Assessment of the climate prediction and analysis of associated precursors signals of 2023 flood season in Zhejiang Province

The 2023 flood season (May to September) in Zhejiang Province was featured by less precipitation,higher temperature,normal high-temperature days,lighter overall typhoon impact compared to the usual condition.In the spring of 2023,the "triple-peak" La Niña just ended and many oceanic and atmospheric precursory signals were in the stage of adjustment and evolution,yielding a big challenge to the climate prediction of 2023 rainy season.This article conducted a retrospective analysis of the result of climate prediction and associated precursory signals using observational data of Zhejiang Province,atmospheric reanalysis from NCEP-NCAR,sea surface temperature from ERSSTv5,etc.The prediction succeeded in accurately capturing the overall trend of less rainfall and higher temperature.Most decadal and interannual signals provided good indications of less-than-normal rainy season precipitation.The anomalous cyclone in the Philippines stimulated by decaying La Niña not only hindered the water vapor transportation in Zhejiang region,but also contributed to the vulnerability of subtropical high to the influence of tropical systems so that the subtropical high broke apart and resulted in normal high-temperature days.However,there were several shortcomings in the prediction:Poor prediction of the location of heavy rainfall regions.This might be due to the incapability of foreseeing the rapid development of El Niño as well as insufficient understanding of the precursory signals associated with the spatial distribution of precipitation anomaly in Zhejiang province during the rainy season.Incorrect judgment of the Meiyu onset trend.This may be related to the reversal of the impact of winter signals compared to spring signals,indicating the need to enhance the ability to prognose spring factors and explore more winter signals with higher credibility.Overestimation of the total influence of typhoons,possibly attributed to the failure to predict substantially less-than-normal number of typhoons generated in the Northwestern Pacific and the South China Sea,which could presumably be explained by decadal variability of typhoon generation frequency,significantly long period of inactive phase of MJO over the Northwestern Pacific.