Simulation the impact of shifts in Southern Ocean westerlies at LGM on ocean physics and atmospheric CO 2 Peter K¨ ohler, Christoph V¨ olker, Xu Zhang, Gregor Knorr, Gerrit Lohmann Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research P.O. Box 12 01 61, D-27515 Bremerhaven, Germany email: [email protected] Abstract We explore the impact of a latitudinal shift in the we- sterly wind belt over the Southern Ocean (SO) on the Atlantic meridional overturning circulation (AMOC) and on 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 in the westerly winds leads to an intensification of the AMOC (northward shift to a weakening). This agrees with other studies (Sijp & England 2009) starting from pre-industrial background, but the responsible processes are different. During deglaciation a gradual shift in westerly winds might thus be responsible for a part of the AMOC enhancement, which is indicated by various studies. The net effects of the changes in ocean circulation lead to a rise in atmospheric pCO2 of less than 10 μatm for both a northward and a southward shift in the winds. For northward shifted winds the zone of upwelling of carbon and nutrient rich waters in the Southern Ocean is expanded, leading to more CO2 out-gassing to the atmosphere but also to an enhanced biological 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 a weakening of the biological pump. A shift in the southern hemisphere westerly wind belt is probably not the domi- nant process which tightly couples atmospheric CO2 rise and Antarctic temperature during deglaciation which is suggested by the ice core data. Motivation 100 150 200 250 300 100 150 200 250 300 CO 2 [ppmv] 800 600 400 200 0 Time [kyr BP] -9 -6 -3 0 3 6 9 12 15 18 T [K] CO2 T 180 200 220 240 260 280 300 180 200 220 240 260 280 300 CO 2 [ppmv] -10 -8 -6 -4 -2 0 2 4 6 T [K] . . . . . . . . . .. .. . . . . . ... . . . . . . . . . . . . . . . . .. . . . . . . . . .. . . . . . . . . . . .... . .. . . . . . . . . . . . ..... . . . . . . . . . . . . . . . . ... . .. . . . . . ... .... . . . . . .. .. . . .. . . . . . . .. . . . . . . . . . . . . . . .. . . .. .. .. . ... ... . . . .... . .. . .. ... . . . . .. . . . . . . . . . . . . . . .. . . . . .. .. .. . . .. . . . . . . . .... . . . . . . . . . .. . . . . . . .. . .. . .. . . . . . . . . . . . . .. .. . . . .. . . . . . . . . .. . . . .. . . . .. . . . . . . . . . . .. . . . . . . . . . . . . . . .. . . . . . . . . .. .. . ... . . . .. . .. . .. . . .. . .. . . . . . . . . . . . . . .. . .. . . . . . . . . . . . . . .. . .. . . . . .. . . . . . . . .. . .. . . . . . . .. . . . . .. . . . .. . . . . . .. . . . . . . .. . . .. . .. .. . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . .. . . . . . . . . . . . . . . . . . . . . . . .. ... . . . . . . . . .. . . . .. . . .. .. . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . .. . . . . . . .. . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . .. . . . . . . . . . . . .. . . . . . . . . . . . . .. . . . .. . . . . .. . . . . . . . . .. . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . .. . .. . . . . . . .. . .. . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . .. . . . . . . . . . r 2 = 75% Ice core data of CO2 and Antarctic temperature. Scenarios ï80 ï60 ï40 ï20 0 ï10 ï5 0 5 10 Latitude ( zonal wind speed (m/s) ï80 ï60 ï40 ï20 0 ï0.1 ï0.05 0 0.05 0.1 0.15 Latitude ( zonal wind stress (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 run of COSMOS (Zhang et al. 2013). (2) Southward shifted westerly winds at LGM increase the AMOC: decrease in temperature and salinity in interme- diate waters (AAIW, SAMW) accompanied by increased northward 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: stronger Agulhas 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 than southern pull). (4) Opposing effects on different carbon pumps: (5) Northward: Extension of upwelling area in SO leads to larger CO2 out-gassing. Enhanced nutrient upwelling & transport north ⇒ stronger biological pump in subpolar region, but less than what was released to the atmosphere further south ⇒ net gain of CO2 in atmosphere. (6) Southward: Contraction of upwelling area in SO reduces amount of upwelling nutrient that travel north, weakening biological pump in the subpolar region. Out-gassing of CO2 is changed only slightly ⇒ atmospheric CO2 rises. Summary - shift10S 70°S 60°S 50°S 40°S 30°S 0° 60°N AMOC+ CO2+ ET+ AL+ ACC+ T- S- weaker biological pump reduced outgassing in compressed upwelling region rise in atmospheric CO2 northwards CO2+ CO2+ winds no shift surface ocean deep ocean sphere atmo- southwards shift10N 70°S 60°S 50°S 40°S 30°S 0° 60°N AMOC- CO2+ + ET- AL- ACC- T+ S+ stronger biological pump in midlatitudes higher CO2 outgassing by a broader upwelling region rise in atmospheric CO2 ACC: Antarctic Circumpolar Current, ET: Ekman transport, AL: Agulhas leakage. Physics Southward (shift10S–LGM) Northward (shift10N–LGM) AMOC (Sv) Temperature (K) Salinity (PSU) Wind Shift Dependencies. ï10 ï5 0 5 10 12 12.5 13 13.5 14 14.5 15 15.5 16 wind shift amplitude (degree latitude) Atlantic meridional overturning (Sv) ï10 ï5 0 5 10 50 100 150 200 250 wind shift amplitude (degree latitude) Drake passage transport (Sv) AMOC ACC@Drake P. -10 -5 0 5 10 260 265 270 275 wind shift amplitude (degree latitude) atmospheric pCO 2 (μatm) -10 -5 0 5 10 3.85 3.9 3.95 4 4.05 wind shift amplitude (degree latitude) particulate export (Pg C/yr) pCO2 export POC -10 -5 0 5 10 0 10 20 30 40 50 60 70 80 wind shift amplitude (degree latitude) Agulhas leakage (Sv) -10 -5 0 5 10 -5 0 5 10 15 20 25 30 35 wind shift amplitude (degree latitude) Northward Ekman transport (Sv) at 55 S Agulhas leakage Ekman transport C Cycle Southward (shift10S–LGM) Northward (shift10N–LGM) DIC (mmol/m 3 ) PO 3- 4 (mmol/m 3 ) Gas exchange (red: outgassing) (gC/m 2 /yr) -40 -30 -20 -10 0 10 20 30 0 o 60 o E 120 o E 180 o W 120 o W 60 o W 0 o 60 o S 30 o S 0 o 30 o N 60 o N -30 -20 -10 0 10 20 30 40 0 o 60 o E 120 o E 180 o W 120 o W 60 o W 0 o 60 o S 30 o S 0 o 30 o N 60 o N Export production (gC/m 2 /yr) -8 -6 -4 -2 0 2 4 6 8 10 0 o 60 o E 120 o E 180 o W 120 o W 60 o W 0 o 60 o S 30 o S 0 o 30 o N 60 o N -10 -8 -6 -4 -2 0 2 4 6 8 10 0 o 60 o E 120 o E 180 o W 120 o W 60 o W 0 o 60 o S 30 o S 0 o 30 o N 60 o N References: d’Orgeville et al (2010) On the control of glacial-interglacial atmospheric CO 2 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 CO 2 , 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.
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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
200
250
300
100
150
200
250
300
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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...
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..
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.
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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.
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