How does the substructure of Arctic extreme seasons look like and what are the dynamical drivers of such seasons?
In this study we define Arctic extreme seasons in the ERA5 data set, using seasonal anomalies of several surface parameters, and analyse their dynamical substructure and driving processes. Based on varying surface conditions, the Arctic is subdivided into several regions for summer and winter, respectively. In each region, extreme seasons are defined as a combination of six different surface parameters, including surface temperature, surface heat fluxes and surface radiation. The substructure of several anomalous seasons is investigated by analysing large-scale features, such as cyclone and blocking frequency, and backward trajectories. First results show a variety of substructures for different seasons. For example, anomalously warm seasons occur as a result of several warm events during one season, driven by different synoptic processes such as transport of relatively warm air from lower latitudes or subsidence-induced adiabatic warming (e.g. DJF 2016/17, see Figures below). But we observe also warm seasons which are just consistently warmer compared to the long-term mean temperature due to prevailing favourable conditions across the whole season.
Daily mean T2m in °C (blue line) and running mean climatology (orange line) during DJF 2016/17 for the Kara-Barents Seas. Blue bars show daily mean coverage of the region by a cyclone. Orange bars show daily mean coverage of the region by blocking (the darker the color the higher the coverage). Black lines mark the dates shown in the synoptic plots.
Daily anomaly of potential temperature at 900 hPa in °C (colored); sea-level pressure (SLP, grey contour), sea-ice edge (yellow line, dashed line shows climatology), cyclone mask (dashed black contour) and blocking mask (dashed green contour) at 00 UTC for (a) 09.01.2017 and (b) 07.02.2017. Region of Kara-Barents Seas is marked with orange grid.