Role of sea surface temperature patterns for the Southern hemispheric jet stream response to CO2 forcing

Abstract:

The Southern Hemisphere (SH) midlatitude jet stream has been shown to shift poleward in response to increased greenhouse gases. This has implications for projected SH regional climate including temperature, rainfall patterns and sea ice extent. In CMIP5 models, the shift in SH midlatitude jet under 4xCO₂ forcing was significantly correlated with equilibrium climate sensitivity (ECS). Many of the latest CMIP6 models have a higher ECS than in CMIP5, which raises the question as to whether they also simulate a larger poleward SH jet shift. However, a recent study (Curtis et al, 2020) shows there is a smaller multi-model mean jet shift under 4xCO₂ in CMIP6. Curtis et al. hypothesise this is related to improved model climatologies, with on average a reduced equatorward jet bias in the CMIP6 models.

Here we test an alternative hypothesis for the reduced poleward expansion of the SH jet in CMIP6. Namely, we investigate the role of differences in the sea surface temperature (SST) patterns simulated by the models. The CMIP6 models show on average larger SST warming in the tropics and at high southern latitudes and weaker warming in the subtropics and midlatitudes, as compared to CMIP5. We impose the CMIP5/6 multi-model mean SST anomaly patterns from the abrupt-4xCO2 experiment into version 4 of the Reading Intermediate General Circulation Model (IGCM4). We focus on the changes in the first decade as this is when almost all of the simulated circulation change occurs. We find that IGCM4 forced only by SSTs reproduces many of the regional differences in 850 hPa zonal wind response between CMIP5 and CMIP6 models, with an annual mean pattern correlation in the midlatitudes of 0.76. We therefore suggest that SST warming patterns have a significant influence on forced changes in the SH midlatitude circulation.

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