Impact of changing sea surface temperatures and sea ice cover on Rossby wave breaking in the Northern Hemisphere

Rossby waves are undulations in the upper tropospheric westerlies that may amplify and break, causing irreversible overturning of the meridional potential vorticity gradient. The resulting anomalies connect to phenomena such as blocking, atmospheric rivers, transitions between weather regimes, and many forms of extreme weather. Rossby wave breaking (RWB) is strongly modulated by the jet streams and the upper tropospheric circulation, in which substantial future changes are expected. This is in part due to changes in the meridional temperature gradient caused by sea surface temperature (SST) warming and Arctic amplification, which manifests through e.g. the melting of polar sea ice. The effect of these two factors on RWB has not been studied before in a manner that considers the entire Northern Hemisphere and its varying flow conditions. 

Our research investigates how projected changes in SST and sea ice cover (SIC) affect the frequencies and spatial distributions of Anticyclonic and Cyclonic Rossby wave breaking (AWB and CWB) in the Northern Hemisphere during the winter and summer seasons. The effects of SST and SIC are studied separately and together using atmosphere-only model simulations performed with two models, OpenIFS and EC-Earth. For present-day climate, the current climatological average is used for annual SST and SIC variations. SST and SIC average annual variations from the CMIP6 scenario SSP5-8.5 are used to simulate a future climate. A Rossby wave breaking detection method based on previous literature is applied to examine the overturning of potential temperature contours on the dynamical tropopause. 

Our results show that present-day AWB and CWB both have two maxima, located respectively at the southern and northern flanks of the Pacific and Atlantic jet exits. The models are generally in good agreement. Differences between the present-day simulations and those with future SST and SIC can be attributed primarily to SST, as the simulations considering SIC changes alone do not demonstrate statistically significant changes. In winter, Pacific AWB is reduced by nearly 70%, and this change is associated with a strengthening and eastward extension of the local jet stream. A slight increase in CWB over northern Pacific and Atlantic basins is collocated with a general increase in zonal wind speed. In summer, the models agree that AWB frequencies decrease by up to 100% on the western flanks of past maxima: this is accompanied by a variety of changes in zonal wind and may be connected to how SST changes affect Northern Hemisphere monsoon circulations. Our results indicate that significant changes to Rossby wave breaking are to be expected. This incentivises further study and validation to understand how the upper troposphere responds to surface warming and how the hazardous phenomena connected to RWB may change in the future.