The Climate Characteristics of the Northeast China Cold Vortex against the Background of Global Warming

Abstract

In this study, by using the ERA5 data of the atmospheric circulation field that was re-analyzed by the ECMWF (European Centre for Medium-Range Weather Forecasts), we revealed the features of the Northeast China Cold Vortex (NCCV) from 1950 to 2020 (including active days, occurrence time of NCCV processes, and process durations). This study focused on a comparative analysis of the differences in the NCCV’s climate characteristics in the cold and warm periods to help future predictions. The results revealed the following: From 1950 to 2020, the NCCV occurred 2961 times on 9782 days. The average annual occurrence time of NCCV processes, annual average of cold vortex days, and average process duration of the NCCVs were 41.7 times, 137.8 days, and 3.6 days, respectively. These indicators of the NCCVs showed an increasing trend, but the trend was not significant. The NCCVs occurred most frequently in May, followed by June, and were located at the southernmost point in June. Therefore, it had the most active days and a relatively long process duration in May and June, significantly impacting Northeast China. During the cold period (1950–1980), the annual occurrence time of NCCV processes, number of cold vortex days, and the process duration of the NCCVs all showed an increasing trend, while in the warm period, these showed a decreasing trend. In addition, the durations of the NCCVs decreased significantly in the warm period, which indicated that the NCCV processes continued to weaken after climate warming. During the warm period (1981–2020), the frequency and active days of the NCCVs throughout the year and most months increased, and its general location was more southerly than in the cold period. Moreover, the annual average occurrence time of NCCV processes, number of active days, and average duration of the NCCV in the warm period were more than those in the cold period. Finally, the NCCVs continued for longer in autumn and winter than in spring and summer, and the durations of the NCCVs increased in warm periods.

1. Introduction

Northeast China is a temperate humid and semi-humid continental monsoon climate zone, with hot and rainy summers and cold and dry winters [1]. The NCCV is an important impact factor in the climate of Northeast China. The NCCV is a high-altitude quasi-stationary cut-off low. The cut-off low mostly occurs in the seasons and regions where the exchange of cold and warm air is active. In spring and autumn, the North Atlantic coast of southwestern Europe, the North Pacific region of North America, and the northwest Pacific coast of northern Asia are the frequent areas of cut-off lows in the northern hemisphere [2,3]. The most common cut-off low affecting China is the NCCV, which mostly occurs in Northeast Asia [4]. It is a crucial weather system that is unique to the climate of Northeast China and is closely related to the advance and retreat of the East Asian monsoon [5,6,7,8]. The NCCV is not only an essential cause of rainstorms, floods, low temperature, cold damage, and drought in Northeast China [9,10,11,12,13,14], but it is also an essential factor affecting the climate in other regions of China [15,16,17]. Since the 1990s, many scholars have conducted research on the identification, climatic characteristics, and cold vortex prediction [18,19,20,21,22,23] and reached many valuable conclusions [3,19,20,21]. Based on the spatiotemporal analysis of 35 years’ worth of data for the NCCV, Sun [20] suggested that the NCCV mainly occurs during the period from April to October, with greater frequencies in May to June and September, and its geographical distribution is clearly clustered. Furthermore, Xie and Bueh [24] found that the frequency of the NCCVs showed an increasing trend in summer (especially during the Meiyu period) from 1965 to 2007. Contrariwise, Hu et al. [7] concluded that the NCCVs from 1958 to 2006 had evident interannual variability and a seasonal cycle, but no apparent long-term trend. Based on an analysis of data from May to August, Liu et al. [25] found that the NCCV activities concentrated mainly in the range of 121° E to 131° E, 48° N to 53° N, and were closely related to the blocking high-pressure system in the middle and high latitudes. Gong et al. found that NCCVs usually form at the border area between China, Mongolia, and Russia, and then develop and move eastward, finally disappearing in the areas of Heilongjiang in China, southeastern Russia, and the Okhotsk Sea; they also found that the number of NCCV occurrences increased in the last 10 years compared to 1999–2008 during the warm season [5]. Liang et al. [9] also claimed that the NCCV occurred with spatial differences in different seasons in an analysis of the location of the cold vortex in different seasons.

