Preprints
https://doi.org/10.5194/wcd-2022-28
https://doi.org/10.5194/wcd-2022-28
 
19 May 2022
19 May 2022
Status: this preprint is currently under review for the journal WCD.

Decadal variability in extratropical Rossby wave packet amplitude, phase, and phase speed

Georgios Fragkoulidis Georgios Fragkoulidis
  • Institute for Atmospheric Physics, Johannes Gutenberg University, Johann-Joachim-Becher-Weg 21, 55128 Mainz, Germany

Abstract. The ongoing global, yet spatially inhomogeneous warming prompts the inspection of decadal variability in the extratropical upper-tropospheric circulation properties. This study provides observational evidence in this regard by utilizing reanalysis data to unveil past trends in the probability distribution of Rossby wave packet (RWP) amplitude (E), phase (Φ), and phase speed (cp) that may creep behind interannual variability. First, a comparison between the NE Pacific and N Atlantic regions indicates that the 300 hPa E probability distribution exhibits a seasonally- and regionally-varying decadal variability. No apparent discrepancy arises between different reanalysis datasets, except from the JJA season where the historical ones systematically underestimate E. Further exploiting the local in space and time character of the employed diagnostics in ERA5 reveals that, while many areas experience pronounced RWP property variations at interannual and/or decadal time scales, patterns of robust trends in the 1979–2019 period do emerge. Notably, the Northern Hemisphere E field exhibits positive trends in N Pacific, NE Atlantic, and S Asia in DJF, whereas negative trends are found in a substantial portion of the extratropics in JJA. In terms of cp, distinct patterns characterize MAM, with positive trends in parts of N Atlantic and most of Europe and negative trends to the north of these regions and parts of N Pacific. The Southern Hemisphere features a poleward shift in the band of climatologically-maximum E values in DJF, widespread positive E trends in MAM, and positive cp trends in large parts of the extratropics in DJF and MAM. Assessing the decadal variability of RWP phase reveals zonally-extended patterns of alternating trends in the trough-ridge occurrence ratio for MAM in the Northern Hemisphere and JJA in both hemispheres. Furthermore, no covariance is observed between area-averaged daily-mean E and cp at decadal time scales, as revealed by the E–cp bivariate distribution trends for the different regions and seasons. Finally, it is shown that many parts of N Pacific and N America experience a shift to increasing occurrence of large-amplitude and/or quasi-stationary RWPs in DJF during 1999–2019, thus reflecting the temporal variation in trends that characterizes areas and seasons that feature pronounced variability at interannual-to-decadal time scales.

Georgios Fragkoulidis

Status: open (until 11 Jul 2022)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse

Georgios Fragkoulidis

Georgios Fragkoulidis

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Short summary
Assessing annual time series of local Rossby wave packet amplitude, phase, and phase speed on reanalysis data reveals that trends outweigh interannual variability in several regions and seasons. While no covariance emerges between amplitude and phase speed at decadal scales, the frequency of DJF large-amplitude quasi-stationary waves increases in several areas during 1999–2019. These outcomes prompt further investigations on the causes and implications of local variations in RWP properties.