Atmospheric variability in the Northern Hemisphere winter in a warm past and a future climate

The Northern Annular Mode (NAM) is the leading mode of atmospheric climate variability in the middle and high Northern latitudes in the present-day climate. Its most prominent regional expression is the North Atlantic Oscillation (NAO), a mode of variability that is well-known and has a strong influence on North Atlantic weather patterns. According to the IPCC AR6 WGI report, the current generation of climate models are ‘skillful’ in simulating the spatial features and variance of the historical and present-day NAM/NAO. However, what kind of NAM or NAO patterns can we expect in a warm future climate?

To answer this question, we have performed equilibrium climate simulations of a warm ‘future’ as well as a warm past climate. Specifically, we have simulated the mid-Pliocene climate, a warm (~400 ppm CO₂) geological period approximately 3Ma ago, using a global coupled climate model (CESM1.0.5). Our simulations compare well to higher latitude sea-surface temperature reconstructions. We have performed sensitivity studies using a pre-industrial and a mid-Pliocene geography, as well as two levels of radiative forcing, as a part of intercomparison project PlioMIP2. But the question remains, to what extent can we treat the mid-Pliocene as an ‘analog’ for a future warm climate?

Looking at Northern hemisphere winter (DJF) sea-level pressure data, we find that the annular ‘belts of action’ move poleward partially due to increase in CO₂, but mainly due to the mid-Pliocene boundary conditions. Over the North Pacific Ocean, sea-level pressure variability slightly increases with CO₂, but greatly reduces due to the mid-Pliocene geography. The NAM seems to behave more ‘annular’ and less ‘sectoral’ or regional due to the mid-Pliocene climate boundary conditions. We will focus on the mechanisms that explain the differences between the past and future simulations.