridional overturning circulation (AMOC) andon the carbon cycle for Last Glacial Maximum back-ground conditions using a state-of-the-art ocean gene-ral circulation model. For this “westerly wind hypothesis”(Toggweiler et al. 2006) we find that a southward shift inthe westerly winds leads to an intensification of the AMOC(northward shift to a weakening). This agrees with otherstudies (Sijp & England 2009) starting from pre-industrialbackground, but the responsible processes are different.During deglaciation a gradual shift in westerly winds mightthus be responsible for a part of the AMOC enhancement,which is indicated by various studies. The net effects of thechanges in ocean circulation lead to a rise in atmosphericpCO2 of less than 10 µatm for both a northward and asouthward shift in the winds. For northward shifted windsthe zone of upwelling of carbon and nutrient rich watersin the Southern Ocean is expanded, leading to more CO2
out-gassing to the atmosphere but also to an enhancedbiological pump in the subpolar region. For southward shif-ted winds the upwelling region contracts around Antarcti-ca leading to less nutrient export northwards and thus aweakening of the biological pump. A shift in the southernhemisphere westerly wind belt is probably not the domi-nant process which tightly couples atmospheric CO2 riseand Antarctic temperature during deglaciation which issuggested by the ice core data.
Motivation
100
150
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300
100
150
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CO
2[p
pmv]
800 600 400 200 0
Time [kyr BP]
-9
-6
-3
0
3
6
9
12
15
18
T[K
]
CO2T
180
200
220
240
260
280
300
180
200
220
240
260
280
300
CO
2[p
pmv]
-10 -8 -6 -4 -2 0 2 4 6
T [K]
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.r2 = 75%
Ice core data of CO2 and Antarctic temperature.
Scenarios
80 60 40 20 0 20 40 60 8010
5
0
5
10
Latitude (deg N)
zona
l win
d sp
eed
(m/s
)
80 60 40 20 0 20 40 60 800.1
0.05
0
0.05
0.1
0.15
Latitude (deg N)
zona
l win
d st
ress
(N/m
2 )
Latitude (deg N) Latitude (deg N)
CTRL:broken, LGM:bold, shift10S:blue, shift10N:red.We shift wind, not wind stress,.
because of fully-prognostic sea-ice model.
Key Points
(1) We used the full OGCM MITgcm, forced with LGM sur-face fields from an atmosphere-ocean coupled GCM runof COSMOS (Zhang et al. 2013).
(2) Southward shifted westerly winds at LGM increase theAMOC: decrease in temperature and salinity in interme-diate waters (AAIW, SAMW) accompanied by increasednorthward Ekman transport ⇒ stronger SO upwelling.AMOC increase is driven by pulled upwelling in the South,not by pushed down-welling in the north.
(3) Same AMOC change in (d’Orgeville et al. 2010) for pre-industrial background, but for different reasons: strongerAgulhas leakage⇒ stronger influx of warm and salty wa-ter in South Atlantic, excess heat lost at northward trans-port, but excess salinity finally leads to stronger deep wa-ter formation in North Atlantic (more northern push thansouthern pull).
(4) Opposing effects on different carbon pumps:
(5) Northward: Extension of upwelling area in SO leads tolarger CO2 out-gassing. Enhanced nutrient upwelling &transport north ⇒ stronger biological pump in subpolarregion, but less than what was released to the atmospherefurther south ⇒ net gain of CO2 in atmosphere.
(6) Southward: Contraction of upwelling area in SO reducesamount of upwelling nutrient that travel north, weakeningbiological pump in the subpolar region. Out-gassing ofCO2 is changed only slightly ⇒ atmospheric CO2 rises.
Sum
mar
y
−
shift10S
70°S60°S50°S40°S30°S 0° 60°N
AMOC+
CO2+
ET+ AL+ACC+
T−
S−
weaker biological pumpreduced outgassing in compressed upwelling regionrise in atmospheric CO2
northwards
CO2+CO2+
winds
no shift
surfaceocean
deepocean
sphereatmo−
southwards
shift10N
70°S60°S50°S40°S30°S 0° 60°N
AMOC−
CO2+
+
ET− AL−ACC−
T+
S+
stronger biological pump in midlatitudeshigher CO2 outgassing by a broaderupwelling regionrise in atmospheric CO2
ACC: Antarctic Circumpolar Current, ET: Ekman transport, AL: Agulhas leakage.
