Download The Surface Warming Trend Over Antarctica

The Surface Warming Trend
Over Antarctica
Jessica Liptak
Atmos 6030
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Outline
• Signs of climate change
• Surface warming trend (Steig et al. 2009)
• Relationship between surface
temperature and the SAM (Marshall 2006)
• Is the warming anthropogenic? (Gillett et al.
2008)
• Summary/Conclusions
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Signs of Warming
2002: Collapse of Larsen Ice Shelf
1998, 2000, 2010: Ronne Ice
Shelf calving events
Turner et al.2008:
2005 Wilkins Ice Shelf Breakup Events
Warming Since 1957
Steig et al. 2009
• Surface temps from thermal
infrared (TIR) satellite and automatic
weather stations (AWS) used as bases
of near-surface temperature
reconstructions from Jan 1957-Dec
2006
•Temperature Trends
•West: 0.17  0.06ºC/decade
East Antarctica
West
Antarctica
•East: 0.10  0.07 ºC/decade
•Peninsula: 0.11  0.04ºC/decade
•Continent: 0.12  0.07ºC/decade
•Global Mean: 0.128  0.06ºC/decade
(1956-2005)
TIR
AVG TIR
AWS
Spatiotemporal Variability
NS= Not Significant
South
Pole
Station
Siple
Station
Vostok
Station
1957-2006 Ann. Mean
Byrd
Station
1969-2000 Ann. Mean
Steig et al. 2009
• Trends are from TIR
reconstruction
•Individual stations show
mean annual upward trends
in West Antarctica
1957-2006 JJA Mean
1957-2006 SON Mean
1957-2006 DJF Mean
1957-2006 MAM Mean
•Greatest warming in
Austral spring (SON) and
winter (JJA)
The Model
Steig et al. 2009
•NASA Goddard Institute for Space Studies (GISS) ModelE
atmosphere-only model and coupled GCM
•Time Periods:
•1957-1981
•1979-2003
•Boundary Conditions:
•5-member ensemble w/observed SST and sea ice
•4-member ensemble w/ observed SST, sea ice, and
atmospheric forcings (changes in GHGs, decrease in
stratospheric O3)
•Observed-climatological sea ice (same atmospheric forcings)
The Results
Steig et al. 2009
Sea ice obs
unreliable
before 1979
SST,sea ice
reproduces
1957-1981
warming over
continent, esp.
West Antarctica
SST, sea ice,
forcings in
general
agreement
w/obs
1979-2003
Warming rate
greatest over
West Antarctica
in springSea
andice
decreases in
winter
west, increases
elsewhere
SST, sea ice
generates
weak cooling
over East
Antarctica
Coupled model
(dynamic ocean) does
not reproduce strong
West Antarctic &
Peninsula warming
The SAM Connection
•Southern Annular Mode (SAM) is first mode of
atmospheric variability in SH
•Positive phase characterized by anomalously strong
circumpolar westerlies
• MSLP is below normal over Antarctica and above normal
over SH mid and high latitudes during positive phase
•Explains ~35% of SH climate variability
•Positive trend in SAM during past 50 years
•Consistent significant relationship between SAM
and Antarctic surface temperature
Correlation Between SAM &
Near-Surface Temperatures
Marshall 2006
Fall (MAM)
Austral
Fall
1957-2004
1957-2004
Adiabatic
cooling
Is Antarctic Warming Anthropogenic?
(Gillett et al. 2008)
• 1950-1999 CRUTEM3 nearsurface station temperature
observations
•Ensemble of 4 CMIP-3
coupled climate models run
from 1950-2008
Warmest NAT
simulation
Warmest ALL
simulation
•NAT=natural forcings
•ALL=natural+anthropogenic
forcings
•Warming trend apparent in
both obs & model output
Coldest ALL
simulation
Coldest NAT
simulation
Grid Cell Temperature Trends
(Gillett et al. 2008)
OBS
ALL
NAT
OBS thru Jul
2008
• Obs show
warming over
peninsula, cooling
over South Pole
•Warming trend
more widespread
for obs extending
thru Jul 2008
•NAT run does not
reproduce warming
trend
SAM Impacts on Temperature
Gillett et al. 2008
• Upward trend in SAM
assc. w/peninsula
warming and cooling
everywhere else
•Effect of obs SAM
trend on temps seems
weaker in this study
than in others
•Simulated SAMcongruent temp trend
is weaker than obs
trend
Obs SAM-congruent T trends
Obs Resid T trends
Model SAM-congruent T trends
1950-1999
Model Resid T trends
The Verdict
Surface temperatures show a detectable
response to anthropogenic warming
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Summary & Conclusions
•There is a significant upward trend in surface
air temperature over most of Antarctica
•The greatest warming has occurred in the
western portion of the continent and the
peninsula
•Anthropogenic forcing is detectable in the
warming signal, and distinct from natural forcing
•Resultant circulation changes are responsible for
the recent decrease in the rate of warming
References
-Gillett, N.P., et al., 2008: Attribution of polar warming to human
influence,Nat. Geosci., 1, 750–754.
-Gillett, N.P., Thompson D.W.J., 2003: Simulation of recent Southern
Hemisphere climate change. Science 302: 273–275.
-Marshall,G.J., 2007: Half-century seasonal relationships between the
Southern Annular Mode and Antarctic temperatures. Int. J. Climatol.
27, 373-383.
-Marshall, G.J., Orr, A., van Lipzig, N.P.M., King, J.C., 2006: The
impactof a changing Southern Hemisphere Annular Mode on Antarctic
Peninsula summer temperatures. J.Clim. 19, 5388-5404.
-Steig, E.J., Schneider, D.P., Rutherford, S.D., Mann, M.E., Comiso,
J.C., Shindell, D.T., 2009: Warming of the Antarctic ice-sheet surface
since the 1957 International Geophysical Year. Nature. 457, 459-463.
-Trenberth, K.E., P.D. Jones, P. Ambenje, R. Bojariu, D. Easterling, A.
Klein Tank, D. Parker, F. Rahimzadeh, J.A. Renwick, M. Rusticucci,
B. Soden and P. Zhai, 2007: Observations: Surface and Atmospheric
Climate Change. In: Climate Change 2007: The Physical Science
Basis. Cambridge University Press, 336 pp.