Heatwave-generating Rossby waves and the persistence of temperature extremes in a changing climate

The frequency and duration of hot extremes is projected to increase over the coming decades. It remains, however, unclear to what extent persistent surface temperature extremes require an anomalously persistent circulation in the upper troposphere. To shed more light on this relationship, we combine idealized model experiments with reanalysis data and assess the zonal phase speed of Rossby waves as a proxy for circulation persistence. In particular, we compare the climatological-mean phase speed spectrum to the properties of heatwave-generating Rossby wave packets.

In the idealized model without thermodynamic feedbacks, a phase speed increase in response to a localized thermal forcing reduces the frequency of heatwaves. Reanalysis data for the Southern hemisphere mid-latitudes shows a similar and significant phase speed increase from the 1980s until today. However, the observed mean phase speed increase does not apply to heatwave-generating Rossby waves and hence does not contribute to a change in heatwave frequency. The Northern hemisphere, on the other hand, does not yet show a clear phase speed trend in reanalysis. But with continued global warming, we expect an acceleration of heatwave-generating Rossby waves and a reduced upper-tropospheric forcing to persistent temperature extremes in the future.