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Things You Have to Know About Tropical Cyclone (TC): Competitions of ENSO and ISO in Modulating TC Activity

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Things You Have to Know About Tropical Cyclone (TC): Competitions of ENSO and ISO in Modulating TC Activity
  • Author(s)

    Jau-Ming Chen
  • Biography

    Prof. Chen is currently in the Department of Maritime Information and Technology at National Kaohsiung University of Science and Technology (NKUST). He is also a joint professor in the Department of Oceanography at National Sun Yat-sen University (NSYSU). His research focuses on climate variability and TC analysis.

  • Academy/University/Organization

    National Kaohsiung University of Science and Technology
  • Edited by

    Prof. Jau-Ming Chen
  • Source

    Jau-Ming Chen*, Pei-Hua Tan, Liang Wu, Hui.-Shan Chen, Jin-Shuen Liu, Ching-Feng Shih, 2018: Interannual variability of summer tropical cyclone rainfall in the western North Pacific depicted by CFSR and associated large-scale processes and ISO modulations. J. Climate, 31,1771-1787.:

    Jau-Ming Chen, Ching-Hsuan Wu, Pei-Hsuan Chung, Chung-Hsiung Sui*, 2018: Influence of intraseasonal-interannual oscillations on tropical cyclone genesis in the Western North Pacific. J. Climate, 31, 4949-4961.:

    Pei-Hua Tan, Jien-Yi Tu, Liang Wu, Hui-Shan Chen, Jau-Ming Chen*, 2018: Asymmetric relationships between ENSO and entrance tropical cyclones in the South China Sea during fall. Int. J. Climatology, 39, 1872-1888.:
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Tropical cyclones (TCs) often cause significant disasters in the Asian-western North Pacific (WNP) region. Although each TC event seems to be independent of the other, the overall TC activity has been found to be systematically modulated by large-scale climate variability, such as intraseasonal oscillation (ISO) and El Niño-Southern Oscillation (ENSO). The relative roles of these two climate variability features vary in different aspects of TC activity. Regarding the TC genesis rate, enhanced TC genesis tends to extend eastward into the eastern section of the WNP in El Niño years, but is constrained in the western section during La Niña years. ISO tends to cause a southeastward (northeastward) extension of enhanced TC genesis during its active (inactive) phase. Overall, ENSO exerts more dominant impacts than ISO in determining the patterns of TC genesis. Regarding TC rainfall, ENSO is more influential in affecting TC movement and frequency from the tropical WNP toward the Taiwan region. Interannual variability of summer TC rainfall near Taiwan is more affected by ENSO than by ISO. Regarding TC movements from the WNP into the South China Sea, ENSO and ISO contribute in-phase effects in modulating TC entrance during La Niña years. In El Niño years, ENSO plays a decisive role in years with reduced TC entrance, but ISO becomes more important in years with enhanced TC entrance.

Tropical cyclone (TC) events normally intensify rapidly from a tropical disturbance and vanish quickly with a lifetime of less than 2 weeks. There is a general impression that each TC is an individual case and independent of others. However, as viewed from a longer timescale, say, monthly or seasonal, gross TC activity does show some recognized patterns. For example, in 2018, TCs in the western North Pacific (WNP) kept moving northward toward Japan, while Taiwan, on the western side, was almost unaffected. In August 2014, TCs were absent from the WNP for the entire month. These longer-timescale variability features suggest possible modulations of TC activity by large-scale background climate variability. Two salient climate variability features in the WNP that can effectively modulate TC activity appear as the El Niño-Southern Oscillation (ENSO) and the intraseasonal oscillation (ISO). The former is an interannual mode with a period of 2-7 years, while the latter is an intraseasonal mode with a period of 20-90 days. It is of interest to explore the competitive roles of ENSO and ISO in the modulating processes of TC activity. 

For interannual variability of summer TC rainfall (TCR), more TCR near Taiwan was found to associate with more TC movements into this region. That is caused by more TC genesis in the tropical WNP under cyclonic anomalies induced by an El Niño-like warm sea surface temperature (SST) anomaly in the tropical eastern Pacific. The formed TCs later move along the cyclonic anomaly northwestward to affect Taiwan. Meanwhile, ISO also propagates from the tropical WNP northwestward toward the Taiwan region. This propagation provides a positive condition to guide TC movements following its path, leading to more TCR in Taiwan. Statistical analyses show that ENSO plays a more important role than ISO in the modulating processes by causing stronger magnitudes and more coherent interannual variability of TCR. 

For the TC genesis rate in the WNP, El Niño causes TC genesis to extend eastward, while it is constrained westward by La Niña. ISO tends to result in an eastward extension of TC genesis with a northeastward extension in its inactive phases and a southeastward extension in its active phases. In the ENSO-ISO combined phases, ENSO is more influential than ISO due to a clear contrast of extended and shrunk patterns in TC genesis variability.

For TCs moving from the WNP into the South China Sea (SCS) during fall, they can increase or decrease in either El Niño or La Niña years, showing an asymmetric relationship. In La Niña years, both ENSO and ISO jointly enhance or suppress TC movements into the SCS, showing coherent and comparable effects in the modulatory processes. In El Niño years, ENSO and ISO exert reverse impacts on TC activity. In years with suppressed TC entry, ENSO acts as the dominant role to cause anomalous flows from the SCS into the WNP that hinder TC movements into the SCS. In years with enhanced TC entry, ISO propagates from the WNP into the SCS to provide favorable conditions for TC movements into the SCS.
The comparisons in the relative roles of ENSO and ISO exhibit a crossing-timescale interaction between interannual and intraseasonal variability. Such comparisons related to TC modulating processes across different timescales have not been comprehensively delineated for many aspects of TC activity. Our studies provide new research approaches for examining the detailed climate features of TC activity. These analyses can identify key climate variability features to monitor, as well as predict, TC activity for the WNP, the SCS, and East Asia. The analysis results can serve as guidance to improve TC forecasts in the aspects of TC genesis and movements, with an extension into TC rainfall.


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