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Cloud–radiation feedback found to be key to diverse tropical Pacific warming projections

New research has uncovered why different climate models offer varying projections of sea surface temperature (SST) changes in the tropical Pacific, a region critical for global climate patterns. The study, published in identifies cloud–radiation feedback as the dominant source behind these differences.

Reliable projections of the tropical Pacific SST warming (TPSW) pattern are crucial for understanding how global climate will change in a warming world.

While the latest climate models from the Coupled Model Intercomparison Project Phase 6 (CMIP6) generally project an El Niño-like warming pattern—characterized by a weakening of the zonal SST gradient between the eastern and western tropical Pacific—the intensity of such weakening varies significantly across models.

Using advanced statistical techniques and heat budget analysis, researchers from China and the UK analyzed data from 30 CMIP6 models. They discovered that the diversity in projecting TPSW patterns stems from two distinct cloud–radiation feedbacks:

• Eastern Pacific: Different cloud–radiation feedback over the eastern Pacific drive varying magnitudes of El Niño-like warming. This is the leading source of uncertainty in the projected TPSW among models, particularly in the far eastern equatorial Pacific.
• Central Pacific: Different negative cloud–radiation feedback over the central Pacific, coupled with ocean–atmosphere interactions including the wind–evaporation–SST (WES) feedback and the Bjerknes feedback, determines the different warming in the western Pacific. Most models underestimate this negative feedback, resulting in projections of stronger warming in the western Pacific than the multi-model average.

"These findings highlight the critical role of cloud–radiation feedback in shaping how different climate models project future warming patterns in the tropical Pacific," said Dr. Jun Ying, lead author of the study, from the Second Institute of Oceanography, Ministry of Natural Resources, China. "Unraveling these mechanisms brings us closer to producing more reliable climate projections."

The study warns that the underestimated negative cloud–radiation feedback in models could mean the real-world tropical Pacific in the future will exhibit even stronger El Niño-like warming than currently projected, which is associated with more severe climate extremes, such as intense storms and prolonged droughts, underscoring the importance of improving climate model projections.

"Previous studies have identified the 'pattern effect' as being important in modifying the magnitude of different climate feedbacks, but here we consider climate feedbacks as being important in shaping the patterns of SST change. Moreover, this is one of the first to consider low-cloud feedbacks as being important in shaping the patterns of SST change," said Prof. Matthew Collins, one of the corresponding authors, from the University of Exeter, UK.

A major challenge lies in reconciling the observed long-term SST trend, which suggests a La Niña-like warming (enhanced west-minus-east SST gradient), with the El Niño-like pattern projected by CMIP6 models. The research team aims to resolve this discrepancy in the next study.

"Our ultimate goal is to refine model projections of the tropical Pacific SST warming pattern, providing a more convincing basis for estimating future climate changes," Dr. Ying added.