The Relation between Atmospheric Intraseasonal Oscillation and Summer Severe Flood and Drought in the Changjiang-Huaihe River Basin

The intraseasonal oscillation (ISO) is studied during the severe flood and drought years of theChangjiang-Huaihe River Basin with the NCEP/NCAR reanalysis data and the precipitation data inChina. The results show that the upper-level (200 hPa) ISO pattern for severe flood (drought) is charac-terized by an anticyclonic (cyclonic) circulation over the southern Tibetan Plateau and a cyclonic (anti-cyclonic) circulation over the northern Tibetan Plateau. The lower-level (850 hPa) ISO pattern is char-acterized by an anticyclonic (cyclonic) circulation over the area south of the Changjiang River, the SouthChina Sea, and the Western Pacific, and a cyclonic (anticyclonic) circulation from the area north of theChangjiang River to Japan. These low-level ISO circulation patterns are the first modes of the ISO windfield according to the vector EOF expansion with stronger amplitude of the EOF1 time coefficient in se-vere flood years than in severe drought years. The analyses also reveal that at 500 hPa and 200 hPa,the atmospheric ISO activity over the Changjiang-Huaihe River basin, North China, and the middle-highlatitudes north of China is stronger for severe flood than for severe drought. The ISO meridional windover the middle-high latitude regions can propagate southwards and meet with the northward propagatingISO meridional wind from lower latitude regions over the Changjiang-Huaihe River Basin during severeflood years, but not during severe drought years.