Driving mechanisms for the onset of the summer melt season at MOSAiC

Onset of surface melt at the MOSAiC Central Observatory (CO) was observed from an Atmospheric Surface Flux Station north of Svalbard at 83.3ºN on 25 May 2020. In winter, the polar vortex was unusually strong and long-lived, but in April the zonal winds weakened and the Arctic Oscillation became more neutral. These conditions permitted air masses to approach from the south and back-to-back warm air intrusions originating from air circulating around low-pressure systems in the Barents and Greenland Seas reached the CO during mid-April. Air temperatures increased to near the freezing point and a brief episode of surface melting was observed on 19 April. Prior to this transition, the temperature was rarely above -20ºC and it rarely fell below -20ºC after. These events, and several subsequent ones provided a punctuated warming of the ice pack in weeks that followed, aided by more sustained warming of the surface beginning 5 May when the net atmospheric forcing of the surface energy budget turned persistently positive. Melt onset on the 25 May was also associated with an intrusion of southerly air of cyclonic origin. Melt initiation was dominated locally by longwave radiative processes and sustained nearly un-interrupted for ~128 hours in the presence of optically thick cloud cover having a thermodynamic temperature near 0 C. These characteristics are similar to the onset documented at both SHEBA in the Beaufort at 76.4ºN on 28 May 1998 and N-ICE2015 at ~80ºN near Svalbard on 1 June 2015. Pre-onset events reduced the surface albedo by ~2% and were followed by colder near-surface air and, when melt occurred, refreezing of the upper snow layer. Conversely, though triggered by longwave forcing, melt onset was associated with an abrupt reduction of surface albedo of >5% which may have helped to sustain melt later. These factors suggest synoptic activity is important for springtime warming of the ice, but also imply resilience of the surface to sustained melt.