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Stuecker, MF, Bitz CM, Armour KC, Proistosescu C, Kang SM, Xie SP, Kim D, McGregor S, Zhang WJ, Zhao S, Cai WJ, Dong Y, Jin FF.  2018.  Polar amplification dominated by local forcing and feedbacks. Nature Climate Change. 8:1076-+.   10.1038/s41558-018-0339-y   AbstractWebsite

The surface temperature response to greenhouse gas forcing displays a characteristic pattern of polar-amplified warming(1-5), particularly in the Northern Hemisphere. However, the causes of this polar amplification are still debated. Some studies highlight the importance of surface-albedo feedback(6-8), while others find larger contributions from longwave feedbacks(4,9,10), with changes in atmospheric and oceanic heat transport also thought to play a role(11-16). Here, we determine the causes of polar amplification using climate model simulations in which CO2 forcing is prescribed in distinct geographical regions, with the linear sum of climate responses to regional forcings replicating the response to global forcing. The degree of polar amplification depends strongly on the location of CO2 forcing. In particular, polar amplification is found to be dominated by forcing in the polar regions, specifically through positive local lapse-rate feedback, with ice-albedo and Planck feedbacks playing subsidiary roles. Extra-polar forcing is further shown to be conducive to polar warming, but given that it induces a largely uniform warming pattern through enhanced poleward heat transport, it contributes little to polar amplification. Therefore, understanding polar amplification requires primarily a better insight into local forcing and feedbacks rather than extra-polar processes.

Li, G, Xie SP, He C, Chen ZS.  2017.  Western Pacific emergent constraint lowers projected increase in Indian summer monsoon rainfall. Nature Climate Change. 7:708-+.   10.1038/nclimate3387   AbstractWebsite

The agrarian-based socioeconomic livelihood of densely populated South Asian countries is vulnerable to modest changes in Indian summer monsoon (ISM) rainfall(1-3). How the ISM rainfall will evolve is a question of broad scientific and socioeconomic importance(3-9). In response to increased greenhouse gas (GHG) forcing, climate models commonly project an increase in ISM rainfall(4-9). This wetter ISM projection, however, does not consider large model errors in both the mean state and ocean warming pattern(9-11). Here we identify a relationship between biases in simulated present climate and future ISM projections in a multi-model ensemble: models with excessive present-day precipitation over the tropical western Pacific tend to project a larger increase in ISM rainfall under GHG forcing because of too strong a negative cloud-radiation feedback on sea surface temperature. The excessive negative feedback suppresses the local ocean surface warming, strengthening ISM rainfall projections via atmospheric circulation. We calibrate the ISM rainfall projections using this 'present-future relationship' and observed western Pacific precipitation. The correction reduces by about 50% of the projected rainfall increase over the broad ISM region. Our study identifies an improved simulation of western Pacific convection as a priority for reliable ISM projections.

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.