The Qusi-biweekly Oscillation Of Meiyu Over Yangtze-huaihe Basin

The low-frequency oscillation of Meiyu over Yangtze-huaihe basin is examined, of which the qusi-biweekly oscillation (QBWO) is discussed in detail. By defining an index QAI to measure the intensity of QBWO, we discussed the annual variability of QBWO, and corresponding circulation characteristics. Then the ability of a regional climate model to simulate the periodic characteristic is verified, and several series of numerical experiments based on the signal of lateral boundary layer are carried out. With respect to the external forcing, the impact of the pre-spring sea surface temperature anomaly on the qusi-biweekly oscillation of Meiyu is studied through diagnostic analysis and numerical simulation, which is conducive to improve the possibility of short-term climate prediction. Also, the longitudinal heterogeneity of Meiyu can be explained to some extent by the propagation of the qusi-biweekly oscillation of Meiyu.The main conclusions are as follows:(1) The 10-25 day oscillation of rainfall is a strong signal which can reflect the temporal heterogeneity of the original rainfall very well. (2) In the lower troposphere, a downstream Rossby wave train extending from the South China Sea to the Western North Pacific is significant, and the phases are transformed when the systems of the wave train keep moving southwestward. The double anti-cyclone and cyclone in the western part modulate the Western Pacific subtropical high to shift southwestward into South China Sea. Coacted with the divergent circulation in the upper troposphere, the convective activity is intensified, thus supports favorable updraft for the rainfall. A mass of water vapor is transported from South China Sea to Yangtze-Huaihe river basin, which forms a convergent belt of water vapor extending zonally. (3) The interannual variability of the intensity of QBWO is significant, and there is an active zone of QBWO from southwest to northeast in atmospherer. The circulation in active years caused more positive rainfall anomaly in phase 3, and a larger negative anomaly in phase7. So the circulation could enlarge the amplitude to intensify the QBWO. (4) The results of the numerical simulation showed that the circulation played more important role in forming the rainfall QBWO than the inner factor, Meiyu front. (5) Through diagnostic analysis and numerical simulation, we find the pre-spring sea surface temperature anomaly has an efffect on the qusi-biweekly oscillation of Meiyu by air-sea interaction. The lasting positive offshore SST anomaly strengthens Meiyu quasi-biweekly oscillation by regulating the Western Pacific Subtropical High to enter and quit South China Sea. (6) During the rainfall longitudinal heterogeneous anomaly years, not only the precQAItation but also the periodic characteristic shows the obvious longitudinal heterogeneous characteristic. Corresponding to the distribution, the 10-25-day filtered water vapor flux, divergence the flux, subtropical high and the vertical circulation all take on the feature of south-north deviation.