The Circulation Response to Volcanic Eruptions: The Importance of Tropical Stratospheric Warming

Paleo-observations and reanalyses indicate that large tropical eruptions are followed by a poleward shift of the boreal jet stream in winter. It has been suggested that the jet shift stems from warming of the tropical stratosphere by volcanic aerosols. Although comprehensive models have generally reproduced this, they are not in uniform agreement. We investigate the impact of stratospheric warming on the circulation using a hierarchy of simplified models: a zonally symmetric model, a dry dynamical core in perpetual winter, and a moist radiative model with an annual cycle.

We find that stratospheric warming suffices to reproduce a poleward jet shift. However, the hypothesized mechanism (a strengthening of the stratospheric winds, in balance with the warming anomaly, which in turn stimulates an annular mode response in the troposphere) does not hold quantitatively. Specifically, we find that the warm anomaly is spread into the subtropics by a Hadley Cell-like response of the zonally symmetric circulation, and then to higher latitudes by changes in wave breaking. Thus, the final response of the stratosphere fundamentally depends on eddy interactions. The tropospheric jet shift also depends critically on changes in wave breaking, evolving in concert with the extratropical stratosphere. The coupling bears some similarity to that of the annular mode—i.e., a stronger polar vortex is associated with a poleward jet shift in the troposphere—but the annular mode cannot explain the seasonal variation of the response, or its sensitivity to the model climatology. Lastly, we discuss implications for comprehensive models.