However, previous studies generally ignored the impacts of global warming on the NCCV, and this requires further investigation. This study divided the period of 1950–2020 into a cold period and a warm period based on global warming events. Moreover, we compared the NCCV’s characteristics during the two periods to clarify the impacts of global warming on the NCCV. The results of this study provide a basis for the prediction business and scientific research regarding the NCCV.

2. Data and Methods

2.1. Data

This study used the data of the ERA5 atmospheric circulation field (500 hPa geopotential height field, wind field, and temperature field) that were re-analyzed by ECMWF from 1950 to 2020. ERA5 has much higher temporal and spatial resolutions than those of previous global reanalyses [26]. The extension of the ERA5 reanalysis back to 1950 supplements the previously published segment covering 1979 to the present. It features the assimilation of additional conventional observations, as well as the improved use of early satellite data. The number of observations assimilated increased from 53,000 per day in early 1950 to 570,000 per day by the end of 1978. Accordingly, the quality of the reanalysis improved throughout the period, generally joining seamlessly with the segment covering 1979 to the present [27]. The spatial resolution of the data is 0.25° × 0.25° (latitude × longitude), and the temporal resolution is 6 h by 6 h. Based on the method described by Fang et al. [28], we set a study area of 30° N–70° N and 95° E–140° E to identify the NCCV (Figure 1), then formed a dataset that included active days, occurrence time of NCCV processes, and process durations.

Objective identification of the NCCV [29]—the NCCV is a weather process that meets the following conditions: (1) at least one closed contour can be analyzed on the 500 hPa weather map and there is a cold center or a low-pressure circulation system with obvious cold trough coordination, (2) cold vortices appear northeast of China, and (3) the life history of the cold vortex in the above-mentioned area is at least 3 days (Figure 2).

2.2. Significance Test Method

2.2.1. Linear Tendency Estimation

In this study, the linear tendency estimation method was used to analyze the time variation characteristics of the northeast cold vortex index series. According to the linear tendency estimation method, a univariate linear regression model of the northeast cold vortex index was established.

$$\widehat{x_i}=a+b{t}_i (i=1,2,\dots ,n)$$

(1)

where x_i is the northeast cold vortex index with sample size n, t_i is the time corresponding to x_i, a is the regression constant, and b is the regression coefficient. a and b can be estimated using the least squares method.

$$b=\frac{n{{\displaystyle \sum }}_{i=1}^n{x}_i{t}_i-\left({{\displaystyle \sum }}_{i=1}^n{x}_i\right)\left({{\displaystyle \sum }}_{i=1}^n{t}_i\right)}{n{{\displaystyle \sum }}_{i=1}^n{x}_i{}^2-{\left({{\displaystyle \sum }}_{i=1}^n{t}_i\right)}^2}$$

(2)

$$a=\overline{x}-b\overline{t}$$

(3)

where

$$\overline{x}=\frac{1}{n}{{\displaystyle \sum }}_{i=1}^n{x}_i, \overline{t}=\frac{1}{n}{{\displaystyle \sum }}_{i=1}^n{t}_i$$

(4)

The regression coefficient b is the tendency value of the northeast cold vortex index x. When b > 0, x shows an upward trend with the increase in time t; when b < 0, it means that x shows a downward trend with the increase in time t. The value of b reflects the degree of tendency to rise or fall.

2.2.2. t-Test

The significance of the linear trend is determined using the t-test method as follows.

