EVALUATION OF MARINE HEATWAVES IN THE NORTH PACIFIC OCEAN SIMULATED BY CMIP6 MODELS

As the global mean sea surface temperature (SST) increases, the frequency and intensity of marine heatwaves (MHWs) are also increasing. A model evaluation is needed to better understand future projections for MHWs. In this study, we evaluated MHWs in a historical simulation of 14 CMIP6 (Coupled Model Project Intercomparison Phase 6) models in the North Pacific Ocean (NPO) where MHW occurs frequently by comparing the OISST (Optimum Interpolation Sea Surface Temperature) reanalysis data for 33 years (1982 similar to 2014). The CMIP6 models overestimated the annual mean cumulative duration of MHWs in the NPO by approximately 25 days compared to the OISST while the frequency was underestimated by 0.3 similar to 0.6 events per year. Over 80% of CMIP6 models underestimated the spatial mean of the mean and maximum intensities, although a majority (60%) of the models overestimated the intensity north of the Kuroshio extension region, presumably by Kuroshio overshooting. Furthermore, because the CMIP6 multi-model ensemble underestimated the increasing trend regarding MHW characteristics (duration, frequency, and intensity) according to warming, it is necessary to investigate the influences of trend underestimation in the characteristics of MHWs on future changes in MHWs. Our results suggest that the MHW biases are still significant in CMIP6 models, demanding an increase in model horizontal resolution.