Impact of Background Dynamic and Thermodynamic States on Distinctive Annual Cycle of Near-Equatorial Tropical Cyclogenes
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Volume 34
AUGUST 2020
Impact of Background Dynamic and Thermodynamic States on Distinctive Annual Cycle of Near-Equatorial Tropical Cyclogenesis over the Western North Pacific Liyuan DENG1 and Tim LI1,2* 1 Key Laboratory of Meteorological Disaster, Ministry of Education/Joint International Research Laboratory of Climate and Environmental Change/Collaborative Innovation Center on Forecast and Evaluation of Meteorological Disasters, Nanjing University of Information Science &Technology, Nanjing 210044, China 2 International Pacific Research Center and Department of Atmospheric Sciences, School of Ocean and Earth Science and Technology, University of Hawaii, Honolulu, HI 96822, USA (Received January 21, 2020; in final form March 24, 2020)
ABSTRACT In a sharp contrast to tropical cyclone (TC) genesis over the main development region of the western North Pacific (WNP), near-equatorial (0°–5°N) TCs exhibit a distinctive annual cycle, peaking in boreal winter and being inactive in boreal summer. The relative roles of dynamic and thermodynamic background states on near-equatorial TCs formation were investigated based on the observational diagnosis of the genesis potential index (GPI) and high-resolution model simulations. It is found that the background vorticity makes a major contribution to the distinctive annual cycle, while mean temperature and specific humidity fields are not critical. Numerical simulations further indicate that seasonal mean cyclonic vorticity in boreal winter has three effects on TC genesis near the equator. First, the environmental cyclonic vorticity interacts with TC vortex to promote a mid-level outflow, which strengthens boundary layer friction induced ascending motion and thus condensational heating. Second, it produces an equivalent Coriolis effect (via enhanced absolute vorticity), which strengthens positive feedback between primary and secondary circulation. Third, it helps to merge small-scale vortical hot towers (VHTs) into a mesoscale core through vorticity segregation process. However, background vorticity in boreal summer has an opposite effect on TC development near the equator. Key words: near-equatorial tropical cyclone (TC), equivalent Coriolis effect, vorticity segregation Citation: Deng, L. Y., and T. Li, 2020: Impact of background dynamic and thermodynamic states on distinctive annual cycle of near-equatorial tropical cyclogenesis over the western North Pacific. J. Meteor. Res., 34(4), 822–835, doi: 10.1007/s13351-020-0007-9.
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Introduction
Gray (1968) indicated that tropical cyclone (TC) genesis requires six necessary thermal and dynamic conditions, one of which is the Coriolis force. Many subsequent studies confirmed the importance of the planetary vorticity in TC formation as it is essential in connecting rotational and convergent flow (Anthes, 1982; Li et al., 2012). Therefore, it seemed logical to assume that TC genesis must occur a few latitude degrees away from the equator so that a region within 500 km north and south of the earth’s equator should be TC-free. Howev
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