The Southern Hemisphere (SH) eddy-driven jet stream has been shown to move poleward in climate models in response to greenhouse gas forcing, but the magnitude of the shift is uncertain. Here we address the fact that the latest Coupled Model Intercomparison Project phase 6 (CMIP6) models simulate, on average, a smaller jet shift in response to an abrupt quadrupling in CO₂ than the predecessor models (Coupled Model Intercomparison Project phase 5 (CMIP5)), despite producing larger global average surface warming. We focus on the response in the first decade when the majority of the long-term jet shift occurs and when the difference between CMIP5 and CMIP6 models emerges. We hypothesise the smaller poleward jet shift is related to the weaker increase in the meridional sea surface temperature (SST) gradient across the southern extratropics in CMIP6 models. We impose the multi-model mean SST patterns alongside a quadrupling in CO₂ in an intermediate complexity general circulation model (IGCM4) and show that many of the regional and seasonal differences in lower tropospheric zonal winds between CMIP5 and CMIP6 models are reproduced by prescribing the SST patterns. The main exception is in austral summer when the imposed SST patterns and CO₂ increase in IGCM4 produce weaker differences in zonal wind response compared to those simulated by CMIP5/6 models. Further IGCM4 experiments that prescribe only SH extratropical SSTs simulate a weaker jet shift for CMIP6 SSTs than for CMIP5, comparable to the full experiment. The results demonstrate that SH SST patterns are an important source of uncertainty for the shift of the midlatitude circulation in response to CO₂ forcing. The study also provides an alternative explanation than was proposed by Curtis et al (2020 Environ. Res. Lett. 15 64011), who suggested model improvements in jet biases could account for the smaller jet shift in CMIP6 models in the extended austral winter season.

在响应温室气体强迫的气候模型中，南半球（SH）涡流驱动的射流已显示向极移，但这种变化的幅度尚不确定。在这里，我们解决了一个事实，即最新的耦合模型比较项目第6阶段（CMIP6）模型平均模拟了较小的射流偏移，以响应CO ₂突然四倍的变化。尽管产生了更大的全球平均表面变暖，但仍比以前的模型（耦合模型比较项目第5阶段（CMIP5））高。我们将重点放在第一个十年的响应中，这是大多数长期喷射位移发生的时间，以及CMIP5和CMIP6模型之间的差异出现的时候。我们假设，在CMIP6模型中，越过南温带的子午海面射流偏移越小，其子午海面温度（SST）梯度的增加越弱。我们将多模型平均SST模式与CO ₂的四倍加在一起在中等复杂度的一般环流模型（IGCM4）中，结果表明CMIP5和CMIP6模型之间的对流层低层纬向风的许多区域和季节差异都是通过规定SST模式来再现的。主要例外是在夏季南方，与CMIP5 / 6模型模拟的结果相比，IGCM4中强加的SST模式和CO ₂增加产生的纬向风响应差异更小。与完整实验相比，其他仅规定SH温带SST的IGCM4实验模拟的CMIP6 SST的射流偏移比CMIP5弱。结果表明，SH SST模式是中纬度环流对CO ₂响应的不确定性的重要来源强迫。这项研究还提供了比提出柯蒂斯另一种解释等人（2020 ENVIRON研究通信。 15 64011），谁建议喷气偏见模型的改进可以解释在扩展南半球的冬季在CMIP6机型较小的射流转变。