Synoptic Conditions Controlling the Seasonal Onset and Days of Heatwaves Over Korea

As global warming gets worse, the extreme heat exposure time is expected to increase. Considering that the heatwave damages increased by the accumulation of heat stress, it is important to understand the heatwave onset and number of heatwave days (HWDs). Here, we show that the end of East Asian summer monsoon activity (i.e., Changma retreat date, CRD) could be an indicator in determining the onset of the heatwave, and the barotropic structure induced by the circumglobal and Pacic–Japan teleconnections is a key factor in lengthening the number of HWDs in Korea. The onset of the heatwave is delayed when CRD belated than the climatology late over Korea due to sucient moisture transportation between the edge of western North Pacic subtropical high and cold polar air mass in July. The number of HWDs from July through August over Korea shows a positive linear relationship with the synoptic stagnation index because the upper-layer anticyclone associated with stagnation is formed around Korea by zonal wave activity. Barotropic anticyclone stabilizes the atmosphere and increases the number of clear sky days, which possibly leads to hot days. Fewer HWDs are observed when there is baroclinic instability due to strong upper-level jet stream and synoptic-scale weather systems move smoothly. By identifying the meteorological mechanism of heatwave occurrence and favorable conditions for sustained hot days over Korea, our results are eventually able to contribute to reducing damages caused by heatwaves.


Introduction
The Korean Peninsula has been recently exposed to an extreme heatwave during summer. In 2018, a strong and long-lasting heatwave with record-breaking surface daily maximum temperature was observed in Korea (Ha et al. 2020). The daily maximum temperature in Seoul, the capital of the Republic of Korea, reached a historical record of 39.6℃. In Hongcheon, a rural area in Gangwon Province, the daily maximum temperature rose to 41.0°C, setting a national record. In addition, the national average of the daily maximum temperature of over 33℃ was recorded for 31.5 days and tropical nights with a daily minimum temperature of over 25°C, which lasted for 17.7 days. The heatwave caused maximum damage to the vulnerable social group with low nancial conditions who cannot afford by using cooling machines ( Most previous studies examined extreme cases of heatwave rather than on a long-term climatic study. . The characteristics of heatwaves could be in uenced by these various mechanisms ). Social damages caused by heatwaves occur when high temperatures continue for a long time rather than sudden shocks of extremely high temperatures. The summertime heatwave day (HWD) is a climatic characteristic, but it shows interannual variations depending on the start date of the heatwave. Therefore, it is important to have an in-depth understanding of the onset and number of HWDs so that potential damages caused by heatwaves could be reduced. The onset of the summertime heatwave in Korea could be determined by the East Asian monsoon activity (Kysely and Kim 2009). In addition, the variability of the HWDs is in uenced by the stagnation of the synoptic weather system and regional atmospheric stability. When a barotropic structure is formed due to the WNPSH extension in the lower layer (Choi and Kim 2019), the Paci c-Japan (PJ) teleconnection pattern by tropical convection (Nitta 1987), and changes in the upper geopotential height by the CGT activity (Ding and Wang 2005), it can cause high-impact heatwaves on the surface.
Although both the East Asian summer monsoon activity and barotropic anticyclones affect the initiation and continuation of the heatwave in Korea, there are few studies on the temporal evolution of heatwaves.
This study aims to identify the climatic mechanisms of heatwave onset and sustainability by examining the Changma and atmospheric stagnation over the Korean Peninsula. As well as the thermodynamically su cient conditions of the middle and lower atmosphere, the effects of the upper layer change caused by large-scale teleconnection are evaluated on the seasonal time scale. Understanding how heatwaves start and why they sustain would be pro table information to predict heatwaves and reduce heat-related disease eventually.
The remainder of this paper is organized as follows. Data and methods used in this study are explained in Sect. 2. Section 3 describes the onset of the heatwave. Section 4 presents the information on the HWDs in Korea and large-scale variability associated with Changma and atmospheric stagnation, respectively. Concluding remarks are provided in Sect. 5.

