Statistical Analysis of the Relationship between Upper Tropospheric Cold Lows and Tropical Cyclone Genesis over the Western North Pacific

Abstract

This study examined the statistical characteristics of tropical cyclones (TCs) for which the cyclogenesis (TCG) process was modulated by upper tropospheric cold lows (UCLs) over the western North Pacific during the 38 years from 1979 to 2016. Among the 965 TCs, 90 TCs (9 %, 2.4 per year) were defined as having TCG influenced by UCLs in the northwest quadrant of the TC region (UL-TCs). Most UL-TCs occurred in the summer, with large variability in the annual occurrence rate of UL-TCs during June to October, ranging from 0 to approximately 30 %. The annual variation was related to the activity of the Tibetan high and the summer temperature anomaly over Japan. The extremely hot summer of 2016 was partly enhanced by the intense Tibetan high, when 4 UL-TCs also occurred. The average location of UL-TCs at the time of TCG and tropical storm formation (TSF) was significantly farther to the north than the average location of TCs not formed under the influence of UCL (N-UL-TCs). Many UL-TCs occurred in lower tropospheric environments associated with the shear line or confluence regions. The UL-TCs tended to move northward, and the occurrence rate of UL-TCs that made landfall in Japan was approximately double that of other countries. The atmospheric environmental parameters around UL-TCs at the time of TCG were more favorable for the development of TCs than those around N-UL-TCs. In contrast, the atmospheric and oceanic environmental parameters around UL-TCs at the time of TSF were less favorable for the development of TCs, such that UL-TCs tended to remain at weak intensity.

Impact of the Boreal Summer Intraseasonal Oscillation on Typhoon Tracks in the Western North Pacific and the Prediction Skill of the ECMWF Model

“…The GPI estimates the favourability of TC formation using large‐scale environmental parameters (e.g., low‐level vorticity, mid‐level relative humidity, the deep‐layer vertical shear of the horizontal wind and the maximum potential intensity, which reflects the vertical thermodynamic structure of the atmosphere). However, answering the question “why is the GPI unable to represent the TC genesis density anomaly in some months?” is beyond the scope of the present study but it is noteworthy that TCs form from tropical disturbances (e.g., mixed Rossby‐gravity waves or tropical depressions; Takayabu & Nitta, 1993) as well as extratropical systems (e.g., upper tropospheric cold lows; Fudeyasu & Yoshida, 2019). Indeed, Fudeyasu and Yoshida (2019) showed that the number of TC geneses from the upper tropospheric cold lows is the highest in August.…”

“…However, answering the question “why is the GPI unable to represent the TC genesis density anomaly in some months?” is beyond the scope of the present study but it is noteworthy that TCs form from tropical disturbances (e.g., mixed Rossby‐gravity waves or tropical depressions; Takayabu & Nitta, 1993) as well as extratropical systems (e.g., upper tropospheric cold lows; Fudeyasu & Yoshida, 2019). Indeed, Fudeyasu and Yoshida (2019) showed that the number of TC geneses from the upper tropospheric cold lows is the highest in August. Yoshida and Fudeyasu (2020) found that the frequency of the easterly wave flow pattern, which often transforms into tropical depressions, is the highest in October.…”

Future Changes in Western North Pacific Tropical Cyclone Genesis Environment in High-Resolution Large-Ensemble Simulations

“…When a contributing factor could not be detected by the TGS algorithm, the flow pattern was categorized as unclassified flow (UCF). Details of the objective TGS algorithm are provided in previous studies [15,17,24].…”

How Significant are Low-Level Flow Patterns in Tropical Cyclone Genesis over the Western North Pacific?

“…They reported that the monsoon SL, monsoon CR, and EW patterns were the most frequent flow patterns over the WNP. The environment of the flow pattern also has an influence on the characteristics (e.g., strength and track) of the tropical cyclones after the TCG (Fudeyasu and Yoshida 2018). Yoshida and Ishikawa (2013) developed a semiobjective analysis method to categorize the five flow patterns for a longterm analysis, with a stable reliable criterion.…”