Projected Future Changes in Westerly Winds Associated the South Pacific Winter Split Jet

Abstract:

Projected changes in the Southern Hemisphere (SH) mid-latitude near-surface westerly winds and associated storm tracks are key influences on Antarctica and the Southern Ocean, with, for example, significant implications for ice sheet stability. Climate model projections from the Coupled Model Intercomparison Project Phase 5 (CMIP5) multi-model ensemble generally exhibit a poleward shift of the SH tropospheric westerly jet under increased greenhouse gas concentrations. However, the picture is more complex over the South Pacific in winter, when the climatological tropospheric jet is split into two components: a sub-tropical jet (STJ) (at around 25-30°S) and a higher-latitude polar front jet (PFJ) (at around 55-60°S) which is more directly related to the main mid-latitude lower-tropospheric storm track. On average the CMIP5 models project an equatorward shift of the winter PFJ which consequently becomes more merged with a poleward-shifted STJ. A key question is the extent to which this ensemble-mean picture is robust across the CMIP5 models, many of which poorly reproduce the observed split jet structure.

To address this question CMIP5 output has been used to conduct the first detailed evaluation of future projections of the winter split jet. The PFJ dominates the surface signature of the split jet, but is too weak in historically-forced simulations of most CMIP5 models. It is found that this has important implications for future projections, since the ensemble mean projected equatorward shift is mainly attributable to models with weak PFJs. A subset of 10 CMIP5 models with the weakest surface expression of the PFJ (on average 39% weaker than reanalysis estimates) exhibit an equatorward shift of 2.5° over the 21^st century following the high emissions RCP8.5 scenario. In contrast, a subset of 10 models with the smallest historical biases (on average 6% weaker than reanalysis) exhibit no shift in the PFJ, but do show a strengthening of approximately 1 m/s that extends polewards to the coast of West Antarctica. The more historically-skilful CMIP5 models therefore exhibit a less distinct merging of the split jet in the future, with results indicating a more general poleward shift and strengthening of the westerly wind structure. Broader implications of these results for the Southern Ocean and Antarctica will be discussed.