Don’t Discount the Tropics
“Challenges to our understanding of the
general circulation: abrupt climate change”
R. Seager and D.S. Battisti 2007
Laura Zaunbrecher
EAS 8801
September 5, 2008
Advances in ACC theory since Broecker 1985
• Spatial Pattern of ACC
– Synchronous in much of N. Hemisphere
– Atmosphere, Surface and Deep Ocean
– No signal in Antarctica
• Mechanisms revolve around the THC
– Rapid switches ‘on’ and ‘off’ of NADW formation
– Using this theory: difficult to explain paleo record
Consider the tropics in ACC
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Spatial footprint of ACC
Seasonality of ACC
Critique the THC-driven theory
Introduce a mechanism
– GLOBAL A-O coupling
– Active role of the tropics
Evidence for ACC from Ice Cores
ACC records in other regions
• Surface Atlantic Ocean
• The Caribbean
• Africa
LAMONT-DOHERTY EARTH OBSERVATORY
From W. Broecker
• Northern Extratropics
• The Tropics
• Southern Hemisphere
Seasonality of ACC
• Modest summer cooling
• Drastic winter cooling (20°C)
• Rapid transition from periods of great
seasonality to more like modern climate
• Sea Ice expanding to S. Britain?
http://www.weatherpictures.nl/seasons.html
Proposed Causes of ACC
• We need a mechanism to explain…
– Cold N. Atlantic
– SST’s in Subtropical N. Atlantic cooling
– ITCZ shifted South of S. America
– Weakened Asian Monsoon
– Cooling in tropical Americas
• THC-driving Theory
Model of THC Shutdown
THC ALONE CANNOT EXPLAIN
OBSERVED CHANGES
Air Temperature Anomalies
Precipitation Anomalies
Atmospheric Circulation Regimes and
Global A-O Coupling
• The large changes in seasonality require seaice to reach as south as the British Isles
• How could sea-ice extend so far south?
• How is there such a large influx of heat in the
spring and summer?
• How do large shifts in seasonality occur?
Required Changes in Atmospheric Circulation
Regimes
• Opposite seasonal cycle of heat transport
• How can winter convergence of heat in mid to
high latitudes be reduced?
– Weak transport of heat, allow sea-ice to expand
– Strong heat transport in summer to melt back ice
• The Atlantic storm track and jet could be
induced to be more zonal, like the Pacific
Shift to zonal circulation in Atlantic
During Winter
• Change in wind stress
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http://www.smh.com.au/ffximage/2007/04/27/Gulf_Stream_070425102608660_wi
deweb__300x300.jpg
pattern
Removal of warm SE
advection into N. Atlantic
Reduce salt influx
Sinking branch of THC shifts
South
Sea Ice can extend further
South
COOLS the N. Atlantic
Abrupt Shifts in Seasonality,
In summer-need a large input of heat into N. Atlantic Region
The Icelandic Low
• SST’s need to warm from
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Image courtesy of MODIS Rapid Response Project at NASA/GSFC;
about freezing to 10°C!
Perhaps summer ice sheets
become a radiative sink
Colder temps draw
atmospheric heat over ice
Strong Icelandic Low, shifted
south, could cause advective
warming of Europe
Tropical Forcing, the ENSO blueprint
El Niño Anomalies
• ACC in the tropics are
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http://www.pmel.noaa.gov/tao/elnino/gif/winter.gif
‘relatively’ as large as those
in the N. Atlantic
Modern climate variations
are linked to tropics
However, ACC patterns do
not match those of El Niño
and La Niña
Persistent tropical changes
could have different spatial
pattern
Tropical Heating and Extratropical Jets and
Storm Tracks
rst.gsfc.nasa.gov/Sect14/jet_streams_a.jpg
Conclusions
• Winter cooling around N. Atlantic must involve a
substantial change in atmospheric circulation, reducing
heat transport – zonal wind
• Summer warming much larger influx of heat from
tropics than we see today
• Change in tropical convection can cause changes to
midlatitude winds
• Models today have yet to produce these abrupt climate
change events
• Much remains uncertain
NOAA, Lamont-Doherty
Earth Observatory