Non‐stationary characteristics of intraseasonal precipitation variability in Northeast Asia during the boreal summer

In this study, we examined the non‐stationary characteristics of precipitation variability in Korea on intraseasonal timescales during the boreal summer (June–July–August) for 1979–2017 by analysing the European Center for Medium‐Range Weather Forecasts ERA‐interim reanalysis data set. We reconstructed the Korean precipitation variability on intraseasonal timescales in each year using three empirical orthogonal function modes of precipitation anomaly representing northward propagating intraseasonal oscillation from the tropics to subtropics. Then, we compared the reconstructed precipitation variability with the total precipitation variability in Korea. We found that the correlation coefficients between two precipitation variabilities change from year to year, indicating non‐stationary characteristics of intraseasonal precipitation variability contributing to the total precipitation variability. Years, when reconstructed precipitation is highly (less) correlated with total precipitation variability, are characterized by a large (small) amount of total precipitation, an early (late) start of the rainy season and enhanced (reduced) precipitation variability on an intraseasonal timescale. We further examined the reason why the relationship between two variabilities is non‐stationary. A large convective forcing in the western tropical Pacific and the associated atmospheric conditions, including the weakening of Western North Pacific subtropical high with its eastward shift and vertical shear of zonal wind affect the characteristic change of intraseasonal precipitation variability in Korea. The schematic diagram showing favourable conditions that are associated with the years when the intraseasonal precipitation originated from the tropics to subtropics largely explains the total precipitation variability in Korea. We found that a large convective forcing in the western tropical Pacific and the associated atmospheric conditions including weakening of Western North Pacific subtropical high with its eastward shift and vertical shear of zonal wind affect the characteristic change of intraseasonal precipitation variability in Korea.