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.
Highlights
A blocking is a quasi-stationary, persistent large-scale atmospheric flow pattern that blocks the typical westerly flow and forces the jet and embedded pressure systems to bypass on its northern and southern sides (e.g., Rex, 1950)
We study climatologies and annually, seasonally and monthly resolved trends of blocking occurrence probabilities with a novel strategy: blocking type classification based on point vortex theory followed by a description of occurrence and transition probabilities with logistic regression and Markov models
For the two-state model with the two states blocking (Yt = B) and no blocking (Yt = nB), we describe transition probabilities changing with year as logit (P (Yt = B|Yt−1)) ∼ Yt−1 ∗ year using the notation for generalized linear models as well the parameter estimation strategies introduced above
Summary
A blocking is a quasi-stationary, persistent large-scale atmospheric flow pattern that blocks the typical westerly flow and forces the jet and embedded pressure systems to bypass on its northern and southern sides (e.g., Rex, 1950). Transitions between the different blocking types can be observed: an example is documented in Schielicke (2017, Appendix A3, Fig. A69) for summer 2010, where long-lasting blocking caused extreme heat and forest fires over Russia, while downstream of the blocks “recordbreaking” floods occurred in Pakistan (Hong et al, 2011; Schneidereit et al, 2012).
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