Have steering flows in the western North Pacific and the South China Sea changed over the last 50 years?

Authors

  • Pao-Shin Chu,

    Corresponding author
    1. Department of Meteorology, School of Ocean and Earth Science and Technology, University of Hawai‘i at Mānoa, Honolulu, Hawaii, USA
    • Corresponding author: P.-S. Chu, Department of Meteorology, School of Ocean and Earth Science and Technology, University of Hawai‘i at Mānoa, 2525 Correa Rd., Honolulu, HI 96822, USA. (chu@hawaii.edu)

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  • Joo-Hong Kim,

    1. Department of Atmospheric Sciences, National Taiwan University, Taipei, Taiwan
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  • Ying Ruan Chen

    1. Department of Meteorology, School of Ocean and Earth Science and Technology, University of Hawai‘i at Mānoa, Honolulu, Hawaii, USA
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Abstract

[1] Long-term trends in steering flows over the western North Pacific (WNP) and the South China Sea (SCS) are examined during the peak typhoon season. A nonparametric and robust trend detection method is employed. Both the NCEP and ERA reanalysis data sets suggest a statistically significant decreasing trend in steering flows in the subtropical region of the western WNP (between 120°E or near Taiwan and 145°E) and the northern SCS during 1958–2001. Over this period, the decrease in the WNP is quite large with a magnitude of 1.1 m s−1 given that the background mean steering flow is only 3.26 m s−1. This decrease corresponds approximately to one third of the mean flow. When the data are extended from 1958 to 2009 the long-term decrease in steering flows in the aforementioned subtropical region are still significant, although more modest at a rate of 0.7 m s−1. Time series of translational speeds averaged over the same subtropical region also exhibit a slow-down of storms' motion over the last 52 years. This is consistent with the weakening of easterly steering flows analyzed from independent data sets. Results of this study imply a longer life span for tropical cyclones and a greater tendency for storms along prevailing typhoon tracks to recurve.

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