Congratulations to Liu hui Passing her Ph.D. Dissertations smoothly
Liu hui successfully passed through the doctoral dissertation defense on May 7th 2015 at the 101 conference room of the IAP scientific research building. Her doctoral thesis title is the circulation modes associated with the East Asian winter temperature change in the last 100 years and the climatic response to Eurasian vegetation change in the future.
Abstract：In the past few decades, global warming has become a fact that cannot be ignored. Climate change has a huge impact on the economic and social life of people; it has become a topic of great concern of the governments and the public. However, there still is a big controversy for the impacts and the reasons of global warming. Especially, the regional climate change and its impacts are of great concerns from the Intergovernmental Panel on Climate Change （IPCC）. In this paper, we based on stations and gridded data of temperature and precipitation in China, sea level pressure field and temperature data from the UK Hadley center and Climate Research Unit （CRU） over the last 100 years, 20th century reanalysis （20CR）, NCEP2 reanalysis and the historical simulation and future projection data of Japan MIROC5（Model for Interdisciplinary Research on Climate Version 5）from CMIP5（Coupled Model Intercomparison Project Phase 5）. From two perspectives as the historical changes and the future projections, use statistical analysis and model projections as two major means, the thesis explores the atmospheric circulation modes of East Asia in the past one hundred years, and the effects of vegetation cover changes on the regional climate in the late 21st century in northern Eurasia. Firstly, we evaluate the 20CR reanalysis data on the circulation climatology of both the northern and southern hemispheres and its performance on centuries scale. Then we use the 20CR data to research the typical warm and cold periods of East Asia winter temperatures and the associated sea level and 500 hPa atmospheric circulation patterns in the last 100 years. This may provide the basis knowledge for East Asia warming in recent decades. Then, we use the high-resolution climate model WRF（The Weather Research and Forecasting Model）, focus on climate change and the impact of vegetation feedback in Eurasian region under RCP8.5 typical concentrations path at the end of 21 century, to provide a reference for a more reasonable estimate of future research on climate and human adaptability. The main conclusions are as follows:
（1） 20CR reanalysis data can well describe the characteristics and seasonal march of main circulation mode in Eurasia and the winter doublet jets in Southern Hemisphere. However, compared with NCEP2 data, there are still some biases. Such as climate field shows systematically higher in the northern high latitudes in Northern Eurasia, systemic lower in the southern part. By comparing the past century climate indexes we found that 20CR reanalysis data can well depict interannual and decadal variability of many winter and summer climate indexes in both hemispheres. Winter temperatures and winter climate indexes are better depicted than summer. But it cannot reproduce the late 1970s weaken of East Asian summer monsoon. In addition, before 1920, there are some large differences between 20CR data and observations.
（2） Based on 20CR data, we identified East Asian winter temperatures changes and its associated atmospheric circulation modes use the SVD method. In the last 100 years, there were two main East Asian winter temperatures change types. One was Southwest - northeast opposite change type, the other was the consistent change type in the south of 40°N. The Eurasia Teleconnection, North Atlantic Oscillation and Siberian High were the main factors that affect the first change type. And the West Pacific Teleconnection and the North Pacific Oscillation modes were the main factors for the second change type. Then we analyzed their respective coupling modes characteristics in three different periods. It was shown the coupled atmospheric circulations above differs in each period and can affect the interannual variability of temperature. In particular, the EU and NAO mode were strongest in the third period. The range of Siberian high began to expand eastwardly and showed close contact with the polar circulation in the late 20th century. This means the polar circulation played an important role in East Asia temperature change. This may account for the warming trend in East Asia in late 20th century.
（3） The WRF model was used to project the future climate in Eurasia in the late 21th century （2089—2099） under RCP8.5 typical concentrations paths. The results showed that, compared to the late 20th century （1996—2005）, a significant warming was found in Eurasia with an annual average warming of 5.85°C. Winter showed greater warming （7.86°C）. And Northern warming rate significantly exceeded the southern parts. Precipitation showed inconsistent changes. More precipitation occurred in Siberian mainland, the Mongolian Plateau, northeast China Plain and less in the Yangtze River valley of China.
（4） After considering the forest expansion in northern Eurasia, compared with the projection results without the forest expansion, significant changes in temperature and precipitation showed in summer, while the other seasons showed no clear changes. In summer, the roughly trends showed increased precipitation and decreases temperature in Siberia and increased temperature in Mongolian plateau. A significant cooling occurs in the eastern region of Siberia, the cooling rate of up to -1.6 °C, the precipitation increased at the same time.
（5） In the forest expansion zone, Eurasia vegetation changes did not bring significant local climate changes. The analysis revealed this may be because the temperatures in northern Eurasia in the 21st century had increased significantly, under this warming environment, local warming trend bought by lower albedo may balance by the cooling trend bought by increased transpiration. It means that the forest expansion in northern Eurasia caused by climate warming may not cause further temperature positive feedback.
（6） The reasons for the decreased/increased temperature/precipitation in middle east Siberia and the increased/decreased temperature/precipitation around Mongolian Plateau that brought by the forest expansion in Eurasia are as follows: The significant cooling in middle east Siberia and the warming trend in Mongolian Plateau may cause by the northward movement of Arctic frontal zone. The forest expansion in northern Eurasia may affect the meridional temperature gradient and further impact the location of Arctic frontal jet and then resulting in a significant decrease in height field over the significant cooling zone, then a significant increase in cloudiness, fewer shortwave radiative fluxes at the surface, and eventually leading to a lower temperature. While low pressure made the moisture convergence, thus precipitation increased over the significant cooling zone. Northward movement of Arctic frontal jet led to northward expansion of the westerly jet, this led to high-pressure anomaly over the Mongolian Plateau, which caused increasing temperature and less rainfall in the Mongolian plateaus. Our results showed that a large area of vegetation may cause atmospheric circulation changes that led to non-local climate response.
（7） The extreme weather events were also examined, we found that the significant cooling zone brought by vegetation change in the late 21th century in the middle east Siberia was more affected by the decreased in daily minimum temperature. The reduction in warm nights and the increase in cold nights were the key factors leading to an increase in the daily minimum temperature. Extreme precipitation significantly increased in northern Eurasia in the end of 21st century. And it differed in a similar pattern as total precipitation after forest expansion. This showed that the vegetation changes can also affect extreme weather in north Eurasia. It has positive meanings for a comprehensive understanding of future climate changes in Eurasia.