Intermodel Diversity of Simulated Long-term Changes in the Austral Winter Southern Annular Mode: Role of the Southern Ocean Dipole

Abstract

The Southern Annular Mode (SAM) plays an important role in regulating Southern Hemisphere extratropical circulation. State-of-the-art models exhibit intermodel spread in simulating long-term changes in the SAM. Results from Atmospheric Model Intercomparison Project (AMIP) experiments from 28 models archived in CMIP5 show that the intermodel spread in the linear trend in the austral winter (June–July–August) SAM is significant, with an intermodel standard deviation of 0.28 (10 yr)⁻¹, larger than the multimodel ensemble mean of 0.18 (10 yr)⁻¹. This study explores potential factors underlying the model difference from the aspect of extratropical sea surface temperature (SST). Extratropical SST anomalies related to the SAM exhibit a dipole-like structure between middle and high latitudes, referred to as the Southern Ocean Dipole (SOD). The role of SOD-like SST anomalies in influencing the SAM is found in the AMIP simulations. Model performance in simulating the SAM trend is linked with model skill in reflecting the SOD-SAM relationship. Models with stronger linkage between the SOD and the SAM tend to simulate a stronger SAM trend. The explained variance is about 40% in the AMIP runs. These results suggest improved simulation of the SOD-SAM relationship may help reproduce long-term changes in the SAM.

摘 要

南半球环状模(SAM)在调控南半球热带外环流中发挥了重要作用。模式在模拟SAM的长期变化时表现出模式不确定性。来自CMIP5的28个模式的AMIP试验结果表明,模式对冬季(6月至8月)SAM的线性趋势模拟差异显著,模式间标准差为0.28 (10 yr)−1,大于0.18 (10 yr)−1的多模型集合平均。本文从热带外海表面温度的角度探讨了模式差异的原因。与SAM有关的热带外海温异常在中高纬度之间表现出偶极子型的结构,称为南大洋偶极子(SOD)。AMIP试验的结果表明,SOD型海温异常可以影响SAM。不同模式模拟的SAM长期趋势的差异,与模式在刻画SOD-SAM关系上的差异有关。在SOD-SAM关系较强的模式中,SAM的长期趋势也更加显著。在AMIP试验中,这一解释方差约为40%。上述结果表明,改进SOD-SAM关系的模拟,可能有助于重现模式中SAM的长期变化。