Calculation of correlation coefficient r:

$$r=\frac{n{{\displaystyle \sum }}_{i=1}^n{t}_i{}^2-{\left({{\displaystyle \sum }}_{i=1}^n{t}_i\right)}^2}{n{{\displaystyle \sum }}_{i=1}^n{x}_i{}^2-{\left({{\displaystyle \sum }}_{i=1}^n{x}_i\right)}^2}$$

(5)

The statistical formula of the statistical value $t$ of the original assumed overall correlation coefficient $\rho =0$:

$$t=\frac{r\sqrt{n-2}}{\sqrt{1-r^2}}$$

(6)

From the given significance level $\alpha$ and degree of freedom $n-2$, the $t$ distribution table is checked to obtain the critical value $t_{\alpha }$. If $t\ge t_{\alpha }$, the original hypothesis is rejected; if $t<t_{\alpha }$, the original assumption is accepted and the linear trend is considered significant [30].

2.3. Division of Cold and Warm Periods

Zhao et al. [31] used the reanalysis data of ECMWF to analyze the variation characteristics of the global annual average surface temperature. They held that the global annual average temperature anomaly had an upward trend year by year, the anomaly in the 1960s and 1970s was negative, and the anomaly in the 1980s and 1990s was positive. In 1980, the global annual average temperature anomaly turned from negative to positive. Therefore, this study defined the cold period as 1950–1980 and the warm period as 1981–2020.

2.4. Methods for Spatial Analysis of NCCVs

The cold vortex center is the grid point with the lowest geopotential height value in the closed isoline of the cold vortex. To more intuitively analyze the geographical distribution of the NCCV centers during the cold and warm periods from 1950 to 2020, we divided the study area into a grid with latitude and longitude intervals of 2°. Then, we interpolated the 6 h position of the cold vortex centers to the fixed grid to obtain the occurrence times of the NCCV processes. Finally, we calculated the multi-year average to generate spatial distribution maps of the occurrence times of the NCCV processes of the NCCV centers (annual average and monthly).

3. Climate Characteristics

3.1. Temporal Variation Characteristics of NCCVs

Based on the statistical analysis of the NCCVs, we found that the NCCV occurred 2961 times on 9782 days from 1950 to 2020, accounting for 37.7% of the calendar days. The annual occurrence time of NCCV processes was 41.7, the annual cold vortex days of the NCCV was 137.8 days, and the average duration of one NCCV process was 3.6 days (

Table 1. Statistical characteristics of the NCCVs from 1950 to 2020.

Month	Cold Vortex Days	Average Cold Vortex Days	Occurrence Time of NCCV Processes	Average Occurrence Time of NCCV Processes	Average Duration of NCCVs (d)
January	826	11.6	228	3.2	4.0
February	738	10.4	205	2.9	3.7
March	722	10.2	208	2.9	3.5
April	865	12.2	287	4.0	3.5
May	1043	14.7	344	4.8	3.7
June	989	13.9	316	4.5	3.7
July	906	12.8	282	4.0	3.7
August	699	9.8	213	3.0	3.2
September	752	10.6	237	3.3	3.4
October	731	10.3	223	3.1	3.5
November	722	10.2	196	2.8	3.8
December	789	11.1	222	3.1	3.7
Year	9782	137.8	2961	41.7	3.6

). The NCCV occurred most frequently in 2010 (58 times) and least frequently in 1968 (21 times). The year 1987 saw the greatest number of cold vortex days (187 days) and 1968 saw the fewest cold vortex days (65 days). The highest average duration of the NCCV process was 4.2 days, found in 2000, and the shortest was 2.1 days, found in 1968. Over the past 71 years, the occurrence time of NCCV processes, cold vortex days, and process durations of the NCCV exhibited noticeable interannual fluctuations and showed a non-significant upward trend (Figure 3).

