| Summary: | <p>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 <span class="inline-formula">%</span> of the air parcels making up these negative PV anomalies experience
lifting and diabatic heating (median 11 <span class="inline-formula">K</span>) in the 6 d prior to the
block. Further, almost 70 <span class="inline-formula">%</span> 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 <span class="inline-formula">%</span> of the 6 d back trajectories
started from these blocks experience diabatic heating, of which 60 <span class="inline-formula">%</span> 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.</p>
|
|---|