Our society is particularly vulnerable to variability and changes in temperature, precipitation, and sea level.  The atmospheric circulation plays a critical role in all of these quantities. On a regional scale, temperature and precipitation are primarily determined by "which way the wind blows", that is, how the large scale flow steers the storms systems, heat waves, and cold fronts that make up our weather.  In the midlatitudes (roughly 30 to 60 degrees latitude), the circulation is dominated by the so-called jet streams, strong currents that blow from west to east in both the Northern and Southern Hemispheres.  Slight shifts in these currents can make the difference between mild winter and freezing temperatures, or between drought and flood on the local level.

Perhaps less intuitively, the atmospheric circulation may also play a critical role in sea level rise.  The melting of the Antarctic ice sheet margins depends critically on the flow of comparatively warm oceanic water onto the ice shelves, which are normally protected by very cold water (which hovers right near the freezing point). Oceanic currents driven by the jet stream of the Southern Hemisphere, and thus may play a significant role in the rate of Antarctic melting.

The research supported by this NSF grant allowed us to explore how the the jet streams are impacted by human induced climate change: greenhouse gases -- primarily CO2, which is produced by the burning of fossil fuels -- and ozone depleting substances, such as chlorofluorocarbons (CFCs).  Emissions of the latter were halted as a result of the Montreal Protocol, but continue to impact the climate.   We focused on the Southern Hemisphere, as this hemispheric has been more greatly impacted by ozone loss (the Antarctic Ozone Hole), but also because circulation changes here are relevant to future sea level changes.

A key finding of our research is that ozone loss over Antartica has been the primary driver of the observed poleward shift of the Southern Hemisphere jet stream.  The slow recovery of the ozone hole over the remainder of this century will help minimize future changes, but could be overwhelmed by greenhouse gas emissions if we choose to follow a high carbon future.  If we choose to aggressively curb future greenhouse gas emissions, however, we have a good chance of limiting any further changes to the position of the Southern Hemisphere jet stream -- and so help reduce the potential impact of global warming on Antarctic ice sheets.

On a more technical level, our research sought to understand and narrow the uncertainty in future projections of climate change.  That is, even if we knew our future emissions of greenhouse gases with high confidence, state-of-the-art climate models disagree on the extent to which the circulation will respond.  We found that roughly half of the uncertainty in Southern Hemisphere circulation trends was associated with differences in the dynamics of the atmospheric flow, and how it is represented in computer models. This helps us focus future efforts on improving and testing climate prediction models.

In particular, we explored the role of coupling between the circulation and precipitation in the troposphere and the changes in the stratospheric circulation. We also highlighted the role of large scale circulation on precipitations changes, providing heuristic models for assessing the changes we observe in complex climate simulations.  This motivated the development of a new, simpler model of the atmospheric circulation, which can be used to test our theories and understand the changes we observe in complex models.

Lastly, this grant supported efforts to understand a large natural forcing of the climate, volcanic eruptions, which can lead to rapid changes in temperature and precipitation on a global scale.  The Volcanic Model Intercomparison Project is part of a world wide assessment of climate models, and will help us improve and understand their response to volcanic eruptions.  As the response of the circulation to volcanic eruptions can be observed, this is a golden opportunity to test our models.  Models that get the right response to natural variability are more likely to make better predictions of the circulation response to human induced forcings.

A final important result of this grant was the training and mentoring of two graduate students and a postdoctoral research scientist.  They were given the opportunity conduct cutting edge research, and to learn how we can use high perfomance computing to advance our understanding of the climate system.

Last Modified: 07/29/2017
Modified by: Edwin P Gerber