Physical mechanisms of the seasonal, subseasonal, and high‐frequency variability in the seasonal cycle of summer precipitation in Korea

Abstract

Three distinct physical mechanisms in the seasonal cycle of the 120 day (19 May to 15 September) summer precipitation in Korea (126°E–130°E, 33°N–38°N) were identified using the 1979–2008 observed precipitation records at 61 Korea Meteorological Administration stations. Detailed space‐time structures of the physical mechanisms of precipitation variability were derived using the daily National Center for Environmental Prediction/National Center for Atmospheric Research reanalysis data over Asia (80°E–180°E, 0°–60°N). The seasonal cycle of summertime precipitation in Korea exhibits three principal temporal scales (seasonal, subseasonal, and high‐frequency components) of variability, each with distinct physical mechanisms. The seasonal component represents the variability associated with the evolution of the Asian summer monsoon, specifically the East Asia summer monsoon, governed primarily by large‐scale circulation as a result of changes in sea level pressure contrasts between the Asian continent and the surrounding oceans. The arrival and the duration of a monsoon front primarily shape the seasonal evolution of precipitation in Korea. The bimodal peaks are due to the low‐level circulation change as a result of redistribution of temperature and, subsequently, of sea level pressure during summer. The subseasonal component has characteristic time scales of 10–30 days and is associated with eastward‐moving upper‐level disturbances at ∼40°N. The upper‐level disturbances affect the meridional circulations, resulting in low‐level convergence/divergence not only underneath but also to the south and to the north of the disturbance. From mid‐July to mid‐August, the subseasonal component is more clearly observable, and the period of oscillations is generally shorter, than during early or late summer. The high‐frequency component with time scales of less than 10 days is associated with midlatitude baroclinic Rossby waves; synoptic‐scale variations of upper‐level geopotential height and low‐level moisture convergence are well correlated with the high‐frequency variability of precipitation in Korea. Positive precipitation anomalies over Korea correspond to an upper‐level divergence with anticyclonic wind anomalies caused by baroclinic Rossby waves with axes around ∼40°N. The vertical structures of key physical variables are well explained in the context of baroclinic instability with the geopotential height anomaly field connecting the upper‐level divergence and the lower‐level convergence tilted westward with height and the temperature anomaly field slanting slightly in the opposite direction.