Robust Identification of Impactful Boreal Summer Anticyclones and Implications for Future Climate

Impactful midlatitude heatwaves are often triggered by persistent anticyclonic atmospheric blocks. Objectively defining such impactful blocking circulations remains a challenge for climate impacts analysis and theoretical understanding, which this work seeks to address. Here, persistent midtropospheric anticyclones over the entire northern hemisphere midlatitude region that lead to anomalously warm surface conditions are identified, independently of specific blocking metrics, with a two-step identification method. This method’s input is daily boreal-summer midtropospheric geopotential height, from reanalysis or from earth-system models (ESMs), over a circumglobal set of midlatitude domains spanning about longitude and latitude. The method’s output is a set of days featuring persistent states that reflect the predominant flow pattern, Archetype 1, extracted using Archetype Analysis. Persistence is defined by high values of a persistence metric, symbolized θ^-1, that reflects how long the atmosphere tends to stay near a specific atmospheric configuration. The high θ^-1 Archetype 1 is a barotropic anticyclonic block with warm surface conditions, lasting about a week. Extending previous European-domain persistence analysis, the archetype analysis filters out less-impactful persistent cyclonic systems associated with anomalously cold conditions. Over land regions, heatwaves are 5-10 times more frequent under persistent Archetype 1 conditions than in the record as a whole. Persistent Archetype 1 patterns are realistically represented in historical ESM simulations. There is a regional increase of θ^-1  by the end of the century, which can signify a continuation of recent trends of the weakening of the boreal summer circulation. Archetype 1 persistent events spatial structure does not change by the end of the century, but their persistence increases by about 7% as part of the overall θ^-1 increase, which can signify that they are getting longer as a result of climate change.