Table 1 shows the characteristics of NCCV activities from 1950 to 2020 (by month). The NCCV occurred 196–344 times from January to December, with a monthly average of 2.8–4.8 times. The highest monthly average frequency occurred in May, followed by June, and the least in November. The monthly cold vortex days varied in the range of 699–1043 from January to December, with an average of 9.8–14.7 days each month. May had the greatest number of cold vortex days, while August had the fewest. The monthly average duration of each NCCV process was 3.2–4.0 days, and the NCCV continued for the longest time in January and the shortest in August. The cold vortex days and average duration of the NCCVs in May and June were relatively long, thus indicating that the NCCVs had the most significant impact in late spring and early summer. This result was consistent with previous research [32,33]. The average duration of the NCCV processes in January showed a significant increasing trend from 1950 to 2020. In addition, the average occurrence time of NCCV processes and the average number of cold vortex days in May and November showed a significant increasing trend (exceeding the 90% confidence level). The occurrence time of NCCV processes, active days, and average durations of the NCCVs in other months exhibited no significant increase or decrease.

Figure 4 illustrates the variation of annual cold vortex days, occurrence time of NCCV processes, and durations of the NCCVs in the cold and warm periods.

Table 2. Variation trends of the NCCVs during the cold and warm periods.

Time	Cold Vortex Days				Occurrence Time of NCCV Processes				Average Duration of the NCCVs (d)
	Cold Period	Warm Period	Average in Cold Period	Average in Warm Period	Cold Period	Warm Period	Average in Cold Period	Average in Warm Period	Cold Period	Warm Period
January	345	481	11.1	12.0	101	127	3.3	3.2	3.8	4.1
February	294	444	9.5	11.1	86	119	2.8	3.0	3.4	4.0
March	317	405	10.2	10.1	92	116	3.0	2.9	3.6	3.5
April	414	451	13.4	11.3	132	155	4.3	3.9	3.6	3.4
May	393	650	12.7	16.3	124	220	4.0	5.5	3.6	3.7
June	418	571	13.5	14.3	139	177	4.5	4.4	3.6	3.7
July	419	487	13.5	12.2	126	156	4.1	3.9	3.7	3.7
August	280	419	9.0	10.5	92	121	3.0	3.0	3.1	3.4
September	323	429	10.4	10.7	96	141	3.1	3.5	3.4	3.5
October	314	417	10.1	10.4	93	130	3.0	3.3	3.6	3.5
November	265	457	8.5	11.4	76	120	2.5	3.0	3.7	3.9
December	356	433	11.5	10.8	100	122	3.2	3.1	3.7	3.7
Year	4138	5644	133.5	141.1	1257	1704	40.5	42.6	3.6	3.7

and

Table 3. Variation trends of the cold vortex days, occurrence time of NCCV processes, and process durations of the NCCVs in cold and warm periods.

Time	Cold Vortex Days		Occurrence Time of NCCV Processes		Average Duration of the NCCVs
	Cold Period	Warm Period	Cold Period	Warm Period	Cold Period	Warm Period
January	+	−	−	+	+	−
February	+	−	+	−	+	−
March	+	−	+	−	+	−
April	+	−	+	+	− *	+
May	+	+	+	+	+	−
June	+	− *	+	− *	+	−
July	+	−	+	−	+	−
August	−	+	−	+	−	− **
September	−	−	−	−	+	−
October	+	−	−	−	+	−
November	−	−	+	+	−	−
December	−	−	−	−	+	−
Year	+	−	+	−	+	− **

Note: * (**) indicates that the annual variation trend reached a 90% (95%) confidence level. + indicates an increasing trend, − refers to a decreasing trend.

show the monthly variations of the characteristics of the NCCVs in different periods.

