Signatures of midlatitude heat waves in global Rossby wave spectra

Abstract. The paper investigates systematic changes of the global atmospheric circulation during midlatitude heat waves. The global balanced circulation is defined in terms of the Rossby wave solutions of the linearized primitive equations. The circulation variability is assessed by the probability density functions (PDFs) of the normalized total energy anomalies with respect to their climatological values. The evaluation is performed in wavenumber space defined by the zonal wavenumbers, meridional modes and vertical structure functions. Heat waves are defined by spatially averaged surface temperatures in Eurasia (bounded by the Ural) above the 95 % percentile on at least three consecutive days. Normalized energy anomalies of the Rossby waves are found to be χ²-distributed with a skewness associated with the number of degrees of freedom. The PDFs of energy anomalies during heat waves are compared with their climatological distributions for the zonal mean flow, for the planetary and for the synoptic scales. During heat waves, the skewness of PDFs of planetary-scale circulation is increased up to a factor of two. The increase is associated with a drastic reduction of the number of degrees of freedom down to 1/4 compared to climatology. This reduction explains the coarse structure of blocking events in the midlatitude troposphere as confirmed by the reconstructed circulation in physical space. The changes in variability are also assessed by investigating submonthly circulation variance across scales. Planetary waves are found to be more persistent during heat waves, while the synoptic waves vary more, which is consistent with the idea of circulation blocking during the extreme heat events. The presented diagnostics can be applied for other extreme events in the atmosphere.