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基于U-Net模型构建热带西太平洋西风爆发的参数化方案及与ENSO的关系分析

A Parameterization Scheme for Westerly Wind Bursts in the Western Tropical Pacific Based on the U-Net Model and its Relationship with ENSO

  • 摘要: 厄尔尼诺-南方涛动 (El Ni?o-Southern Oscillation, ENSO) 是影响热带太平洋海气耦合系统的最强年际气候模态,热带太平洋西风爆发 (Westerly Wind Bursts, WWBs) 是ENSO现象的重要前兆因子。人们通常使用基于统计学和动力学方法建立的模型对ENSO事件进行模拟和预测。然而,模型在合理地表征WWBs存在一定的困难,导致其对ENSO事件的模拟和预测性能受到了限制。本研究利用深度学习技术和神经网络模型构建了一种由海气多变量数据集驱动的U-Net模型重构热带太平洋海表纬向风异常场,进一步识别WWBs的时空演变特征并分析了WWBs与ENSO之间的联系。结果表明,在测试时段内 (2003-2022年),U-Net模型能很好地模拟出与观测值较为一致的WWBs的时空分布特征,模型重构的WWBs发生次数和累计天数与观测值的偏差小于4.0%, WWBs发生概率 (P) 的时间序列与观测值之间的相关系数达到了0.87。同时,U-Net模型能较好地捕捉WWBs与ENSO之间的非线性响应关系。观测和模型重构结果均显示,WWBs与ENSO之间存在显著的超前-滞后关系,且WWBs的累计日数、纬向宽度和平均最大振幅等物理量均在El Ni?o事件期间达到峰值,模型重构的P值在El Ni?o年出现的峰值与观测值之间也更为接近。因此,与基于传统的WWBs与海表温度场之间近似线性关系所建立的WWBs参数化方案相比,U-Net模型在模拟WWBs方面具有显著优势,可得到更接近真实的参数化表征。

     

    Abstract: El Ni?o-Southern Oscillation (ENSO) is the strongest interannual climate mode in the atmosphere-ocean coupling system over the tropical Pacific. Westerly wind bursts (WWBs) are an important precursor of the ENSO events. In the past few decades, several statistical and dynamics-based models have been used to simulate and predict ENSO events. However, these models have some difficulties in simulating WWBs, leading their simulation and prediction performance for ENSO events that are also limited. This study constructs U-Net models that are driven by several atmospheric and oceanic data for zonal wind anomalies (ua) in the tropical Pacific. Furthermore, the spatiotemporal characteristics of WWBs are identified and their relationship with ENSO phenomenon are analyzed. The results indicate that the U-Net model can effectively simulate the spatiotemporal distribution characteristics of WWBs during the testing period (2003-2022). The deviation between the occurrence frequency and accumulated days of WWBs reconstructed by the model and the observed values is less than 4.0 %, and the correlation coefficient between the time series of WWBs occurrence probability (P) simulated by the model and observed values 0.87. Meanwhile, the U-Net model can effectively capture the non-linear relationship between WWBs and ENSO, and there is a significant lead-lag correlation between WWBs and ENSO in both U-Net models and reanalysis data. Besides, the duration, zonal width and the average maximum amplitude of WWBs reach their peak during El Ni?o events, and the peak of probability (P) of WWBs during El Ni?o events reconstructed by the U-Net model is much closer to that of observation. In conclusion, compared with the traditional WWBs parameterization scheme that relies on establishing an approximate linear relationship between WWBs and sea surface temperature fields, the U-Net model has significant advantages in representing WWBs.

     

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