Linking jet stream and Rossby wave spectra changes within internal variability and climate change responses

The occurrence and magnitude of extreme events have been linked to quasi-stationary waves (QSW). However, the response of QSWs to climate change is uncertain. Here, we gain insight into the forced QSW response by looking at internal variability in QSW activity. The Rossby wave spectra is highly influenced by the location and strength of the background jet stream. It is known that the poleward shift of the jets in response to external forcing resembles internal variability in the jet such as the Southern Annular Mode. Although open questions remain on the driving mechanisms of these jet responses, we can identify common changes in the Rossby wave spectra within internal variability and the climate change response. 

Using the daily meridional velocity from the Community Earth System Model 2 Large Ensemble, we calculate a space-time spectral decomposition over the midlatitudes, revealing changes in the wavenumber-phase speed structure of synoptic Rossby waves. We investigate the climate change response of the spectra and use maximum covariance analysis between the spectra and the vertically integrated zonal wind to find co-varying patterns of internal variability. Under the SSP3-7.0 scenario in the Southern Hemisphere, we observe a polewards shift of the jet, faster jet speeds, and a corresponding shift of the spectra perpendicular to the barotropic Rossby wave dispersion relationship. This results in a decrease in power in higher wavenumbers and an increase in lower wavenumbers across all phase speeds, including quasi-stationary ones, corresponding to a decrease in stationarity (i.e. wave power with near-zero phase speed). We find this relationship holds on monthly timescales and in response to climate change. The response in the Northern Hemisphere is more complex and differs between the Atlantic and Pacific basin. Our results provide a simple explanation for the wavenumber-dependent changes in Rossby waves and the reduced stationarity of QSWs in response to climate change, which have implications for future changes in weather extremes.