Recent trend of cold winters followed by hot summers in South Korea due to the combined effects of the warm western tropical Pacific and North Atlantic in spring

This study finds a significant negative correlation between the December-February mean surface temperature (Ts_DJF) and the following June-August mean surface temperature (Ts_JJA) in South Korea for the period 1991-2017. This indicates that colder winters tend to precede hotter summers with extreme seasonality, while mild winters generally precede mild summers. This winter-to-summer association can be attributed to persistent atmospheric circulation anomalies on the Eurasian continent during the preceding winter and spring characterized by cyclonic circulations in Europe and East Asia and anti-cyclonic circulation in the Arctic regions. Resembling a negative Arctic Oscillation (AO) pattern combined with a negative Polar/Eurasia (PE) pattern, these atmospheric patterns tend to cause colder winters in South Korea and to increase the springtime sea surface temperatures in the western tropical Pacific (WTP) and in the North Atlantic (tripole pattern, NATRI). High WTP and NATRI values induce summertime anti-cyclonic circulations and then hotter summers in Korea with different pathways, the former via northward Rossby wave propagation in response to strong convection over a warm Philippine Sea and the latter via both extratropical Rossby wave propagation from the North Atlantic to East Asia and tropical connections from the tropical Atlantic to the Indian Ocean and then increased summer precipitation in South Asia. Under the opposite conditions (e.g. positive AO and PE phases in winter and negative WTP and NATRI), mild summers are preceded by mild winters. Since the early 1990s, the aforementioned atmospheric circulation anomalies during winter have shown greater persistence, creating the negative correlation between Ts_DJF and Ts_JJA. These findings provide useful information for the long-lead prediction of summer temperatures and heat waves in South Korea.