Centennial-scale Climate Change from Decadally-paced Explosive Volcanism

Transcription

1 Centennial-scale Climate Change from Decadally-paced Explosive Volcanism Yafang Zhong and Gifford Miller INSTAAR, University of Colorado at Boulder, USA Bette Otto-Bliesner, Caspar Ammann, Marika Holland, David Bailey and David Schneider NCAR, USA Aslaug Geirsdottir University of Iceland, Iceland CCSM Workshop on June. 29, 2010

2 NH climate of the past millennium NH Temperature LIA MWP Colder 1950AD 1300AD 800AD Time Crowley (2000) The past millennium is characterized by Medieval Warm Period (MWP), Little Iage Age (LIA), and 20 th century warming.

3 Step-like ice cap expansion during the cooling into the Little Ice Age 1950A D Ice cap expansion on N Baffin Island 1300A D Time Step-like changes at AD, ~1450AD Anderson et al. (2008) LIA MWP NH Temperature Colder 1950AD Time 1300AD Crowley (2000) 800AD

4 Possible causes of the Little Ice Age Solar irradiance? Uncertainty in reconstruction. Ice cap expansion on N Baffin Island Anderson et al. (2008) Volcanic forcing? Centennial-scale response? Time

5 Possible causes of the Little Ice Age Question: Solar Could irradiance? the decadally sequenced volcanic eruptions cause centennial-scale response in NH sea ice? Ice cap expansion on N Baffin Island Volcanic forcing? Centennial-scale response?

6 Experiments using the CCSM3 Medieval control run: 950AD orbital forcing Solar constant is 1365 W/m2 1000AD Greenhouse Gas level 280.6ppm CO 2, 684.3ppb CH 4, ppb N 2 O

7 Experiments using the CCSM3 (Cont) Volcanism experiments Volcanic forcing: decadally sequenced tropical eruptions in the second half of the 13 th century Time No volcanic forcing for the 14 th century

8 Experiments using the CCSM3 (Cont) Volcanism experiments Four runs, randomly selected initial states. Control run (3) (1) (2) (4)

9 Centennial-scale response in NH sea ice in volcanism experiments SW radiation Gray CTRL Red EXP1 Blue EXP2 Mar ice volume Sep ice volume Mar ice area Sep ice area Time

10 Largest anomalies in September occur in the Atlantic sector, which agrees with paleo reconstructions. NH sea ice concentration anomalies SEP MAR ANN AD AD

11 Mass budget of ANN NH sea ice Mass budget of NH annual sea ice Solid Volcanism EXP Dashed CTRL Basal growth Basal melt Time Sea ice expansion is driven by decreased ice-ocean heat exchange.

12 Ice mass budget for Barents Sea region Solid Volcanism EXP Dashed CTRL Basal growth Basal melt Sea ice expansion is driven by decreased ice-ocean heat exchange. Time

13 Sea ice mass budget for west of Greenland Solid Volcanism EXP Dashed CTRL Basal growth Basal melt Sea ice expansion is driven by decreased ice-ocean heat exchange. Time

14 Surface cooling in the North Atlantic and Arctic oceans Strong cooling at NH high latitudes Zonal mean ocean temperature anomalies. Shading: significant at 95% confidence level Heat flux anomaly is into the Arctic Ocean, not responsible for the ocean cooling

15 Surface cooling in the NH high-latitude and Arctic oceans The Arctic Ocean cooling is not caused by surface heat flux changes, but rather by reduced ocean heat transport poleward. Strong cooling at NH high latitudes Zonal mean surface heat flux anomaly Into-the-ocean anomaly

16 Cooling! Changes in the surface ocean

17 Changes in the surface ocean Cooling! Sustained for >100 years!

18 Changes in the surface ocean Increased sea ice export (freshwater flux) to and thus weakened convection in the subpolar N Atlantic! Cooling! Freshening! Lightening!

19 Decreased heat/salt advection associated with the slowed-down subpolar gyre circulation Color: ANN sea ice thickness anomaly Contour: Sea surface height anomaly Vector: Current anomaly

20 Summary The sequenced volcanism could produce an expanded NH sea ice that is sustainable for >100 years after the removal of volcanic aerosols from the stratosphere. A coupled sea ice-ocean mechanism: Sea ice expands and thickens in response to volcanic aerosol radiative forcing More ice is exported through Fram Strait and the Canadian Achipelago, and melts in the subpolar North Atlantic Cooling and freshening of the surface water in the subpolar N Atlantic, in aid of weakened convection and decreased heat/salt advection Anomalously cold water is advected into the Arctic Ocean and reduces ice-ocean heat exchange Preserves the expanded sea ice The coupled sea ice-ocean mechanism may be sensitive to the pre-volcanic mean states, such as mixing layer depth in the subpolar N Atlantic.

21 Thank you!

22 Summary Sequenced volcanism produced an expanded NH sea ice that sustained for >100 years after the removal of volcanic aerosols from the stratosphere. The largest concentration anomalies occurred in the Atlantic sector of the Arctic, agreeing with paleo data. A coupled sea ice-ocean mechanism: Sea ice expands and thickens in response to volcanic aerosol radiative forcing More ice is exported and melts in the subpolar North Atlantic Cooling and freshening of the surface water in the subpolar N Atlantic, in aid of weakened convection and decreased heat/salt advection

23 The volcanism experiments showing no centennial-scale response Pre-volcanic Shallow mixing layer depth in the subpolar N Atlantic, already weak convection Post-volcanic Convective feedback ineffective in the subpolar N Atlantic. Rapid sea ice growth, increased brine rejection, positive density flux in marginal seas strengthened Atlantic MOC, increased heat transport into the Arctic Sea ice recedes

24 Summary Sequenced volcanism produced an expanded NH sea ice that sustained for >100 years after the removal of volcanic aerosols from the stratosphere. The largest concentration anomalies occurred in the Atlantic sector of the Arctic, agreeing with paleo data. A coupled sea ice-ocean mechanism

25 ncreased sea ice export to subpolar N Atlantic Net freshwater flux Freshening effect on the subpolar North Atlantic!

26 Centennial-scale response in NH air temperature ANN JJA DJF Time

27 Medieval ontrol run

28 Ocean temperature anomaly AD AD

29 Atlantic MOC in volcanism exps LRSP1, 2 SRSP1 SRSP AD AD AD AD AD

30 Control run Vs. LRSP1

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33 AD realistic volcanic forcing

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37 Decreased oceanic heat transport into the Arctic Colder water into the Arctic!

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39 Weakend Atlantic Meridional Overturning Circulation