Transient anticyclonic eddies and their relationship to atmospheric block persistence

Abstract

Abstract. Atmospheric blocking is a circulation pattern that describes the presence of large-scale, persistent anticyclones, which have the potential to bring severe impacts at the surface. However, the dynamical behaviour of blocks is still not fully understood. For example, the factors that determine the persistence of blocking events are not clear. In this study, the relationship between blocks and smaller-scale transient anticyclonic eddies is examined, with a particular focus on the impact of transients on the persistence of a block. Analysis is performed in two areas: the Euro-Atlantic and North Pacific, which are locations with both high blocking frequency and potential for severe impacts. Geopotential height anomalies at 500 hPa are used to identify blocking events and the anticyclonic transient eddies. This allows for a Eulerian definition of blocking, as well as a Lagrangian perspective on the eddies. It is found that anticyclonic eddies experience a northward acceleration prior to entering a block, which is indicative of ridge building ahead of the block but could also potentially provide evidence for the previously proposed selective absorption mechanism for block maintenance. A general pattern is found whereby longer blocks interact with more anticyclonic transients than less persistent blocks at all times of the year. This effect is strongest in winter and weakest in summer, which agrees with the fact that blocks are most persistent in winter and least persistent in summer. However, the strength of the anticyclonic eddy that interacts with a block, measured by its maximum 500 hPa geopotential height anomaly, has a more complicated relationship with block persistence. The strength of anticyclonic transient eddies is a more determining factor of block persistence in the North Pacific than in the Euro-Atlantic region. In the North Pacific the longest blocks interact with stronger eddies than the shortest blocks in all seasons except summer, when the reverse is true. By contrast, longer Euro-Atlantic blocks only result from stronger anticyclonic eddies in autumn and winter. We therefore conclude that the number of anticyclonic eddies that interact with a block is most important in determining its persistence, with the strength of the eddies having a more variable effect.

Bidimensional climatology and trends of Northern Hemisphere blocking utilizing a new detection method

“…However, the persistent BE is only reported around the Chukchi Sea for the PV– θ index in summer, up to 8.5 days. Frankly speaking, many previous studies found that BE is the most persistent in winter while the least persistent in summer (Wiedenmann et al ., 2002; Suitters et al ., 2022). We can also understand the shorter‐lived blocking in summer by the large climatological $$$ {\mathrm{PV}}_y $$$ (Figure S2b in Data S1), which behaves as an unfavourable context for long‐lived and large‐amplitude blocking high.…”

Life cycle dynamics of Greenland blocking from a potential vorticity perspective

“…A further development of the algorithm allows the detection of splitting and merging events along a tracked PVA − . This enables an analysis which examines the role of transient anticyclonic anomalies in feeding a block and is strongly inspired by the work of Shutts (1983), Yamazaki and Itoh (2013) and Suitters et al (2023).…”

“…The breaking of upper-level Rossby waves (further referred to as RWB) has been found as a formation mechanism for blocking over Greenland with predominantly cyclonic RWB towards Greenland (Benedict et al, 2004;Woollings et al, 2008; results on the importance of moist-baroclinic processes by adopting the Eulerian perspective of Teubler et al (2023) into a new quasi-Lagrangian perspective. Previous studies have partly used quasi-Lagrangian approaches to investigate the role of eddy feedback on blocks and, in particular, the role of transient anticyclonic eddies (e.g., Shutts, 1983;Yamazaki and Itoh, 2013;Suitters et al, 2023). By tracing back negative, upper-tropospheric PV anomalies linked to a block over Europe and investigating the relative contributions of dry and moist dynamics using the same piecewise PV tendencies of Teubler et al (2023), the quasi-Lagrangian approach of Hauser et al (2023b) revealed a non-local development of the negative PV anomaly over the western North Atlantic.…”