High Climate Sensitivity Suggested by Satellite Observations: the Role of Circulation and Clouds AGU Fall Meeting, San Francisco, CA, 9-13 December, 2013 Hui Su 1, Jonathan H. Jiang 1, Chengxing Zhai 1, Janice T. Shen 1 David J. Neelin 2, Graeme L. Stephens 1, Yuk L. Yung 3 1 Jet Propulsion Laboratory, California Institute of Technology 2 University of California, Los Angeles 3 California Institute of Technology

 Model differences in cloud feedback is a leading contributor to the uncertainty of climate sensitivity.  What process drives the inter-model spread in cloud feedbacks?  Satellite observations have been used extensively to evaluate model performance of present-day climate.  Can satellite observations provide indications of future climate change? Motivation

Cloud Feedback and Climate Sensitivity Cess et al. (1989) Stephens (2005) “one step toward unraveling the complex nature of cloud feedback lies ultimately in understanding the influence of the large scale atmospheric circulation on clouds.” - Stephens (2005) A “Three Cs” Investigation CLIMATE SENSITIVITY

Changes of the Hadley Circulation, Clouds and Cloud Radiative Effects in the RCP4.5 ∆ = in “RCP4.5” – in “historical run” Multi-model-mean from 15 CMIP5 coupled models

The Equatorial Tropics (around 5°S to 5°N) S W W W W S S ∆ = in “RCP4.5” – in “historical run”

The Flanks of Deep Tropics (around 5° to 15°N/S) S W W W W S S ∆ = in “RCP4.5” – in “historical run”

Equator-ward side of Descent Zone (about 15°-30°N/S) S W W W W S S ∆ = in “RCP4.5” – in “historical run”

Poleward-side of Descent Zone (about 30°-45°N/S) S W W W W S S ∆ = in “RCP4.5” – in “historical run”

Quantifying the Model Differences in Circulation and Relation with Cloud Radiative Effect Changes The amplitudes of the 1 st EOF mode differ by two orders of magnitude. The explained variance by the 1 st EOF is 57%

Direct Circulation Response and T as Mediated Change The total circulation change in the RCP4.5 scenario is highly correlated with the circulation response to direct CO 2 forcing, at a greater extent than its correlation with global-mean surface temperature change. Direct circulation response explains 76% of the variance of the total circulation change, compared to 37% by surface warming From 4xCO2 −1xCO2 fixed SST experiments (Bony et al., 2013)

Circulation Response, Cloud Feedback and Climate Sensitivity Fast circulation response to direct CO 2 forcing and the total circulation change are both correlated with cloud feedbacks Circulation Response Cloud Feedback Climate Sensitivity “ Three Cs” CO 2 Forcing

Comparing to Observations Zonal mean ω: the Hadley Circulation CloudSat/CALIPSO Cloud Fraction and AIRS/MLS Relative Humidity WARMEST 8 models: ECS > 3.4 K COOLEST 7 models: ECS < 3.4 K OBSERVATIONS

New Model Performance Metrics to Represent the Hadley Circulation Structure OBS The best estimates of ECS range from 3.6 to 4.7°C, with a mean of 4.1°C and a standard deviation of 0.3°C, compared to the multi-model-mean of 3.4°C and a standard deviation of 0.9°C.

Conclusions Changes of the Hadley Circulation exhibit latitudinally alternating weakening and strengthening structures, with nearly equal contributions by the weakening or strengthening segments to the integrated cloud radiative effect changes within the Hadley Cell. Model differences in the circulation change at the end of 21 st century in the RCP4.5 scenario is highly correlated with fast circulation response to direct CO 2 forcing and less correlated with surface warming. Circulation response is strongly correlated with cloud feedback and model difference in circulation change explains about 50% of the inter-model spread in cloud feedback. High sensitivity models simulate better the spatial variations of clouds and relative humidity in association with the Hadley Circulation than the low sensitivity models, consistent with earlier studies (Fasullo and Trenberth, 2012; Klein et al., 2013). Su et al, Weakening and Strengthening Structures in the Hadley Circulation Change under Global Warming and Implications for Cloud Response and Climate Sensitivity, J. Geophys. Res., in review, 2013.