Study On The Relationship Between Anomalous Large-scale Atmospheric Circulation Patterns And Climate Anomalies Over East Asia Based On Self-organizing Map Neural Network

Anomalous large-scale atmospheric circulation is considered as one of the main drivers causing the regional climate anomalies.The application of data mining and statistical analysis on the re-analysis data is very useful for identifying the atmospheric circulation characteristics and the rules of climate extremes.In this thesis,the self-organizing map(SOM)method was firstly used to classify the large-scale anomalous atmospheric circulation patterns over East Asia during 1979-2017.The relationships between atmospheric circulation patterns and surface air temperature variations then were established to explore the possible formation mechanism of climate changes in China.Secondly,the anomalous atmospheric circulation patterns were further related to the temperature anomalies and precipitation anomalies over the East Asia Monsoon region to reveal the climate extremes under the combing effect of atmospheric circulation and monsoon.Finally,in order to explore the characteristics of climate changes and their large-scale circulation background,the atmospheric circulation anomaly patterns and sea surface temperature anomaly patterns in the strong positive/negative phase of Western Pacific mode(WP),Arctic Oscillation(AO)and El Ni?o-Southern Oscillation(ENSO)were composited to examine the role of teleconnections on the climate fluctuations in local areas in terms of atmospheric and oceanic process.The main conclusions in this study were drawn as follows:(1)The large-scale anomalous atmospheric circulation patterns over East Asia could be identified based on SOM method.(2)AO severely contributed to the surface air temperature variations during the winter within China,especially in northeastern area,while its effect was weakened prominently in summer.A La Ni?a event in winter enhanced the anomalous warm throughout the whole region of China,whereas an El Ni?o event was linked to the anomalous cold over northern area.The temperature anomalies in summer within China were less correlated with ENSO.(3)Difference of pattern frequencies and extreme occurrence intensity(EOI)in winter and in summer for the two periods of 1979-1998 and 1999-2017 were calculated to investigate the possible mechanism of changes of temperature extremes.Our analysis implied that the changes of EOI were mainly favored by the thermodynamics factor,whereas dynamic factor has little contribution on temperature extreme changes,and sometimes even has a negative effect.(4)The precipitation anomalies and extremes associated with anomalous large-scale atmospheric circulation patterns over East Asia during the winter and summer were presented in the fourth chapter.The precipitation extremes in winter were mostly enhanced by the joint effect of the horizontal and vertical atmospheric circulations.By contrast,the precipitation anomalies in summer were merely explained by the upward atmospheric motion.(5)The strong East Asian Summer Monsoon(EASM)enhanced the cold anomalies and extremes over most of the region within East Asia.Nevertheless,the relationship between the East Asian Winter Monsoon(EAWM)and precipitation extremes depended on the geographical locations.Additionally,it was not satisfied with the ability of SOM method to capture the typical features of anomalous atmospheric circulation patterns in the strong/weak phase of EASM.(6)The WP and AO patterns played an essential part in the prevalence of prominent Tmin fluctuations over East Asia in winter,while the effect of ENSO was relatively weak.Besides,the daily cold extremes were primarily triggered by the local-synoptic atmospheric circulations embedded in the large-scale teleconnection modes,while the monthly cold extremes have a closer relationship with these low-frequency modes.