Skillful Seasonal Forecasts of Summer Surface Air Temperature in Western China by Global Seasonal Forecast System Version 5

Abstract

Variations of surface air temperature (SAT) are key in affecting the hydrological cycle, ecosystems and agriculture in western China in summer. This study assesses the seasonal forecast skill and reliability of SAT in western China, using the GloSea5 operational forecast system from the UK Met Office. Useful predictions are demonstrated, with considerable skill over most regions of western China. The temporal correlation coefficients of SAT between model predictions and observations are larger than 0.6, in both northwestern China and the Tibetan Plateau. There are two important sources of skill for these predictions in western China: interannual variation of SST in the western Pacific and the SST trend in the tropical Pacific. The tropical SST change in the recent two decades, with a warming in the western Pacific and cooling in the eastern Pacific, which is reproduced well by the forecast system, provides a large contribution to the skill of SAT in northwestern China. Additionally, the interannual variation of SST in the western Pacific gives rise to the reliable prediction of SAT around the Tibetan Plateau. It modulates convection around the Maritime Continent and further modulates the variation of SAT on the Tibetan Plateau via the surrounding circulation. This process is evident irrespective of detrending both in observations and the model predictions, and acts as a source of skill in predictions for the Tibetan Plateau. The predictability and reliability demonstrated in this study is potentially useful for climate services providing early warning of extreme climate events and could imply useful economic benefits.

摘要

我国西部地区夏季气温的变化对水循环, 生态系统和农作物产量都有着重要影响. 本文利用英国气象局的GloSea5预测系统, 评估了我国西部气温的季节预测技巧和可靠性. 研究发现, 模式对我国西部地区近地面气温的预测技巧较高. 对应气候模式的预测结果与观测的时间相关系数在西部大部分地区超过了0.6, 包括西北和青藏高原地区. 这些高预测技巧有两个重要的物理来源:西太平洋地区海温的年际变化和热带太平地区海温的趋势变化. 其中, 近二十年来热带太平洋的海温趋势, 包括热带西太的增暖和东太的偏冷, 对我国西北地区气温的预测技巧贡献显著. 而青藏高原地区气温的高预测技巧主要受到西太平洋海温年际变化的影响. 西太平洋海温可以影响海洋性大陆地区的对流, 调制青藏高原地区环流年际变化, 进而对地表气温产生重要影响. 这个调制过程在观测和模式中都得到了很好的表现, 为青藏高原气温预测提供了重要的技巧来源. 本研究所展示的可预测性和可靠性度有助于我国气候服务业务的开展, 可以为极端气候事件提供有效的前期预警, 并进一步保护人民的生命和经济财产损失.