Sunlight is one of the main drivers of polar landscapes, which rely on the Sun’s rays for warmth and energy.
Researchers now have quantified how sunlight permeates Arctic sea ice and reaches marine ecosystems. They found distinct seasonal patterns in light transmission, with the presence of ice, snow, and melt ponds dictating how much light makes it into the ocean. These results reveal interactions among the atmosphere, sea ice, and ocean, data that are important for climate modeling, the team suggests.
The Ice Decides
Stefanie Arndt, a sea ice physicist at the Alfred Wegener Institute in Bremerhaven, Germany, and her colleagues used measurements obtained with the R/V Polarstern, a research icebreaker operating out of Bremerhaven. Over the course of six expeditions between 2011 and 2017, researchers sent remotely operated vehicles (ROVs) under Arctic sea ice in 45 different locations. The sites were more or less randomly selected, said Arndt. “The route [of R/V Polarstern] is dependent on the ice conditions.”
Instruments on the ROVs measured light levels between wavelengths of 320 and 950 nanometers. By comparing measurements obtained from under the sea ice and measurements taken on top of the sea ice, Arndt and her collaborators calculated light transmission as a function of wavelength. In total, they analyzed more than 35,000 spectra obtained between May (spring) and September (autumn) through ice ranging from a few centimeters thick to more than 3 meters thick.
Snow, Ice, and Melt Ponds
The Arctic landscape follows distinct seasonal patterns, said Arndt. It snows in the winter, but the snow melts in the springtime. As temperatures continue to increase, the exposed sea ice begins to melt as well. Melt ponds form and enlarge, and they can extend all the way through the ice into the underlying water. In the autumn, snow accumulates, and the process begins again.
The scientists found the lowest transmission levels in May—less than 1% at all wavelengths. That’s not surprising because snow still blankets much of the ice at that time of the year, said Arndt. “Snow is really opaque.”
But as the snow disappeared, some of the surface ice melted, ponds of meltwater started to accumulate, and more light was transmitted. In August, more than 10% of the incident sunlight was transmitted to the ocean below, Arndt and her colleagues found. That sunlight transports heat and energy, said Arndt. For instance, it warms the water and powers the metabolism of algae.
One explanation for the increased transmittance is meltwater ponds that melt all the way through the ice. These conduits, which can measure up to a few hundred meters across, link the atmosphere and the ocean and are important for transmitting light, the scientists suggest. “They’re a window from the atmosphere to the upper ocean,” said Arndt.
These results reveal how the atmosphere, sea ice, and ocean are entwined, said Arndt. “That’s poorly understood in climate models.”
Thriving Brown Algae
Arndt and her colleagues also found evidence that sunlight was being used by marine life in the Arctic. The researchers measured a spectral shift in data collected in July: There was lower transmittance at wavelengths beyond 500 nanometers. That’s likely due to brown algae that thrive on the underside of sea ice, the team concluded. These algae grow by absorbing light.
“Decreased early summer light transmission through sea ice moderated by sea ice algae might be a widespread feature in the Arctic,” the researchers wrote in the Journal of Geophysical Research: Oceans in July.
“Light transmission through sea ice is central in both sea ice energy balance and ecosystem studies,” said Hanna Kauko, a biological oceanographer at the Norwegian Polar Institute in Tromsø not involved in the research. “The novelty and significance of this work lie in the extensive data set it presents.”
Arndt and her colleagues are looking forward to collecting data on a special R/V Polarstern cruise that begins this month (September). During the expedition, called Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC), the icebreaker will purposefully anchor itself to an ice floe and drift around the Artic for a year.
The project will shed new light on how the Arctic is responding to climate change and the links among the atmosphere, sea ice, and liquid water. Arndt plans to be aboard R/V Polarstern for at least one of MOSAiC’s legs. She’ll arrive via helicopter or plane if a runway can be cleared, she said. “That’ll be a real expedition.”
—Katherine Kornei (@katherinekornei), Freelance Science Journalist