What is the meaning of “Arctic melt onset”? What is it, where, when and how does it occur? And do atmospheric rivers play a role for the timing of the melt?  

These were questions addressed at the 1st ARTofMELT Science Workshop 22-24 April, 2024. The workshop was held at Stockholm University as a lunch through lunch meeting starting on Monday 22nd and ending on Wednesday 24th. For one full day and two half days, almost 50 in person participants and collaborators met with ~10 online participants for a first discussion on the scientific results from the ARTofMELT (Atmospheric rivers and the onset of Arctic melt) expedition to the Arctic on the Swedish research icebreaker Oden in May and June 2023.

The first half-day was spent as a summary of all activities with presentations from all Work Package leaders, however, started with a presentation of the onboard artist sharing her experience as a non-scientist and ending with a review of the outreach and media activities. This session was also open to a wider community within the Bolin Centre for Climate Research. The full day was devoted to submitted science presentation from all fields of science on the expedition, while the second half day was devoted to planning issues; where to go next, what science to prioritize, upcoming publications and database issues. Science-to-policy was also addressed. Of those participating in person in the workshop, almost half were Early Career Researchers (ECRs) and majority of the science presentations were presented by ECRs. Support from the IASC Atmospheric Working Group helped eight of these to cover parts of their travel costs to participate. The workshop was also sponsored by the International Meteorological Institute at Stockholm University.

The rationale behind the ARTofMELT expedition was twofold. First, to study processes in the ocean, ice, snow and atmosphere relevant for the transition from the winter freeze to the summer melt. Second, to explore the role of so-called atmospheric rivers for the timing of the melt onset. The Oden expedition departed from Svalbard on 7 May and returned on 15 June; a time period when the ice is the most difficult to navigate. A special circumstance for this expedition was the attempt to navigate Oden into favorable locations using weather forecasts, meaning that the majority of all measurements had to be based onboard. Nevertheless, two ice camps were also launched for shorter periods, making it possible to deploy instruments on the ice, and meanwhile the helicopter was used to ferry scientists to surrounding ice for sampling. The scientific strategy was to characterize a column from ~500 meters into the ocean, through the ice and snow and to the top of the troposphere. The expedition was atmosphere-dominated, but also had research teams covering oceanography, cryosphere and biogeochemistry.

Only two warm-air-intrusions were captured. The first in late May when the temperatures in the lower atmosphere exceeded the melting point for half a day. The second, that also initiated the persistent surface melt, occurred on 10 June; unexpectedly late. The main discussion topic at the workshop became how to define the melt onset, as there was evidence that the ice thickness had started to decline already well before the melt finally started at the surface and there was also signs of an algal bloom starting under the ice well over a week before the melt on 10 June. So, the question on when the melt starts and what triggered it first needs a more informed definition. Operationally, the date for the melt onset is determined either by time-averaged near-surface air temperature (often from reanalysis) or from surface changes measured by satellite. ARTofMELT results indicate these methods may be less than adequate. Another observation surprising the science team was the speed by which the surface melt was manifested by melt ponds. Locations for melt ponds were obvious already on the first day of the melt and by two days later, fully developed melt ponds were present.

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