Exploratory analysis of extremely low tropical cyclone activity during the late‐season of 2010 and 1998 over the western North Pacific and the South China Sea

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Abstract
This study attempts to understand why the tropical cyclone (TC) frequency over the western North Pacific and the South China Sea was so low in 2010 and 1998 even though a strong La Niña signal occurred in both years. We found that the TC frequency during the late‐season (October to December), not in the peak season (July to September), makes 2010 a record low year; the next lowest year is 1998. Specifically, four TCs were observed over the South China Sea (SCS) in the late‐season of 1998, but no TCs occurred over the SCS in the same season during 2010. The genesis potential index is used to help diagnose changes in environmental conditions for TC genesis frequency. Results indicate that the decreased low‐level vorticity makes the largest contribution to the decreased TC formation over the SCS. The second largest contribution comes from the enhanced vertical wind shear, with relatively small contributions from the negative anomaly in potential intensity and reduction in midlevel relative humidity. These observational results are consistent with numerical simulations using a state of the art model from the Meteorological Research Institute (MRI‐AGCM 3.2 Model). Numerical experiments show that the unfavorable conditions for sharply decreased TC formation during the late‐season over the SCS in 2010 mainly results from the sea surface temperature anomaly over the western North Pacific basin. This effect is partly offset by the sea surface temperature anomaly in the South Indian Ocean and Northern Indian Ocean basins.
Funding Information
  • National Natural Science Foundation of China (41305050, 41275093, 4175091)
  • National Basic Research Program of China (2013CB430103, 2015CB452803)
  • Jiangsu Province and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD)
  • P. C. Hsu is supported by the NSFC (41375100)
  • Natural Science Foundation of the Jiangsu Province, China (BK20140046)
  • Research Project of Chinese Ministry of Education (213014A)
  • SOUSEI program of the Ministry of Education, Culture, Sports, Science, and Technology (MEXT) of Japan

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