Role of tropical variability in driving decadal shifts in the Southern Hemisphere summertime eddy-driven jet

Abstract:

The Southern Hemisphere summertime eddy-driven jet and storm tracks have shifted poleward over the recent few decades. The role of external forcing and internal variability are isolated by using a hierarchy of Community Earth System Model Version 1 (CESM1) simulations, including Pacemaker runs for each ocean basin. Model simulations suggest that in the early 21st Century, both external forcing and internal observed tropical Pacific SST variability were important in driving a positive Southern Annular Mode (SAM) phase and 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. In response to both the external forcing and negative phase of IPO, the transient eddy momentum flux convergence belt in the middle latitudes experiences a poleward migration due to an enhanced meridional temperature gradient at around 55°S, leading to a zonally symmetric southward migration of the eddy-driven jet. However, the mid-latitude circulation change is not entirely zonally symmetric; in the South Pacific Ocean it is dominated by a jet intensification rather than a poleward shift. An atmosphere-only simulation Pacific-Ocean-Global-Atmosphere (POGA) is designed to examine this SH zonal asymmetric variation and the role of sea-air coupling. Coupled Pacific pacemaker and uncoupled POGA comparisons imply that the influence of IPO on the jet is mainly via direct atmospheric processes for the South Pacific in all seasons, while for the South Atlantic and Indian jet, interbasin interactions are crucial in driving the poleward summertime shift.