Different characteristics of the quasi-biweekly oscillation over the South China Sea in two boreal summer stages

Abstract

This study investigates different evolutions and spatial structures of the quasi-biweekly oscillation (QBWO) over the South China Sea (SCS) during the early (May–June, MJ) and late (August–September, AS) summers of 1991–2010. During the MJ stage, a local QBWO in convection is dominant over the SCS, which is strengthened by the southwestward extension of QBWO anomaly from the east of Japan. In contrast, the QBWO during the AS stage originates from the east of the Philippines and propagates northwestward and then westward until attaining maturation over the SCS, followed by a gradual decay while moving westward. Regarding the circulations, the QBWO during the MJ stage is accompanied by a wave train of lower-level circulation anomalies extending from the SCS to the North Pacific. However, it is found that a wave train exists from the equator to the north of the SCS during the AS stage. In the vertical structures, the QBWO exhibits a northeast tilted relative vorticity with height during the MJ stage, while a vorticity anomaly extending from the surface up to near 300 hPa, but with an opposite above during the AS stage. In general, the QBWO active phases during both stages are characterized by the low-level convergence and cold anomaly and upper-level divergence and warm anomaly, with a strong upward motion. However, the cold anomaly at the low level is prominent during the MJ stage in contrast to that during the AS stage. And the warm anomaly at the upper level reveals a distinct enhancement from the MJ to AS stage.