Evaluation of North Pacific Intermediate Water Simulated by HadGEM2-AO

We analyzed the North Pacific Intermediate Water (NPIW) that was simulated in 25 coupled general circulation models (CGCMs) using historical and Representative Concentration Pathway 4.5 (RCP4.5) scenario experiments of Coupled Model Intercomparison Project Phase 5 (CMIP5), focusing on the evaluation of the performance of HadGEM2-AO. A large inter-model diversity in salinity, density, and depth of the NPIW exists even though the multi-model ensemble mean (MME) is comparable to observations. It was found that the depth of the NPIW tends to be deeper in the models in which the NPIW is relatively saltier. HadGEM2-AO simulates the lightest NPIW having the lowest salinity at shallower depth, compared with other CGCMs. Future projections of the NPIW show that the temperature of the NPIW increases, but the density decreases in all CMIP5 models. It was shown that the salinity of the NPIW decreases in most models and the decrease tends to be larger in models simulating the lighter NPIW. The HadGEM2-AO projects moderate changes in the temperature and density of the NPIW out of the CMIP5 models.