Physical processes controlling earlier and later onset of a typhoon season in the western North Pacific
The interannual variability of typhoon onset date in western North Pacific (WNP) during 1979–2015 is investigated. The average date in the earlier (later) onset group is early April (late July). A diagnosis of the genesis potential index shows that the key factor affecting the earlier and later onset lies in the background moisture field. According to their evolution feature, the earlier onset cases are further divided into two sub-groups. In the first sub-group, positive specific humidity anomalies were confined in the western WNP and caused primarily by circulation anomalies associated with a La Niña in the equatorial eastern Pacific. In the second sub-group, positive specific humidity anomalies were confined in the eastern WNP and caused by anomalous convection associated with the Pacific Meridional Mode (PMM) in the tropical and subtropical eastern Pacific. In the later onset composite, negative specific humidity anomalies appear in the WNP, and were accompanied by an anomalous anticyclone in situ and a V-shape negative SSTA pattern in the western Pacific. In addition, the change of background vertical wind shear (VWS) also influences the TC onset date. A decrease (increase) in the VWS magnitude and an easterly (westerly) shear anomaly occur in the earlier (later) onset composite. Physically it is argued that both the magnitude of the VWS and the sign of anomalous zonal wind shear may affect TC formation in the WNP.
KeywordsEarlier or later onset of a typhoon season Environmental moisture change Vertical wind shear
This work was jointly supported by China National Key R&D Program 2017YFA0603802 and 2015CB453200, NSFC Grants 41630423, 41475084 and 41575043, NSF Grant AGS-1565653, ONR Grant N00014-16-12260, NRL Grant N00173-16-1-G906, Jiangsu Project BK20150062, Jiangsu Shuang-Chuang Team R2014SCT001, and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). This work was supported in part by the scholarship from China Scholarship Council (CSC) under the Grant CSC N201708320288 and the Postgraduate Research and Practice Innovation Program of Jiangsu Province KYCX17_0877. This is SOEST contribution number 10288, IPRC contribution number 1300, and ESMC number 199.
- Emanuel KA, Nolan DS (2004) Tropical cyclone activity and global climate. Preprints, 26th Conf. on Hurricanes and Tropical Meteorology, Miami, FL, Amer. Meteor. Soc., pp 240–241Google Scholar
- Ge XY, Li T, Zhou XQ (2007) Tropical cyclone energy dispersion under vertical shears. Geophys Res Lett 34:99–100Google Scholar
- Gray WM (1979) Hurricanes: their formation, structure and likely role in the general circulation. In: Shaw DB (ed) Meteorology over the tropical oceans. Royal Meteorological Society, pp 155–218Google Scholar
- Li T, Zhang YS, Chang CP, Wang B (2012) Synoptic and climatic aspects of tropical cyclogenesis in western North Pacific. In: Oouchi K, Fudevasu H (eds) Cyclone: formation, trigger and control. Naval Science Publishers, pp 61–94Google Scholar
- Liebmann B, Smith C (1996) Description of a complete (interpolated) outgoing long-wave radiation dataset. Bull Am Meteorol Soc 77:1275–1277Google Scholar