Local versus remote origin of wintertime extreme surface energy budget anomalies in the Arctic

Recent studies have emphasized the relevance of synoptic-scale atmospheric processes in driving extreme events in the Arctic. In particular, warm extremes and episodes of anomalous sea ice melting have been attributed – in part – to intrusions of warm and moist air from lower latitudes associated with mid-latitude weather systems such as cyclones and blocks. At the same time, also local processes such as subsidence in anticyclones and diabatic heating have been found to play an important role in some cases. Accordingly, the relative importance of remote vs. local processes remains a matter of debate.

Here, we focus on the atmospheric components of the surface energy budget (SEB) over Arctic sea ice. Specifically, we present a novel methodology to identify and track extreme (> 95^th percentile) anomalies of the SEB in space and time in ERA5 reanalysis data (extended winters 1979 – 2020). The resulting 142 so-called SEB life-cycle events are predominantly caused by enhanced downward longwave radiation and turbulent heat fluxes. A synoptic analysis reveals that most of these events occur in narrow gateways in the Atlantic and Pacific sectors and are associated with a poleward deflection of the storm tracks and with blocks over northern Eurasia and Alaska.

Kinematic backward trajectories from the tropospheric column collocated with the events show that mid- and upper tropospheric air has predominantly been transported poleward from mid-latitudes, thereby ascending along slanted isentropes. Lower tropospheric air, in contrast, is largely of Arctic origin and subject to substantial (diabatic) air mass transformations (warming and moistening) during the days preceding the events. Despite the differences in origin of the air at different altitudes, the entire tropospheric column shows a pronounced warm anomaly (~ 10 K) building up over 2-4 days prior to the events. A decomposition of the temperature anomalies emphasizes the relevance of horizontal advection and latent heating for the generation of the mid- and upper tropospheric temperature anomalies, whereas subsidence and heating by surface heat fluxes over ocean are important for the transformation of the originally cold Arctic air at lower altitudes.

Thus, this work emphasizes that a rich chain of processes is required for the generation of the most extreme Arctic SEB anomalies, resulting from a combination of air mass injections from (remote) lower latitudes and local air mass transformations in the Arctic.