Characteristics and origin of quasi‐biweekly oscillation over the western North Pacific during boreal summer

Abstract

This study investigates the structure, energetic, and origin of quasi‐biweekly oscillation (QBWO) over the western North Pacific (WNP), using NCEP reanalyses for the years 2000–2007. In the context of vorticity there appears to be a significant QBWO mode over the WNP during the summer. QBWO emerges from the equatorial region and propagates northwestward. Its horizontal structure exhibits a slight southwest‐northeast tilt but mainly longitudinal elongation. In the vertical the QBWO has a northwest tilt with height that gives rise to a structure of the first baroclinic mode. The centers of vorticity and vertical motion near the equator show a phase lag of about one‐quarter wavelength, consistent with the characteristics of equatorial waves, whereas the cyclonic circulation is tightly coupled with anomalous convection as the wave moves away from the equator. Energetic analysis of the QBWO reveals that diabatic heating in the tropics and baroclinic processes in the subtropics play important roles in the generation of eddy available potential energy (EAPE). In turn, the conversion from EAPE to eddy kinetic energy (EKE) and the barotropic conversion are major sources for EKE to compensate the loss by EKE redistribution and dissipation. Tracing the QBWO to equatorial disturbances, our results show some features of equatorially trapped n = 1 Rossby mode, such as phase speed and group velocity. This mode is generally characterized by a zonal planetary wave number of about 6 and nearly symmetric circulation about the equator. A typical case from 2002 is chosen to illustrate that the origin of the QBWO is closely associated with the theoretical equatorial Rossby wave.