CMIP6 Models Properly Simulate the Emergence of Global Ocean Warming Pattern

The global sea surface warming pattern emerged since the early 1980s is characterized by a boomerang-shape warming in the western Pacific, the basin-wide warming in the Indian Ocean north of 30°S, and a triple-stripe warming in the North Atlantic. This pattern can be obtained with or without El Niño/La Niña signals, indicating the independence of El Niño/La Niña, and is the leading EOF with the El Niño/La Niña signals removed. A negative phase of this pattern started emerging in the early 1980s, switched to positive phase in the 1990s, and has been becoming more prominent for the past few years.

CMIP models have been found to have difficulty simulating observed global sea surface temperature (SST) trend, especially the cooling trend in the tropical eastern Pacific. However, the cooling trend in the tropical eastern Pacific in the past four decades is statistically insignificant in our trend analysis adopting a more stringent signal detection method (namely, the False Discovery Rate, FDR). By applying the same trend detection and EOF approach to the simulated SST in the historical simulations of forty CMIP6 models by removing El Niño/La Niña signals, we detected in the ensemble mean SST a trend pattern closely resembling the observed, which also changes from negative to positive phases in the late 1990s and continues becoming more positive into 2014. Whereas each model has slightly different performance in simulating this trend pattern, the ensemble time series of corresponding trend pattern in each model correctly reflects the emergence and enhancement of the warming pattern in the past four decades. However, this model ability seems to be masked by the large fluctuations of El Niño/La Niña, an intrinsic climate mode contributing large internal variability to the global domain, and its temporal fluctuations cannot be synchronized in the coupled models in the historical experiments, which are strongly driven by continuously increasing radiative effect of greenhouse gases concentration. On the other hand, The models seem to be capable of simulating the emergence of the global ocean warming pattern in response to the prescribed increasing greenhouse gas concentration.