The Impact of Intra-Seasonal Oscillation on Westward Track Deflection of Super Typhoon Fitow (2013)

Xinxin Bi,Guanghua Chen,Weican Zhou

doi:10.3390/atmos13030474

Abstract

Typhoon Fitow (2013) took an unusual westward track deflection after a lengthy northward movement over the western North Pacific (WNP). Based on observation and wave analysis, it is found that the track deflection of Fitow is attributed to the transition of environmental flow from meridional to zonal orientation, which is closely associated with a low-frequency intra-seasonal oscillation (ISO). Furthermore, the impact of ISO on tropical cyclone (TC) unusual movement is investigated using the Advanced Research version of Weather Research and Forecasting (WRF-ARW) model. The control simulation (CTL) reproduces well the synoptic pattern and track deflection of the TC. The TC moves straightly westward and northwestward without track deflection in the sensitivity experiments with the removal of total ISOs and the west-propagating ISO component, while keeping the recurving track with the removal of east-propagating ISO, which suggests that the west-propagating ISO plays a dominant role in the westward track deflection. In the experiment of removing west-propagating ISOs, an anomalous southeast–northwest-oriented wave train around the TC is modified, the mid-latitude trough decays, and the enhanced zonally elongated subtropical high is responsible for the straight northwestward motion of the TC. However, after removing a weaker convection anomaly associated with east-propagating ISOs in the form of a southwest–northeast oriented dipole circulation, the TC is affected by a sustained shallow mid-latitude trough and a west-extended ridge of subtropical high to keep the cyclonic track turning analogous to the counterpart in CTL. The piecewise potential vorticity inversion diagnosis further assesses the contribution of the different ISO components to TC track deflection.

Highlights

In recent years, major research on tropical cyclone (TC) movement has focused on unusual TC tracks, such as left and right sudden turning, looping track, rotating track, and zigzagging track [1]
When the ISOrelated components are removed before the track deflection happens, it is clear that the mid-latitude trough in the first stage of NOISO is much shallower than that in control simulation (CTL), a weaker TC maintains a westward track during the whole simulation (Figure 8c,d)
The possible dynamical influences from the different intra-seasonal oscillation (ISO) modes are examined based on observational analysis, model simulation, and potential vorticity (PV) diagnosis

Summary

Introduction

Major research on tropical cyclone (TC) movement has focused on unusual TC tracks, such as left and right sudden turning, looping track, rotating track, and zigzagging track [1] These TC cases are found to cause tremendous challenge to operational weather forecasts and numerical simulations due to complicated dynamic process and interaction with multiple scale disturbances. It can be hypothesized that there may be multiple ISO wave modes with different spacing–time scale that pose substantial influence on our concerned westward track deflection, and their individual roles could be investigated by conducting model experiments, thereby giving deeper understanding of the existing mechanisms for the unusual track deflection of TCs over the WNP region. The quantitative contribution associated with the ISOs, and the corresponding dynamical process responsible for Fitow’s unusual westward deflection are detailed

Model and Experiment Design—Method

Sensitivity of TC Track Deflection to the Zonal Propagation of ISO

Piecewise PV Inversion Diagnosis

Summary