Simulations of three natural decadal climate variability phenomena in CMIP5 experiments with the UKMO HadCM3, GFDL-CM2.1, NCAR-CCSM4, and MIROC5 global earth system models

Abstract

The ability of four Earth System Models (ESMs) to simulate major attributes of sea-surface temperature (SST) manifestations of the Pacific Decadal Oscillation (PDO), the tropical Atlantic SST gradient (TAG) variability, and the West Pacific Warm Pool (WPWP) SST variability was assessed. Data from simulation experiments conducted under the CMIP5 with the CM2.1, HadCM3, MIROC5, and CCSM4 ESMs from 1861 to 2005 were used. A Multi-Model Ensemble was also formed by combining data from the four ESMs. Aerosol optical depths and solar radiation were specified in these experiments. The simulation of PDO spatial pattern, annual cycle, and preferred timescales is the closest to observed attributes in these ESMs. The simulated and observed TAG SST patterns are generally similar, but their average annual cycles are very different and simulated TAG variability does not have a preferred decadal timescale unlike observed variability. No one of the ESMs is able to simulate the WPWP SST pattern. Simulated and observed average annual cycles of the WPWP SST are similar, and the dominant timescale of WPWP variability is close to 8 years in all ESMs and observations. Simulation skills of the three phenomena fluctuate from decade to decade in all ESMs. Major volcanic eruptions substantially influence observed and simulated indices and simulation skills of the three phenomena. The mixing of net surface heat flux anomalies to 200 m depth in the tropical and mid-latitude Pacific, and the WPWP region causes a delayed and multiyear response of the PDO and WPWP indices to volcanic eruptions.