Polar climate and future projections
Tom Bracegirdle
NCAS Earth System Science Spring School (ES4)
Thursday 3rd April 2014
World distribution of ice
<1%
90%
9%
Rest of world
Antarctica
Greenland
30 Million cubic km of ice
= 65 m SLE
3 Million cubic km of ice
= 6 m SLE
0.33 Million cubic km of ice
= 70 cm SLE
SLE = sea level equivalent
Key processes in the
Polar Regions
Growth via precipitation
Ice loss via iceberg calving,
snow blowing off the
continent and melting at the
coast
Growth via precipitation
Ice loss via iceberg calving,
snow blowing off the
continent and melting at the
coast
Limitations to understanding Antarctic climate
•
Vast expanse and harsh conditions
making it difficult and expensive to
conduct measurements
–
•
Satellite measurements only
available since early 1980s
–
•
temporally and spatially sparse
coverage of in-situ observations
This period is anomalous due to ozonehole and greenhouse gases (GHGs)
The Antarctic Peninsula is a region
of steep high mountains that are
not accurately captured in most
climate models
A coupled system (atmosphere, ocean, ice)
Contents
• Introduction to Antarctic climate
• Southern hemisphere atmospheric variability
• The ozone hole and its impacts
• Recent changes in sea ice
• Projections of future change
Introduction to Antarctic climate
Key Characteristics
•
Antarctica is the coldest continent
– coldest measured temperature of –89.2°C
(July 21st, 1983 at Vostok Station)
•
3
4
2
1
The total precipitation, averaged over the entire
continent, is about 166 mm per year.
– <250 mm classified as deserts
– as little as 50 mm in the high interior
•
High albedo of snow and ice (99% of the continent)
–
•
reflects 80‐90% of incident solar radiation back to space
Temperature drop suddenly in winter as we tend
towards the polar night.
–
4
3
Coreless Winter
2
1
Global energy transport
• Tropics  poles
• Heat flux to poles
dominated by atmospheric
rather than oceanic
transport
• Clear annual cycle
- greater transport winter
months
Trenberth and Caron (2001)
1 petawatt (PW) = 1015 watts
The Southern Ocean & the Thermohaline Circulation
•
Southern Ocean linked to
all oceans
•
Formation region for the
densest water in the world
ocean
– Antarctic Bottom Water
http://maps.grida.no/go/graphic/thermohaline-circulation1
Formation of deep bottom water
Katabatic winds
(wind drainage)
•
Cold dense air drains off the elevated Antarctic interior,
gradient accelerates air downslope towards the coasts,
steered by orography and Coriolis force
Van Lipzig et al. (2004)
•
Caused by continuous radiational cooling of the surface of
the Antarctic Plateau
•
Clearly defined regions of confluence
•
•
•
At the coast, annual mean speeds can be tens of km/h
NOAA NCEP Animation
http://earth.nullschool.net/#current/wind/surface/level/orth
ographic=-1.23,-86.08,446
Atmospheric variability
SH Annular Mode (SAM)
(aka “High Latitude Mode” and “Antarctic Oscillation”)
•
The principal mode of variability in the
atmospheric circulation of the Southern
Hemisphere (SH)
– accounts for ~30-40% of interannual
variability
•
Associated with westerly winds
(eastward flow) around continent
EOF1 from HadGEM climate model
Wave patterns
Low frequency wave patterns (e.g., 1-4) modulated by...
