Abstract
Abstract. Atmospheric blocking can influence Arctic weather by diverting the mean westerly flow and steering cyclones polewards, bringing warm, moist air to high latitudes. Recent studies have shown that diabatic heating processes in the ascending warm conveyor belt branch of extratropical cyclones are relevant to blocking dynamics. This leads to the question of the extent to which diabatic heating associated with mid-latitude cyclones may influence high-latitude blocking and drive Arctic warm events. In this study we investigate the dynamics behind 50 extreme warm events of wintertime high-Arctic temperature anomalies during 1979–2016. Classifying the warm events based on blocking occurrence within three selected sectors, we find that 30 of these events are associated with a block over the Urals, featuring negative upper-level potential vorticity (PV) anomalies over central Siberia north of the Ural Mountains. Lagrangian back-trajectory calculations show that almost 60 % of the air parcels making up these negative PV anomalies experience lifting and diabatic heating (median 11 K) in the 6 d prior to the block. Further, almost 70 % of the heated trajectories undergo maximum heating in a compact region of the mid-latitude North Atlantic, temporally taking place between 6 and 1 d before arriving in the blocking region. We also find anomalously high cyclone activity (on average five cyclones within this 5 d heating window) within a sector northwest of the main heating domain. In addition, 10 of the 50 warm events are associated with blocking over Scandinavia. Around 60 % of the 6 d back trajectories started from these blocks experience diabatic heating, of which 60 % undergo maximum heating over the North Atlantic but generally closer to the time of arrival in the block and further upstream relative to heated trajectories associated with Ural blocking. This study suggests that, in addition to the ability of blocks to guide cyclones northwards, Atlantic cyclones play a significant role in the dynamics of high-latitude blocking by providing low-PV air via moist-diabatic processes. This emphasizes the importance of the mutual interactions between mid-latitude cyclones and Eurasian blocking for wintertime Arctic warm extremes.
Highlights
The positive trend observed in global-mean surface temperatures is unequally distributed, with greater and more rapid surface warming seen over the Northern Hemisphere highlatitude regions
The 10 Scandinavian cases, on the other hand, show an average of only four cyclone crossings per event; comparison with climatology using random sampling of 10 pentads shows that this average lies within the interquartile range and cannot be considered exceptional. These results indicate that serial cyclone clustering (Pinto et al, 2013) can be important for generating Arctic warm events, at least in those cases associated with Ural blocking
We have investigated the dynamics behind the 50 most extreme wintertime high-Arctic warm anomalies, focusing on the importance of Ural and Scandinavian blocking preceding the warm extremes
Summary
The positive trend observed in global-mean surface temperatures is unequally distributed, with greater and more rapid surface warming seen over the Northern Hemisphere highlatitude regions. This phenomenon, observed in winter during recent decades, is known as Arctic amplification (e.g., Serreze and Barry, 2011; Cohen et al, 2014). Local processes comprise snow- and ice-albedo feedbacks (e.g., Screen and Simmonds, 2010a; Pithan and Mauritsen, 2014), enhanced ocean–atmosphere heat exchanges (e.g., Screen and Simmonds, 2010b; Boisvert et al, 2016), temperature feedbacks and related changes of water vapor content and cloud cover (e.g., Serreze et al, 2012), and circulation changes within the Arctic (e.g., Sorteberg and Walsh, 2008; Ding et al, 2017), while remote processes include increased oceanic
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