The number of HWDs is important as well as heatwave onset. In this study, the number of HWDs is analyzed because the high-heat exposure time (i.e., HWDs) is more closely related to the heatwave damage (Laaidi et al. 2012; Mishra et al. 2017; Rey et al. 2009). Heatwave exposure in Korea is strongly related to the variation in the barotropic high-pressure system, which makes sustaining the clear sky over Korea. Large-scale climate variabilities such as CGT, PJ pattern, WNPSH, and PDO can induce high pressure around Korea and make heatwaves last for a long time (Ding and Wang 2005; Yeo et al. 2019, Choi and Kim 2019; Liu et al. 2019).
A definite positive relationship was identified by analyzing the stagnation index and the number of HWDs. The correlation coefficient is 0.70 which is statistically significant at the 99% confidence level (Fig. 5). Thus, the number of HWDs in Korea from July through August varies according to the atmospheric stagnation. We compared five years with the highest HWDs (i.e., 1990, 1994, 2013, 2016, and 2018) and five years with the lowest HWDs (i.e., 1980, 1987, 1993, 1998, and 2003) and examined the effects of stagnation on the heatwave in Korea. These five years on the basis of the number of HWDs were different from those five years used in the CRD analysis.
The zonal wave trains across the mid-latitudes affect the heatwave formation in Korea by spreading wave energy (Yeo et al. 2019). The barotropic structure tends to be constructed if there are higher 200-hPa geopotential height anomalies in Korea during the summer. Such a position of the barotropic structures slows the eastward-moving weather systems around East Asia and blocks the upper-level trough migration, creating favorable conditions for the prolonged heatwave (Ding and Wang 2005). Normalized 200-hPa zonal and meridional winds over Korea were examined together with HWDs from July through August to establish the relationship between the stagnation of atmospheric flow and heatwaves (Fig. 6). The number of HWDs and zonal wind speed at 200 hPa are closely related, considering that the red dots tend to be located on the left side, whereas the blue dots appear on the right side of the diagram. For four years out of five, the low number of HWDs indicated that the zonal wind was stronger than one standard deviation. It means that the weather system was flowing smoothly in that years. The years of the high number of HWDs tend to show weakened upper-level westerly winds, resulting in blocking patterns, clear weather, and lots of HWDs. However, atmospheric stagnation is not the only factor that determines the number of HWDs as it is influenced by various other factors, such as udden local rainfall or horizontal heat advection.
The years with a high number of HWD show a positive geopotential height anomaly at 200 hPa in East Asia (Fig. 7). This positive anomaly over Korea is the result of wave propagation from the upper troposphere in mid-latitude continental Asia and corresponds to a pattern reported in a previous study (Wu 2017). Overall, the atmospheric flow is stagnant, and the weather system is maintained due to the strengthening of the upper-level high pressure around East Asia. During the years of high HWDs, the core of anticyclone in the upper atmosphere is generally located in the northern part of the Korean Peninsula, and its low-level core appears in the southern part of the Korean Peninsula (Fig. 7a). Therefore, the barotropic and stable atmospheric structure is constructed over Korea. The anomalous barotropic high over Korea was observed in August of 1994, 2016, and 2018 (Figs. 7e, 7i, and 7k), when favorable conditions for the development of the heatwave and led to prolonged sunny weather were created in the East Asian continent. In 1990 and 2013, the barotropic anticyclone anomaly was not noticeable in Korea due to different geopotential height changes in the upper and lower levels. The horizontal and meridional wind velocities are also close to the climatology (close to value ‘0’ in Fig. 6) in those years. Increases in the number of HWDs in those years are presented by a small number of rainfall events rather than the atmospheric stagnation.
The years with the lowest number of HWD show a negative geopotential height anomaly at 200 hPa over Korea (Fig. 7b). For the 850-hPa geopotential height anomaly, negative geopotential height anomalies were also identified in all low HWD years (Figs. 7d, 7f, 7h, 7j, and 7l). In all five years, the anomalous low was located on the northern side of the Korean Peninsula, thus strengthening the upper-level jet stream over Korea. Therefore, the eastward movement of the weather system is activated and makes baroclinic instability over Korea. Various weather phenomena such as cloudy weather or continuous rainfall could occur rather than lasting the clear sky and hot weather.
Anomalies of the 2 m temperature, 850-hPa wind, and mid-level geopotential height were investigated to understand the mechanisms for the five years of the highest and lowest HWDs, respectively (Fig. 8). If the barotropic change happens in the geopotential height over Korea due to the atmospheric stagnation, it may interact with neighboring climate systems such as the WNPSH and control summer weather in East Asia. The development of an anticyclone over Korea lead to an increase in surface air temperature (i.e., the temperature at 2 m), provoke clear skies, and stabilize atmospheric conditions. The low-level anomalous anticyclonic circulations around the Korean Peninsula partially suppress the rainfall events which relieves heatwaves by reducing moisture transport in high HWD years. In the north-south direction, the signs of temperature anomaly and geopotential height between the mid- and low-latitude regions are opposite (Figs. 8a and 8b). These dipole changes are estimated to be the influence of convection related with the PJ pattern.
In the years of the highest five HWDs, environmental conditions leading to the prolonged HWDs were observed in August with a positive anomaly of the 2 m temperature in most regions of East Asia, including Korea. The warm anomaly region is generally consistent with the 500-hPa geopotential height anomaly and has actively induced anticyclonic circulations around Korea. Extreme heatwave cases, 1994, 2016, and 2018, can be classified as the cluster 3, 2, and 1 of heatwaves covering the entire Korean Peninsula as reported in previous studies, respectively (Yoon et al. 2020). However, in the five years with the lowest number of HWDs, the negative temperature at 2 m and geopotential height anomaly were found in Korea. The changes in geopotential height at 500 hPa change were associated with the contraction of the WNPSH boundary. Anomalous wind from the ocean resulted in more moisture transport into Korea and a lower number of HWDs.