Abstract
Size of tropical cyclone (TC) is often asymmetric in nature. Yet, there is a lack of systematic, clean, and intuitive definition/expression to specify the asymmetry of TC size. Here, we introduce a novel index, TC size asymmetry index (SAI), which specifies both the degree and pattern of the asymmetry synthetically. In particular, the symbolic form of SAI is vividly designated for identifying the latter. The SAI proposes 1 quasi-symmetric pattern and 28 asymmetric patterns in total. The 41-year (1979–2019) global climatology of SAI shows that the distribution of the degree of TC size asymmetry is trimodal. Elementarily, the degree and pattern of asymmetry are found to be TC intensity, TC movement, time, and space dependent. The introduction of SAI provides an insight into the subject and lays an important foundation for future applications and research. Furthermore, besides meteorology, it could inspire other fields to index the geometric asymmetries of other kinds.
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Data availability
The ERA5-derived global TC size database is available at Zhang and Chan (2023). The Niño 3.4 index is extracted at https://psl.noaa.gov/gcos_wgsp/Timeseries/Nino34/. The AMM index is obtained at https://psl.noaa.gov/data/timeseries/monthly/AMM/. The NAO index is retrieved at https://psl.noaa.gov/gcos_wgsp/Timeseries/NAO/. The AO index is available at https://psl.noaa.gov/gcos_wgsp/Timeseries/AO/. The PDO index is extracted at https://psl.noaa.gov/gcos_wgsp/Timeseries/PDO/. The AMO index is obtained at https://psl.noaa.gov/data/correlation/. The IOBW index is retrieved at http://cmdp.ncc-cma.net/Monitoring/cn_nino_index.php?product=cn_nino_index_iobw. The SAM index is extracted at https://climatedataguide.ucar.edu/climate-data/marshall-southern-annular-mode-sam-index-station-based. Data that supports the findings of this study is available in Supplementary Information or from the corresponding author on request.
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Funding
This study was supported by the National Key R&D Program of China (2019YFC1510400), the National Natural Science Foundation of China (41975052), the Guangdong Basic and Applied Basic Research Foundation (2023A1515010741), the Innovation Group Project of the Southern Marine Science and Engineering Guangdong Laboratory (Zhuhai) (311021001), and the Guangdong Province Key Laboratory for Climate Change and Natural Disaster Studies (2020B1212060025).
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KTFC conceived the idea, performed the analyses, wrote the manuscript, and established the KTFCHAN font. KZ and LX performed the analyses. All authors contributed to the discussion.
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Significance statement
Size of tropical cyclone is often asymmetric in nature, but there is a lack of systematic, clean, and intuitive metric to specify the asymmetry which is strongly urged. A novel size asymmetry index (SAI) is therefore proposed to specify both the degree and pattern of asymmetry synthetically in this study. It is practically useful for assessing and expressing the size structure of tropical cyclone, especially for the parties in applications and research.
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Chan, K.T.F., Zhang, K. & Xu, L. Tropical cyclone size asymmetry index and climatology. Clim Dyn 61, 5049–5064 (2023). https://doi.org/10.1007/s00382-023-06840-5
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DOI: https://doi.org/10.1007/s00382-023-06840-5