Solar Transmittance in the Arctic as a Seasonally Ice-covered Sea

In this project, we're working to gain a better understanding of how sunlight and the thinner, younger sea-ice cover that is now found in the Arctic interact with each other. A better understanding of the processes that are now important for the climate system in the Arctic will be important for understanding how the Arctic will continue to change under further climate changes and for evaluating and trying to improve the models that are used to study climate and to fill in gaps in observations. We measured sunlight over and under different types of sea ice (first-year/multiyear ice) in the Arctic using autonomous platforms with real-time data transfer and using mobile platforms that can observe the significant spatial and temporal variations in the sunlight's partitioning (how much is reflected, absorbed by the ice, or transmitted through the ice to the ocean). In spring 2014, the first two autonomous platforms were deployed in sea ice north of Alaska. One of these had a failure in the light sensor beneath the ice after 50 days, but we received good data from the other through the whole melting season. Two more platforms were deployed in the same region in late-summer/fall 2014, in the hope that they would survive the winter and be ready to measure when the sun returns to the Arctic in 2015. Both managed to send data into the following summer. We worked with spectral data and measurements of spatial variability from earlier projects, and made preparations for an ambitious and important field season from April to June 2015. During this time, we worked north of Svalbard, where we installed many sensors above, within, and under the ice, and also took them under the ice with divers and ROVs (remotely operated vehicles). Since the major field campaign in the first half of 2015, we spent 2 years performing extensive analysis of these and earlier observations to study the properties and processes that have the strongest effects on the transmission of sunlight through younger, thinner sea ice and within the water below. In the end, this project contributed to the publication of at least 18 research articles in prominent international journals during the project period. One interesting finding has been the strong effect an under-ice algae bloom had on light absorption in the upper ocean in an area still covered by a large fraction of sea ice with thick snow, a situation generally thought to have too little light for algae to live. We showed that small areas of open water and thin ice let enough light into the ocean to support the growth of algae and that the algae then cause an increased warming of the uppermost 10 m of the ocean water. We also worked on several articles with biologists to help them interpret observations and run models of primary production within and under the sea ice. Good measurements of and methods for modelling the light levels in and under the ice are important for these type of studies, and we benefited here from good interdisciplinary collaboration. Other articles showed new methods for modelling light in the ice and ocean, aspects of such models that need improvement and how important it is to choose the right method to measure light in the ocean, depending on whether the measurements will be used for biological or physical studies. We continue to work to use the observations to evaluate models and global data products that calculate how much sunlight reaches the surface over the whole Arctic. The observations from this project are important for such evaluations since there is very little data from the Arctic. Eventually, we will also use the observations, together with radiative transfer modelling to perform analyses of changes to the solar radiation budget over the entire ice-covered Arctic Ocean, both those that have already occurred, and those that are expected in the future. Our social media pages show regular updates from our activities in this and related projects: https://instagram.com/oceanseaicenpi/ https://twitter.com/oceanseaicenpi https://www.facebook.com/oceanseaicenpi .

Sea ice covers the majority of the Arctic Ocean for at least some of the year, reducing both the amount of sunlight reaching the ocean (low transmittance) and the overall amount taken up by the Arctic (high albedo). Advances have been made in quantifying the seasonal evolution of sea-ice albedo; however, transmission observations under ice, especially seasonal ice, in the high Arctic are far more limited, so a good overview of the seasonal evolution of the transmission of sunlight to the ocean beneath is still lacking. Through measurement and modeling experiments, this project will result in a better understanding of the processes in first-year sea ice in the high Arctic that affect its solar transmittance. Such first-year ice has become the predominant ice type across the entire Arctic Ocean. It is predicted that the Arctic will become seasonally ice covered within this century, drastically changing the fate of solar energy reaching the surface. Measurements will be made from autonomous drifters to see seasonal variations in transmission at many locations in the high Arctic. Field campaigns will be used to observe small-scale spatial variability during ice melt. The result will be a clear understanding of the seasonal evolution of first-year ice transm ittance, along with the spatial variability overlain on the mean. Using this detailed understanding of solar transmittance, a study will be performed, combining radiative transfer modeling and Arctic climate data, to examine recent and potential future ch anges in solar energy reaching the Arctic Ocean due to sea-ice changes. Additionally, the wealth of incident solar radiation observations will allow for an assessment of these data in the reanalysis products often relied upon in the Arctic. The result wi ll be a detailed understanding of processes that affect the availability of sunlight in the Arctic Ocean, valuable to biologists, sea-ice modelers, and those studying natural and human systems impacted by sea ice.