High-frequency transient and oscillation features related to wet and dry summer monsoons over East China

Abstract

Using precipitation observations over China, the zonal wind of NCEP-NCAR reanalysis and outgoing longwave radiation (OLR) of NOAA, the features of high-frequency transient (HFT) and oscillation for eight continuous summer monsoons from 1997 to 2004 are studied. (1) The standard deviation analysis reveals the large variability of precipitation is always located in the Yangtze River valley and/or South China. (2) With the wavelet analysis of precipitation, the 2–16-day HFT is most effective over both wet and dry regions of East China. Only in 1998, the 30–40-day oscillation plays an important part in heavy precipitation over the Yangtze River valley. (3) After filtering, the 2–16-day HFT of precipitation does not display northward propagation over East China in 1998 and 1999. The heavy precipitation is closely related with the phase locking of 2–16-day HFT and 30–40-day oscillation in 1998. However, the phase locking is not a necessary condition for the heavy precipitation. In the dry year (2001), the intensity of effective HFT is much weaker than in the wet year (1998 and 1999). (4) The 30–40-day/40–60-day oscillation of zonal wind presents over South China in 1998/the northern part of South China Sea in 1999 and the southern part of South China Sea in 2001. The comparison of locations between the anomalous rainband and the effective oscillation of zonal wind indicates that the south–north location of anomalous rainband is closely associated with the location of 30–40-day/40–60-day oscillation of zonal wind. (5) OLR cannot present the spatial discontinuity or break of precipitation anomalies over East China. Also, with the wavelet analysis of OLR departure time series, any relationship between the variabilities of OLR and wet or dry summer monsoon cannot be concluded over East China.