Inter-decadal and inter-annual variability of meridional tropical cyclone activity during September–October in the northwestern North Pacific after 1998

This study examines inter-decadal and inter-annual variability of meridional tropical cyclone (TC) activity during September–October (SO) in the northwestern North Pacific (NWNP) around Taiwan after 1998. From inter-decadal period 1 (1980–1997) to period 2 (1998–2017), the Pacific Decadal Oscillation (PDO) changes its phase from positive to negative, characterizing cold sea surface temperature (SST) anomalies in the tropical eastern Pacific and warm SST anomalies in the tropical western Pacific. An induced low-level convergent centre around the Maritime Continent evokes a cyclonic anomaly on its northwestern side overlying Taiwan that facilitates TC formation in the region north of 20°N. These TCs later move northwards/northwestwards in association with a westwards extension of the western Pacific subtropical high, leading to inter-decadal increase of northwards TC movements in the 120°–130°E region. For inter-annual variability, TC passage frequency in the NWNP tends to decrease during El Niño events but to increase or decrease in La Niña-related events. In La Niña-related conditions, the low-level anomalous convergent centre situating to the west of 130°E (east of 135°E) tends to increase (decrease) TC passage frequency in the NWNP around Taiwan. This region is influenced by an anomalous anticyclone (cyclone) on its eastern side that induces anomalous southeasterly (northeasterly) steering flows to facilitate (hinder) TC movement from the tropical western Pacific into this region. Inter-annual and inter-decadal variability of TC passage frequency in the NWNP is highly and positively connected with TC passage frequency and TC counts affecting Taiwan. More (less) TC passage frequency in the NWNP thus tends to relate to increased (decreased) TC rainfall in Taiwan.