In the cold period, the NCCVs occurred 1257 times on 4138 days, accounting for 36.5% of the calendar days. The average annual occurrence time of the NCCVs was 40.5 times, and the average duration of one NCCV process was 3.6 days. The average number of annual cold vortex days was 133.5 days (Table 2). The annual occurrence time of NCCV processes showed an increasing trend (Figure 4c). The occurrence time of NCCV processes decreased in January, August, September, October, and December, and increased in the other months, but none of the trends passed the significance test (Table 3). The annual cold vortex days showed an increasing trend (Figure 4a) from 1950 to 1980. As shown in Table 3, the average number of cold vortex days in August, September, November, and December showed a decreasing trend, with an increasing trend in other months, but none of the trends passed the significance test (Table 3). The annual duration of the NCCVs showed an increasing trend (Figure 4e) from 1950 to 1980. The average duration of the NCCVs had a decreasing trend in April, August, and November, and an increasing trend in the other months. However, only the decreasing trend of the average duration of the NCCV processes in April passed the significance test (Table 3).

In the warm period, a total of 1704 NCCVs occurred during the warm period on 5644 days, accounting for 38.6% of the calendar days. The average number of annual cold vortex days from 1980 to 2020 was 141.1 days. The average annual occurrence time of NCCV processes was 42.6 times, and the average duration of NCCVs was 3.7 days (Table 2). The annual occurrence time of NCCV processes showed a decreasing trend (Figure 4d). The occurrence time of NCCV processes increased in January, April, May, August, and November, and decreased in the other months. The decreasing trend of the occurrence time of NCCV processes in June had a confidence level of 90% (Table 3). The annual number of cold vortex days had a decreasing trend (Figure 4b). The number of monthly cold vortex days increased in May and August and decreased in other months. The decreasing trend was significant in June and had a confidence level of 95% (Table 3). The annual average duration of the NCCVs had a decreasing trend (Figure 4f). Except for the increasing trend in the average duration of the NCCVs in April, the other months showed a decreasing trend.

The annual average duration of the NCCV processes had a significant decreasing trend, and the same was true in August, with a reliability higher than 95%. However, the monthly variation in process duration in the cold and warm periods showed an opposite trend, with the exceptions of August and November (Table 3). The numbers of cold vortex days and occurrence time of NCCV processes in May and November showed significant increasing trends from 1950 to 2020. Additionally, the average duration of the NCCVs in January had a significant increasing trend in the same period. However, these trends were not apparent in either the cold period or the warm period. The annual cold vortex days, annual occurrence time of NCCV processes, and annual average duration in the cold period had increasing trends (Table 2), while these climatic characteristics in the warm period showed decreasing trends, and the result of the annual average duration of NCCVs had a confidence level of 95% in the warm period. The NCCVs were more likely to occur in May, September, October, and November in the warm period than in the cold period, while the pattern was contrary to this in other months. In the warm period, the NCCV durations in January, February, and November were significantly longer than in the other months. Compared with spring and summer, when the NCCVs were prone to occur, the NCCVs in the warm period in autumn and winter were more likely to continue for longer.

3.2. Spatial Variation Characteristics of NCCVs

The spatial distribution map of the annual average occurrence time of NCCV processes of the NCCV centers in Figure 5 shows that there were two extremum centers to the south and north of 60° N, near 65° N and 50° N in the cold period, respectively. However, in the warm period, the northern (65° N) center’s size and frequency decreased, while those of the southern (50° N) center increased. These variations indicated that the NCCV centers in the warm period moved southward. To further elucidate the variations in the two centers of the NCCVs, we performed a spatial analysis of the occurrence time of NCCV processes (Figure 6a) on the northern and southern sides of 60° N. In the warm period, the NCCVs on the southern side of 60° N were distributed between 118°~138° E, and reached the maximum value at 131° E, while in the cold period, it was concentrated between 121°~137° E, and reached the maximum value at 128° E. The annual average occurrence time of the NCCV processes of the southern cold vortex center along each longitude in the warm period was higher than that in the cold period, with frequency increases mainly occurring in the area between 116°~131° E; this result is consistent with Liu et al. [25] and Gong et al. [5]. The northern center was larger than the southern center during the warm period (cold period), and the maximum frequency increase appeared at 116° E (117° E). Along each longitude, the variation amplitude of the frequency in the north center was less than that of the south center in the warm period, and the area with the maximum value of frequency decreases mainly appeared in the range of 110°~117° E. Two extremum centers of the number of occurrences of NCCV processes (Figure 6b) were located at 65° N and 52° N in the warm period, which respectively moved southward to 63° N and 51° N in the cold period. In the range of 30°~57° N, there was a great difference in NCCV frequency between the cold and warm periods, while there was only a slight change to the north of 57° N. The occurrence time of NCCV processes of the southern center increased both longitudinally and latitudinally during the warm period. In contrast, the occurrence time of NCCV processes of the northern cold vortex decreased longitudinally in the range of 110°~117° E in the warm period.