Data
Daily maximum temperatures from 45 automated synoptic observing system (ASOS) weather stations in Korea were obtained from the Korea Meteorological Administration (KMA) (Fig. 1). These manned observatories provide reliable data subject to the quality control process. In addition, daily geopotential height, zonal and meridional winds, the temperature at 2 m, total precipitation, and speci c humidity were obtained from the global atmospheric reanalysis (ERA-interim) produced by the European Centre for Medium-Range Weather Forecasts with the 0.75° × 0.75° horizontal resolution (Dee et al. 2011). All the data were analyzed for the period from 1979 to 2019.
The main climatic factors affecting the formation and evolution of the Changma front over Korea are known as the in ux of low-level moisture, expansion of the WNPSH, and the reinforcement of the upperlevel jet stream creating baroclinic instability (KMA, 2011). Thus, the end of Changma over Korea is determined through a comprehensive analysis including the low-level moisture ux, ground temperature, location of the rainband front, solar radiation, sunshine time, and wind elds at 200 hPa. Information on the annual Changma retreat date (CRD) is provided by the KMA for each station. In this study, the averages of annual CRDs in 45 ASOS weather stations were analyzed.

Methodology
Although many de nitions of a heatwave exist in previously published studies, herein, a heatwave was de ned by the daily maximum temperature of over 33°C, in accordance with KMA standard temperature. The annual HWD was calculated as the average of the number of days when the maximum temperature The main conclusion of this study is not sensitive to analyzing this threshold using a relative one such as 90th (i.e., 32.7°C) or 95th (i.e., 33.9°C) ( gure not shown). The temperature values averaged over the Korea region were used when analyzing the reanalysis data. The Korea region was de ned as the smallest box that contained 45 ASOS stations (Fig. 1).
The CGT phenomenon was proposed by Ding and Wang (2005) and has been known as one of the major modulators of the summertime synoptic weather pattern in the mid-latitude region. Because the summertime weather over the Korean Peninsula varies depending on the phase of CGT, the CGT was . In other words, the stagnation index is calculated by the sum of 200-hPa geopotential height averages over the East Asia box and northwestern India area. The correlation coe cient (r) between this stagnation index and the CGT index is 0.83, which means the variability of the stagnation index is very similar with the CGT variation at a 99% con dence level. However, the stagnation index is found to better explain the heatwave over Korea (r = 0.70) than the CGT index (r = 0.42) during the 1979-2019 period, because the stagnation index more focuses on the series of anticyclones which locates not only over the northwestern India box but also over the Korean Peninsula. In addition, by representing the barotropic structure in Korea, this stagnation index can be considered comprehensively not only the impact of zonal teleconnection but also meridional variation such as PJ pattern.

Heatwave Onset
Climatically, the averaged HWD values over Korea are 0.6 days (5%) in June, 4.2 days (38%) in July, 5.8 days (54%) in August, and 0.2 days (2%) in September. However, the interannual time series of HWD beginning in June and lasting through September shows a large HWD variability from 0 days (1993) to 30 days or more (1994 and 2018) (Fig. 2). Much of this variability is caused by the number of HWDs in July (r = 0.86), associated with the start of the heatwave, as well as HWDs in August (r = 0.87). The number of HWDs in July depends on the onset of the heatwave, which might be related to CRD (Yoon et al. 2018). After the Changma period, Korea is predominantly in uenced by warm and humid airmass with plentiful insolation. Therefore, the onset of the heatwave is suggested to be determined by CRD, which shows large interannual variability.
The number of HWDs from June through July shows a signi cant negative relationship with CRD (Fig. 3).
Except for the extreme cases of 1994 and 2018, the slope is approximately − 1 HWD week − 1 , and the correlation coe cient is − 0.46 in the linear regression analysis, showing a statistically signi cant relationship of 99%. We compared the large-scale environmental elds collected over ve years (i.e., 1981, 1984, 1994, 2011, and 2018) with early CRD and other ve years (i.e., 1980,1987,1991,1993,2013) with late CRD to identify the differences between two groups. As a result, this analysis is expected to be able to provide characteristics of the early and late CRD years.
The summertime heatwaves frequently occur when strong solar radiation continues in a clear sky and there is less rain event. Therefore, if there is a su cient supply of moisture to the Korean Peninsula, the heatwaves could be alleviated as it rains. We examined precipitation and mid-level geopotential height anomaly in July according to the composite analysis by CRD (Fig. 4). The Changma front is formed between a cold polar air mass (represented by 5820 gpm, in blue line) and a warm, humid region, which is depicted as the boundary of WNPSH (represented by 5880 gpm, in red line) (KMA 2011). In the years when the Changma ended early, the gradient of 500-hPa geopotential height was small and there is little precipitation around the Korean Peninsula in July (Fig. 4a). However, there are more precipitation and a large gradient of 500-hPa geopotential height in the 30°-35°N during the years when the rainy season ended late (Fig. 4b).
When the Changma front retreats quickly, the polar air mass moves further north than its climatology (Figs. 4c, 4g, and 4k), which explains the northward migration of the Changma front. Moreover, when the development of WNPSH is weaker than the climatology, the Changma front is not explicitly formed, and there is relatively little systematic moisture transport to the Korean Peninsula (Figs. 4e and 4i). In 1994, during the record-breaking heatwave, the amount of precipitation in July was minimal, caused by the early CRD (Fig. 4g). However, Korea was overwhelmingly covered by the expanded WNPSH starting from early summer in 2018 so that clear weather prevailed, and the heatwave occurred (Fig. 4k).
In general, when the WNPSH boundary is expanded further westward than the climatology and moisture is transported to Korea along the WNPSH boundary, much precipitation occurs around Korea. When a meridional geopotential height gradient is large, the baroclinic wave tends to be developed in the vicinity of the boundary between WNPSH and cold air mass (Li 2006;Murakami and Matsumoto 1994). Eventually, it results in stronger frontogenesis (i.e., Changma) and the late onset of the heatwave. As a result, a large amount of precipitation can be seen between the explicitly well-developed WNPSH boundary and the cold air mass in all years (Figs. 4d, 4f, 4h, 4j, and 4l).

