A High Latitude Convective Cloud Feedback

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A High Latitude Convective Cloud Feedback

• Dorian S. Abbot
• PhD Oral Exam
• August 25, 2008

Future Climate
Equable Climates
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Outline

• Motivation what is an equable climate?
• Convective Cloud Feedback high-latitude warming
  leads to convection, leads to convective clouds,
  causing more positive cloud radiative forcing,
  causing further warming
• Onset and Strength of CC Feedback
• CC Feedback and Maximum Seasonal Sea Ice at High
  CO2
• Future Work

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Equable climate warm poles, mild winters

• High global mean temperature
• Very high polar temperatures
• above freezing winter temperatures _at_ 60N,
  interior of N. America (now -30C)
• No significant ice
• Warm deep ocean 15C
• Weak high-lat seasonality
• Tropical SSTs gt modern
• Low Equator-pole temperature difference 25C
  (now 45C).

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The good news? State-of-the-art fully coupled
GCMs cannot reproduce Eocene (50Myr) proxy
observations? When some of them get close, the
mechanism is not clear
Coupled GCM study of Huber and Sloan, 2001, for
example. Uses best guess Eocene

• Bathymetry
• Topography
• Land Surface
• Vegetation
• CO2 (560 ppm)

Sheillito et al 2003 AGCM at 2000 ppm gets
closer, mechanism not clear
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In SimpleIntermediate Models, CC Feedback
Mechanism Leads to High Latitude Warming and
Convection
This positive feedback supports 2 states (1)
Equable (high lat deep convection, high clouds
warm) (2) present-day-like, deep convection only
at equator
warmer surface ? unstable air column ? deep
convection ? high clouds ? greenhouse effect
warmer surface
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Onset and Strength of CC Feedback

• Critical CO2 of ONSET determines if feedback
• was active in past and if could be in future
• TS increase caused by feedback (STRENGTH)
• helps to establish relevance

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Simple Two-Level High-Latitude Climate Model
LW
Free Troposphere
Fa
Pressure Pa Emissivity ?a?0??
LW
Fc
LW
SurfaceBoundary Layer
Fs
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Onset Critical Emissivity
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Strength Surf. Temp. Increase
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NCARs Single Column Atmospheric Model


Results are confirmed in SCAM
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SCAM Confirms Main Two-Level Model Conclusions

• Roughly Modern
• Heat Transport
• CO22000 ppm

Polar Night, 2 States, Initial Conditions ONLY
Difference
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CC Feedback and Maximum Seasonal Sea Ice at High
CO2

• Evidence for CC feedback in IPCC AR4 GCM runs
• CC feedback may help determine maximum seasonal
  (spring) sea ice in these runs
• CAM snapshot sensitivity runs establish
  importance of CC feedback

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CC Feedback Active During Winter in GCMs
NCAR GFDL 3D CPLD O-A GCMs, x4 CO2 minus
pre-indust.
NCAR
Cloud CRF up/ unchanged
Convection up/ unchanged
Srfc Temp up/ unchanged
Sea ice gone/ unchanged
GFDL
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CC Feedback in Arctic for all IPCC Models
CC Feedback associated with winter sea ice loss
Effect of feedback on spring sea ice?
?NDJF CRF W m-2
?March Sea Ice
?NDJF Sea Ice
?NDJF CRF W m-2
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CAM runs to Compare CC OHT Feedback

• Increase CO2 to 1120 ppm
• and do four runs
• 1. LO OHT and LO CRF
• 2. LO OHT and HI CRF
• 3. HI OHT and LO CRF
• 4. HI OHT and HI CRF

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BOTH Feedbacks Needed to Eliminate March Sea Ice!!
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Future Work

• Using Modern Observational Evidence
• to Constrain the CC Feedback
• 2. CC Feedback and Continental Interiors

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Evidence for CC Feedback in Modern Climate
Winter Arctic CRF seems tied to sea ice in
modern climate
Yearly sea ice anomalies tied to yearly CRF
anomalies
Can this constrain feedback so that models are
more useful?
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Speculation on continental interiors
Continental interiors not addressed in our
study. Speculation Drifting moisture high
clouds from above-ocean convection may provide
greenhouse effect over continental interiors
weaker eddies in equable climate ? less drying by
vertical eddy motions.
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? in CAMs DJF Climate Per CO2 DoublingCRF
Increase Over BOTH Ocean and Land
CO2 Doubling
1
2
3
4
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Important Increase in Moisture Transport
Vert. Int. Cond. At 60N W m-2
200
Is transport zonal or meridional?
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Land Fraction
0
Longitude
Thanks to Gabriel Bousquet!
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Central Conclusions

• Main Point Found a simple, interesting
  unexpected convective cloud feedback on
  high-latitude climate change high-latitude deep
  atm convection high tropospheric clouds result
  in an equable-like climate
• Onset of CC feedback depends strongly on heat
  transport
• magnitude and vertical distribution. Strength
  does not.
• x4 CO2 3D state-of-the-art coupled ocean-atm IPCC
  runs and
• CAM sensitivity runs indicate CC feedback will
  help determine
• future maximum seasonal (spring) sea ice
• Remaining Work observations and explicit
  mechanism for continental interiors

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Thanks useful conversations John Dykema, Ian
Eisenman, Kerry Emanuel, Brian Farrell, John
Higgins, Peter Huybers, Zhiming Kuang, Richard
Lindzen, Dan Schrag, Jacob Sewall, Chris
Walker For helpful reviews Rob Korty, Adam
Sobel, Richard Seager.
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Thanks Especially to Professor Superman!