Fig.1

Interannual variation in the frequency (a) and day number (b) of Northeast China Cold Vortex, and occurrence frequency of cold vortex with different duration (c) during the spring sowing period in Heilongjiang province from 1961 to 2021"

Fig.2

Interannual variations in the average geopotential height in the presence of the Northeast China Cold Vortex during the spring sowing period in Heilongjiang province from 1961 to 2021"

Fig.3

Interannual variation of precipitation during spring sowing period (a) and during cold vortex processes (b), and precipitation contribution rate of cold vortex (c) in Heilongjiang province from 1961 to 2021"

Table 1

Day numbers and occurrence frequency of precipitation during cold vortex processes, and the precipitation frequency of cold vortex during the spring sowing period from 1961 to 2021"

Fig.4

Average air temperature during spring sowing period and average air temperature with and without cold vortex processes in Heilongjiang province from 1961 to 2021"

Table 2

Air temperatures at different periods during the spring sowing period and during cold vortex process at different periods in Heilongjiang province from 1961 to 2021 ℃"

Fig.5

Interannual variation of minimum air temperatures during the spring sowing period, cold-vortex process and the non-cold-vortex process in Heilongjiang province from 1961 to 2021"

Fig.6

Distribution of correlation coefficients between the intensity of cold vortex and the 500 hPa geopotential height during the spring sowing period in Heilongjiang province from 1961 to 2021"

Fig.7

Composite of anomaly geopotential height field at 500 hPa (a) and its frequency of positive anomaly (b) at the same period in years with high cold vortex activity and that in low cold vortex activity (c-d) during the spring sowing period in Heilongjiang province from 1961 to 2021"

Fig.8

Difference in the composite anomaly geopotential height field at 500 hPa during the spring sowing period in years with high and low cold vortex activities (a) and the results of the t-test (b) in Heilongjiang province from 1961 to 2021"

Table 3

Average temperatures during the spring sowing period in Heilongjiang province under different circulation situations from 1961 to 2021"