ALBERT

Description: The present work investigates the statistical linkage between upper-tropospheric transient Rossby wave packets (RWPs) and lower-tropospheric temperature extremes on the Northern Hemisphere during the period 1979–2015. Data from ERA-Interim reanalyses are used for the diagnosis of RWP amplitude and 2-metre/850 hPa temperature anomalies as well as the systematic examination of their connection. Areas of large RWP amplitude are found to be associated with an increased probability of lower-tropospheric temperature extremes in many regions of the mid-latitudes. Although a seasonal and inter-regional variability is apparent, this link is always stronger compared to an analysis using a circumglobal waviness metric based on Fourier wavenumber amplitudes. During summer, large 300 hPa RWP amplitudes are associated with an increased probability of 850 hPa temperature extremes in many regions of the mid-latitudes. Up to 55% of these temperature extremes concur with the highest 20% of RWP amplitudes, with large regional differences. Similarly, the correlation between 850 hPa and 2-metre temperature extremes shows large regional differences. As a consequence, the correlation between 300 hPa RWP amplitudes and 2-metre temperature extremes is locally fairly high, but varies greatly with location. During winter, the linkage between the three fields is generally weaker. The non-circumglobal nature of RWPs and our novel perspective regarding the investigation of temperature extremes are further highlighted through a comparison with a hemispheric waviness metric based on a Fourier series expansion of the 300 hPa meridional wind, that results in much lower correlations. Further insight is gained by complementing the climatological results with an investigation of the two most severe heat waves in the recent history of Europe, viz. during the 2003 and 2010 summers. The statistical results are complemented by an investigation of two recent heat waves in the summers of 2003 and 2010. Both events are found to be associated with conspicuous non-circumglobal RWPs, but differences between the two events suggest that the mechanisms linking RWPs and temperature extremes are case dependent. The aforementioned results underscore the important role of upper-troposphere dynamics and unveil avenues for future research on heat waves and cold spells at both weather and climate time scales.