The Role of Local and Remote Processes for Wintertime Surface Energy Budget Extremes over Arctic Sea Ice

Arctic warm extremes and anomalous sea ice melting have been linked to episodic injections of warm and moist air from midlatitudes, as well as airmass transformations inside the Arctic. However, the relative importance of remote and local processes for such events remains unclear. Here, we focus on events with extreme positive daily-mean surface energy budget (SEB) anomalies over Arctic sea ice in ERA5 data during extended winters (November-March during 1979-2020). Kinematic backward trajectories from the tropospheric column collocated with the SEB anomalies show that near-surface air is of Arctic origin, whereas air farther aloft is transported poleward from the midlatitudes and ascends. Despite the different origin of the air, the entire tropospheric column shows pronounced potential temperature anomalies (on the order of 10 K) building up along air-parcel trajectories over 2-4 days. Quantifying the contributions of horizontal and vertical transport as well as diabatic processes to the generation of these potential temperature anomalies emphasizes the relevance of horizontal advection across the climatological potential temperature gradient for the generation of the anomalies at all levels. Anomalies aloft are further enhanced by latent heating and those near the surface by subsidence, respectively. Surface heat fluxes over subpolar and polar oceans are key for warming and moistening the near-surface air of predominantly Arctic origin and maintaining a positive potential temperature anomaly, which due to its proximity to the surface unfolds the strongest impact on the SEB. This suggests that Arctic airmasses and their local transformations are crucial for generating the most extreme SEB anomalies.