Because of global warming and abnormal climate change, record-breaking heat wave events have been increasing. Therefore, it is necessary to analyze and predict the mechanisms for generating and developing heat waves. In this study, the impacts of synoptic and local factors on heat waves over the southeastern region of South Korea in 2015 are investigated. First, Heat wave events for 1980-2016 defined by spatiotemporal criteria are classified into three clusters using the K-means clustering analysis. Heat wave events in 2015 are compared to clusters of different synoptic conditions, which are strongly related to the western North Pacific subtropical high around South Korea. Second, numerical experiments using Weather Research and Forecasting model are performed to examine the effect of local conditions on 2015 heat waves. Four types of sensitivity experiments were conducted: a CTL experiment with no change in topography and soil moisture, TOPO experiment with lowered terrain, and WET and DRY experiments with wet and dry anomalies in initial soil moisture. The CTL experiment simulates higher temperature on the lee side compared to the TOPO experiment because of the Foehn effect. In particular, the effect is prominent when dry westerlies passing through mountains and the lower level atmosphere over the windward region are in a statically stable condition (i.e., nighttime). Due to the less (more) evaporation, the DRY (WET) experiment with less (more) soil moisture simulates higher (lower) maximum temperature than the CTL experiment during the daytime when solar radiation is relatively intense.Various studies have considered the impacts of large-scale factors on heat waves to identify mechanisms generating and developing heat waves. One major factor that causes heat wave events is the presence of large-scale anticyclone circulation over a specific area, producing low cloudiness, clear skies, and warm advection (Black et al., 2004;García-Herrera et al., 2010;Meehl & Tebaldi, 2004). The expansion of the western North Pacific subtropical high (WNPSH) has an important role in influencing summer extreme high temperatures over the East Asia summer monsoon region, including East China, South Korea, and Japan. Xue-zhao and Dao-yi (2002) determined

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This research investigates the impact of local sea surface temperature (SST) on the 2-month (January and February) accumulated snowfall over the Yeongdong (YD) region. The YD region is strongly affected by synoptic-scale factors such as the East Asian winter monsoon (EAWM). The relationships of snowfall over the YD region to the EAWM and local SST are examined based on observational analyses and sensitivity experiments using a regional climate model. In the sensitivity experiments, local SST is replaced with the 33-yr mean winter SST (1982–2014). The observational analysis shows that both the synoptic environment and local SST are important factors for the occurrence of anomalous heavy snowfall over the YD region. The favorable synoptic environments can be characterized by eastward expansion of the Siberian high over Manchuria and corresponding enhancement of easterly anomalies over the YD region. These conditions are more frequently observed during the weak EAWM years than during the strong EAWM. Furthermore, warm SST over the East Sea contributes to heavy snowfall over the YD region by providing heat and moisture in the lower troposphere, which are important sources of energy for the formation of heavy snowfall. Warm SST anomalies over the East Sea enhance low-level moisture convergence over the YD region, while cold SST anomalies lead to reduced moisture convergence. Sensitivity experiments indicate that local SST can significantly affect snowfall amount over the YD region when the synoptic environments are favorable. However, without these synoptic conditions (expansion of the Siberian high and easterly inflow), the impact of local SST on the snowfall over the YD region is not significant.

South Korea’s heat wave events over 39 years (1980–2018) were defined by spatiotemporal criteria, and their quantitative characteristics were analyzed. The duration and intensity of these events ranked the highest in 2016 and 2018. An examination of synoptic conditions of heat wave events in 2016 and 2018 based on a reanalysis dataset revealed a positive anomaly of 500-hPa geopotential height, which could have induced warm conditions over the Korean Peninsula in both years. However, a difference prevailed in that there was a blocking high over the Kamchatka Peninsula and a continental thermal high over northern China in 2016, while the expansion of the western North Pacific subtropical-high was mainly associated with 2018 heat wave events. Numerical experiments using the Weather Research and Forecasting model (WRF) were conducted to (1) evaluate how distinct meteorological characteristics of heat wave events in 2016 and 2018 were reproduced by the model, and (2) investigate how they affect extreme temperature events. Typical synoptic features of the 2016 heat wave events (i.e., Kamchatka blocking and continental thermal high) were not captured well by the WRF model, while those of 2018 were reasonably reproduced. On the contrary, the heat wave event during late-August 2016 related to the Kamchatka blocking high was realistically simulated when the blocking was artificially persisted by applying the spectral nudging. In conclusion, the existence of a blocking high over the Kamchatka region (i.e., northern Pacific region) is an important feature to accurately predict long-lasted heat waves in East Asia.

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