Group seminar on 09. November, 14:15 CET
Rossby Wave Energy Distributions and Heat Waves in the Mid-latitudes
Iana Strigunova
Surface extreme events in the mid-latitudes like heat waves can be the result of special atmospheric circulation conditions, for example, the well-known enhanced intensity of quasi-stationary Rossby waves causing blocking and summer heat waves (Petoukhov, 2013). We identify the circulation in terms of Hough modes, which represent an orthogonal and complete set of eigenfunctions of the global primitive equations (Žagar et al., 2015). This decomposition allows for a clear physical separation in balanced and unbalanced modes (Rossby and inertia-gravity waves) and the direct diagnostics of the total (kinetic and potential) energies. This diagnostic is the major advantage of our projection method compared to the common Fourier decomposition along a latitude belt.
In my talk, I will present a diagnostic tool and the interpretation of mid-latitude surface extremes as anomalies in the global atmospheric circulation. The method is based on determining the probability distributions of energies in the zonal mean flow and in planetary and synoptic wavenumber regimes. To examine the robustness of our results, we compare the four reanalyses ERA5, ERA-Interim, JRA-55 and MERRA. We find that the distributions of the averaged energy anomalies in the wavenumber regimes are non-Gaussian with almost the same positive skewness in the four reanalyses. During persistent heat waves, we find an increase of the skewness of all energy anomaly distributions, however with less consistency among the reanalyses. The reconstructed circulation during surface heat waves is represented by a large-scale anticyclonic system in northeastern Europe with zonal wavenumbers 2 and 3.
References
Petoukhov, Vladimir et al. (2013). “Quasiresonant amplification of planetary waves and recent Northern Hemisphere weather extremes”. In: Proceedings of the National Academy of Sciences 110.14, pp. 5336–5341.DOI:10.1073/pnas.1222000110
Žagar, N., A. Kasahara, K. Terasaki, J. Tribbia and H.L. Tanaka, 2015: Normal-mode function representation of global 3D datasets: open-access software for the atmospheric research community. Geosci. Model Dev., 8, 1169-1195.