Zaplotnik, Ž , N. Žagar and N. Semane, 2023: Flow-dependent wind extraction in strong-constraint 4D-Var. Q. J. R. Meteorol. Soc., accepted
Strigunova, I., Blender, R., Lunkeit, F., and Žagar, N., 2022: Signatures of midlatitude heat waves in global Rossby wave spectra, Weather Clim. Dynam., 3, 1399–1414, https://doi.org/10.5194/wcd-3-1399-2022
This paper investigates systematic changes in the global atmospheric circulation statistics during Eurasian heat waves (HWs). The investigation of Rossby wave energy anomalies during HWs is based on the time series of Hough expansion coefficients representing Rossby waves with the troposphere–barotropic structures through the extended boreal summer in the European ERA5, ERA-Interim, Japanese 55-year Reanalysis (JRA-55) and Modern-Era Retrospective analysis for Research and Applications (MERRA). The climatological Rossby wave energy distribution is shown to follow a χ2 distribution with skewness dependent on the zonal scale.
The applied multivariate decomposition reveals signatures of the Eurasian HWs in the probability density functions (PDFs) of the Rossby wave energy across scales. Changes in the PDFs are consistent with changes in the intramonthly variance during HWs. For the zonal-mean state (the zonal wavenumber k=0), a decrease in skewness is found, although it is not statistically significant. A reduction in skewness hints to an increase in the number of active degrees of freedom, indicating more independent modes involved in the circulation. A shift in the spectral distribution of the k=0 intramonthly variance is shown to describe a weakening of the mean westerlies near their core at 45∘ N and their strengthening at high latitudes. At planetary scales (k= 1–3), the skewness in the troposphere–barotropic Rossby wave energy significantly increases during HWs. This coincides with a reduction of intramonthly variance, in particular at k=3, and persistent large-scale circulation anomalies. Based on the χ2 skewness, we estimate a reduction of the active degrees of freedom for the planetary-scale Rossby waves of about 25 % compared to climatology. At synoptic scales (k= 4–10), no change in skewness is detected for the Eurasian HWs. However, synoptic waves k= 7–8 are characterised by a statistically significant increase in intramonthly variance of about 5 % with respect to the climatology. In addition, a shift of the entire Rossby wave energy distribution at synoptic scales, along with amplification, is observed during HWs.
Karwat, A., Franzke, C. L. E., and Blender, R., 2022: Long-Term Trends of Northern Hemispheric Winter Cyclones in the Extended ERA5 Reanalysis. Journal of Geophysical Research: Atmospheres, 127, e2022JD036952. https://doi.org/10.1029/2022JD036952
Fix, F., Buehler, S. A., & Lunkeit, F., 2022: How certain are El Niño–Southern Oscillation frequency changes in Coupled Model Intercomparison Project Phase 6 models? International Journal of Climatology, 1– 12. https://doi.org/10.1002/joc.7901
Denamiel, C., S. Vasylkevych, N. Žagar, P. Zemunik, I. Vilibić, 2022: Destructive potential of planetary meteotsunami waves beyond the Hunga Tonga–Hunga Ha’apai volcano eruption. Bull. Amer. Meteor. Soc., in press, https://doi.org/10.1175/BAMS-D-22-0164.1