Assessing Rossby Wave Breaking Activity in the CESM Large Ensemble 2 from the Historical Period to the Late 21st Century

Amplified Rossby waves, when subject to strong horizontal shear, can undergo a process known as Rossby wave breaking (RWB). These events generate strong dynamical forcing which can influence the speed and position of the upper-tropospheric jet stream. As a result, substantial sensible weather impacts ranging from the development of atmospheric blocking to extreme precipitation events can be associated with RWB events. Given the relationship between RWB, the jet, and episodes of impactful sensible weather, it is important to understand the representation of RWB activity in Global Climate Models (GCMs). Here, we utilize the Community Earth System Model version 2 Large Ensemble (CESM-LENS2) to detect Northern Hemisphere RWB events on the dynamic tropopause (DT) in December, January, and February (DJF) 1980-2100. RWB activity during the historical period (i.e., 1980-2014) is evaluated using the LENS2 and European Centre for Medium Range Weather Forecasts Reanalysis version 5 (ERA5) datasets. Regions with frequent wave breaking activity (i.e., RWB surf zones) were observed in nearly identical locations between the LENS2 and ERA5 datasets. However, a statistically significant underrepresentation in RWB activity was found in the LENS2 relative to ERA5. This signal in the frequency of RWB occurrence may be associated with differences in the representation of the DJF-mean DT jet which was found to be 3-5 m s^-1 faster in the LENS2 relative to ERA5. Understanding these model biases can help inform our interpretation of future projections of RWB activity. Similar analysis was completed for the early (2030-2045), mid (2046-2070), and late (2071-2100) 21^st century. An overall reduction in wave breaking was observed in the Pacific surf zone while a general downstream shift in RWB activity was found in the Atlantic surf zone. These projected changes in the frequency of wave breaking will be discussed in the context of Rossby wave theory and identified future changes to the DT jet.