Pliocene climate variability: Northern Annular Mode in models and tree-ring data

The Northern Annular Mode (NAM) and its regional expression, the North Atlantic Oscillation (NAO) are the primary interannual oscillatory systems in the Northern Hemisphere. In the modern climate, NAM has been linked with a number of weather extremes, including large terrestrial temperature increases and extreme snowfalls. Its connection to climate is more controversial, although a change in its behaviour between 1970 and 2000 coincided with rapid temperature increases in the Northern Hemisphere. The North Atlantic and Nordic Seas are a key component in Pliocene climate, showing the largest increases from modern sea surface temperature. To understand these changes and the ability of climate models to reproduce them, we must consider simulations of the NAM. Here we show that existing mid-Pliocene simulations exhibit large changes to the mean state and variability in the North Atlantic and a significant dampening of the NAO. Through sensitivity experiments this change is primarily attributed to the impact of the lowering of the Rocky Mountains. As the timing of Rocky Mountains uplift is still disputed, simulations of the North Atlantic region contain significant uncertainty, particularly relating to interannual variability and its climate feedbacks. New high temporal resolution climate proxy data is required to test these model reconstructions. Here we report new annual resolution data and an analysis of climate variability from fossil tree-rings. These fossils, from sites in the Canadian Arctic, provide support for a strong North Atlantic Oscillation during the Pliocene.