Greenhouse gas and aerosol forcings contribute differently to changes in the tropical Pacific sea surface temperature gradient in models and observations

The zonal gradient in tropical Pacific sea surface temperatures (ΔSST_west-east) plays a major role in global climate, from modulating the rate of global warming and ocean-atmosphere CO₂ fluxes, to influencing tropical cyclone genesis and regional precipitation patterns. While observations estimate that this gradient has strengthened over the historical record, most coupled climate models, from all generations up to the most recent (CMIP6), simulate a forced weakening of the zonal SST gradient over this period, a discrepancy that has been variously attributed to representations of internal variability, model mean-state biases and incorrect forced responses. In a previous study, we demonstrated that CMIP6 models fail to capture most recently-ending observed trends in ΔSST_west-east and identified signs that the observed strengthening is consistent with a forced response to rising atmospheric greenhouse gases. We additionally classified the 14 analyzed CMIP6 models into two groups according to whether they simulate a forced weakening of the gradient or a more ambiguous response over the historical period. In this study, we characterize how the forced trends in these model groups evolve under different 21st-century Shared Socioeconomic Pathways and find that even models that simulate slight gradient strengthening over the historical period ultimately simulate weakening gradients under most projections, a transition that occurs roughly in the contemporary period. To better understand why the two model groups show contrasting gradient behavior over the historical period, and more consistent behavior under projected scenarios, we conduct an empirical orthogonal function analysis to investigate the contributions of greenhouse gas (GHG) and aerosol forcings to ΔSST_west-east changes in the historical period. We further validate these analyses through use of single forcing large ensembles. We find that both greenhouse gas and aerosol modes are near ubiquitous within the model group, with these modes contributing in opposite senses to gradient changes between the model groups. A similar analysis on 4 observational products provides evidence for the influence of both GHG and aerosol modes on changes in observed ΔSST_west-east. Taken together, these findings quantify contributions from both GHG and aerosol forcing to changes in observed and modeled ΔSST_west-east, providing increased understanding of real-world ΔSST_west-east changes and the origin of model responses that yield unrealistic gradient changes over the historical period.