Variability of the Pacific subtropical cells under global warming in CMIP6 models

The Pacific subtropical cells (STCs) are shallow meridional overturning circulations connecting the tropics and subtropics, and are assumed to be an important driver of the tropical Pacific decadal variability. The variability of STCs under global warming is investigated using multimodal outputs from the latest phase of the Coupled Model Inter-comparison Project (CMIP6) and ocean reanalysis products. Firstly, the volume transport diagnostic analysis is employed to evaluate how coupled models and ocean reanalysis products reproduce interior STC transport. The variation of heat transport by the interior STC under the high-emissions warming scenarios is also analyzed. The results show that the multimodal-mean linear trends of the interior STC transport along 9°S and 9°N are −0.02 Sv/a and 0.04 Sv/a under global warming, respectively, which is mainly due to the combined effect of the strengthened upper oceanic stratification and the weakening of wind field. There is a compensation relationship between the interior STC and the western boundary transport in the future climate, and the compensation relationship of 9°S is more significant than that of 9°N. In addition, compared with ocean reanalysis products, the coupled models tend to underestimate the variability of the interior STC transport convergence, and thus may lose some sea surface temperature (SST) driving force, which may be the reason for the low STC-SST correlation simulated by the model. The future scenario simulation shows that the heat transport of interior STC is weakened under global warming, with a general agreement across models.