CMIP5 diversity in southern westerly jet projections related to historical sea ice area; strong link to strengthening and weak link to shift

A major feature of projected changes in Southern Hemisphere climate under future scenarios of increased greenhouse gas concentrations is the poleward shift and strengthening of the main eddy-driven belt of mid-latitude near-surface westerly winds (the westerly jet). However, there is large uncertainty in projected twenty-first century westerly jet changes across different climate models. Here the World Climate Research Programme’s Coupled Model Intercomparison Project phase 5 (CMIP5) models were evaluated to assess linkages between diversity in simulated sea ice area (SIA), Antarctic amplification and diversity in projected 21st century changes in the westerly jet following the Representative Concentration Pathway 8.5 scenario (RCP8.5). To help disentangle cause and effect in the coupled model analysis, uncoupled atmosphere-only fixed sea-surface experiments from CMIP5 were also evaluated. It is shown that across all seasons approximately half of the variance in projected RCP8.5 jet strengthening is explained statistically by inter-model differences in simulated historical SIA, whereby CMIP5 models with larger baseline SIA exhibit more ice retreat and less jet strengthening in the future. However, links to jet shift are much weaker and only statistically significant in autumn and winter. It is suggested that a significant cross-model correlation between historical jet strength and projected strength change (r = -0.58) is, at least in part, a result of atmospherically-driven historical SIA biases, which then feed back onto the atmosphere in future projections. The results emphasize that SIA appears to act in concert with proximal changes in sea-surface temperature gradients in relation to model diversity in westerly jet projections.

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