Heatwave Days
The number of HWDs is important as well as heatwave onset. In this study, the number of HWDs is A de nite positive relationship was identi ed by analyzing the stagnation index and the number of HWDs. The correlation coe cient is 0.70 which is statistically signi cant at the 99% con dence level (Fig. 5).
Thus, the number of HWDs in Korea from July through August varies according to the atmospheric stagnation. We compared ve years with the highest HWDs (i.e., 1990, 1994, 2013, 2016, and 2018) and ve years with the lowest HWDs (i.e., 1980HWDs (i.e., , 1987HWDs (i.e., , 1993HWDs (i.e., , 1998HWDs (i.e., , and 2003 and examined the effects of stagnation on the heatwave in Korea. These ve years on the basis of the number of HWDs were different from those ve 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 ow 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 ve, the low number of HWDs indicated that the zonal wind was stronger than one standard deviation. It means that the weather system was owing 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 in uenced 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 ow 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 identi ed in all low HWD years (Figs. 7d, 7f, 7h, 7j, and 7l). In all ve 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 ve 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 in uence of convection related with the PJ pattern.
In the years of the highest ve 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 classi ed 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 ve 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.

Concluding Remarks
Heatwave studies have been mostly focused on understanding extreme cases or the analysis of daily maximum temperature. Many climatic indices, such as the ENSO, PDO, Indian Ocean dipole, ISM, and AO, are used to diagnose statistical relationships with the summertime temperature over East Asia and to understand the mechanism of heatwave development. Although the effects of large-scale climate variability predominantly impact the changes in summer weather in East Asia, it is di cult to investigate extreme daily weather solely using long-term climate variability. The time scale of rain showers caused by atmospheric instability is very short relative to the aforementioned climate variabilities. Moreover, the approaches that rely on the large-scale climate variability limit the understanding of the overall phenomena and reasons, especially with ENSO and AO being predominant over winter.
This study investigated the characteristics of the summer heatwaves by identifying the meteorological factors that control the onset and days of the heatwave. Statistical analysis showed that the onset of the heatwave is related to CRD in July, whereas the prolonged heatwave in August is mainly due to the anticyclone over Korea and stagnation of the weather system by the large-scale teleconnections. If climate models well simulate the East Asian monsoon activity such as CRD or the variability of WNPSH in the future, the performance of heatwave prediction on a seasonal time scale will be improved.
The determination of the end date of the heatwave concerning its sustainability is crucial. Notably, interannual variability at the end of the heatwave is negligible. During the past 41 years, most of the heatwaves in Korea consistently ended in late August. However, as global warming continues, the variability in WNPSH and the end of hot summer weather may change. As such, trend monitoring and future studies on the timing of WNPSH weakening in autumn are required. As another issue, although the amplitude of the interannual variability of CRD in the current climate is greater, the long-term trends of CRD changes should also be considered. Further studies on the changes in the Changma activity associated with the position of the jet stream are necessary along with the studies on the jet positional change and various climatic systems. Such studies could ultimately form the basis for the understanding of the heatwave characteristics and heatwave prediction in the future.