Abstract
Arctic sea ice is shifting from a year-round to a seasonal sea ice cover. This substantial transformation, via a reduction in Arctic sea ice extent and a thinning of its thickness, influences the amount of light entering the upper ocean. This in turn impacts under-ice algal growth and associated ecosystem dynamics. Field campaigns have provided valuable insights as to how snow and ice properties impact light penetration at fixed locations in the Arctic, but to understand the spatial variability in the under-ice light field there is a need to scale up to the pan-Arctic level. Combining information from satellites with state-of-the-art parameterizations is one means to achieve this. This study combines satellite and modeled data products to map under-ice light on a monthly time-scale from 2011 through 2018. Key limitations pertain to the availability of satellite-derived sea ice thickness, which for radar altimetry, is only available during the sea ice growth season. We clearly show that year-to-year variability in snow depth, along with the fraction of thin ice, plays a key role in how much light enters the Arctic Ocean. This is particularly significant in April, which in some regions, coincides with the beginning of the under-ice algal bloom, whereas we find that ice thickness is the main driver of under-ice light availability at the end of the melt season in October. The extension to the melt season due to a warmer Arctic means that snow accumulation has reduced, which is leading to positive trends in light transmission through snow. This, combined with a thinner ice cover, should lead to increased under-ice PAR also in the summer months.
Highlights
The Arctic is undergoing a period of profound transformation in response to anthropogenic warming, with the loss of the sea ice cover one of its starkest changes
Since there is little to no light during the winter months (November, December, and January), we focus on the months of October, February, March, and April for which there is some daylight along the sea ice pack margins (October and February) up to the point where there is daylight on a monthly pan-Arctic scale (March and April)
Arctic marine ecosystems have adapted to extremes in light conditions, taking advantage of the short time-period over which primary production can occur
Summary
The Arctic is undergoing a period of profound transformation in response to anthropogenic warming, with the loss of the sea ice cover one of its starkest changes. While the 2012 sea ice extent minimum was striking, variability is increasing, in relation to a thinning ice cover (Goosse et al, 2009) and recent departures from average conditions during the transition seasons have become even more anomalous than in summer: Arctic sea ice extent (defined as the area with at least 15% sea ice concentration, SIC) in May and November 2016 fell nearly 4 standard deviations below the 1981–2010 long-term average (Stroeve and Notz, 2018) This represents a distinct change in the seasonality of the Arctic Ocean as the melt onset is happening earlier and the fall freeze-up later (Stroeve et al, 2014; Stroeve and Notz, 2018; Lebrun, 2019)
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