Characteristics And Causes Of The Cold Summer Climate In The Southern Part Of China

Using the daily data from China Meteorological Administration and those from NCEP/NCAR reanalysis,we have analyzed the features of the low temperature events occurred in the southern part of China,especially in the middle and lower reaches of the Yangtze River as well as the related circulation changes,during summer from 1960 through to 2004.The results are summarized as follows:(1)The temperature trends in the southern part of China have a clear transition in the late 1970s.The temperature is lower than normal before the late 1970s and higher after that time.(2)The low temperature events occurred more frequently in the middle and lower reaches of the Yangtze River.The three leading EOFS shows very interest features in temperature changes.EOF1 shows the same signed temperature anomalies in Yangtze River region with the obvious inter-annual and centurial variations in temperature.EOF2 shows an east-west oppositely signed pattern with periodicities of inter-annual and long-term trend.EOF3 shows a south-north oppositely signed pattern with temperature varying on inter-annual and inter-decadal.(3) Both the general circulation changes at different levels and the precipitation along with the total cloud cover are responsible for the formation of low temperature events during boreal summer seasons.Larger amount of precipitation,increased cloud cover,intensified mei-yu front,and weaker subtropical high to the south of its normal position facilitate the low temperature events.(4)The general circulation,the precipitation,and the heating fields are different in typical high and low temperature years.During the summertime of 1999,the vertical thermal advection is negatively anomalous and the horizontal one is almost equal to zero.On the other hand,the diabatic heating is positively anomalous. Therefore,the dynamic cooling related to vertical thermal advection is the cause of low temperature events in summer in Yangtze River region.(5)During an ELNino year,it is likely to be colder in the middle and lower reaches of the Yangtze River during boreal summer season.Both Ninol+2 and Nino3 indices which reflect SSTA in the Pacific Ocean have good simultaneous correlations with the temperature whereas Nino4 index shows good leading and lagging correlations with the temperature in Yangtze River region.All these suggest that the SSTA in tropical Pacific is an important indicator of the anomalous temperatures.