Multiscale Interaction and Barotropic Instability at the Subtropical High Lead to the Sudden Typhoon Recurvature in the Northwestern Pacific

Using a recently developed mathematical apparatus, namely, multiscale window transform (MWT), and the MWT-based theory of canonical transfer, we investigate, based on the ECWMF ERA-40 dataset, the multiscale interactions within the atmospheric system over the Northwestern Paicific as tropical cyclones (TCs) recurve during the period of January 1995-December 2016. A total of 350 TCs have been identified. We focus on those with relatively long life span (≥ 7 d) and high intensity (≥ that of severe tropical storm), and have found that there are 32 TCs with sudden track change (beyond the 5% percentile; twin typhoons and those with loop tracks excluded). The atmospheric fields are first reconstructed onto three scale windows, namely, basic flow window, TC-scale window, and cumulus convection-scale window. The multiscale transports and canonical transfers are computed for the 32 suddenly recurved typhoons and then composited. It is shown that, normally the environmental flow simply transports the TC-scale kinetic energy (KE) toward the northwest; the flow coincides with the KE transport on the TC scale. The sudden recurvature takes place at the westernmost ridge of the subtropical high, where a strong barotropic instability occurs in the northeast of the composited typhoon about 6 hours ahead of time. The resulting canonical transfer provides a new energy source for the TC to grow, favoring a sharp reflection of the track toward the northeast, and, indeed, the TC-scale KE transport now directs northeastward, albeit the environmental flow is still toward northwest. The discrepancy between the directions of the environmental flow and the TC-scale KE transport indicates that it is the intrinsic barotropic instability-induced second flow that deflects the typhoon track and causes the sudden recurvature.

Keywords: Typhoon, recurvature, subtropical high, canonical transfer, multiscale window transform, barotropic instability, environmental flow