Modulation of tropical cyclone tracks over the western North Pacific by intra-seasonal Indo-western Pacific convection oscillation during the boreal extended summer

摘要

This study investigates the effect of the intra-seasonal Indo-western Pacific convection oscillation (IPCO) on tropical cyclone (TC) tracks over the western North Pacific (WNP) during the boreal extended summer (May-October). The number of west- and northwest-moving TC tracks is found to sharply increase over the WNP in the positive intra-seasonal IPCO phase. Recurving tracks have greater weight in the negative intra-seasonal IPCO phase. Possible physical mechanisms are further examined in terms of steering flow, energy conversion, and energy propagation. When the intra-seasonal IPCO phase is positive, the first-order moment term of perturbation potential energy (PPE1) converts into perturbation kinetic energy (PKE) at lower latitudes. The pressure trough spreads farther to the east. Meanwhile, Rossby waves emanating from the convective centers of the intra-seasonal IPCO over the WNP (Wave(WNP)) and EEIO (Wave(EEIO)) travel into the trough region, thereby deepening the trough. These features enhance the westward and northwestward steering flow between 20 degrees N and about 30 degrees N, sharply increasing the number of straight west- and northwest-moving TC tracks over the WNP. When the intra-seasonal IPCO is in a negative phase, conversion from PPE1 to PKE at lower latitudes is suppressed and the trough weakens. More PPE1 converts to PKE in the climatological western Pacific subtropical high (WPSH) region and the WPSH is intensified. Moreover, Wave(EEIO) intensifies the north-south ridge of the WPSH over the southern Indian Peninsula. Meanwhile, part Wave(WNP) propagate northeastward. These features favor northeastward motion of TCs over the WNP.