Long-term changes of physical mechanisms in the seasonal cycle of the summertime precipitation variability in Korea

In order to understand the physical mechanisms affecting the Korean summertime (May 19-September 15) precipitation, Kim et al. (2010) divided the seasonal cycle of summertime precipitation variability into three distinct physical mechanisms: the seasonal, the subseasonal, and the high-frequency variations. Delineating distinct physical mechanisms are effective in analyzing the Korean summertime precipitation variability. In order to see if and how the nature of physical mechanisms has varied with climate changes, we analyzed observed precipitation records for the 1979-1995 pre period (PRE), and for the 1996-2008 post period (POS) at 61 Korea Meteorological Administration stations. Detailed space-time structures of the physical mechanisms of precipitation variability were derived via CSEOF analysis using the daily NCEP/NCAR reanalysis data over East Asia (80°-180°E, 0°-60°N). The seasonal component represents the variability associated with the evolution of the East Asian summer monsoon governed primarily by the 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 with a bimodal pattern. The first peak has increased significantly and the second peak has broadened in POS years. Similarly, the subseasonal oscillations, which are associated with the northward migration of precipitation from lower latitudes with 10 to 30-day time scales, exhibit a physical structure associated with heavy rainfall in Korea. The strength of the subseasonal component of variability has nearly doubled in POS years. Relatively strong subseasonal precipitation activity with an averaged period of ~15 days has shifted from late June through mid July in PRE years to early August through early September in POS years. The high-frequency component with time scales less than 10 days is associated with the mid-latitude baroclinic Rossby waves. Upper-level geopotential height anomalies and low-level moisture convergence anomalies are well correlated with the high-frequency precipitation variability, which has increased in POS years by ~25% particularly in early- to mid-August in Korea.