Occurrence and transition probabilities of omega and high-over-low blocking in the Euro-Atlantic region

Abstract

Abstract. Stationary, long-lasting blocked weather patterns can lead to extreme conditions such as anomalously high temperatures or heavy rainfall. The exact locations of such extremes depend on the location of the vortices that form the block. There are two main types of blocking: (i) a high-over-low block with a high located poleward of an isolated low and (ii) an omega block with two lows that lie southeast and southwest of the blocking high in the Northern Hemisphere. In this work, we refine a novel method based on the kinematic vorticity number and the point vortex theory that allows us to distinguish between these two blocking types. Based on the National Centers for Environmental Prediction–Department of Energy (NCEP–DOE) Reanalysis 2 data, we study the trends of the occurrence probability and the onset (formation), decay (offset) and transition probabilities of high-over-low and omega blocking in the 30-year period from 1990 to 2019 in the Northern Hemisphere (90∘ W–90∘ E) and in the Euro-Atlantic sector (40∘ W–30∘ E). First, we use logistic regression to investigate long-term changes in blocking probabilities for full years, seasons and months. While trends are small for annual values, changes in occurrence probability are more visible and also more diverse when broken down to seasonal and monthly resolution, showing a prominent increase in February and March and a decrease in December. A three-state multinomial regression describing the occurrence of omega and high-over-low blocking reveals different trends for both types. Particularly the February and December changes are dominated by the omega blocking type. Additionally, we use Markov models to describe transition probabilities for a two-state (unblocked, blocked) and a three-state (unblocked, omega block, high-over-low block) Markov model. We find the largest changes in transition probabilities in the summer season, where the transition probabilities towards omega blocks significantly increase, while the unblocked state becomes less probable. Prominent in winter are decreasing probabilities for transitions from omega to high-over-low and persistence of the latter. Moreover, we show that omega blocking is more likely to occur and to be more persistent than the high-over-low blocking pattern.

Quantifying the statistical dependence of mid-latitude heatwave intensity and likelihood on prevalent physical drivers and climate change

“…Areas with (mostly non-significant) negative coefficients are located south-east and south-west of the grid points associated with (significant) positive coefficient estimates, resembling the omega blocking pattern (e.g. Detring et al, 2021), but the orientation and spatial extent vary across climate model datasets. It should also be noted that temporally integrated effects, for example the progressive accumulation of heat in a persistent atmospheric high-pressure situation (as described by Miralles et al, 2014), cannot be explicitly accounted for with this model, as only temporally co-occurring 7 d averaged geopotential height fields are analysed.…”

Atmospheric conditions favouring extreme precipitation and flash floods in temperate regions of Europe

“…Both remarkable flash flood series from 2016 and 2018 occurred during atmospheric blocking situations (Mohr et al, 2020;Piper et al, 2016) that stymied the movement of the atmosphere, ultimately causing weather constellations to last longer and, thus, creating extreme situations. In recent years, such situations have been increasingly observed, especially in summer (Detring et al, 2021;Kreienkamp et al, 2021;Lupo, 2020). This could hint at a change in the intra-annual distribution of precipitation, while the number of precipitation events as well as their maximum 5 min and hourly intensity stayed -apart from their large intra-annual variations -at a similar level between 2001 and 2020.…”

Constraints on simulated past Arctic amplification and lapse-rate feedback from observations

“…This approach was used by e.g. Detring et al (2021) to study recent trends in blocking probabilities, but it is also suitable for the multi-class problem of describing P i in dependence of some covariates. The basic idea of MLNR is to describe the log-odds (defined as the logarithm of the chance of observing a distinct regime with respect to a predefined base-line regime) as a linear combination of the covariates.…”