Interannual Variability Of Quasi-Biweekly Oscillation Of Yangtze-Huaihe Precipitation During The Meiyu Period

Based on the observed daily precipitation data sets during 1952-2006 from APHRO, atmospheric reanalysis data sets from NCEP/NCAR and sea surface temperature(SST) data from Hadley, we analyze the climate feature of quasi-biweekly oscillation(QBWO)of precipitation during Meiyu at Yangtze River. Daily-empirical orthogonal function (D-EOF) decomposition is employed to reveal the main interannual space-time variability mode of QBWO during Meiyu at Yangtze River and its related background air-sea patterns. The evolutions of quasi-weekly precipitation anomalies during Meiyu at Yangtze River and atmospheric circulation anomalies are also discussed.We find that precipitation anomalies during Meiyu over Yangtze-Huaihebasin display a prominentcharacter of low-frequency variability, which consists of two mainly spectral bands. One is 20-50 days, which propagates northward with Meiyu belt. And the other is 10-20 days’ quasi-biweekly oscillation, which mainlyoscillates locally. Besides, the interannual variabilities of the two bands are most prominent during Meiyu at Yangtze-HuaiheBasin.However, characteristics of low-frequency variability of precipitation during Meiyu are quite different year by year. The first mode of D-EOF features a pattern of "triple floods and one drought", its major oscillation center locates north of 30°N with a period of 20-50 days. The second mode of D-EOF features a "triple floods and four droughts"QWBO which mainly occurs during June and July in the whole Yangtze-Huaihe basin. The third mode consists of two rain belts at Huai River basin and north of Yangtze River, the former occurs from early June to late July with 20-50 days period and the latter occurs between middle of June and early July, respectively. In addition, those two rain belts are out-of-phase during June.We further discuss the two prominent characteristics of the interannual variability of QWBO, that is, consistent pattern at Yangzte-Huaihe basin and out-of-phase pattern from south to north, which correspond to the first and second modes of D-EOF of interannual variability of QWBO, respectively. The rain belt of consistent pattern locates at 27°N-32°N Yangtze-Huaihe basin, whose strongest oscillation occurs between middle of June and June. Positive phase of the first mode of D-EOF of interannual variability of QWBO almost comes up with the development of La Nina, when the SST of summer central tropical Pacific is colder,850hPa is dominant by anomaly anticyclone circulation above northwest Pacific during June and July, resulting in flood at Yangtze-Huaihe but drought at South China. Negative phase of the first mode of D-EOF generally occurs during development of El Nino, when SST of central tropical Pacific and Indian Ocean are warmer during June and July, 850hPa is dominant by anomaly cyclonic circulation above northwest Pacific, resulting in a bit more rainfall at Yangtze River and less rainfall at south of Yangtze and North China, but fail in reliability test. While the rain belts of "out-of-phase" pattern locates Yangtze River basin nearby 30N and south of Yangtze, whose strongest oscillation occurs from late June to late July and from early June to early July. Two rain belts are out-of-phase during late June to early July and the rain belt at south of Yangtze is more stronger. The positive phase of second mode of D-EOF corresponds to warmer SST of summer central tropical Pacific and stronger water vapor flux from bay of Bengal, giving rise to flood at Yangtze and South China during June and July. But thenegative phase corresponds to warmer SST of summer tropical Indian Ocean and west Pacific, prominent anomaly anticycloniccirculation above northwest Pacific 850hPa, giving rise to more rainfall at the south of the lower reaches of the Yangtze River but less rainfall at North China.The variability of QBWO of Meiyu over Yangtze-Huaihe basin corresponds to raw precipitation well, heavy rainfall almost occurs in extreme wet phase while brreak of the consecutive rainfall occurs in extreme dry phase, which are results of interaction between upper and lower levels circulation. As for consistent pattern, QBWO of lower level circulation propagates from west tropical Pacific to the South China Sea, and additionally from southeast of Japan to Yangtze-Huaihe basinthrough northeast China during the positive phase. In the upper layer, anomalous wave train in the high latitude over Eurasia moves eastward before the extreme wet phase of the Meiyu over Yangtze-Huaihe Basin,then moves westward after that during positive phase, moving in the opposite direction of the subtropical high over northwest Pacific. During the negative phase the anomaly wave train extends from mid-latitude north Pacific to Yangtze-Huaihe basin along the coast of East Asia. However, during the positive phase of "out-of-phase" pattern, QBWO of lower level circulation propagates from west tropical Pacific to the South China Sea, while from Bay of Bengal to Arabian Sea accosiated transition of cyclonic and anticyclonic anomalous circulation over northwest Pacific. In the upper layer, anomalous wave train in the high latitude propagates southeastward from Eurasia to Yangtze-Huaihe basin during positive phase, while anomalous wave train propagates eastward before positive phase of "south wet to north dry" then westward later during negative phase.