CMIP5模式对近30年沃克环流强度变化模拟的不足及成因分析 *

太平洋沃克环流(Pacific Walker Circulation, PWC)是热带太平洋上空至关重要的大气环流系统, 但其在全球变暖背景下的长期变化仍存在争议, 换而言之, 沃克环流增强或减弱仍是有待回答的科学问题之一。观测表明近30年PWC呈增强趋势, 而气候模式无法得出观测的趋势。文章分析了参加第五次耦合模式比较计划(Coupled Model Inter-comparison Project Phase 5, CMIP5)的18个耦合模式模拟的PWC变化。结果表明, 大部分耦合模式能够较好地再现PWC的气候态分布特征, 但不能给出其加强的趋势。究其原因, 主要取决于模式对海表温度(SST)变化的模拟能力, 能模拟出PWC加强的耦合模式, 其模拟的SST趋势分布与观测相近即类拉尼娜(La Niña)型], 但仍存在一定差异; 而模拟出PWC减弱的耦合模式, 其模拟的SST趋势分布表现为类厄尔尼诺(El Niño)型, 这与观测不符。对于后者, 如果用观测的SST驱动其大气模式却能够模拟出PWC的加强, 从另一方面也说明了SST变化对于PWC长期变化的主导作用。因此, CMIP5模式要想合理地预估PWC在全球变暖背景下的变化, 需要提高对于热带太平洋SST变化的模拟能力。

Deficiency of CMIP5 models in simulating changes of Pacific Walker circulation in recent three decades: the role of Sea Surface Temperature *

The Pacific Walker circulation (PWC) is the most important atmospheric system over the tropical Pacific Ocean, and the cause of the long-term change of the PWC in response to global warming still remains debatable. The observations consistently indicate that the PWC has significantly strengthened in the past three decades. We examine the changes of the PWC in 18 climate models participated in phase 5 of the Coupled Model Intercomparison Project (CMIP5). Most CMIP5 models have produced successful simulations for the climatological spatial distribution features of the PWC, but no one can simulate the trend of significant enhancement of the PWC as observed. The deficiency of the models to simulate the trend of the PWC depends mainly on the capability of the models to simulate changes in sea surface temperature (SST). The trend pattern of SST is similar to that of the observation (i.e., La Niña-like) in the coupled models that simulate a strengthening PWC, but there are still some differences between the two. However, the distribution of SST shows an El Niño-like trend pattern in the coupled models that simulate a weakening PWC, which does not match that of the observation. For the latter models, if the observed SST is used to drive its corresponding atmospheric models, it can simulate the enhancement of the PWC, which fully demonstrates the leading role of SST change in the long-term change of the PWC. Therefore, to reasonably predict the change of the PWC in the context of global warming, the CMIP5 models need to improve their simulation capability for SST changes in the tropical Pacific.