Cloud–radiation feedback found to be key factor in conflicting model projections of tropical Pacific warming

The study says that ‘inter-model uncertainty is one of the most severe problems in global climate change projections’. That begs the question: how many (known) severe problems are there? The lead author of the study says the ultimate goal is ‘providing a more convincing basis for estimating future climate changes’. Solving the tropical Pacific problems described in this study would be a step forward for climate models, but meanwhile how much significance can they realistically claim?
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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, says Phys.org.

The study, published in Advances in Atmospheric Sciences, 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.”
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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.

Full article here.
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Study: Causes of Differences in the Tropical Pacific SST Warming Pattern Projected by CMIP6 Models

Quote from the study:
However, there has been a long-standing uncertainty as to whether the zonal SST gradient in the tropical Pacific will be strengthened or weakened under global warming (Seager et al., 2019; Watanabe et al., 2021). On the one hand, the majority of climate models, such as those from phase 5 of the Coupled Model Intercomparison Project (CMIP5), project a future weakened east-west SST gradient—the so-called “El Niño-like” warming pattern (Huang and Ying, 2015)—which lies in stark contrast to observations that depict a strengthened east-west SST gradient in recent decades—the “La Niña-like” warming pattern (Seager et al., 2022). This model–observation discrepancy lowers the reliability of model-projected results, as part of the discrepancy is believed to be caused by different responses to external forcing (Lee et al., 2022; Heede and Fedorov, 2023). On the other hand, there are also prominent inter-model differences in the projected tropical Pacific SST warming (TPSW) pattern under the same future emission scenario. Although most climate models project an El Niño-like warming pattern, a few models still project a La Niña-like warming pattern (Kohyama et al., 2017) or a zonally uniform pattern (DiNezio et al., 2009). Such inter-model uncertainty is one of the most severe problems in global climate change projections (Ma and Xie, 2013; Cai et al., 2021). Here, we focus on the inter-model difference in the projected TPSW pattern while leaving the model–observation discrepancy in the TPSW pattern to be examined in other studies. [bold added]