North Atlantic variability in a warmer world: what can the Pliocene tell us?

The Late Pliocene (3 million years ago) is the last period of sustained warmth characterised by elevated carbon dioxide (~400 ppmv), smaller ice sheets and warmer temperatures (~3.2°C above pre-industrial), with a similar to modern continental configuration. This period gives us an insight into how the climate system behaves in a warmer than present state. The majority of research on the Late Pliocene focuses on long term mean states, but examining variability and extreme events provides a deeper understanding of the response of the climate to different forcings, and how these changes are captured across different climate models.

Here, we present an overview atmospheric circulation in the North Atlantic in the Late, including changes to the jet stream and the North Atlantic Oscillation (NAO).

We use data from the Pliocene Model Intercomparison Project Phase 2 (PlioMIP2), a multi-national modelling effort consisting of 17 climate models. We find that the NAO tends towards a more positive phase of the NAO and this shift can explain the mean state precipitation pattern change observed in the PlioMIP2 ensemble. We investigate the drivers of the change using the Hadley Centre Coupled Climate Model, Version Three (HadCM3) to separate out the impacts of Pliocene CO₂, orography and ice sheets on the NAO.

This work highlights the benefit of using past climates to improve understating of the climate system and shows the need to consider a multi-model, multi-centennial viewpoint when examining higher frequency variability in past climates.