Characteristics of QBWO over the East Asian Monsoon Region Presented by Different Elements

Different variables present discrepancy in characteristics of the quasi-biweekly oscillation (QBWO), which is a dominant sub-seasonal signal in the East Asian monsoon regime. However, there is limited investigation about similarities and differences in features of QBWO presented by varying variables. In order to fill this gap, adopting the empirical orthogonal function (EOF) and composite analysis, such variables as outgoing longwave radiation (OLR), 500 hPa potential vorticity (PV), 850 hPa relative vorticity, 850 hPa zonal wind, 850 hPa meridional wind and 750 hPa specific humidity are compared, regarding of the spatial-temporal distribution, intensity and propagation of QBWO over the East Asian monsoon region. It is found that all these variables show significant QBWO across the region with similar spatial and temporal variation. And the strongest QBWO is observed over the South China Sea (SCS) with all variables. QBWO in OLR propagates north-westward over the East Asian monsoon regime. Centres of active (suppressed) QBWO convection correspond to positive (negative) PV anomalies at 500 hPa level and cyclonic (anticyclonic) vortex at 850 hPa level. These circulations form a northwest-southeast tilted wave train. Two leading modes of QBWO in 500 hPa PV, 850 hPa relative vorticity and 850 hPa zonal wind have greater meridional magnitude than those of OLR. QBWO in three variables also propagate north-westward, but spread faster to north. Oppositely, two leading modes of QBWO in 850 hPa meridional wind are characterized by the zonal dipole pattern and the westward propagation is evident. Actually, its speed of northward propagation is the slowest of all. Different from all others, QBWO in 750 hPa specific humidity propagates south-eastward, and variances explained by QBWO of 750 hPa specific humidity is the smallest. As for the intensity of QBWO, except for 750 hPa specific humidity, other variables have consistent inter-annual variation. Totally, affected by such complex physical processes as transformation of precipitation state, release of heat and so on, characteristics of QBWO is hardly captured by 750 hPa specific humidity. However, OLR, 500 hPa PV, 850 hPa relative vorticity, 850 hPa zonal wind and 850 hPa meridional wind can well characterize QBWO over the East Asian monsoon region. Of all variables compared in this analysis, 500 hPa PV and 850 hPa relative vorticity are highly consistent in describing QBWO over the East Asian monsoon region. Specific causes that lead to different characteristics of QBWO over the East Asian monsoon regime presented by different variables need further discussion, which can provide a new reference for selecting monitoring indices for QBWO over the East Asian monsoon region.