• Circumpolar winds
• Interruption of these winds by the arrangement, shape and elevation of the
continents (South America, Africa, Australia, Antarctica)
•Tropical asymmetric forcing
•
e.g., Wave number 3 is easy to see in
monthly/seasonal pressure fields
•
Two climatological regions of low pressure off
the East Antarctic coast (stationary)
•
Amundsen-Bellingshausen Seas Low (more
mobile)
– Modulated by ENSO variability and other
tropical teleconnections
Autumn MSLP highlighting the wave-number 3 pattern
El Niño Southern Oscillation (ENSO)
•
•
Wave-train of anomalies can extend from the tropics to polar latitudes
El Niño leads to more blocking near Peninsula
Karoly (1989)
Ozone Hole and its impacts
Reduction of stratospheric ozone concentrations
• Stratospheric ozone hole has been the
single biggest influence on Antarctic
atmospheric circulation over the last
few decades
• CFCs, and other chlorinated
compounds are broken down in the
stratosphere by UV radiation
liberating chlorine
• Ozone depletion events are less
frequent over the Arctic
– greater meridional component
in NH due to orography
(e.g., The Rockies and Himalayas)
NASA/Goddard Space Flight Center
Scientific Visualization Studio
Reduction of stratospheric ozone concentrations
• Stratospheric ozone hole has been the
single biggest influence on Antarctic
atmospheric circulation over the last
few decades
• CFCs, and other chlorinated
compounds are broken down in the
stratosphere by UV radiation
liberating chlorine
• Ozone depletion events are less
frequent over the Arctic
– greater meridional component
in NH due to orography
(e.g., The Rockies and Himalayas)
Courtesy of Jonathan Shanklin, BAS
Polar vortex and its impact on surface climate
• Ozone loss intensifies
vortex which
propagates down to
impact the
troposphere and
surface climate
Orr et al, 2012
Trends in the SAM and its relationship with temperature
•
Strong positive trend from the mid‐1960s to the end of the 20th century
•
Pressures become lower over Antarctica and
higher over mid-latitudes
•
Increasing the circumpolar westerlies
– by 15% in recent decades
Seasonal values of the SAM index calculated from
station data (Marshall, 2003). The smooth black
curve shows decadal variations
The contribution of the SAM to changes in surface
temperature December to May 1969 -2000 (Thompson
and Solomon, 2002)
Arblaster and Meehl (2006)
Why has the SAM changed?
Prevailing circumpolar westerlies
Larsen B
Courtesy of Tony Phillips
Ice shelf collapse on the Peninsula
•
Stronger westerly winds cause
more air to flow over the
Peninsula (rather than around it)
– Air is warmed adiabatically upon
descent on the east side
– Contributed to the Larsen B ice
shelf collapse
31st January – 7th March 2002
Ice shelf collapse on the Peninsula
•
Stronger westerly winds cause
more air to flow over the
Peninsula (rather than around it)
– Air is warmed adiabatically upon
descent on the east side
– Contributed to the Larsen B ice
shelf collapse
van Lipzig et al. (2008)
Recent changes in sea ice
Antarctic sea ice
maximum in September ≈ 19.0 × 106 km2
minimum in February ≈ 3.5 × 106 km2
area of Antarctica
≈ 13.8 × 106 km2
•
Seasonal cycle has a major impact on albedo  radiative balance and climate
•
Very little multi-year sea ice
(in contrast to the Arctic)
–
•
most Antarctic sea ice has a thickness
less than 1 m
Sea ice reduces air‐ocean fluxes
Sea ice climatologies (1981-2010) – Source NSIDC
Sea-ice extent trends
Source?
Changes in sea-ice and circulation
Holland & Kwok, 2012
Surface wind pattern
correlates with ice
motion vectors
The future
Stratospheric ozone
Ozone at ~20 km above Antarctica
Cionni et al. (2011)
Greenhouse gases
CO2 950 ppm
CO2 394 ppm
Future surface westerlies
• Average of 29 different climate models
• High emissions scenario (RCP8.5)
Climate models
(Observations)
Summer
(DJF)
10% increase over 21st century
Westerly wind strength (m s-1)
Westerly wind strength (m s-1)
5% increase over 21st century
Climate models
(observational)
(Observations)
CMIP5
Annual
RCP4.5
mean
Future surface westerlies
• Average of 29 different climate models
• Medium emissions scenario (RCP4.5)
Climate models
(Obervations)
Summer
(DJF)
3% increase over 21st century
Westerly wind strength (m s-1)
Westerly wind strength (m s-1)
1% increase over 21st century
Climate models
Observations
Annual
mean
Implications of wind changes
• Ongoing research on:
– Southern Ocean: possible
reduced effectiveness at
absorbing heat and carbon
– Antarctic Peninsula: possible
further warming
– West Antarctic ice sheet:
possible acceleration of
glaciers
Bracegirdle et al. (2013)
Polar winter warming
21st century change
Antarctic
Arctic
Bracegirdle and Stephenson (2012)
Future changes - temperature
21st century temperature change
AR4 models (Frierson et al., 06)
• Polar amplification shallow.
• Clear increase of vertical gradient.
• Slow warming of Southern Ocean.
Surface temperature change
Impact on Southern Ocean krill
• Spatial pattern of projected
change in Antarctic krill habitat
by the late 21st Century.
RCP4.5
• The parameter shown is gross
growth potential (GGP).
• Significant decreases of large
regions of the Southern Ocean.
Hill et al. (2013)
RCP8.5
Summary
• The ozone hole has had a major
influence on Antarctic climate
• The effects of greenhouse gas
increases are likely to emerge
strongly over the 21st century
• Projected changes have
implications across the earth
system
Thank you