Relations between the quasi-biweekly oscillation over the East Asian monsoon region and the East Asian tropical monsoon depressions

An empirical orthogonal function analysis was applied to 34 years of anomalous daily 10-20 day filtered outgoing longwave radiation data to determine the north-westward propagating mode of quasi-biweekly oscillations (QBWOs) over tropical East Asia during boreal summer. The centres of active QBWO convection coincided with positive potential vorticity (PV) anomalies at 500 center dot hPa and cyclonic gyres at 850 center dot hPa, whereas the centres of suppressed convection were superimposed upon negative PV anomalies and anticyclonic gyres. These circulations formed an inclined northwest-southeast wave train. The vertical dynamic and thermodynamic structures of the QBWO mode evince two centres of PV at 500 and 750 center dot hPa, respectively, and an almost upright structure in the troposphere up to 300 center dot hPa. The QBWO's convective cell has a warm-over-cold structure in the troposphere. These features have several similarities to East Asian tropical monsoon depressions (EAMDs), implying a close relationship between the north-westward propagating mode of QBWOs and EAMDs. Further investigation indicates that EAMDs are prone to appear in the region of deep convection and move north-westward with the QBWO circulation. The westernmost position of the western Pacific subtropical high (WPSH), which is linked to the QBWO phase, strongly influences the route of EAMDs. An EAMD generated in the easterly winds to the south of WPSH moves westward under the influence of the horizontal adiabatic advection of PV. More eastward-moving and turning EAMDs appear when WPSH weakens and retreats eastward due to enhancement of QBWO convection over the South China Sea (SCS) and western North Pacific. Most EAMDs reach peak intensity when moving over the SCS, and show a strong relation to the vigorous convection. However, there is no significant relation between the QBWO convection and the points of disappearance of the EAMDs.