Equilibrium climate sensitivity (ECS) - the Earth's long-term surface warming response to CO2 doubling - is one of the most fundamental benchmarks for evaluating climate change projections. ECS is governed by the Earth's energy budget, where most of the uncertainty arises from radiative feedbacks (e.g. clouds, sea ice, water vapor, lapse rate). Recent work has shown that estimates of ECS vary with time, primarily because radiative feedbacks are sensitive to the spatial pattern of sea-surface temperature, which itself evolves with time. This phenomenon has been referred to as the "pattern effect". 

What are the key mechanisms of the pattern effect? To what extent does the pattern effect contribute to the ECS uncertainty? Can GCMs well represent the observed pattern effect? What are the implications of the pattern effect for constraining ECS and transient global warming? 

Selected papers: 

• Dong et al. (2019): The preeminence of the western Pacific, J. Climate

• Dong et al. (2020). Intermodel spread in the pattern effect and its contribution to climate sensitivity in CMIP5 and CMIP6 models. J. Climate 

• Dong et al. (2021). Biased estimates of equilibrium climate sensitivity and transient climate response derived from historical CMIP6 simulations. GRL

• Andrews et al. (2022). On the effect of historical SST patterns on radiative feedback. JGR: Atmospheres

Climate model projections of long-term climate change under CO2 increase generally feature an El Niño-like tropical SST pattern (more warming in the eastern Pacific than in the western Pacific), a weakening of the Walker Circulation, and enhanced warming in the Southern Ocean. While this aligns with early theories of how the atmosphere and ocean respond to CO2 forcing, what we've observed over the recent decades has been a La Niña-like tropical SST pattern (with cooling trends over the eastern Pacific), strengthening of the Walker Circulation, and cooling over the Southern Ocean. Meanwhile, coupled models under historical forcings (the "historical" simulations) generally fail to reproduce the observed trend patterns. Why? What causes the discrepancies between observations and models/theories? Are we observing "forced" climate change signals or random natural variability? Are coupled models reliable? What have we missed? 

Selected papers: 

• Wills et al. (2022): Systematic climate model biases in the large‐scale patterns of recent sea‐surface temperature and sea‐level pressure change. GRL

• Watanabe et al. (2023). Two competing drivers of the recent Walker circulation trends. GRL

While It has long been appreciated that the tropics can influence the extratropics via an "atmospheric bridge" (Rossby dynamics), the "teleconnection" from the extratropics to the tropics has not been extensively investigated and yet appears to modulate the tropical climate significantly across timescales. In particular, the interactions between the Southern Ocean and the tropical Pacific have been found to play an important role in setting the global surface warming pattern over recent decades, which drives an anomalously low climate sensitivity through the pattern effect. 

What are the atmospheric and oceanic pathways linking the Southern Ocean to the tropical Pacific? How do these teleconnections modulate the tropical Pacific SST variability and long-term trends? How do these processes change in response to different forcings? 

Selected papers: 

• Dong et al. (2022). Two-way teleconnections between the Southern Ocean and the tropical Pacific via a dynamic feedback. J. Climate 

• Dong et al. (2022). Antarctic Ice‐Sheet Meltwater Reduces Transient Warming and Climate Sensitivity Through the Sea‐Surface Temperature Pattern Effect. GRL

Despite continuous global warming under increased greenhouse gases, the Southern Ocean has experienced multi-decadal cooling since 1980s, along with a slight expansion of Antarctic sea ice and long-term surface ocean freshening. These observed changes, however, are not being accurately simulated by state-of-the-art GCMs. 

What caused the observed multi-decadal Southern Ocean surface cooling and why GCMs fail to reproduce that? Is it natural variability or forced climate change? Will the observed cooling persist to the next century or should we expect to see enhanced Southern Ocean warming in the future as GCMs predicted? 

Selected papers:

• Dong et al. (2023). Recent multi-decadal Southern Ocean surface cooling unlikely caused by Southern Annular Mode trends. GRL

• Dong et al. (2022). Antarctic Ice‐Sheet Meltwater Reduces Transient Warming and Climate Sensitivity Through the Sea‐Surface Temperature Pattern Effect. GRL