The Importance of Anticyclonic Transient Eddies for Atmospheric Block Persistence

Atmospheric blocking is often responsible for high-impact surface weather conditions such as heatwaves, cold spells, and droughts. Very long blocking events are particularly problematic due to the persistence of hazardous surface conditions. Despite their many impacts, understanding the dynamical behaviour of blocking events is still incomplete despite recent progress. Here, the relationship between block persistence and anticyclonic eddies contributing towards these blocks is investigated. Anticyclonic eddies that contribute to blocking are defined as large positive 500 hPa geopotential height (Z500) anomalies, obtained by filtering in both space and time, that pass through a blocked region. These features are then followed using an objective feature tracking algorithm. It emerges that stationary blocking conditions are often the result of more than one transient anticyclonic eddy entering the block itself, which is a reflection of the selective absorption mechanism (SAM) for block maintenance, proposed by Yamazaki and Itoh (2013). A relationship is found between the number of anticyclonic eddies contributing to a block and the persistence of the block itself, with longer-lasting blocks typically absorbing more eddies than less persistent events; this behaviour is particularly noticeable in winter. The contribution of the smaller eddies to the blocks via the SAM is also observed as the anomalies speed up slightly just before entering the blocking region, before intensifying, becoming slow-moving, and sometimes reversing in direction inside the block itself. In addition to this climatological viewpoint, case studies have also been analysed to obtain a more detailed view of the process. From these, it is observed that some of the Z500 anomalies that contribute to blocking events originate from a long way upstream and travel along the wave guide until they are absorbed into a block, and this is again most evident in winter. The results from this work suggest there is an inherent link between repeated block maintenance and the persistence of block events, and also provides evidence that block maintenance processes may differ according to time of year.