Interdecadal Variations Of Autumn Cold Surge Paths Over North China And Its Possible Mechanism

Based on 1961-2014 daily surface temperature data collected at 72 stations in North China,NCEP/NCAR reanalysis data and Hadley sea-ice concentration data,this study uses HYSPLIT model to track the paths of autumn cold surges affecting North China and classified them by clustering analysis,its interdecadal variations and possible causes are investigated.The main conclusions are as follows:(1)Our results indicate that autumn cold surges affecting North China can be classified into three types according to their paths: the north type,west type,and northwest type.The north type cold surge has strongest intensity,but the northwest and west type are weaker.There was an interdecadal decrease in the intensity of autumn cold surges in the early 1970s;this was mainly due to the interdecadal decrease in north cold surges.On the other hand,the decrease of the northwest cold surges may be the main reason for the interdecadal decrease in the frequency of autumn cold surges in the early 2000 s.(2)Associated atmospheric circulation evolution and prior signal of three types cold surges are investigated: the north type is associated with a blocking type of atmospheric circulation pattern,with an enhanced stretching northeast ridge over the Ural Mountains and a transverse trough over Lake Baikal.However,the northwest type is characterized by a ridge-trough-ridge wave-train pattern that is located over the Barents Sea,West Siberian Plain,and Sakhalin Island,respectively.The west-type cold surge is related to a conversion type: a blocking system over the Ural Mountains forms four days before the cold surge occurrence,after which it becomes a wave-train type.The atmospheric signals detected prior to the occurrences of the three types of cold surges are also explored.The main signal of the north-path cold surges is that the energy propagates eastward from the Azores Islands to the Ural Mountains,and then forms a blocking high over the Urals.However,for the northwest-path cold surges,there is a weak trough over the Ural Mountains that gradually strengthens because the blocking high collapses over the Norwegian Sea.The key signal of the formation of the west-path cold surges is a blocking high over the Norwegian Sea's continuous enhancement and extension to Novaya Zemlya.(3)In the 1970 s,the decrease of summer and autumn sea ice in the Laptev sea was themain cause of the interdecadal decrease of the north cold surges.In the northern Barents Sea in 2000 s,the sea ice was abnormally low in summer and autumn,which was the main cause of the interdecadal decrease of the northwest cold surges.The NAO negative phase is conducive to the occurrence of the north and northwest cold surge,and their intensity are strong;while the NAO positive phase is favorable for the occurrence of the west cold surge,and its intensity is weak.The 1960 s and 1990 s were dominated by the north and northwest cold surge respectively,which was caused by the negative phase of NAO;while the 1970 s and2000s were dominated by the west cold surge,it was caused by the NSO positive phase.