| Using Hadley SST data,the NCEP/NCAR reanalysis data,and the daily precipitation data from China 753 station from 1961 to 2016,with the methods of power spectrum analysis,Hilbert transform,a little lag correlation,and TN flux,we analyze the characteristics of summer precipitation in the middle and lower reaches of the Yangtze River(MLRYR)in Non-ENSO years and the following years of El Ni(?)o and La Ni(?)a,and the characteristics of Rossby wave packet activities and its influences on severe precipitation events in the middle and lower reaches of the Yangtze River.Results are summeriged as follows.In Non-ENSO years,there exist strong interannual variations of summer precipitation in middle and lower reaches of Yangtze River,while the heavy rainfall events occurred frequently in the early 90s.In 1993,the heavy precipitation is mainly concentrated in the late June to early July.The appearance of a strong,westerly jet upstream from the upper troposphere is beneficial to the spread of baroclinic wave package.The wave packets originat mainly from the south of Caspian Sea and traveled downstream along the path from the west to the east and eventually reached the middle and lower reaches of the Yangtze River.The wave flux indicates that wave energy can propagate from west to the east and downstream to the middle and lower reaches of the Yangtze River to be convergence,which can eventually lead to the development of heavy precipitation events in the region.In El Ni(?)o years,there exist strong interannual and interdecadal variations of summer precipitation in middle and lower reaches of Yangtze River.Among them,1983,1998,and 2016 all show significant positive precipitation anomalies.From the significant differences of the distribution of westerlies in strong and weak precipitation years,we find that the jet stream regions in the strong precipitation years are mainly located in the[40°N-45°N]and[45°N-60°N]latitude bands,providing a channel for the propagation of Rossby baroclinic waves.The wave packets mostly originate from the northwestern side of the Caspian Sea and travel downstream along the northwest-southeast path during periods of strong precipitation.The energy brought about by the baroclinic waves play a significant role in inducing strong precipitation at the base point and nearby areas.In La Ni(?)a years,there also exist strong interannual and interdecadal variations of summer precipitation in middle and lower reaches of Yangtze River.1996,1999,and 2011 are the years when significant positive precipitation anomalies occur.There are significant differences in the distribution of westerlies between strong and weak precipitation years.It is revealed that five times of heavy wave packet propagate downstreamward.The wave packets propagate respectively from the north and south sides of jet stream axis,originating from the western side of the Eurasian continent.The energy brought about by the baroclinic waves has a significant impact on loading to severe precipitation at the base point and nearby areas.Comparing the characteristics of baroclinic Rossby wave packets in Non-ENSO years,in the following years of El Ni(?)o and La Ni(?)a events,we find that the wave packets all propagate from the upstream to the middle and lower reaches of the Yangtze River,resulting in the occurrence of heavy precipitation events there.However,the differences are clear.The propagation path of Rossby baroclinic wave packets are mainly from west to east in Non ENSO years while the main path is northwest-southeast in the following years of El Ni(?)o.In the following years of La Ni(?)a,the baroclinic Rossby wave packets propagate in two ways from northside and southside of the Asian jet axis downstreamward to MLRYR.These results are conducive to a deep understanding of the formation mechanism of heavy rainfall events in the middle and lower reaches of the Yangtze River during the Non-ENSO year,El Ni(?)o years,and La Ni(?)a years. |
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