The Late Pliocene jet stream: Changes and drivers of the mean state and variability

Abstract. The Late Pliocene has frequently been used as a way to improve our understanding of the climate system in a warmer state. Larger scale features of Late Pliocene climate, such as Arctic Amplification, will impact global circulation including the jet stream. To date, the majority of Late Pliocene studies have focused on long term mean climate, however, considering interannual variability is important to fully understand the response of the climate system to different forcings. Using data from the Pliocene Model Intercomparison Project Phase 2, we find a more poleward, yet weaker jet stream in the North Pacific during winter months and increased interannual jet stream variability in the Late Pliocene compared to the pre-industrial control. This result is consistent across the majority of models, although there is variation in the magnitude of change across the ensemble. Using new simulations from the Hadley Centre Climate Model Version 3 (HadCM3), we find that changes in jet stream variability are due to orography and vegetation boundary conditions and are correlated with sea ice feedbacks. Carbon dioxide has little impact on the interannual variability in HadCM3. These differences in jet stream variability are linked to a decrease in meridional temperature gradient driven by an enhanced Atlantic Meridional Overturning Circulation. This is important as these differences might suggest a shift in the distributions of climate variables, such as temperature and precipitation, which could have implications for how proxy data and model simulations are compared. These changes in variability, and how the changes are represented in climate models, suggest the Pliocene is not an analogue for future jet stream interannual variability.