Impact of the Spring SST Gradient between the Tropical Indian Ocean and Western Pacific on Landfalling Tropical Cyclone Frequency in China
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The present study identifies a significant influence of the sea surface temperature gradient (SSTG) between the tropical Indian Ocean (TIO; 15°S–15°N, 40°–90°E) and the western Pacific warm pool (WWP; 0°–15°N, 125°–155°E) in boreal spring on tropical cyclone (TC) landfall frequency in mainland China in boreal summer. During the period 1979–2015, a positive spring SSTG induces a zonal inter-basin circulation anomaly with lower-level convergence, mid-tropospheric ascendance and upper-level divergence over the west-central TIO, and the opposite situation over the WWP, which produces lower-level anomalous easterlies and upper-level anomalous westerlies between the TIO and WWP. This zonal circulation anomaly further warms the west-central TIO by driving warm water westward and cools the WWP by inducing local upwelling, which facilitates the persistence of the anomaly until the summer. Consequently, lower-level negative vorticity, strong vertical wind shear and lower-level anticyclonic anomalies prevail over most of the western North Pacific (WNP), which decreases the TC genesis frequency. Meanwhile, there is an anomalous mid-tropospheric anticyclone over the main WNP TC genesis region, meaning a westerly anomaly dominates over coastal regions of mainland China, which is unfavorable for steering TCs to make landfall in mainland China during summer. This implies that the spring SSTG may act as a potential indicator for TC landfall frequency in mainland China.
Key wordstropical cyclone landfall sea surface temperature gradient air–sea interaction
本文揭示了春季热带印度洋和西太平洋暖池的海表温度梯度(SSTG)对夏季登陆中国大陆热带气旋频次的显著影响. 1979-2015年, 春季正SSTG诱导了一个纬向跨洋环流异常. 此异常表现为: 低层辐合风, 中层上升气流和高层辐散风出现在热带印度洋中西部上空, 而相反的环流情况出现在西太平洋暖池上空, 这些环流场造成热带印度洋和西太平洋暖池间出现了低层东风异常和高空西风异常. 并且, 这个纬向环流异常可通过驱动暖海水向西移动来维持热带印度洋东西部的暖状态, 而通过引发本地冷海水上翻来维持西太平洋暖池的冷状态, 从而有利于在夏季维持此纬向环流异常. 这使得夏季西北太平洋大部地区出现了低层负涡度, 强垂直风切变和低层反气旋异常, 减少了热带气旋的生成频次. 同时, 夏季西北太平洋热带气旋的主要生成区域上空出现了一个中层反气旋异常, 导致中层西风异常控制了中国大陆沿海地区, 不利于引导热带气旋登陆中国大陆. 上述分析意味着春季SSTG可以作为登陆中国大陆热带气旋频次的潜在预报因子.
关键词热带气旋登陆 海表温度梯度 海-气相互作用
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The authors are grateful to the anonymous reviewers for their helpful comments. This study was supported by the National Natural Science Foundation of China (Grant Nos. 41461164005, 41375065 and 41475074).
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