Objective identification method and variation characteristics of the Northeast China cold vortex from May to August of 1979—2018

Northeast China cold vortex (NECV) is an important weather system in Northeast China. Based on the ERA-Interim four times daily reanalysis data provided by ECMWF, the existing objective identification method is improved and the idea of weather map analysis is applied to the objective identification of cold vortex. 516 Northeast China cold vortex processes in the summers (May to August) from 1979 to 2018 are extracted in this study. In order to study the difference in the NECV initial position, the active region of the NECV is divided into four quadrants with 45°N as the X-axis and 125°E as the Y-axis. Results show that in summer, the frequency of NECV formation is the highest in the second quadrant (northwest) and the lowest in the fourth quadrant (southeast). The NECVs are mainly moving to the east. The average persistence time of the NECV in summer is 3.2 d, and the persistence times are quite different for NECVs generated in different quadrants. The longest persistence time is found for NECVs generated in the second quadrant (3.5 d on average) while the shortest is found for those generated in the fourth quadrant (2.9 d on average). 72.1% of the NECVs have a lifespan less than 4 d. The frequency of the NECV generatation in the second quadrant in each month of the summer is much more than that in the other three quadrants, and decreases month by month, demonstrating an obvious characteristic of intraseasonal variation. The initial position and the frequency of the NECV both have obvious interdecadal variation characteristics. In May, the NECV initial position is located further north from 1998 to 2007 and further south from 2008 to 2018. In June, it moves southward from 1980 to 1998 and northward from 2008 to 2018. It is also located further east from 1993 to 2003 and further west from 2004 to 2018. In July, it moves northward from 1998 to 2010, westward from 1985 to 2000 and eastward from 2005—2012. In August, it is located further north from 2002 to 2010 and further east from 2009 to 2018. In terms of frequency, especially in the second quadrant, the interdecadal variation of the NECV is obviously inconsistent within the season. There are more NECVs from 1979 to 1985 and fewer from 1986 to 1992 in May; more from 1979 to 1992 and fewer from 1993 to 2018 in June; more from 1980 to 1991 and fewer from 1992 to 2000 in July.