An assessment of the North Atlantic Oscillation in the Late Pliocene and its impact on European climate

Abstract. The North Atlantic Oscillation (NAO) heavily influences European climate and weather and is projected to change with future warming. Despite it's strong links to climate, there has not yet been an analysis of the NAO using the most recent climate model simulations of the Late Pliocene. Using data from the Pliocene Model Intercomparison Project Phase 2 (PlioMIP2), the NAO and North Atlantic atmospheric circulation are analysed. In winter an increased meridional sea level pressure gradient contributes to an increase in the multi-model mean NAO index. However, this is model dependent with only a few models displaying changes in the NAO index. Using forcing factorisation simulations, the differences in sea level pressure are found to be driven by non‑CO₂ boundary conditions such as ice sheets and orography. These changes in the NAO also relate to shifts in the zonal winds, with North Atlantic Westerlies being more poleward in winter, and more equatorward in summer. When examining European climate, Northern Europe is on average wetter in the Late Pliocene compared to the pre‑industrial, but this is not linked to a difference in NAO behaviour. The simulations examined all show increased mean precipitation over Northern Europe in the Late Pliocene compared to the pre-industrial. Shifts in the distribution of wettest month rainfall are not linked to any change in the NAO index. This could imply changes in Northern European Late Pliocene precipitation extremes, but that these differences are may not be related to differences in the NAO. These shifts in rainfall distributions demonstrate the importance of considering multiple models when examining variability, especially in a palaeoclimate context.