Don’t Discount the Tropics
“Challenges to our understanding of the general circulation: abrupt climate change”
R. Seager and D.S. Battisti 2007
Laura ZaunbrecherEAS 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
• 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
• Northern Extratropics• The Tropics• Southern Hemisphere
LAMONT-DOHERTY EARTH OBSERVATORYFrom W. Broecker
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 sea-ice 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
• Change in wind stress 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
http://www.smh.com.au/ffximage/2007/04/27/Gulf_Stream_070425102608660_wideweb__300x300.jpg
During Winter
Abrupt Shifts in Seasonality, In summer-need a large input of heat into N. Atlantic Region
• SST’s need to warm from 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 EuropeImage courtesy of MODIS Rapid Response Project at NASA/GSFC;
The Icelandic Low
Tropical Forcing, the ENSO blueprint
• ACC in the tropics are ‘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
http://www.pmel.noaa.gov/tao/elnino/gif/winter.gif
El Niño Anomalies
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
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