Centennial Trend and Decadal-to-Interdecadal Variability of Atmospheric Angular Momentum in Climate Model Simulations

Wednesday, May 14, 2014 - 07:00
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The centennial trend and decadal-to-interdecadal variability in atmospheric angular momentum (AAM) are analyzed for all models in CMIP3 and CMIP5. The climatology and trend of AAM from the 20th century simulations in CMIP3 and CMIP5 are diagnosed and validated with the Twentieth Century Reanalysis (20CR). It is found that CMIP5 models produced a significantly smaller bias in the 20th-century climatology of the relative and mass (omega) angular momentum compared to CMIP3. The CMIP5 models also produced a narrower ensemble spread of the climatology and trend for both components of atmospheric angular momentum. Both CMIP3 and CMIP5 simulations consistently produced a positive trend in AAM for the 20th and 21st centuries. The trend for the 21st century is much greater, affirming the role of greenhouse gas (GHG) forcing in inducing the trend. The simulated increase in the relative AAM for the 20th century is consistent with reanalysis. Both CMIP3 and CMIP5 models produced a wide range of magnitudes of decadal and interdecadal variability of AAM compared to 20CR. The ratio of the simulated standard deviation of decadal or interdecadal variability to its observed counterpart ranges from 0.5 to over 2.0 for individual models. Nevertheless, the bias is largely random and ensemble averaging brings the ratio to within 18% of the reanalysis for decadal and interdecadal variability, for both CMIP3 and CMIP5. The majority of CMIP models underestimate the amplitude of interdecadal variability. This reflects the general absence of large-amplitude interdecadal climate shift events, comparable to the observed 1976 climate shift, in the model simulations. The 21st-century simulations from both CMIP3 and CMIP5 produced only a small trend in the amplitude of decadal or interdecadal variability, which is not statistically significant. Thus, while GHG forcing induces a significant increase in the climatological mean of atmospheric angular momentum, it does not significantly affect its decadal-to-interdecadal variability in the 21st century.

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