Intra-seasonal characteristics of wintertime extreme cold events over South Korea

In this study, the characteristics of extreme cold events over South Korea in the boreal winter (November to March) are investigated in terms of month-to-month variations of frequency, duration, amplitude, and atmospheric circulation pattern. To explain the mechanism for these month-to-month variations, all extreme cold events are classified into three types based on an agglomerative hierarchical clustering method, and their linkages with large-scale variabilities, such as the Siberian High (SH), the Arctic Oscillation (AO), and the Madden–Julian Oscillation (MJO), are analysed. “Wave-train” type does not tend to occur under the typical favourable conditions for the East Asian cold extremes, e.g., strong SH intensity, strong negative AO phase, and MJO phases 2–3, hence resulting in weak and short events. On the other hand, strong and long-lasting “Ural blocking” and “subarctic blocking” types are accompanied with the aforementioned favourable large-scale conditions. Thus, November and March with the most frequent occurrences of the wave-train type are the months in which extreme cold events have weaker amplitude and shorter duration than other months. Compared with the wave-train type, the Ural blocking type occur frequently in January and February, while occurrences of the subarctic blocking type are concentrated in December and January. It contributes to relatively powerful and sustained extreme cold events in mid-winter (from December to February). In particular, extreme cold events during March have the lowest frequency, weakest amplitude, and shortest duration due to a lack of strong SH activity, strong negative AO phase, and MJO phases 2–3, but strong activities of these factors in January force the most frequent, strongest, and longest extreme cold events in the boreal winter.