PhysicsSouthward (shift10S–LGM) Northward (shift10N–LGM)
AM
OC
(Sv)
Tem
per
atu
re(K
)Salin
ity
(PSU
)W
ind
Shift
Dep
enden
cies
.
10 5 0 5 1012
12.5
13
13.5
14
14.5
15
15.5
16
wind shift amplitude (degree latitude)
Atla
ntic
mer
idio
nal o
vertu
rnin
g (S
v)
10 5 0 5 1050
100
150
200
250
wind shift amplitude (degree latitude)
Dra
ke p
assa
ge tr
ansp
ort (
Sv)AMOC ACC@Drake P.
−10 −5 0 5 10260
265
270
275
wind shift amplitude (degree latitude)
atm
osph
eric
pC
O2 (
µatm
)
−10 −5 0 5 103.85
3.9
3.95
4
4.05
wind shift amplitude (degree latitude)
part
icul
ate
expo
rt (
Pg
C/y
r)
pCO2 export POC
−10 −5 0 5 100
10
20
30
40
50
60
70
80
wind shift amplitude (degree latitude)
Agu
lhas
leak
age
(Sv)
−10 −5 0 5 10−5
0
5
10
15
20
25
30
35
wind shift amplitude (degree latitude)
Nor
thw
ard
Ekm
an tr
ansp
ort (
Sv)
at 5
5 S
Agulhas leakage Ekman transport
C CycleSouthward (shift10S–LGM) Northward (shift10N–LGM)D
IC(m
mol/
m3)
PO
3−
4(m
mol/
m3)
Gas
exch
ange
(red
:outg
ass
ing)
(gC/m
2/yr
)
−40 −30 −20 −10 0 10 20 30
0o 60oE 120oE 180oW 120oW 60oW 0o
60oS
30oS
0o
30oN
60oN
−30 −20 −10 0 10 20 30 40
0o 60oE 120oE 180oW 120oW 60oW 0o
60oS
30oS
0o
30oN
60oN
Exp
ort
pro
duct
ion
(gC/m
2/yr
)
−8 −6 −4 −2 0 2 4 6 8 10
0o 60oE 120oE 180oW 120oW 60oW 0o
60oS
30oS
0o
30oN
60oN
−10 −8 −6 −4 −2 0 2 4 6 8 10
0o 60oE 120oE 180oW 120oW 60oW 0o
60oS
30oS
0o
30oN
60oN
References:d’Orgeville et al (2010) On the control of glacial-interglacial atmospheric CO2 variations by the Southern Hemisphere westerlies,. Geophysical Research Letters 37:L21703.Sijp & England (2009) Southern Hemisphere Westerly Wind Control over the Oceans Thermohaline Circulation,. Journal of Climate, 22:1277pp.Toggweiler et al (2006) Midlatitude westerlies, atmospheric CO2, and climate change during the ice ages,. Paleoceanography, 21:PA2005.Zhang et al. (2013) Different ocean states and transient characteristic in LGM simulations and implications for deglaciation,. Climate of the Past, in press.
Literatur
[d’Orgeville et al. 2010] d’Orgeville, M., Sijp, W. P., England, M. H., & Meissner, K. J. 2010. On the control of glacial-interglacialatmospheric CO2 variations by the Southern Hemisphere westerlies. Geophysical Research Letters, 37:L21703.
[Sijp & England 2009] Sijp, W. P. & England, M. H. 2009. Southern Hemisphere Westerly Wind Control over the Ocean’s ThermohalineCirculation. Journal of Climate, 22:1277–1286.
[Toggweiler et al. 2006] Toggweiler, J. R., l. Russell, J., & Carson, S. R. 2006. Midlatitude westerlies, atmospheric CO2, and climatechange during the ice ages. Paleoceanography, 21:PA2005, doi: 10.1029/2005PA001154.
[Zhang et al. 2013] Zhang, X., Lohmann, G., Knorr, G., & Xu, X. 2013. Different ocean states and transient characteristic in LGMsimulations and implications for deglaciation. Climate of the Past, Page in press.