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Laura Zaunbrecher EAS 8801 September 5, 2008

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

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Laura Zaunbrecher EAS 8801 September 5, 2008

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  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 Anomalies Precipitation 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 During Winter • 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

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

  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 El Niño Anomalies http://www.pmel.noaa.gov/tao/elnino/gif/winter.gif

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

  15. 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

  16. NOAA, Lamont-Doherty Earth Observatory

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