Near-equatorial tropical cyclone formation in western North Pacific: peak season and controlling parameter
The near-equatorial (0°–5°N) tropical cyclones (TCs) in the western North Pacific (WNP) exhibit distinctive seasonal variability, with a peak in the boreal winter, as opposite to that in the main TC development region over the WNP. The mechanism behind such a distinctive annual evolution is investigated through the diagnosis of the genesis potential index (GPI). By isolating the effect of various environmental parameters, we found that the increase of the near-equatorial GPI in the boreal winter is primarily attributed by the low-level absolute vorticity. As the season progresses from the boreal summer to winter, the northeasterly trade wind turns anticlockwise near the equator, leading to maximum low-level cyclonic vorticity near 5°N. In addition, the mean flow advection also plays a role in “allowing” more time for perturbations to grow in the near-equatorial zone in DJFMAM than in JASO. The seasonal changes of other environmental conditions, such as relative humidity and sea surface temperature, are not as critical. While the effect of area-averaged vertical wind shear is small due to the opposite signs between western and eastern sectors of the WNP in the boreal winter, a moderate vertical shear over 140–160°E, 2–5°N may favor the development of TC-like disturbances in the region. Our analysis results suggest that dynamic parameters are more important for the formation of near-equatorial TCs.
KeywordsNear-equatorial tropical cyclone Tropical cyclone genesis Genesis potential index
This study is jointly supported by the China National 973 projects (2017YFA0603802 and 2015CB453200), the NSF of China (Grants 41775058 and 41630423), NRL grant N00173-16-1-G906, NSF AGS-16-43297, Jiangsu NSF project (BK20150062), and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). This is SOEST contribution number 10407, IPRC contribution number 1324 and ESMC number 219.
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