Rossby wave packets driving concurrent and non-concurrent heatwaves in the Northern and Southern Hemisphere mid-latitudes

Abstract

Heatwaves that occur simultaneously over several regions, termed concurrent heatwaves, pose compounding threats to society and the environment. Amplified quasi-stationary circumglobal Rossby wave patterns (CGWPs) and high-amplitude transient non-circumglobal Rossby Wave Packets (RWPs) have been proposed as two possible explanations for the occurrence of heatwaves. The relation of these mechanisms for heatwaves has been investigated over different timescales, but their relevance for concurrent and non-concurrent heatwaves remains unexplored. In the present study we focus on daily time scales and investigate the relevance of the global CGWP amplitude and of the local RWP amplitude for the occurrence of concurrent and non-concurrent heatwaves over the Northern Hemisphere (NH) and Southern Hemisphere (SH) mid-latitudes. To distinguish between concurrent and non-concurrent heatwaves we apply a k-means clustering algorithm on all heatwaves detected in ERA5 reanalysis data within the 1959–2021 period. We identify 42 spatial clusters of heatwaves in the NH and 53 in the SH. In all identified clusters, mid-latitude heatwaves typically occur at the leading edge of RWPs where Rossby wave breaking takes place in the form of ridge building or block formation. No specific zonal wavenumber is more frequently related to the concurrent or to the non-concurrent heatwave category. However, for high global CGWP amplitudes concurrent heatwaves occur more often in the NH when the dominant zonal wavenumber is k = 7, and non-concurrent heatwaves occur more often in the SH for k = 5. The mid-latitude regions exhibiting increased heatwave probabilities under the influence of either global or local high wave amplitude, include western North America, central Europe, Black Sea, Tibet, the southwest coast of Australia, as well as the southern Indian and Atlantic Oceans. Over those regions, the local high amplitude RWPs increase heatwave probabilities by a factor ranging from 4 to 7, whereas the maximum factor for high global CGWP amplitude is 2. These results emphasize the importance of the daily RWP amplitude and the weak association of the global CGWP amplitude to heatwave occurrence over the NH and SH mid-latitudes. This research for the first time investigates the underlying atmospheric dynamical processes that contribute to the development of concurrent and non-concurrent heat extremes, a crucial step towards improving our understanding and ability to predict heatwave variability at weather and longer time scales.