The extremum of occurrence time of NCCV processes was southernmost in June in both the cold and warm periods (Figure 7). The NCCV centers moved southward from January to June and northward from June to December. This phenomenon may be related to the seasonal advance and retreat of the East Asian monsoon. In the warm period, the NCCVs moved southward in January, May, June, and November, northward in February, and occurred more frequently in the northeast. A higher frequency of NCCVs occurred south of 45° N in April, May, June, August, September, November, and December. Above all, the NCCVs were more southerly in the warm period than in the cold period and significantly impacted the occurrence time of NCCV processes.

4. Conclusions and Discussion

Based on the data on the abrupt change in global annual average temperature, this study divided the period of 1950–2020 into two periods, namely, a cold period and a warm period, and compared the climatological characteristics of the NCCVs. The conclusions of the study were as follows:

(1) From 1950 to 2020, the NCCVs occurred 2961 times on 9782 days. The annual average occurrence time of NCCV processes was 41.7, with an average duration of 3.6 days. The annual average of cold vortex days was 137.8 days. Over the past 71 years, the occurrence time of NCCV processes, cold vortex days, and duration of the NCCVs showed no significant increasing trends.

(2) From 1950 to 2020, the monthly cold vortex days and monthly number of occurrences of NCCV processes in May and November underwent an increasing trend, while the process duration of the NCCVs in January showed a noticeable increasing trend. At the same time, May and June most frequently had NCCVs, indicating a significant impact in late spring and early summer.

(3) In the cold period, the annual average occurrence time of NCCV processes had two extremum centers around 65° N and 50° N. In the warm period, the size and frequency of the northern center decreased, while those of the southern center increased, indicating a southward movement of the centers during the warm period. The monthly average occurrence time of NCCV processes also exhibited this phenomenon.

(4) The annual average occurrence time of NCCV processes, annual average active days, and annual average process duration of the NCCVs in the warm period were all greater than those in the cold period, thus indicating that it occurred more frequently in the warm period.

(5) The monthly cold vortex days and occurrence time of NCCV processes in May and November showed a significant increasing trend from 1950 to 2020. Moreover, the average process duration of the NCCVs in January showed a significant increasing trend. However, those patterns were not significant in either of the two periods. In the cold period, the annual occurrence time of NCCV processes, annual active days, and annual process duration of the NCCV had increasing trends, while in the warm period, the patterns were the opposite, and the decreasing trend of the NCCVs’ durations reached a significant level.

(6) There were apparent interannual and interdecadal differences in the activity characteristics of the NCCVs. The average process duration of the NCCVs in January, February, and November in the warm period became longer, which indicated that the NCCVs were more likely to last longer in the autumn and winter of the warm period.

Some of the conclusions of this study were consistent with those of previous studies. The average annual occurrence time of NCCV processes, annual average number of cold vortex days, and average process duration of the NCCVs showed an increasing trend, but the trend was not significant; these results are consistent with Hu et al. [7] and Jiang et al. [14]. The most active days and a relatively long process duration occurred in May and June [14,21], which is in line with our conclusion. The main active area of the NCCVs was roughly consistent with the studies of Liu et al. [25] and Gong et al. [5]. However, due to the differences in the objective identification methods and data of the NCCVs, there may be differences present in the climatological characteristics of the NCCVs. Therefore, the specific comparative work still requires further analysis.