On the Linkage Between Rossby Waves and Heatwaves

Abstract

This thesis examines the relationship between upper-level atmospheric circulation and heatwaves. We studied the role of planetary Rossby waves in the observed increase in intensity, frequency, and duration of extreme temperature events in the mid-latitudes. Focusing on the summer season, we tested the proposed hypothesis that links recent changes in the Rossby wave amplitudes and propagation speed to the increased occurrence of extreme events. We applied a Fourier decomposition technique to separate the large-scale Rossby waves, defined a fitting amplitude-like metric, and developed a peak-tracking algorithm to estimate their zonal propagation speed.

Heatwaves were identified using a percentile-based definition that enables a straightforward comparison of heatwave magnitude across different regions. We carried out a statistical analysis of the Rossby wave speeds and amplitudes during heatwaves and formulated a hypothesis regarding the latitude-dependent circulation mechanisms that potentially favor heatwaves.

We found that heatwaves in the southern mid-latitudes are more often associated with a low-amplitude setting and non-disrupted zonal flow, while heatwaves in the northern mid-latitudes are accompanied by high-amplitude Rossby waves. We confirmed that this trend further intensified in the last decade. Our analysis did not reveal any overall hemispheric trends in the Rossby wave speed or amplitudes. We did, however, find significant regional trends that possibly affect the heatwave activity through the proposed mechanism.