Climate Variability in East Asia in Simulations and Reconstructions in the Last 1200 Years

The climate variability during 800-2005 AD is investigated using ensemble simulations performed with the COSMOS-Atmosphere-Surface(Land)-Ocean-Biogeochemistry model ECHAM5/MPIOM/JSBACH forced by both anthropogenic and natural forcings. The millennium run is designed as two ensembles forced by a solar forcing with weak variability (E1, 5 members) and a solar forcing with strong variability (E2, 3 members). The study also includes a detailed comparison of the simulated results with reconstructed data in East Asia. First, the temperature variability in China is analyzed by a mutual assessment of three reconstructed and two simulated data sets which are obtained from ensembles E1 and E2. The model uncertainty is assessed by the ensemble spread as maximum and minimum in five members. After the comparison in terms of temperature anomaly and long term memory analysis, ensemble E1 and one reconstructed data set are selected for further comparison. In the three regions in China, ensemble E1 and the reconstruction reveal weak long-term anomaly periods known as the Medieval Climate Anomaly (MCA), the Little Ice Age (LIA), and the Modern Warming (MW), whereas reconstructed data hint to higherdecadal variability inWest China and centennial variability in Northeast China. Cold periods are found in 1200-1300 and in 1600-1900 AD in whole China. The coldest anomalies which are caused by volcanic eruptions in the beginnings of the thirteenth and the nineteenth centuries are only partly consistent with reconstructed data. After 1800, the annual cycle reduces in the Northeast and on the Tibetan plateau, whereas the eastern Pacific Ocean shows an enhanced summer-winter contrast. Secondly, the simulated precipitation is compared with various reconstructed drought indices in China. The mutual assessment reveals less consistency compared to the surface temperature. The evolution of precipitation is dominated by random unpredictable internal dynamics in four considered regions. The distinguishing decreasing trend in the simulated precipitation, which is not captured in the reconstructions, is analyzed through internal forcing (sensitivity experiments) and dynamical mechanism (the Asian summer monsoon system) to pursue the possible causes. The most plausible answers are attributed to the anthropogenic land cover change (ALCC), the rising fossil fuel CO2 emission and the weakening Asian summer monsoon in the last century. The analysis is further completed by the variability of the precipitation and runoff in Yangtze catchment (the biggest river in China) as well as its relationship with the snow cover in the Tibetan Plateau, El Nino/Southern Oscillation (ENSO) and the Asian summer monsoon. In the third step, the impacts of volcanic eruptions and ENSO and their cooperative effects on the climate are obtained at a global scale and compared to the reconstructed data in East Asia. Volcanoes without ENSO events cause a global cooling and a non-significant El Ni no-like warming in the tropical Pacific Ocean one year after the eruption. The following winter is warmer in Siberia and central Asia. The recovery times for the volcano induced-cooling vary globally between one and 12 years. In East Asia, the simulated temperature and drought index from E1 and E2 are compared with reconstructed data sets. Volcanoes without ENSO event in the following winter cause a dramatic cooling in West China and a drought in East China at the year one after the eruption. The reconstructed data show similar cooling patterns with smaller magnitudes in China, and similar droughts in East China. In both ensembles, there is no significant increase of El Nino events after volcanic eruptions. El Nino events in the winters after eruptions compensate the cooling in most regions of China (consistent with reconstructions), while La Nina events intensify the cooling. The simulated and reconstructed drought indices both show a tripole pattern disturbed by El Nino. The simulated impact of the eruption of the Tambora in 1815, which caused the ‘year without summer’ in 1816 in Europe and North America and a three-year famine in China, depends crucially on the ENSO state of the coupled model. The Tibetan Plateau is the biggest and highest plateau in the world and has evident dynamic and thermodynamic effects on the general circulation and the climate in East Asia. Since the ALCC turns out to be one of the factors that cause the summer precipitation deficits in the Tibetan Plateau and in eastern China, the effects of surface albedo changes on the climate in the Tibetan Plateau is analyzed using a model of intermediate complexity, Planet Simulator. The experiments include one control simulation and two sensitivity experiments. The climate change in the Tibetan Plateau and its remote influence on the summer monsoon precipitation in eastern China are analyzed, and the underlying merchanism is discussed and compared to general circulation models.