Close linkage between quasi-biweekly oscillation and tropical cyclone intensification over the western North Pacific

Forecasting tropical cyclone (TC) intensity is challenging, particularly at the sub-seasonal scale. In this study, we found that compared to the 30-90-day Madden-Julian oscillation (MJO), the activity of 10-30-day quasi-biweekly oscillation (QBWO) has a closer connection with the intensity change of TCs over the tropical western North Pacific. Both the TC growth rate (24-hr increases in maximum sustained wind speed) and the probability of TC intensification during the active phases of QBWO increase significantly compared to those during the inactive QBWO phases. The effects of QBWO on TC intensification take place mainly through alteration of the large-scale dynamic conditions. During the active convection phase of QBWO, the 10-30-day cyclonic vorticity and convergence in the lower troposphere, divergence in the upper troposphere, and the circulation-induced moistening anomalies provide a favorable environment for TC growth. The thermodynamic conditions of atmospheric instability, sea surface temperature (SST), and ocean heat potential show negative but insignificant contributions to TC intensification at the quasi-biweekly time scale. These results not only aid our understanding of the influences of QBWO-related environmental conditions on TC intensification, but also suggest the potential predictability of TC intensity at an extended range (10-30 days) once the QBWO has been successfully predicted.