1. 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

2. 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

3. 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

4. Evidence for ACC from Ice Cores

5. ACC records in other regions Surface Atlantic Ocean The Caribbean Africa Northern Extratropics The Tropics Southern Hemisphere LAMONT-DOHERTY EARTH OBSERVATORY From W. Broecker

6. 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

7. 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

8. Model of THC Shutdown THC ALONE CANNOT EXPLAIN OBSERVED CHANGES Air Temperature AnomaliesPrecipitation Anomalies

9. 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?

10. 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

11. 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_wi deweb__300x300.jpg During Winter

12. 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 Europe Image courtesy of MODIS Rapid Response Project at NASA/GSFC; The Icelandic Low

13. 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

14. Tropical Heating and Extratropical Jets and Storm Tracks rst.gsfc.nasa.gov/Sect14/jet_streams_a.jpg

16. 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

18. NOAA, Lamont-Doherty Earth Observatory