Antarctic climate change
over the 21st century
Tom Bracegirdle
[email protected]
Outline
• Global perspective
• Key questions
– What drives regional Antarctic temperature and
precipitation change?
– How will regional winds/circulation change in the future?
– What is the importance of internal variability?
Future projections
• A robust feature that emerges
from climate model simulations
is that high latitudes are
projected to become warmer
and ‘wetter’ under future
warming scenarios
• The Southern Ocean is
projected to warm relatively
slowly
Source: IPCC AR5 Figure 12.10 (p1061)
The current generation of climate models
• The most comprehensive dataset from the latest generation
of global climate models is the CMIP5 dataset
• The CMIP5 dataset was used extensively in the IPCC AR5
report and comprises approximately 50 different models and
their variants
• Atmospheric grid boxes
of typically 100-200 km
across in the horizontal
Source: IPCC AR5 Figure 1.14
Scenarios for climate projections
CO2 950 ppm
CO2 394 ppm
Meinshausen et al. (2011)
Stratospheric ozone
Eyring et al. (2013)
Outline
• Global perspective
• Key questions
– What drives regional Antarctic temperature and
precipitation change?
– How will regional winds/circulation change in the future?
– What is the importance of internal variability?
Antarctica
Nakamura and Oort (1988)
• Atmospheric component dominates over Antarctic continent
• Local radiative forcing has been found to be relatively weak or even
negative at high altitudes (Krinner et al., 2014; Schmithusen et al., 2015)
• Energy fluxes from lower latitudes are affected by surrounding SST
and sea ice conditions
Ice loss via iceberg calving,
snow blowing off the continent
and melting at the coast
Growth via precipitation
Glacier mass balance and atmospheric circulation. By NASA.
From Wikimedia Commons
• Surface mass balance (SMB) is the net balance between the processes of
accumulation and ablation on a glacier’s surface
• Antarctic SMB dominated by net precipitation, which is precipitation
minus evaporation (sublimation)
21C percentage change
Precipitation change
• Medium to high emissions
scenarios give a 20% to 50%
increase in annual precipitation
• The resulting change in surface
mass balance is equivalent to a
negative contribution to sea level
of between 0 and 7 cm over the
21st century (IPCC AR5, Table 13.5).
Updated from Bracegirdle et al. (2008)
21C Temperature change (°C)
Temperature change
• Ensemble mean warming of
approximately 2 to 4°C in medium
to high emissions scenarios.
• Polar amplification not strong in
annual mean (but evident in winter
over areas of ice retreat).
Updated from Bracegirdle et al. (2008)
• Projections of 21st
century Antarctic
temperature change
from different climate
models following the
same (RCP8.5)
emissions scenario
• Differing spatial
patterns linked to
historical sea ice extent
Model sea ice concentration
climatology (1970-1999)
Model with
excessive sea ice
in winter
Model with
too little sea ice
in winter
Projected temperature change
(2070-2099 minus 1970-1999)
Regional and hemispheric CMIP5 projections
Surface temperature
Precipitation
Large historical SIE
Small historical SIE
Bracegirdle et al. (2016)
• Models with large historical sea ice extent (SIE) exhibit more 21st century warming
• Cross model correlation between tropical and high latitude warming is only 0.22
Sea ice trends
• CMIP5 models generally project approximately 40% to 25% reduction in annual mean
ice area over 21st century (RCP8.5, RCP4.5)
• However, some question over representation of recent trends (Turner et al., 2013; Zunz
et al., 2013; Gagne et al., 2015)
• Clear priority on understanding processes that drive sea ice and Southern Ocean trends
Zunz et al. (2013)
Bracegirdle et al. (2016)
Outline
• Global perspective
• Key questions
– What drives regional Antarctic temperature and
precipitation change?
– How will regional winds/circulation change in the future?
– What is the importance of internal variability?
