The Role of Coupled Feedbacks in the Decadal Variability of the Southern Hemisphere Eddy‐Driven Jet.

Recent work has suggested that tropical Pacific decadal variability and external forcings have had a comparable influence on the observed changes in the Southern Hemisphere summertime eddy‐driven jet over the satellite era. Here we contrast the zonally asymmetric response of the Southern Hemisphere eddy‐driven jet to tropical Pacific decadal variability by designing an atmosphere‐only PAC‐A experiment using the Community Atmosphere Model version 5 (CAM5) and comparing it with the fully coupled Community Earth System Model Version 1 (CESM1) tropical Pacific pacemaker (PAC‐C) experiments. In both frameworks, the tropical Pacific sea surface temperature (SST) anomalies are identical (model climatology plus observed anomalies), which allows the PAC‐C and PAC‐A experiments to be used to estimate the impact of coupling on teleconnections from the tropical Pacific to the Southern Hemisphere extratropics. The observed summertime South Pacific jet intensification is reproduced in both coupled and uncoupled experiments, indicating that the central and eastern tropical Pacific (hereafter, tropical Pacific) SST impacts the South Pacific jet mainly via direct atmospheric teleconnections. By contrast, only the coupled PAC‐C captures the summertime poleward shift of the South Atlantic‐Indian jet, suggesting that air‐sea coupling is essential in driving the teleconnections between tropical Pacific SST anomalies and South Atlantic‐Indian jet variations. Plain Language Summary: There is a zonally asymmetric response of the Southern Hemisphere summertime eddy‐driven jet to tropical Pacific decadal variability, with a poleward migration observed in the South Atlantic‐Indian basin and an intensification for the South Pacific basin. By designing an atmosphere‐only experiment and comparing it with a fully coupled tropical Pacific pacemaker run, we find that the South Pacific jet responds to tropical Pacific sea surface temperature (SST) variability via direct atmospheric processes. By contrast, air‐sea coupling is notably important for the South Atlantic‐Indian jet response to tropical Pacific SSTs as it is conducive to the Pacific–South American Rossby wave propagation into the Atlantic. Key Points: Atmosphere‐only experiments are designed to contrast with coupled pacemakers runs to understand the influence of tropical Pacific decadal variability on the Southern Hemisphere eddy‐driven jetThe South Pacific jet responds to tropical Pacific sea surface temperature variability via direct atmospheric processesAir‐sea coupling is crucial for reproducing the observed poleward migration of the South Atlantic‐Indian jet [ABSTRACT FROM AUTHOR]