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Li, G, Xie SP, Du Y, Luo YY.  2016.  Effects of excessive equatorial cold tongue bias on the projections of tropical Pacific climate change. Part I: the warming pattern in CMIP5 multi-model ensemble. Climate Dynamics. 47:3817-3831.   10.1007/s00382-016-3043-5   AbstractWebsite

The excessive cold tongue error in the equatorial Pacific has persisted in several generations of climate models. Based on the historical simulations and Representative Concentration Pathway (RCP) 8.5 experiments in the Coupled Model Intercomparison Project phase 5 (CMIP5) multi-model ensemble (MME), this study finds that models with an excessive westward extension of cold tongue and insufficient equatorial western Pacific precipitation tend to project a weaker east-minus-west gradient of sea surface temperature (SST) warming along the equatorial Pacific under increased greenhouse gas (GHG) forcing. This La Nia-like error of tropical Pacific SST warming is consistent with our understanding of negative SST-convective feedback over the western Pacific warm pool. Based on this relationship between the present simulations and future projections, the present study applies an "observational constraint" of equatorial western Pacific precipitation to calibrate the projections of tropical Pacific climate change. After the corrections, CMIP5 models robustly project an El Nio-like warming pattern, with a MME mean increase by a factor of 2.3 in east-minus-west gradient of equatorial Pacific SST warming and reduced inter-model uncertainty. Corrections in projected changes in tropical precipitation and atmospheric circulation are physically consistent. This study suggests that a realistic cold tongue simulation would lead to a more reliable tropical Pacific climate projection.

Lintner, BR, Langenbrunner B, Neelin JD, Anderson BT, Niznik MJ, Li G, Xie SP.  2016.  Characterizing CMIP5 model spread in simulated rainfall in the Pacific Intertropical Convergence and South Pacific Convergence Zones. Journal of Geophysical Research-Atmospheres. 121:11590-11607.   10.1002/2016jd025284   AbstractWebsite

Current-generation climate models exhibit various errors or biases in both the spatial distribution and intensity of precipitation relative to observations. In this study, empirical orthogonal function analysis is applied to the space-model index domain of precipitation over the Pacific from Coupled Model Intercomparison Project Phase 5 (CMIP5) simulations to explore systematic spread of simulated precipitation characteristics across the ensemble. Two significant modes of spread, generically termed principal uncertainty patterns (PUPs), are identified in the December-January-February precipitation climatology: the leading PUP is associated with the meridional width of deep convection, while the second is associated with tradeoffs in precipitation intensity along the South Pacific Convergence Zone, the Intertropical Convergence Zone (ITCZ), and the spurious Southern Hemisphere ITCZ. An important factor distinguishing PUPs from the analogy to time series analysis is that the modes can reflect either true systematic intermodel variance patterns or internal variability. In order to establish that the PUPS reflect the former, three complementary tests are performed by using preindustrial control simulations: a bootstrap significance test for reproducibility of the intermodel spatial patterns, a check for robustness over very long climatological averages, and a test on the loadings of these patterns relative to interdecadal sampling. Composite analysis based on these PUPs demonstrates physically plausible relationships to CMIP5 ensemble spread in simulated sea surface temperatures (SSTs), circulation, and moisture. Further analysis of atmosphere-only, prescribed SST simulations demonstrates decreased spread in the spatial distribution of precipitation, while substantial spread in intensity remains. Key Points Systematic spread in CMIP5 simulation of Pacific region rainfall is investigated by using empirical mode reduction techniques Two significant modes of model spread are identified for the DJF rainfall climatology These modes are interpreted in terms of spread in simulated patterns of SST and circulation

Long, SM, Xie SP.  2016.  Uncertainty in tropical rainfall projections: Atmospheric circulation effect and the ocean coupling. Journal of Climate. 29:2671-2687.   10.1175/jcli-d-15-0601.1   AbstractWebsite

Uncertainty in tropical rainfall projections under increasing radiative forcing is studied by using 26 models from phase 5 of the Coupled Model Intercomparison Project. Intermodel spread in projected rainfall change generally increases with interactive sea surface temperature (SST) warming in coupled models compared to atmospheric models with a common pattern of prescribed SST increase. Moisture budget analyses reveal that much of the model uncertainty in tropical rainfall projections originates from intermodel discrepancies in the dynamical contribution due to atmospheric circulation change. Intermodel singular value decomposition (SVD) analyses further show a tight coupling between the intermodel variations in SST warming pattern and circulation change in the tropics. In the zonal mean, the first SVD mode features an anomalous interhemispheric Hadley circulation, while the second mode displays an SST peak near the equator. The asymmetric mode is accompanied by a coupled pattern of wind-evaporation-SST feedback in the tropics and is further tied to interhemispheric asymmetric change in extratropical shortwave radiative flux at the top of the atmosphere. Intermodel variability in the tropical circulation change exerts a strong control on the spread in tropical cloud cover change and cloud radiative effects among models. The results indicate that understanding the coupling between the anthropogenic changes in SST pattern and atmospheric circulation holds the key to reducing uncertainties in projections of future changes in tropical rainfall and clouds.

Kamae, Y, Ogura T, Watanabe M, Xie SP, Ueda H.  2016.  Robust cloud feedback over tropical land in a warming climate. Journal of Geophysical Research-Atmospheres. 121:2593-2609.   10.1002/2015jd024525   AbstractWebsite

Cloud-related radiative perturbations over land in a warming climate are of importance for human health, ecosystem, agriculture, and industry via solar radiation availability and local warming amplification. However, robustness and physical mechanisms responsible for the land cloud feedback were not examined sufficiently because of the limited contribution to uncertainty in global climate sensitivity. Here we show that cloud feedback in general circulation models over tropical land is robust, positive, and is relevant to atmospheric circulation change and thermodynamic constraint associated with water vapor availability. In a warming climate, spatial variations in tropospheric warming associated with climatological circulation pattern result in a general weakening of tropical circulation and a dynamic reduction of land cloud during summer monsoon season. Limited increase in availability of water vapor also reduces the land cloud. The reduction of land cloud depends on global-scale oceanic warming and is not sensitive to regional warming patterns. The robust positive feedback can contribute to the warming amplification and drying over tropical land in the future.

Zhou, ZQ, Xie SP.  2015.  Effects of climatological model biases on the projection of tropical climate change. Journal of Climate. 28:9909-9917.   10.1175/jcli-d-15-0243.1   AbstractWebsite

Climate models suffer from long-standing biases, including the double intertropical convergence zone (ITCZ) problem and the excessive westward extension of the equatorial Pacific cold tongue. An atmospheric general circulation model is used to investigate how model biases in the mean state affect the projection of tropical climate change. The model is forced with a pattern of sea surface temperature (SST) increase derived from a coupled simulation of global warming but uses an SST climatology derived from either observations or a coupled historical simulation. The comparison of the experiments reveals that the climatological biases have important impacts on projected changes in the tropics. Specifically, during February-April when the climatological ITCZ displaces spuriously into the Southern Hemisphere, the model overestimates (underestimates) the projected rainfall increase in the warmer climate south (north) of the equator over the eastern Pacific. Furthermore, the global warming-induced Walker circulation slowdown is biased weak in the projection using coupled model climatology, suggesting that the projection of the reduced equatorial Pacific trade winds may also be underestimated. This is related to the bias that the climatological Walker circulation is too weak in the model, which is in turn due to a too-weak mean SST gradient in the zonal direction. The results highlight the importance of improving the climatological simulation for more reliable projections of regional climate change.