On the atmospheric transport pathways of heat into the Arctic in winter and spring

The atmospheric meridional energy transport is an important component in the global climate system, as it transports moisture and heat from the midlatitudes into the Arctic, balancing the regional energy loss to space. Narrow and intense injections of warm and moist air, so-called warm-air intrusions, have a large local imprint on the surface through various processes acting on different scales, such as cloud formation, turbulent fluxes and atmospheric large-scale circulation. Despite their infrequency, they substantially alter near-surface temperatures and surface energy fluxes, promote and advance sea-ice melt and delay ice growth, thereby influence the annual ice evolution in the Arctic. Changes in the atmospheric energy transport are suggested as one of the remote mechanisms behind Arctic amplification, i.e. the phenomenon that the Arctic is warming substantially faster than the global average. Climate models fail to capture the observed magnitude of the Arctic warming, indicating that the driving mechanisms are not well understood. To properly predict the changing Arctic climate, a better knowledge of the current processes and more in-situ observations supporting our understanding are needed. In this thesis, winter- and springtime atmospheric processes associated with these warm-air intrusions over Arctic sea ice are explored. Lagrangian backward trajectories are utilized to study airmass origin, pathways and transformation. Reanalysis data are used to describe the synoptic situation and key processes associated with identified intrusion events or regions of interest. Observations from the Arctic Expedition MOSAiC in mid-April 2020 are also analyzed. The role and the mechanisms behind the formation of atmospheric blocking associated with wintertime warm extreme events in the high Arctic are explored. We find that the majority of these events are preceded by blocking over Eurasia and that 60 % of air-parcels ending up in the upper-level blocks experience diabatic heating. Most of this heating is associated with cloud-processes ahead of midlatitude cyclones, indicating that the interplay between cyclones and blockings are important for meridional transport and resulting warm extremes. A new method for detecting Arctic extreme events based on coherent regions of positive anomalies in the surface energy budget (SEB) is presented. Life cycles and pathways of such wintertime events with Pacific or Atlantic origin are described. Variations in the anomalies are shown to be associated with variations in turbulent fluxes. These extremes are linked to warm-air intrusions, despite that, they differ from temperature extremes in the Arctic as local processes also can produce SEB anomalies. Expanding from winter to spring, these extreme SEB anomaly events are utilized to investigate the importance of atmospheric processes and airmass origin in controlling the spatiotemporal variability of Arctic melt onset dates. Surface temperature and satellite-based observations are used as melt indicators. Indicative for early melt onset dates are inflow from Pacific and high frequency of SEB events both at melt and three weeks prior to melt, whereas later melt dates are favored by continental airmass origin, a low frequency of SEB events at melt and clear-sky conditions until melt