A New Combined Detection Algorithm for Blocking and Subtropical Ridges

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Blocks are high-impact atmospheric systems of the mid-/high latitudes and have been widely addressed in meteorological and climatological studies. However, the diversity of blocking definitions makes comparison across studies not straightforward. Here, we propose a conceptual model for the life cycle of high pressure systems that recognizes the multifaceted and transient characteristics of these events. A detection scheme identifies and classifies daily structures, discriminating between subtropical ridges and different types of well-established blocking patterns (omega and dipole-like Rex). This is complemented by a spatiotemporal tracking algorithm, which accounts for transitions between patterns, providing a global catalog of events for 1950-2020. Criteria rely on simple metrics retrieved from one single-level field, and allow implementation in different datasets and climatic realms. Using reanalysis data, we provide illustrative examples, the first global and seasonal climatological assessment of the diversity of high pressure events, their associated impacts, and recent frequency changes. Results reveal that ridge and blocking events affect widespread regions from the subtropics to high latitudes. We find remarkably distinct regional impacts among the considered types, which had been hindered in previous studies by restricted focus on Rex-like structures. This plethora of high pressure systems is much less evident in the Southern Hemisphere, where activity is dominated by subtropical ridges and secluded blocking-like patterns. We report increasing frequencies of low-latitude systems, although with hemispheric and seasonal differences that can only be partially interpreted as a consequence of subtropical expansion. Blocking frequency trends exhibit more heterogeneous and complex spatial patterns, with no evidence of generalized significant changes.
© 2021 American Meteorological Society. FP. M. Sousa and R. M. Trigo acknowledge financial support from FCT-UIDB/50019/2020-IDL. P. M. Sousa was supported through the following FCT project: HOLMODRIVE-North Atlantic Atmospheric Patterns Influence on Western Iberia Climate: From the Late Glacial to the Present (PTDC/CTA-GEO/29029/2017). D. Barriopedro and R. García-Herrera were supported by the Spanish government through the PALEOSTRAT (CGL2015-69699-R) and JEDiS (RTI2018-096402-BI00) projects. R. M. Trigo was supported by project INDECIS, which is part of ERA4CS, an ERA-NET initiated by JPI Climate, with co-funding by the European Union (Grant 690462). We are grateful to three anonymous reviewers, who made very constructive comments and helped improve the manuscript.
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