Moving beyond the mean state: Jet stream variability in the Pliocene

The mid-Piacenzian Warm Period (mPWP), specifically Marine Isotope Stage KM5c, has been the focus of many palaeoclimate studies due to its potential analogy to the future climate. The similar to modern continental configuration, higher CO₂ concentrations of 400ppm, and similar to modern orbit provides the opportunity to examine the world in a warmer climate with relevance to our future.

To date, the majority of studies have focussed on changes to the mean state, however, changes to higher frequency climate variability are crucial to assess to understand both the potential for the mPWP to be used as an analogue for extremes, and to understand the distribution of data that may be recorded in the palaeo record. Here, we use data from the Pliocene Model Intercomparison Project Phase 2 and move beyond the mean state to shorter temporal scales. This aims to improve our understanding of the change in extreme events in the mPWP and the drivers of these changes.

We start broad, looking at the mean state and examining changes to the Northern Hemisphere jet stream and find that in boreal winter months, the jet stream exhibits a poleward shifted state, with a dipole pattern in the speed of the winds, in the mPWP compared with the pre-industrial control. We then move beyond the mean state analysis to consider the variability of the speed and position of the northern hemisphere jet stream in the mPWP and how these changes relate to extreme events.

To further understand the mechanisms behind these changes in the jet stream we present new forcing factorisation simulations using Hadley Centre Coupled Model version 3 (HadCM3) which aim to understand the different contributions of ice sheets, orography, and CO₂ forcings to jet stream behaviour.