Were Winds Weird in a Warmer World? A Data-Model Comparison of the Pliocene Westerlies

The midlatitude jet streams, frequently referred to as ‘the westerlies’, are an important facet of the climate system that critically impact global hydroclimate, ocean circulation, and aerosol distributions. Recent observations hint at changes in both the position and intensity of the westerlies due to ongoing anthropogenic climate change.  However, the historical significance of these shifts, as well as the magnitude and implications of projected future westerly changes, remains highly uncertain given the short time span of modern observations. Here we use output from the Pliocene Model Intercomparison Project Phase 2 (PlioMIP2) to evaluate changes in the position, intensity, and variability of the westerlies during the mid-Pliocene (~3.3 to 3.0 Ma), a warm period in Earth’s history that is considered an analogue for climate expected in the coming decades to century. We focus on comparing the Pliocene results to output from corresponding Pre-industrial simulations for three societally relevant ocean basins: the North Atlantic, the North Pacific, and the Southern Ocean. For the annual mean, we find considerable cross-model disagreement, amounting to near-negligible multi-model mean (n = 17) shifts of ≤1˚ and <0.5 m s^-1 in the position and intensity of the westerlies, respectively, during the Pliocene. While these are considerably smaller than those indicated by prior (PlioMIP1) results, we show that annual averaging obscures a much larger seasonal divergence in Pliocene westerly wind variability, which appears broadly consistent across models and oceans basins. This divergence is especially evident in the North Atlantic westerlies, which show a poleward migration and intensification of nearly 2˚ and ~1 m s^-1, respectively, during boreal winter, and an equatorward shift and diminishing of ~4˚ and ~1 m s^-1 during boreal summer. To better contextualize and further validate these findings, ongoing efforts are now comparing the PlioMIP2 results to available paleoclimate dust flux, precipitation, and sea-surface temperature records that are potentially relevant for characterizing the Pliocene westerlies.