Congratulations to Sun bo Passing his Ph.D. Dissertations smoothly
Sun bo successfully passed through the doctoral dissertation defense on May 7th 2015 at the 101 conference room of the IAP scientific research building. His doctoral thesis title is the moisture sources of precipitation in different regions of China and related climate dynamics .
Based on observational station data from data archives of China Meteorological Administration and National Climate Centre and reanalysis data from NCEP and ECMWF, the moisture sources of precipitation in different regions of China (including Northeast China, North China, the middle and lower Yangtze River valleys, South China, and the semi-arid grassland areas) are investigated in this paper using statistical analysis methods and Lagrangian particle dispersion model FLEXPART, results of which contribute to a better understanding of the atmospheric water cycle over China. In addition, this paper analyzed the inter-decadal variations of precipitation and water vapor transport over East China during 1951‒2012, and further revealed the inter-decadal transition of the leading EOF (Empirical Orthogonal Function) mode of the inter-annual variability of summer precipitation in East China and its underlying mechanisms, which is related to the tropical air-sea interaction activities. The results are summarized as follows.
(1) The moisture supplied to the widespread snowfall in Northeast China in winter is mainly from East China, the Bohai Sea, the Yellow Sea, and the Sea of Japan. Water vapor carried into Northeast China is mainly from East China and its adjacent seas (the Bohai Sea and the Yellow Sea) in the 24 hours before the widespread snowfall outbreaks, whereas the Bohai Sea, the Yellow Sea, and the Sea of Japan are the most important sources of moisture taken into Northeast China after the widespread snowfall outbreaks. There is a large amount of moisture accumulated over Northeast China in the 24 hours before the widespread snowfall outbreaks due to southerly water vapor transport, which is crucial for the occurrence and persistance of the widespread snowfall in Northeast China. The water vapor transport concurrent with the widespread snowfall solely cannot provide sufficient moisture to the widespread snowfall, which consumes a vast mass of moisture. Moreover, the evolution of budget of the atmospheric moisture transport over Northeast China leads the precipitation in Northeast China for approximately 18 hours.
(2) The simulation results of the Lagrangian particle dispersion model FLEXPART indicate that the moisture contributing to the precipitation over the semi-arid grassland areas in China is mainly from the Eurasian continent, in both summer and winter, whereas moisture from oceans makes little contribution. This is because the semi-arid regions in China are far away from oceans. A large portion of moisture from the oceans is lost on its way to the semi-arid grassland areas of China due to precipitation or topography obstruction, and thus only a limited amount of moisture from the oceans can be taken into the semi-arid regions and becomes precipitation. On the other hand, oceanic moisture from the Indian Ocean and the western Pacific is quite important for precipitation in East China. In summer, moisture from the Indian Ocean and moisture from the western Pacific both make substantial contribution to the precipitation in South China; whereas, moisture from land evaporation (mainly East China and South China) is the major contributor to the precipitation in North China and the Yangtze River basin. In winter, the western Pacific is the most important moisture sources for precipitation in South China, whereas moisture from land area (mainly East China and South China) still makes a major contribution to the precipitation in the Yangtze River basin and North China.
(3) During 1951–2012, the summer precipitation in East China underwent four inter-decadal changes which occurred around the mid-1960s, the end of 1970s, the early 1990s, and the end of 1990s in sequence. The inter-decadal changes at mid-1960s and the end of 1970s are both featured by significantly weakened meridional water vapor transport over East China, which is mainly caused by the weakening of East Asian summer monsoon. In contrast, the inter-decadal changes around the early 1990s and the end of 1990s are featured by anticyclonic and/or cyclonic water vapor transport anomalies over the subtropical western Pacific and East Asia, which is related to the inter-decadal changes of the western Pacific subtropical high. The most significant inter-decadal change of winter precipitation in East China occurred around the mid-1980s, which is featured by southeasterly water vapor transport anomalies and widespread increases in winter precipitation over East China. This inter-decadal change is essentially caused by the significant weakening of East Asian winter monsoon after mid-1980s. Moreover, it seems that the winter precipitation in East China underwent two more inter-decadal changes around the end of 1970s and 2000. The former inter-decadal change is related to the temporary strengthening of the East Asian winter monsoon, and the latter inter-decadal change is speculated to be related to the teleconnection wave-train over the Eurasia, which needs further research.
(4) The leading EOF mode of the inter-annual variability of summer precipitation in East China as well as its related water vapor transport is found to have been changed in recent decades. During 1951–2000, the leading EOF mode of the summer precipitation in East China features a “– + –” meridional pattern, whereas during 2001–2012 it shifted to a “– +” pattern. Correspondingly, the water vapor transport over East Asia related to the leading mode of the summer precipitation in East China changed from an “anticyclone-cyclone” dipole structure to an anticyclonic monopole structure. This inter-decadal transition is inferred to be related to the tropical air-sea interaction activities. The inter-annual variability of Sea Surface Temperatures (SSTs) in the tropical Indian Ocean, western Pacific, and central-east Pacific are all below normal after 2000, which may result in weakened impacts on the inter-annual variability of East Asian climate exerted by tropical air-sea interaction activities. Thus the weakened forcing of tropical SST anomalies failed to motivate a strong teleconnection wave-train which can induce “anticyclone-cyclone” structured water vapor transport anomalies over East Asia, and consequently the leading mode of the summer precipitation changed.