While the influence of anthropogenic forcings on the poleward movement of the Southern Hemisphere’s summertime westerly winds is well-established, the role of natural variability has received relatively little attention. We used the CESM1 coupled model pacemaker simulations to isolate the influence of natural variability from tropical sea surface temperatures (SSTs) on the position of the Southern Hemisphere westerly wind belt and contrast it to that due to climate change using the CESM1 Large Ensemble, understanding the dynamical mechanisms behind such shifts.
The position and strength of the westerly winds over the Southern Ocean have profound impacts on the ocean circulation and Antarctic sea-ice and ice sheets. Understanding the influence of tropical sea surface temperature variability and the mechanisms that create these wind shifts has significant implications for the prediction of Southern Hemisphere mid-latitude wind changes expected over the coming decades.
We used the CESM1 pacemaker simulations to isolate the role of anthropogenic forcings from internal variability on observed decadal changes in the Southern Hemisphere summertime westerly winds. We find that both external forcing and internal tropical Pacific SST variability are important in driving a poleward migration of the eddy-driven jet. Tropical Pacific SST variability, associated with the negative phase of the Interdecadal Pacific Oscillation (IPO), acts to shift the jet poleward over the South Indian and Southwest Pacific and intensify the jet in the Southeast Pacific basin, while external forcing drives a significant poleward jet shift in the South Atlantic basin.