Abstract
Seasonal forecasts for Yangtze River basin rainfall in June, May–June–July (MJJ), and June–July–August (JJA) 2020 are presented, based on the Met Office GloSea5 system. The three-month forecasts are based on dynamical predictions of an East Asian Summer Monsoon (EASM) index, which is transformed into regional-mean rainfall through linear regression. The June rainfall forecasts for the middle/lower Yangtze River basin are based on linear regression of precipitation. The forecasts verify well in terms of giving strong, consistent predictions of above-average rainfall at lead times of at least three months. However, the Yangtze region was subject to exceptionally heavy rainfall throughout the summer period, leading to observed values that lie outside the 95% prediction intervals of the three-month forecasts. The forecasts presented here are consistent with other studies of the 2020 EASM rainfall, whereby the enhanced mei-yu front in early summer is skillfully forecast, but the impact of midlatitude drivers enhancing the rainfall in later summer is not captured. This case study demonstrates both the utility of probabilistic seasonal forecasts for the Yangtze region and the potential limitations in anticipating complex extreme events driven by a combination of coincident factors.
摘要
利用英国气象局 GloSea5 系统, 本文分别对 2020 年 6 月、 5-7 月和 6-8 月的长江流域降水进行了预报. 其中, 三个月的预报主要利用动力预测得到的东亚夏季风 (EASM) 指数建立线性回归模型转换到区域平均降水; 6 月份长江中下游的预报则基于降水的线性回归. 经检验, 至少提前三个月, 预报结果都一致明确地预报出长江流域降水偏多的特征. 然而, 长江地区整个夏季都受到异常强降雨的影响, 导致观测值超出了三个月预报的 95% 预测区间降水数值. 本文的预测结果与其他关于 2020 年东亚夏季风降水的研究结果一致, 较好地预测了初夏梅雨锋的增强, 但没有捕捉到中纬度驱动因素对后期夏季降水增强的影响. 该个例研究结果表明, 概率季节预报对长江地区降水预报有一定的效用, 但在预报由多因素共同驱动的复杂极端事件方面存在潜在的局限性.
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Acknowledgements
This work and its contributors (Philip BETT, Gill MARTIN, Nick DUNSTONE, Adam SCAIFE, and Hazel THORNTON) were supported by the UK-China Research & Innovation Partnership Fund through the Met Office Climate Science for Service Partnership (CSSP) China as part of the Newton Fund. Chaofan LI was supported by the National Key Research and Development Program of China (Grant No. 2018YFC1506005) and National Natural Science Foundation of China (Grant No. 41775083). This paper contains modified Copernicus Climate Change Service information (2021), and neither the European Commission nor ECMWF is responsible for any use that may be made of that Copernicus information or data.
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Article Highlights
• Seasonal forecasts for Yangtze rainfall in June, MJJ, and JJA 2020 are presented.
• The forecasts correctly predicted above-average rainfall with high confidence.
• The observed values lie outside the 95% prediction interval of the three-month forecasts.
• This partial success is consistent with the event being driven by teleconnections from multiple sources, not all of which were predicted.
This paper is a contribution to the special issue on Summer 2020: Record Rainfall in Asia—Mechanisms, Predictability and Impacts.
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Bett, P.E., Martin, G.M., Dunstone, N. et al. Seasonal Rainfall Forecasts for the Yangtze River Basin in the Extreme Summer of 2020. Adv. Atmos. Sci. 38, 2212–2220 (2021). https://doi.org/10.1007/s00376-021-1087-x
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DOI: https://doi.org/10.1007/s00376-021-1087-x