Relations Between The Quasi-biweekly Oscillation Over The East Asian Monsoon Region And The East Asian Tropical Monsoon Depressions

Based on the daily outgoing longwave radiation data and ERA-Interim reanalysis dataset at six-hour interval,an empirical orthogonal function and composited analysis are applied to determine the characteristics of quasi-biweekly oscillations(QBWOs)in different region over the Asian monsoon area during boreal summer.Focusing on the East Asian monsoon region,we reveal activities and three-dimensional structures of the QBWO over the East Asian monsoon region,and further investigate how it modulates the activities of East Asian tropical monsoon depressions(EAMDs).The main results are as follows.The 10–20 day oscillation,quasi-biweekly oscillation(QBWO),is a dominant sub-seasonal oscillation signal of convection in Asian monsoon region during the boreal summer,especially in the South China Sea(SCS).The activities of QBWO over Asian monsoon area show some regional differences.The signal of convective QBWO mode over the equatorial Indian Ocean area propagates eastward from western Arabian Sea,and finally disappears over the western tropical Pacific;the QBWO of the Indian monsoon region forms over the southern Bay of Bengal,and displays a north-westward propagating mode.The characteristics of QBWO over the tropical and subtropical western Pacific regions in the East Asian monsoon area are similar.The original signal of this convective QBWO mode is present over the tropical Pacific,and then propagates north-westward.The features of the QBWO over the East Asian monsoon region characterized by different elements are different.Using OLR data shows a north-westward propagating QBWO in the East Asian monsoon region clearly.The centres of active(suppressed)QBWO convection coincide with positive(negative)potential vorticity(PV)anomalies at 500 h Pa and cyclonic(anticyclonic)gyres at 850 h Pa.These circulations form an inclined northwest-southeast wave train.The QBWO characterized by 500 h Pa PV,850 h Pa relative vorticity and 850 h Pa zonal wind also indicate a north-westward propagating mode,but northward moving is more obvious.The QBWO circulation characterized by the 850 h Pa meridional wind implies a clear westward propagation.The characteristics of the QBWO can not be shown better by 750 h Pa specific humidity.Therefore,we choose the dominant mode of the QBWO characterized by OLR to analyze the three-dimensional structures of the QBWO.The vertical dynamic and thermodynamic structures of the QBWO mode evince two centres of PV at 500 and 750 h Pa,respectively,and an almost upright structure in the troposphere up to 300 h Pa.The QBWO's convective cell has a warm-over-cold structure in the troposphere.These features have several similarities to EAMDs,implying a close relationship between the north-westward propagating mode of QBWOs and EAMDs.Further investigation indicates QBWO modulation at different stages of the EAMDs and in different track groups.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 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.