Circumpolar westerlies
Annual mean westerly wind (1979-2002) (ERA-40)
Consistent projected poleward shift in Southern
Hemisphere mid-latitude westerlies
• CMIP5 models exhibit a
projected poleward shift of
the latitude of the annual
mean surface westerly jet
under medium-to-high
emissions scenarios
• Opposing impacts of
stratospheric ozone
depletion and greenhouse
gas increases are evident in
the annual mean, but most
prevalent in summer
Surface zonal wind stress
Swart and Fyfe (2012)
Consistent projected poleward shift in Southern
Hemisphere mid-latitude westerlies
• CMIP5 models exhibit a
projected poleward shift of
the latitude of the annual
mean surface westerly jet
under medium-to-high
emissions scenarios
• Opposing impacts of
stratospheric ozone
depletion and greenhouse
gas increases are evident in
the annual mean, but most
prevalent in summer
10 m zonal wind
Bracegirdle et al. (2013)
Offset between GHG increases and recovery of
stratospheric ozone
• CMIP5 models exhibit a
projected poleward shift of
the latitude of the annual
mean surface westerly jet
under medium-to-high
emissions scenarios
• Opposing impacts of
stratospheric ozone
depletion and greenhouse
gas increases are evident in
the annual mean, but most
prevalent in summer
10 m zonal wind
Bracegirdle et al. (2013)
Equatorward shift over Pacific in winter (JJA)
• An equatorward shift
emerges in CMIP5
projections for the Pacific
in winter (RCP8.5)
• Changes this region are
potentially important for
Antarctica, motivating a
more detailed evaluation
across the models
Surface zonal wind stress
Simpson et al. (2014)
Winter split jet in
South Pacific
• The polar front jet (PFJ) is
close to East Antarctic coast
and dominates the nearsurface fields
• Sub-tropical jet (STJ) at lower
latitudes and is strongest in
the upper troposphere
PFJ
STJ
ERA-Interim climatology (1979-2014)
Winter split jet in
South Pacific
• The polar front jet (PFJ) is
close to East Antarctic coast
and dominates the nearsurface fields
• Sub-tropical jet (STJ) at lower
latitudes and is strongest in
the upper troposphere
PFJ
STJ
ERA-Interim climatology (1979-2014)
Unpublished work
Outline
• Global perspective
• Key questions
– What drives regional Antarctic temperature and
precipitation change?
– How will regional winds/circulation change in the future?
– What is the importance of internal variability?
Large internal variability
Variability in annual mean MSLP
(ECMWF reanalysis 1979-1993)
• Standard deviation of
year-to-year mean sealevel pressure variability
shows large maximum
over Amundsen Sea
“Pole of variability”
- Connolley, Clim. Dyn. (1997)
Internal variability
Hawkins and Sutton (2012)
Annual mean 10m zonal wind climatology
ERA-Interim 1979-2012
• The box shows a region
important for stability of the
West Antarctic ice sheet (after
Thoma, 2008)
• UAS denotes the spatial average
of westerly surface winds in
this region
• UAS is usually westerly with
with occasional easterlies
Bracegirdle et al. (2014)
Annual mean UAS from CMIP5 following RCP4.5 and RCP8.5
One standard deviation of decadal
variability – ‘Internal variability’
One standard deviation of inter-model
spread in response – ‘Model uncertainty’
Bracegirdle et al. (2014)
Summary
•
•
•
•
Two factors (wind and sea ice) have major implications for future projections (wind
most important for regional coastal impacts and sea ice (SO) most important for
temperature and precipitation over the Antarctic continent).
1. Under scenarios of increased emissions climate models indicate Antarctic warming
and increased precipitation in association with retreat of sea ice
– Large uncertainty in the amount of ice retreat
– Models with large global warming do not necessarily exhibit large Antarctic
warming – i.e. regional process dominate Antarctic transient response
2. Broad poleward shift of the westerlies is projected under high emissions scenarios
– Compensation between greenhouse gas increases and stratospheric ozone
recovery in medium emissions scenarios, mainly in summer
– Zonal mean annual mean picture is possibly too simplified since there are
important regional and seasonal contrasts
– No poleward shift in winter over the South Pacific, but CMIP5 models indicate
wind increases near the coast of East Antarctica
Large internal variability is a key consideration in regional Antarctic climate change,
especially over West Antarctica and the Amundsen Sea.
Antarctic climate change
over the 21st century
Tom Bracegirdle
[email protected]