Planetary-scale variations in the atmospheric circulation are the principal cause of longitudinal variations in Earth’s surface climate (e.g., the warmth and wetness of Northern Europe relative to similar latitudes in eastern North America). It is therefore critical to predict future atmospheric circulation changes, both anthropogenically forced and due to processes internal to the climate system, in order to reduce uncertainty in future projections of regional climate. I will discuss recent progress in understanding two important influences on the large-scale atmospheric circulation in Northern Hemisphere winter: (i) planetary-scale waves forced by large mountain ranges; and (ii) internal atmosphere-ocean decadal variability.
I simulate orographically forced planetary-scale waves by adding a single Gaussian mountain range to an idealized general circulation model with no land-sea contrasts and simulate global warming therein by adjusting the longwave optical depth. I find that moist processes, in particular latent heating on orographic slopes, help to set the strength of orographic forcing. The increase in atmospheric moisture content with global warming leads to an increase in latent heating and reduces the amplitude of orographically forced planetary-scale waves in warm, moist climates.
Observed changes in the large-scale atmospheric circulation are notably different from the changes predicted by climate models, but it is unclear whether this discrepancy is the result of unpredictable internal variability or climate model biases. In order to study the mechanisms of internal atmosphere-ocean variability and quantify its impact on atmospheric circulation trends, I apply a pattern recognition method to isolate the signal of decadal variability from amongst the various types of noise in climate model simulations. I demonstrate the fundamental importance of wind-driven ocean circulation changes for decadal sea-surface temperature variability and identify the atmospheric response to these slow ocean changes. This variability has likely contributed to observed atmospheric circulation changes, but I will argue that there are aspects of the observed changes that are characteristic of an externally forced response and remain to be fully understood.