<scp>ISO</scp> Modulations on Landfalling Tropical Cyclones in the Philippines

ABSTRACT Landfalling tropical cyclones in the Philippines (LTC PH ) occur most frequently in October–December (OND) than July–September (JAS). LTC PH concentrate in the region north of 14° N during JAS, but increase evidently in the region south of 14° N during OND. This study examines modulations of the 30–60‐day intraseasonal oscillation (ISO) on LTC PH in JAS and OND during strong convective (CON) and non‐convective (N‐CON) ISO phases. In both JAS and OND, the major ISO anomaly during the CON phase is an elongated anomalous cyclone across the South China Sea (SCS) and tropical western North Pacific (WNP). The WNP tropical cyclones (TCs) were driven by anomalous southeasterly flows along the interior of this anomalous cyclone toward the Philippines. For the N‐CON phase, the dominant ISO anomaly is an anomalous anticyclone extending southwestward from the oceans east of Taiwan toward the SCS. LTC PH were steered by anomalous easterly or northeasterly flows in the southern part of this anomalous anticyclone. Both the anomalous cyclone and anticyclone and their steering flows expand more southward in OND than JAS. This assists more LTC PH to move along a more southern track toward the region south of 14° N during OND. Numbers of LTC PH in the CON phase are about 2.9 (3.4) times as many as those in the N‐CON phase during JAS (OND) facilitated by ISO anomalies of increased moisture content and decreased vertical wind shear. During the CON phase, the dominating anomalous cyclone connects with a southeastward intensification of the monsoon trough during JAS and a northwestward intensification of the equatorial trough during OND. During the N‐CON phase, the governing anomalous anticyclone associates with a southwestward amplification of the western Pacific subtropical high (WPSH) during JAS and zonal and meridional intensifications of the WPSH and Siberian high during OND.