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

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 examine the past interannual-to-decadal variability and unveil trends in the probability distribution of Rossby wave packet (RWP) amplitude (E), phase (φ), and phase speed (cp). First, a comparison between the NE Pacific and N Atlantic regions indicates that the 300hPa 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 two historical reanalyses systematically underestimate E compared to three modern-era reanalyses. Further exploiting the local-in-space and local-in-time character of the employed diagnostics in ERA5 reveals that, while many areas experience pronounced RWP property variations at interannual and/or decadal timescales, patterns of statistically significant trends in the 1979-2019 period do emerge. Notably, the Northern Hemisphere E field exhibits positive trends in the N Pacific, the 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 the N Atlantic and most of Europe and negative trends to the north of these regions and parts of the 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 timescales, as revealed by the bivariate probability distribution trends in the E-cp domains for the different regions and seasons. Finally, it is shown that many parts of the N Pacific and N America experience a shift to increasing occurrence of large-amplitude and/or quasi-stationary RWPs in DJF during 1999-2019, which is a manifestation of the pronounced interannual-to-decadal variability that characterizes the E and cp seasonal distributions in some areas and seasons. Overall, this study underscores that substantial seasonal and regional variations characterize the past decadal variability and trends that emerge in the seasonal probability distributions of key RWP properties.