Atmospheric Feedbacks Over the Pacific Cold-tongue: Results From Models and Observations
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Transcript of Atmospheric Feedbacks Over the Pacific Cold-tongue: Results From Models and Observations
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Atmospheric Feedbacks Over the Pacific Cold-tongue: Results From Models and
Observations
De-Zheng Sun and Tao Zhang
CIRES/Climate Diagnostics Center
& ESRL/Physical Science Division
http://www.cdc.noaa.gov/~ds
In collaboration with
Curt Covey and Steve Klein
Lawrence Livermore National Laboratory
Bill Collins, Jim Hack, Jeff Kiehl, and Jerry Meehl
National Center for Atmospheric Research
Isaac Held
Geophysical Fluid Dynamics Laboratory
Max Suarez
National Aronics and Space Administration
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Why feedbacks?
• Feedbacks determine the sensitivity of the climate system to an external perturbation
• Feedbacks determine the amplitude of the natural variability in the climate system
• Feedbacks determine the equilibrium state of the climate system--the time-mean climate
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Why feedbacks over the Pacific cold-tongue?
• The cold-tongue plays a key role in the heat and carbon balance of the climate system and is a major source region of climate variability
• Climate models do not simulate the cold-tongue properly: they have a tendency to develop an excessive cold-tongue.
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The Excessive Cold-tongue in coupled models Annual Mean SST from observations and CGCMs
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Ideas about the causes of the excessive cold-tongue
• Insufficient resolution of the ocean model (Stockdale et al. 1998, Kessler et al. 1998)
• Lack of phytoplankton in the ocean model (Murtugudde et al. 2002)
• A weaker regulating effect from the model atmosphere (Sun et al. 2003)
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The idea of a weaker regulating effect from the
model atmosphere?
Could it be that the strength of one or more negative feedbacks in the model underestimated? Or alternatively, could it be that the strength of the one or more positive feedbacks in the model is over estimated?
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The effect of a less negative net atmospheric feedback on the cold-tongue SST bias
€
T −T0 =H(T0)
4σT03 − (
∂Ga∂T
+∂Cl∂T
+∂Cs∂T
+∂Da∂T
) −∂Do∂T
Atmospheric Feedbacks
€
∂Do∂T
= −13wm−2K−1
€
(∂Ga∂T
+∂Cl∂T
+∂Cs∂T
+∂Da∂T
)
M
SST Bias
Net flux error at T0
Ocean feedback
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A further assessment of the effect of feedback errors on the cold-tongue
SST
Perturbed run with feedbacks from CAM1
Perturbed run with observed feedbacks
Control run
Coupled model used: NCAR Pacific basin model coupled with an empirical atmosphere (Sun 2003)
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Methodology
• Use ENSO as the forcing signal and obtain the feedbacks by examining the response of various energy fluxes to SST changes associated with El Nino and La Nina
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Data
•Observational data: ERBE, NCEP/NCAR reanalysis, ISCCP
•Model outputs: AMIP runs of climate models
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The Physical Processes
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The Water Vapor Feedback
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The Dynamical Feedback
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The Cloud Feedbacks
ERBE
CAM1CAM1
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The Cloud Feedbacks
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The Cloud Feedbacks
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Response of Cs to El Nino warming
(W/m
2/K)
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Response of Precipitation to El Nino Warming
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Response of Cloud Cover in CAM1 CAM1
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Atmospheric feedbacks in observations and the NCAR models
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How are other models doing in simulating the feedbacks?
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Precipitation Response to El Nino Warming
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Summary and Conclusion• These results confirm the suspicion that
an underestimate of the regulating effect from the equatorial atmosphere is a prevalent problem in climate models.
• Most models underestimate the strength
of the negative feedbacks of cloud albedo and atmospheric transport. All models overestimate of the positive feedback from water vapor.
• The results highlight the need to validate the feedback from the ocean transport.
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Water vapor response
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Oceanic Issues
• What’s the effect of ENSO on the time-mean upper ocean temperature?
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Atmospheric Issues
• What are the processes that control the equatorial precipitation?
• How is the vertical distribution of clouds determined?
• Why does convection in the models have a stronger moistening effect on the upper troposphere?
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El Nino in climate models
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La Nina in Climate Models
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The Asymmetry between El Nino and La
Nina
Sk.=0.62
Sk.=0.30
Sk.=0.05
Sk.=-0.10
Sk.=-0.20
Sk.=0.29