Landfalling tropical cyclone characteristics and their multi‐timescale variability connected to monsoon and easterly formation environments over the western North Pacific

Abstract

This study analyses the impacts of tropical cyclones (TCs) that formed in monsoon and easterly environments on the major coastlines of the western North Pacific (WNP) basin during 1981–2009. The TC formation processes, defined as the development of tropical cloud cluster to TC, associated with monsoon environments (monsoon trough, monsoon confluence, and north of monsoon trough) are categorized into monsoon‐type TCs (monsoon‐TCs). Similarly, those associated with easterly flow environments (easterly flow west and southwest of subtropical high) are categorized into easterly‐type TCs (easterly‐TCs). Monsoon‐TCs form farther westward and at lower latitudes than easterly‐TCs, contributing to higher landfall proportion on the coastal countries for monsoon‐TCs. Monsoon‐TCs have a higher probability of affecting southern China, Taiwan and Vietnam, while easterly‐TCs tend to affect eastern China, southern Japan and the Philippines. Monsoon‐TCs have more widely dispersed rainfall and slower translation speed during landfall than easterly‐TCs. These characteristics are consistent with stronger environmental moisture transport and weaker steering flow in monsoon environments. Landfalling TC intensity and size are not different between easterly‐ and monsoon‐TCs. Both easterly‐ and monsoon‐TCs have interannual (1–4 years) and interdecadal (8–11 years) variability, which are related to variability of the large‐scale monsoon trough. El Niño–Southern Oscillation is significantly correlated with the interannual variability of easterly‐ and monsoon‐TCs, and changes in the monsoon‐TC landfall proportion and easterly‐TC landfall intensity. The interdecadal variability mainly affects the background vorticity and cyclonic circulation, leading to changes in the formation number of easterly‐ and monsoon‐TCs. In summary, this study provides evidence for connections between multiscale variability of the large‐scale monsoon and easterly patterns, TC formation environments, and TC impacts on the WNP coasts.

A New Approach to Skillful Seasonal Prediction of Southeast Asia Tropical Cyclone Occurrence

“…Predicator two explains well the remote TCs, which is associated with the genesis‐related conditions in the central tropical Pacific and the steering flow to the north. Thus, we confirm that the three derived predictors represent three different genesis and track clusters of SCS TCs (e.g., Camargo et al., 2007; Chen et al., 2017; Kim & Seo, 2016; Teng et al., 2021).…”

“…We anticipate plausible links between the MT and other predictors for PHL and SCS TCs (not described here). All these are consistent with the previous findings that a strengthening or an eastward shift of the MT encourages more TCs to form in the south-eastern sector of the basin, and a weakening or westward-shift of the MT favors more TCs to form in the western sector (Basconcillo et al, 2021;Chen et al, 2019;Molinari & Vollaro, 2013;Teng et al, 2021;Wu et al, 2012). The merit of the TC predictors is that they are not only associated with the genesis conditions (e.g., the MT) but also related to the background steering winds in which TCs are embedded.…”

“…The predictor 2‐associated environment is depicted by warm SST and low‐level westerly wind anomalies in the central tropical Pacific, suggesting enhanced zonal wind‐SST feedback with a deeper oceanic thermocline and less upwelling (Cane et al., 1986). The warm SST and westerly wind anomalies could correspond to an eastward shift of the MT (Basconcillo et al., 2021; Chen et al., 2019; Molinari & Vollaro, 2013; Teng et al., 2021; Wu et al., 2012), and they are closely related to the PMM pattern (Chiang & Vimont, 2004). The correlation between predictor 2 and the preseason PMM index is r = 0.69.…”

“…From the preseason to peak‐season, the predictor 1‐related environmental conditions strengthen (Figures 6e–6h). In the tropical Pacific, the east‐to‐west SST gradients and low‐level easterly wind anomalies become stronger in the peak‐season, indicating a strengthening or westward shift of the summer Monsoon Trough (MT) (e.g., Basconcillo et al., 2021; Chen et al., 2019; Li et al., 2017; Teng et al., 2021). In short, with positive anomalies of predictor 1, the peak‐season thermodynamic and ventilating conditions are evidently more favorable for TCs to form in the SCS and PHL regions.…”

Importance of Midlevel Moisture for Tropical Cyclone Formation in Easterly and Monsoon Environments over the Western North Pacific

“…Both Fu et al (2012) and Kerns and Chen (2013) suggested that the TC formation probability is more sensitive to the environmental dynamic conditions (e.g., low-level vorticity) for the WNP, while the thermodynamic conditions (e.g., water vapor content) appear to be more important for the North Atlantic. This is because TCs forming in the WNP are mostly associated with the monsoon system but associated with easterly waves in the North Atlantic (Teng et al 2019). However, given that TCs can also form under easterly conditions in the WNP, it is important to understand the contribution of each environmental parameter to TC formation separately for easterly-and monsoon-type TCs.…”