年代际气候预测系统IAP DecPreS的海洋同化试验在西北太平洋的海温偏差及其对亚洲夏季风的影响

SST Bias over the Northwest Pacific in Oceanic DataAssimilation Experiments with the Interdecadal Climate Prediction System IAP DecPreS and Its Impacts on the Asian Summer Monsoon Simulation

本文分析了中国科学院大气物理研究所年代际气候预测系统IAP DecPreS的海洋同化试验(简称EnOIIAU试验)在西北太平洋地区的海表面温度(SST)年循环的模拟技巧,并通过对比IAP DecPreS系统自由耦合历史气候模拟试验结果,在包含海气耦合过程的框架下讨论了耦合模式中西北太平洋夏季SST模拟差异,及其对亚洲季风区夏季季风降水模拟的影响。结果表明,EnOI-IAU试验较好地模拟出了西北太平洋各个季节的SST空间分布,并显著减小了原存在于历史气候模拟试验中持续全年的SST冷偏差。混合层热收支诊断分析表明,包含同化过程在内的海洋过程的模拟差异对西北太平洋海温的模拟提升有重要贡献。夏季,EnOI-IAU试验模拟的印度季风伴随的低层西风较观测偏东、偏强,且高估了赤道西太平洋区域的降水量值、低估了印度洋区域的降水量值。水汽收支分析显示,气旋式环流异常造成的水汽辐合异常是造成亚洲季风区降水模拟差异的主要原因。研究表明,较之历史模拟试验,EnOI-IAU试验中夏季西北太平洋地区SST增暖造成局地对流增强,进而使得局地产生异常上升运动,水汽辐合增强,造成西北太平洋地区降水模拟偏多,激发出低层西风异常及赤道外气旋式环流异常。该低层西风异常导致了北印度洋地区低层辐散异常,减小了原存在于历史试验中印度洋地区的正降水偏差。西北太平洋气旋式环流异常一方面增强了印度夏季风伴随的低层西风,使得更多的水汽从阿拉伯海输送到西太平洋暖池区域,增强了该区域的降水量;另一方面,该气旋式环流异常减小了历史模拟试验中中国南部区域偏强的低层风速,进而提升了模式对东亚低层西南风的模拟能力。

Based on the oceanic data assimilation experiments(the merge of the Ensemble Optimal Interpolation and Incremental Analysis Update, hereafter EnOI-IAU) using the near-term climate prediction system of the Institute of Atmospheric Physics(IAP DecPreS), the authors first evaluate the model skill for annual cycle simulation of sea surface temperature(SST) over the Northwest Pacific. By comparing with the historical simulation without oceanic data assimilation, the authors investigate the impacts of differences in simulated summer SST over Northwest Pacific on the Asian summer monsoon simulation in the framework with ocean-atmosphere coupling. The results show that the spatial distribution of SST over the Northwest Pacific is generally captured by the EnOI-IAU experiment and the cold SST biases throughout the year in the historical simulation are significantly reduced. According to the mixed-layer heat budget analysis, oceanic processes including the assimilation process are primarily responsible for the improvement in the simulation of SST. During the summer, low-level westerlies associated with the Indian summer monsoon in the EnOIIAU experiment are stronger and shifted eastward compared with observations. Meanwhile, excessive precipitation is found over the western equatorial Pacific while dry biases are seen over the Indian Ocean region. The moisture budget analysis demonstrates that the anomalous moisture convergence due to changes in circulation plays a crucial role in the simulated rainfall changes over the Asian monsoon region. Compared with the historical simulation, warmed summer SST over the Northwest Pacific in the EnOI-IAU experiment enhances local convection and induces anomalous local ascending motions, which are favorable for the development of anomalous low-level westerly flow and off-equatorial cyclonic circulation anomalies. The low-level westerly anomalies induce low-level divergence anomalies over the Indian Ocean and partly reduce the corresponding wet biases in the historical simulation. The cyclonic circulation anomalies over the Northwest Pacific, on the one hand, enhance low-level westerlies associated with the Indian summer monsoon,which transport more water vapor form the Arabian Sea to the western Pacific warm pool and increase precipitation over this region. On the other hand, the enhanced low-level westerlies reduce the overly strong low-level southerly winds over southern China in historical simulation, and subsequently improve the simulation of low-level southwesterly winds over East Asia.