A spectral perspective on the existence of hemispheric circulation regimes

While the existence of regional weather regimes (e.g., over the North Atlantic) is a known result, the presence of preferred circulation patterns at the hemispheric scale is still disputed. Space-time spectral analysis can offer a different perspective to tackle this problem, as it provides a compact representation of the large-scale flow evolution. It can objectively extract the most relevant harmonics, in terms of spatial wavenumbers and temporal frequencies, that dominate the hemispheric Rossby wave pattern at a given time and is easily applicable to gridded data sets as Reanalysis or the output of general circulation models.

With the aim to highlight the existence of clusters in the spectral space, we build a data set of spectra of upper-level meridional wind over midlatitudes (35°N-75°N) in the wavenumber/phase-speed domain for the 1979-2019 Reanalysis period. A spectrum is assigned to each day being located in the center of a sliding 61-days time window. This data set contains interesting information about the stationarity and the persistence of the hemispheric Rossby wave pattern. The most persistent harmonics are the ones related to quasi-stationary or westward propagating waves, as confirmed by an analysis of the dominant harmonics during atmospheric blocking events.

Cluster analysis is performed using self-organizing maps (SOMs) on this data set. To assess its significance, the same procedure is applied to an artificially generated red noise with the same mean, variance and lag-1 covariance as the real data. This cross-check does not highlight a preferred number of circulation regimes in the spectral space. However, a subjective classification of the spectral patterns highlighted by the SOM analysis in four different groups can be attempted: 1) a ground state, with no particular deviation from climatology; 2) a state characterized by rapidly propagating, high wavenumber waves; 3) a state characterized by slowly propagating, low wavenumber waves; and 4) a state with a clear, dominant wavenumber. Spectral patterns corresponding to each of these groups are present regardless of the chosen number of SOMs.