Contribution of Natural Decadal Variability to Global-Warming Acceleration and Hiatus

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Monday, 5 January 2015: 2:00 PM
122BC (Phoenix Convention Center - West and North Buildings)
Masahiro Watanabe, Atmosphere and Ocean Research Institute, The University of Tokyo, Kashiwa-shi, Chiba, Japan; and H. Shiogama, H. Tatebe, M. Hayashi, M. Ishii, and M. Kimoto

Reasons for the apparent pause in the rise of global-mean surface air temperature (SAT) after the turn of the century has been a mystery, undermining confidence in climate projections. Recent climate model simulations indicate this warming hiatus originated from eastern equatorial Pacific cooling associated with strengthening of trade winds. Using a climate model that overrides tropical wind stress anomalies with observations for 19582012, we show that decadal-mean anomalies of global SAT referenced to the period 19611990 are changed by 0.11, 0.13, and 0.11 K in the 1980s, 1990s, and 2000s, respectively, without variation in human induced radiative forcing. They account for about 47%, 38%, and 27% of the respective temperature change. The dominant wind stress variability consistent with this warming/cooling represents the deceleration/acceleration of the Pacific trade winds, which can be robustly reproduced by atmospheric model simulations forced by observed sea surface temperature (SST) excluding anthropogenic warming components. Results indicate that inherent decadal climate variability contributes considerably to the observed global-mean SAT time series, but that its influence on decadal-mean SAT has gradually decreased relative to the rising anthropogenic warming signal.