Cold surge pathways in east Asia and their tropical impacts

Cold surge occurrences are one of the robust features of winter monsoon in East Asia and are characterized by equatorward outbreaks of cold air from the high latitudes. Beside greatly affecting weather variability across the Far East, cold surges are of importance for Southeast Asian countries because they can propagate far to the tropics and excite convective activities. However, the tropical responses highly depend on the downstream pathways of the surges. To better understand how cold surges influence tropical weather, we investigate 160 cold surges identified using a quantitative approach during 40 winters from 1979/80 to 2018/19, and then classify them into several groups based on their prominent pathways. At the midlatitudes, we find two groups: one for surges that show clear equatorward propagation of cold air to lower latitudes and the other for surges that turn eastward and bring cold air to the North Pacific. These groups arise due to the strength difference of the Siberian high expansion controlled by cold air blocking near the Tibetan Plateau. The tropical impact is evident in the former group. We perform further classification on this group and find four types of surges based on their pathways in the low latitudes: 1) South China Sea (SCS) surges, 2) Philippines Sea (PHS) surges, 3) both SCS and PHS surges, and 4) blocked surges. They exhibit distinct precipitation signatures over the Maritime Continent, which are driven by interactions between the surges and the pre-existing synoptic conditions over the tropics, particularly the Madden-Julian oscillation (MJO).