Richard Leeding: Understanding Wintertime Pan-Atlantic Compound Weather Extremes: Statistical and Dynamical Insights

Abstract

This thesis investigates the connections between extreme winter weather in North America and Europe through large-scale atmospheric patterns. The focus is on the co-occurrence of North American cold spells and European wet or windy extremes. The European extremes occur more frequently than normal during North American cold spells, yet the specific timing and location is dependent on the location of the cold spells. Three North American regions are identified where cold spells coincide with specific footprints of European extremes:

1. Eastern Canada: Cold spells predominantly precede wind extremes over the British Isles and Northern Europe.

2. Central Canada: Cold spells are predominantly preceded by wind and precipitation extremesin Iberia.

3. Eastern United States: Cold spells are both preceded and followed by wind and precipitationextremes in Iberia.

These cold spells are associated with an intensified and more zonal North Atlantic jet stream, increasingthe frequency, and in some cases, intensity of extratropical cyclones affecting Europe. This in turn leads to an increased occurrence of wet and windy extremes. 

However, cold spells in a single North American region can arise due to different large-scale atmospheric circulations. Stratifying the cold spells by wave activity flux intensity indeed reveals differing atmospheric dynamics, referred to as "dynamical pathways".

Further analysis using weather regimes and Fourier decomposition identifies five dynamical pathways, which produce with varying frequencies, cold spells across all three North American regions. Three pathways, associated with the North Atlantic Oscillation (NAO), result in specific footprints of wind extremes in western Europe. The remaining two pathways, associated with Scandinavian blocking, do not produce such extremes. The regional preferences of these pathways provide a mechanistic explanation for the observed spatial and temporal relationships between cold spells and European extremes.

Climate models are found to effectively replicate these pathways, though they underestimate the frequency of NAO+ pathways. Under a high-emission future climate scenario, the frequency and dynamics of the pathways are not projected to change significantly. However, the model simulations point to both North American cold spells and western European wind extremes diminishing in severity.