| Summary: | Wintertime cold air outbreaks are predicted to decline in frequency and intensity as a result of climate change, but in spite of a robust warming trend over the last few decades it is unclear whether this decline has been observed. Some studies have found no trend or even a slight increase in North American cold air outbreaks, which is particularly remarkable when one considers the enhanced warming signal of 2-3 degrees C already observed in the wintertime Arctic, where most air masses resulting in mid-latitude cold air outbreaks originate. But with evidence from warmer climates in the far distant past, we know that cold air outbreaks should decline with global warming. Fossils of frost-intolerant species dating back to the Eocene warm climate period and found in the interior of North America indicate that the wintertime temperature never dropped below freezing while the average temperature was only 10 degrees C warmer than it is today, implying that cold extremes warmed by 2-3 times the average. This suggests that some mechanism may be acting to maintain cold air outbreaks in the modern climate in spite of the overall warming trend. Climate models have long-standing problems matching proxy records for winter temperatures at high latitudes during the Eocene, indicating that such a mechanism may be missing or improperly represented in models. Winter weather can also be mediated by teleconnections with geographically and dynamically distinct features. The stratospheric polar vortex has been hypothesized to exert a downward influence on surface weather, and more specifically to affect the frequency of cold air outbreaks, but the time scale and nature of this influence remains elusive. My dissertation is therefore presented in two parts: chapters one and two concern the mechanisms driving cold air outbreaks in near-modern and paleo climates, while chapter three considers the nature of teleconnections between the troposphere and the stratosphere in winter. The first chapter looks at cold air outbreaks in a ...
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