Characteristics Of Low Frequency Oscillation Over Yangtze And Huaihe River Basin In The Drought/Flood And Drought-Flood-coexistence Years

Using monthly precipitation data at160observational stations and daily precipitation data at753observational stations of China provided by the National Climate Center of China Meteorological Administration, TRMM3b42daily rainfall, OLR data from National Oceanic and Atmospheric Administration(NOAA) and global reanalysis data provided by NCEP/NCAR, the characteristics of low frequency oscillation during summer in drought/flood and drought-flood-coexistence (DFC) years over Yangtze and Huaihe River basin are investigated. The main results are as follows:(1)The characteristics of low frequency oscillation of precipitation and convection during the drought/flood and drought-flood-coexistence (DFC) years over the Yangtze-Huaihe basin are analyzed. Results show that summer rainfall over the Yangtze-Huaihe basin has multi-scale periods, they are remarkably different in drought/flood and DFC years. The8-16d quasi-biweekly oscillation (QBWO) plays an important role in drought years, while in flood years, there exists16-32d period as well as QBWO. The16-32d oscillation also occurs in DFC and common years accompanied with comparatively weak QBWO, which seems more significant in DFC. Thus, QBWO may be regarded as a vital factor for summer rainfall variation. On the other hand, OLR’s oscillation period is highly correlated with summer rainfall’s. As regard to the propagation of low frequency convection, it is found that in typical drought years, the8-16d low frequency convection propagates mostly on a southward direction, while in typical flood, the convection’s zonal traveling with a westward direction is more remarkable.(2)The phenomenon of the drought-flood abrupt transition during summer in the middle and lower reaches of the Yangtze River, especially the multi-scale low frequency oscillations and their effects on the drought-flood abrupt transition are investigated. Results show that the most obvious drought-flood abrupt transition event occurred in2011, and the associated atmospheric circulation shows significant differences before and after this transition. The short-term activity of western Pacific subtropical high and the low tough over the Bay of Bengal region act as an important role during the period of transition. Summer precipitation in2011has several different timescale periods, in which10—20d and30—60d are the most remarkable low-frequency components. The two low-frequency oscillations affect summer rainfall in different ways, that is,10—20d low-frequency systems are active mostly over the South China Sea and the western Pacific, typically with an10—20d low frequency anticyclone developing and strengthening, while the other side,30—60d low-frequency convection strongly deepens over the Arabian Sea and the Bay of Bengal. The short-term activity of western Pacific subtropical high is the key factor to the drought-flood abrupt change in2011. Corresponding with the10—20d anticyclone in the South China Sea, the western Pacific subtropical high strengthens and moves westward, and the10—20d anticyclone is out of phase with the30—60d low frequency convection over the Bay of Bengal. The latter, as an important source of heat, plays a guiding role in the westward movement of subtropical high. These two low-frequency oscillations work together, and both reach their peak phase in the early June of2011, when the serious drought-flood abrupt transition appears.