Tuesday, 24 January 2017
The Earth has warmed significantly over the past 60 years, under anthropogenic forcing. We are confident about the robustness of the global surface temperature warming pattern, which is primarily controlled by thermodynamics; however, we have much less confidence in our understanding of the atmospheric circulation aspect of climate change, which is mainly determined by these dynamics. Attribution of the cause of global circulation change over the last 60 years is hindered by our ability to separate intrinsic low-frequency variability from the forced response of the climate system to anthropogenic forcing. In this study, we characterize the spatiotemporal characteristics of intrinsic variability by comparing average Coupled Model Intercomparison Project Phase 5 (CMIP5) historical simulations to the observed trend estimated by five reanalysis data (ERA-Interim+ERA40, ERA-20c, NOAA-20th, NCEP1 and JRA55). For upper-level circulation change, both CMIP5 models and reanalysis data show that the human-caused long-term trend predominates in the tropics, and a weaker trend exists in the high latitudes of the northern and southern hemispheres. After removing this long-term trend from the circulation field, the classical El Niño Southern Oscillation (ENSO) appears to be the leading factor driving global circulation variability on an interannual time scale. On longer timescales, the tropical impact on global circulation is greatest over the polar regions and likely responsible for the destabilization of some ice shelves in the West Antarctic in the 1990s as well as for about half of the remarkable warming over Greenland and northeast Canada in the past 15 years and during the 1950–1960s. These findings suggest that circulation changes in the Arctic and Antarctic over the last 60 years have largely been internally driven as a result of the interdecadal-scale sea surface temperature (SST) variability in the tropics. Finally, I will suggest a way forward so that we can better understand the cause of global climate variability in the past 1000 years and improve projections of future global climate change.
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