Western North Pacific Tropical Cyclone Activity in 2018: A Season of Extremes

Abstract

The 2018 tropical cyclone (TC) season over the western North Pacific (WNP) underwent two extreme situations: 18 TCs observed during June-August (JJA) and ranked the second most active summer in the satellite era; only 5 TCs that occurred during September-October (SO), making it the most inactive period since the late 1970s. Here we attribute the two extreme situations based on observational analyses and numerical experiments. The extremely active TC activity and northward shift of TC genesis during JJA of 2018 can be attributed to the WNP anomalous low-level cyclone, which is due primarily to El Niño Modoki and secondarily to the positive phase of the Pacific Meridional Mode (PMM). Overall, the extremely inactive TC activity during SO of 2018 is due to the absence of TC formation over the South China Sea and Philippine Sea, which can be attributed to the in-situ anomalous low-level anticyclone associated with the positive phase of the Indian Ocean Dipole, although the positive PMM phase and El Niño Modoki still hold. Scientific RepoRtS | (2020) 10:5610 | https://doi.Model version 5.3 (CAM-5.3) 29 was used to perform numerical experiments. The model has T31 horizontal resolution (approximately 3.75° × 3.75°) and 26 vertical levels. Parameterization schemes adopted by the model include the deep convection scheme from Zhang and McFarlane 30 , the moist turbulence scheme from Bretherton and Park 31 , the shallow convection scheme from Park and Bretherton 32 , the stratiform cloud microphysics scheme by Morrison and Gettelman 33 , the Rapid Radiative Transfer Method for GCMs (RRTMG) radiation scheme 34 , etc. Details of the experiment design can be found in section 4.2. All the experiments were integrated for 100 years, simulations during the last 80 years were analyzed. Scientific RepoRtS | (2020) 10:5610 | https://doi.

Characterizing the highest tropical cyclone frequency in the Western North Pacific since 1984

“…For these reasons, we select the WNP and JJA as our regional domain and seasonal scale of analysis, respectively. While our choice of scaling is similar to the latest report 4 , we demonstrate in the succeeding discussions that our conclusion deviates from their reported primary cause of the highest tropical cyclone frequency record in JJA 2018. In particular, we iterate that our aim is to contribute (rather than to discount the merit of the said previous reports) to the existing knowledge pool in understanding the large-scale drivers behind above normal TC frequency and activity in the WNP.…”

“…Questions were expectedly raised on what caused such unprecedented increased TC frequency in the WNP hence it is not surprising that a number of reports have already contributed to the existing knowledge pool 1 – 4 . Of the said reports, the first two studies investigated the TC frequency in the entire North Pacific (i.e.…”

“…Meanwhile, the other two papers focused on the WNP TC frequency but they also differ in their choice of seasonal scaling. The third paper selected June–November 3 while the most recent report divided the WNP TC frequency into two seasons: JJA and September–October 4 . However, these choices of regional scaling and seasonal clustering do not totally account for the variability that is inherent to each basin and season.…”

Variations in Seawater pCO2 Associated With Vertical Mixing During Tropical Cyclone Season in the Northwestern Subtropical Pacific Ocean

“…Another possibility is that an unidentified factor, such as large-scale oceanic and atmospheric variabilities, could favor more frequent TCs and enhanced mixing. In the study region (22 • N-28 • N, 135 • E-145 • E), however, interannual variations in pCO 2 nt during TC months were not correlated with the Pacific Meridional Mode or Central Pacific El Niño, which have been reported to be related to basin-wide TC occurrence in the western North Pacific Ocean (Gao et al, 2020).…”

“…However, the uneven occurrence of TCs was also observed in 2018, when 14 and five TC events were generated during July-August and September-October, respectively. These abnormal cases, which are associated with positive phases of the Pacific Meridional Mode and Central Pacific El Niño (Gao et al, 2020), may amplify the interannual variability in the summertime pCO 2 and air-sea CO 2 flux. Another mixing-related factor that may affect pCO 2 variability is the recovery of the TC-induced SST drop with time.…”