Season‐Dependent Modulation of Pacific Meridional Mode on Tropical Cyclone Genesis Over the Western North Pacific

Abstract

As the most active basin of tropical cyclone (TC) activity, the western North Pacific (WNP) brings up more than 30% of global TC activity (Chan, 2005;Gray, 1968), which causes enormous disasters to coastal Asia (Zhang et al., 2009). Thus, understanding the climate variabilities in TC genesis over the WNP would greatly benefit the seasonal forecasting and disaster migration of TC activity (Camargo et al., 2007;Klotzbach et al., 2019;Wang et al., 2019a;Zhao et al., 2022), and thus be of prodigious societal and scientific concerns.Climate variabilities in TC genesis over the WNP are usually interpreted by variations in sea surface temperature (SST) across the tropical and subtropical oceans through modifying the local large-scale circulation over the WNP (Wang & Wang, 2019). Among them, the influences of El Niño-Southern Oscillation (ENSO) on the WNP TC

Asymmetric influence of the Pacific meridional mode on tropical cyclone formation over the western North Pacific

“…Compared with negative PMM years, there is an almost basinwide enhancement of TC formation during positive PMM years, except for a small region near the southern Philippines (Figure 2b). This result is consistent with TC formation density directly regressed onto the PMM index as reported in previous publications (e.g., Fu et al, 2023; Zhang et al, 2016). In particular, over the region from 0°–25°N and 140°–155°E, TC formation density is significantly greater during positive PMM years than during negative PMM years.…”

“…Figure 1a highlights a significant positive relationship between WNP TC frequency during ASON and the simultaneous PMM index from 1961 to 2021 ( r = 0.49, p < 0.01). Consistent with Fu et al (2023), during June and July, we find a weak correlation between TC frequency and the PMM index ( r = −0.01, p = 0.94). Given that TC frequency during ASON (16.8) is nearly three times that during June and July (5.8) climatologically, the significant PMM–TC frequency relationship during June–November as found in previous publications (e.g., Wu et al, 2020; Zhang et al, 2016) is dominated by the significant relationship during ASON.…”

“…Figure 2 displays the spatial distributions of both the long‐term climatology and the PMM‐induced changes in WNP TC formation density. On average, a majority of ASON TCs form east of the Philippines (10°–20°N, 120°–150°E; Figure 2a), which is similar to the TC formation distribution during August–December (Fu et al, 2023). Compared with negative PMM years, there is an almost basinwide enhancement of TC formation during positive PMM years, except for a small region near the southern Philippines (Figure 2b).…”

“…The WNP TC‐active season is usually defined as June–November, during which ~85% of the annual total number of WNP TCs form (Song & Klotzbach, 2019). Recently, Fu et al (2023) found that there was a season‐dependent modulation of PMM on WNP TC formation, with a significant positive correlation in January–April and August–December but an insignificant relationship in May–July. Therefore, we focus on the 4‐month period of August–November (ASON), which includes ~63% of the annual total number of TCs and ~74% of the TCs forming during June–November.…”

“…Previous publications have shown that the modulation of WNP TC activity by the PMM can be explained by changes in large‐scale environmental conditions (Fu et al, 2023; Gao et al, 2018, 2020; Hong et al, 2018; Wu et al, 2020; Zhang et al, 2016, 2020). Figure 3 displays the anomalies of environmental variables during positive PMM years relative to the 1961–2021 climatology.…”