Future changes in association between atmospheric circulation anomalies and extreme temperature events

With increasing global temperatures, there has been an observed increase in the quantity and intensity of extreme weather events, particularly heat extremes in the midlatitude regions. Some recent studies have attributed this increase at least partially to an amplification of upper tropospheric jet stream waves. Whilst there is significant scientific uncertainty over causes of recent trends in jet stream waviness, the impact atmospheric waves have on extreme events is clear. Therefore it is key to quantify whether the relative importance of jet stream waviness on the formation of extreme temperature events changes in the future.

We achieve this by studying the probability ratio between co-occurring high magnitude geopotential height anomalies at 500 hPa, and coincident surface temperature extremes. We calculate this for the historical period (1980-2015) and the future (2065-2100), and compare how this probability ratio - the association between atmospheric circulation and surface temperature extremes - changes between these two periods. To understand the changes seen, we also look at projected changes in the frequency of high magnitude geopotential height anomalies.

Results from three large ensembles show that cold extremes in boreal winter (December-February) exhibit a clear decrease in association between the historical to the future period, indicating that cold extremes at the end of the 21st century become less associated with strong atmospheric circulation anomalies compared to the current historical period. Conversely, hot extremes in boreal summer (June-August) exhibit small regional changes in association for the future period but hemispherically show no clear trend. We further explore the boreal winter trend in CMIP6 models, and explore mechanisms for this trend by comparing across different models with different changes.