Abstract
The quasi-stationary, large-scale, persistent weather phenomena known as atmospheric blocking is the focus of this work. Blocks represent a meridional reversal of midlatitude flow with a high-pressure system shifted north and the jet stream split around it. This shift of the storm track, accompanied with blocking size and persistence, often lead to prolonged weather extremes. With the impacts of human and natural systems at large, not only is the need to better understand blocking mechanisms important, but also the need to better predict them. However, climate models are notorious for poor representation of atmospheric blocking even with recent generations and improvements (Davini and D’Andrea, 2020).
Here, blocking representation in the Community Earth System Model (CESM) is dissected by leveraging an in-depth evaluation on the sensitivity and usefulness of blocking analysis tools and the relationship of blocking to a leading mode of variability, the North Atlantic Oscillation (NAO). Two blocking detection methods were applied, compared, and contrasted on how useful they are in blocking analysis. From this, a comprehensive 40-year Northern Hemisphere climatology of blocking frequency across all seasons was developed using ERA5 reanalysis. One of the detection methods, the PV-θ index, has a complimentary direction of breaking (DB) index (Masato et al., 2012, 2011) allowing for blocking events to be classified as the type of Rossby wave breaking driving their formation. Consideration of classification as cyclonic and anticyclonic, season, and additionally, breakdown of analysis into North Atlantic subregion allowed for a better representation of blocking impacts to highly populated areas such as Europe. It also led to insight on jet stream dynamics influencing blocking formation over various locations. Overall, it was largely found that different indices will lead to varying results of the same blocking analysis.