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