Response of atmospheric variability in the Northern Hemisphere winter to past climate conditions and elevated CO2 levels

A specific feature where future climate projections fail to see a consistent response to increasing CO₂ levels is Northern Hemisphere winter atmospheric dynamics and variability. This holds specifically for the Northern Annular Mode (NAM) and its regional expression, the North Atlantic Oscillation (NAO). The lack of consensus in future projections is caused in part due to the large internal variations of these modes of atmospheric variability compared to the response to elevated CO₂.

The response of interannual and decadal climate variability to warm conditions can be isolated in climate simulations equilibrated at elevated CO₂ concentrations. However, we cannot perform a future model-data comparison. Fortunately, we can turn to the past. The last time the Earth saw similar CO₂ concentration as the present day was approximately 3 million years ago, in the mid-Pliocene epoch. The mid-Pliocene is often considered the ‘best analog’ to an equilibrated climate at present or near-future CO₂ levels. However, can the mid-Pliocene be used to assess the response of Northern Hemisphere winter atmospheric variability, such as the NAO and NAM, to a warm climate?

To answer this question, we have performed a set of sensitivity experiments using a global coupled climate model (CESM1.0.5). We have performed sensitivity studies using a pre-industrial and a mid-Pliocene geography, as well as two levels of radiative forcing (280 ppm and 560 ppm), as a part of intercomparison project PlioMIP2. Our mid-Pliocene simulations generally compare well to proxy reconstructions of sea-surface temperature.

We consider the sea-level pressure (SLP) and zonal wind at 200 hPa using 200 years of January-mean data, and perform principal component analysis. In response to the mid-Pliocene boundary conditions (other than CO₂), we find a large increase in the mean SLP along with a decreased variance over the North Pacific Ocean. This is accompanied with a weakened jet stream over the western North Pacific, as well as increased occurrence of a split jet condition over the eastern North Pacific. These findings are connected to a regime shift in the modes of atmospheric variability in the Northern Hemisphere, where the so-called North Pacific Oscillation (NPO) becomes the most dominant mode of variability. We do not see tendencies towards similar behavior in the CO₂ doubling experiment indicating that the Pliocene boundary conditions are the main driver of the observed shifts in variability. This suggests that the mid-Pliocene is not a good analog for a warm future climate when considering Northern hemisphere winter atmospheric variability.