On tropospheric rivers

In this thesis, we investigate atmospheric water vapor transport through a distinct synoptic phenomenon, namely, the Tropospheric River (TR), which is a local filamentary structure on a daily map of vertically integrated moisture flux. Firstly, an automated procedure for identifying and tracking these rivers (named TRICKS, i.e., the Tropospheric River Identifying and traCKing Scheme) is described and its performance is evaluated. This procedure enables the maxima of moisture flux (socalled TR cores) to be detected and accurately located. The relationships among the adjacent TR cores are then evaluated to construct the axes of rivers. A river is tracked from birth to termination and its life cycle properties are recorded, thus allowing various statistics of TR distributions and movements to be estimated. All these stages of the scheme are performed without intervention once a number of governing constants have been decided upon. We then apply the scheme to the vertically integrated moisture flux calculated from 43 years of 6-hourly NCEP/NCAR reanalyses and present a climatology of mean TR behavior. On average, there are 4 5 rivers per analysis in the Northern Hemisphere and 5 in the Southern Hemisphere. Northern Hemisphere TRs form and intensify near the eastern seaboards of Asia and North America. They move eastward and poleward during their lives before weakening in the two principal graveyards: over the Gulf of Alaska and the region to the southeast of Greenland. In comparison, Southern Hemisphere TRs are more evenly distributed and tend to form in a band extending from the southeast coast of South America into the Atlantic, across the Indian Ocean, and throughout much of middle latitudes of the Pacific sector. The corresponding genesis regions are also found to be adjacent to (or slightly equatorward to) the maximum SST gradients in these regions. It appears that both TR genesis and termination maxima tend to be displaced near the upstream equatorward flanks of cyclogenesis and lysis maxima. We suggest that the TR formation and termination could be a leading predictor for the occurrence and decaying of extratropical cyclones. TR axis length appears to be longer during the warmer season and in the Southern Hemisphere. The distance traveled by TR systems shows a broad distribution and a sizeable fraction (-25%) of systems travel in excess of 3000 km. One unique feature is that although TRs occur very actively over the Indian ocean sector with highly densed tracks, large translational speeds, and intensities, they contribute little to meridional water vapor transport, while the reverse situation can be found in the South Pacific where southward transport is sometimes comparable to those over or off the east coast of South America. The rivers seem to account for a substantial fraction of the total meridional moisture transport in both middle and subtropical regions. Finally, we complete the TR climatology with an analysis of the variability and trends exhibited by many aspects of rivers during the 43-year period. The annual average number of TRs per analysis has undergone an overall increase during the last couple of decades and is more significant in the Southern Hemisphere. The greatest increases occur in the 50'-30'S and 50'-70'N belts. We suggest that warming and cooling trends in surface air temperature are accompanied, respectively, by increases and decreases in TR frequencies (particularly in the Southern Hemisphere). We also examined the extent to which changes in TR frequency and strength might be related to the variations in broader atmospheric system such as the ENSO phenomenon. The results of the mean and variability of TR behavior have added to our knowledge of atmospheric water vapor transport and represent a data set which may be used to aid in the understanding of the complex mechanisms governing the atmospheric circulation. Thesis Supervisor: Dr. Reginald E. Newell Title: Professor of Meteorology Acknowledgments Writing this thesis has incurred many debts. I owe a special debt to Professor Reginald E. Newell, supervisor of this thesis, for his insights, encouragement, support, and guidance throughout my studies at MIT. I greatly benefited from his knowledge of climate changes, atmospheric chemistry, meteorological data, etc. Without his enthusiasm, inspiration, and support, this work could not have been done. I extend my sincere thanks to my other thesis committee members, Drs. Alan Plumb, Kerry A. Emanuel, and Elfatih A. B. Eltahir, for their constructive criticism and helpful suggestions during the final course of this work, and their close and perceptive reading of the thesis manuscript. I have benefited from scientific communications with Drs. Ian Simmonds and Ross J. Murray of the University of Melbourne in Australia, Mark R. Sinclair of Embry-Riddle Aeronautical University in Arizona, and Isabel F. Trigo of the University of East Anglia in UK. All of them kindly provided the original codes of their cyclone finding and tracking schemes that were very helpful in developing the numerical scheme in the thesis. I also have enjoyed both the academic and social interactions with many people at MIT. I would like to thank all of my friends presently or formerly at EAPS and CE. In particular, Dr. Zhongxiang Wu, Dr. Jingfeng Wang, Dr. Xinyu Zheng, Dr. Jeremy S. Pal, Dr. Jeff Scott, Mr. Jiyong Wang and his wife Haiyan Zhang, Chris Winkler, and Bruce Kuo. I am grateful to Dr. John Y. N. Cho for proofreading the thesis manuscript and many inspiring conversations on various topics, Will Heres for computer supports, and Dorothy Frank and Mary Elliff for valuable administrative assistance. The most important people in this whole endeavor have been my parents, Zhimin Hu and Aiqing Ye. They encouraged me to pursue my true interests and would have been supportive of any path I chose. I can't express how grateful I am to my parents whose unbounded love has continuously flowed across the Pacific. Appreciation is extended also to my parents-in-law, Zhongshun Gao and Mei Chen, and my sister-in-law, Prof. Haiyan Gao. I owe a great deal to my mentors and dearest friends, Liang Fei and Xingzhang Tang at the Shanghai Meteorological Bureau, who encouraged me to challenge the new world and always be there when I need advice. Finally, I wish to express my deepest affection to my wife, Yang Gao, for her love, joy, patience, and understanding. Our love story was among one of the beautiful romantic stories that occurred on the campus. Without her sacrifice, endurance, and unconditional support during my late years at MIT, my study at MIT would not have been so smooth and colorful. It is to her that this work is dedicated.