Linking midlatitude atmospheric circulation to extreme temperature events : present and future relationships

Extreme temperature events are strongly influenced by large-scale atmospheric circulation variability, particularly in the Northern Hemisphere midlatitudes where Rossby waves and jet streams shape the occurrence, persistence, and severity of hot and cold extremes. However, uncertainties remain in how circulation anomalies contribute to temperature extremes, and how they may change under future climate change. This thesis investigates links between midlatitude atmospheric waviness and extreme temperatures across different waviness metrics, temporal scales, and future climate projections. First, this thesis evaluates how waviness metrics relate to extreme temperatures. ERA5 reanalysis shows that metrics differ in the structure and strength of their co-located associations with temperature extremes. Local Wave Activity (LWA), separated into cyclonic and anticyclonic components, shows the strongest association with extremes, as does a simple 500 hPa geopotential height (Z500) zonal anomaly metric. Neural network analysis shows that including nonlinear, non-co-located relationships strengthens this association, confirming that LWA and Z500 remain the most strongly associated metrics. Second, this thesis examines how the association between atmospheric waviness and temperature extremes depends on persistence. ERA5 and CMIP6 simulations show that the Z500-temperature extreme relationship strengthens from daily to multi-day timescales, with the largest increases on synoptic timescales. Persistent amplified waviness overlaps strongly with atmospheric blocking, but blocking represents a narrower subset of these anomalies, while amplified waviness captures more temperature extremes. CMIP6 models reproduce the timescale dependence, with little change in future simulations. Finally, this thesis investigates how the daily circulation–cold extreme relationship changes under future warming. CMIP6 and large ensembles show that the historical association between negative Z500 anomalies and winter cold extremes weakens, particularly at high latitudes. This weakening accompanies reduced cold-extreme severity, weaker negative Z500 variability, and fewer cold extremes under negative Z500 anomalies. Inter-model differences are linked more strongly to reductions in the meridional temperature gradient, indicating that Arctic amplification weakens this association primarily by reducing the Arctic–midlatitude temperature contrast. Overall, this thesis demonstrates the importance of atmospheric waviness for understanding midlatitude temperature extremes. It links metric sensitivity, persistence, climate model representation, and future change to clarify how large-scale circulation anomalies shape temperature extremes across present and future climates.