Studying the sea ice from the northern tip of Alaska
Norwegian Polar Institute (NPI) researchers are working in collaboration with their American colleagues to study the processes important to the melt of sea ice in the Arctic. Based in Barrow, Alaska, the northernmost town in the USA, they are measuring the energy fluxes that are driving the melt of the ice just offshore, in the Chukchi Sea.
An Iñupiat town
Barrow, located on the north coast of Alaska, at its northernmost point, where the Chukchi and Beaufort Seas meet, is a town of about 4200 people. The majority of the residents are Iñupiat, the indigenous people of Alaska’s northern and northwestern region, who have lived in the Barrow area for many centuries, calling it Ukpeagvik, which in the Iñupiaq language means “place where snowy owls are hunted.”
Today Barrow is the administrative center of Alaska’s North Slope Borough, an area the size of the United Kingdom, with a population of just under 10,000. While government services, oil and gas production, tourism and research support provide many jobs, subsistence hunting of local birds, caribou, seals, bowhead whales, and other animals remains vital to many of the locals.
Located at a similar latitude to Norway’s North Cape, one could get to Barrow from Tromsø by travelling north over Svalbard and the North Pole, and then continuing in a straight line (now going south) to the coast of Alaska, a journey of about 4400 km. Reaching it by commercial flights, however, takes a journey of over 14,000 km. Despite being just barely north of northern Norway, the climate is much colder, with temperatures in the -30s common in winter, ice covering the ocean from November until July, and an average daily high temperature in July of only 8°C. These more extreme conditions, together with its easy accessibility (by Arctic standards), make it an excellent base for studying sea ice that is similar to what covers most of the Arctic Ocean.
What drives the ice melt
It’s not surprising that the sea ice warms up and starts to melt in spring and summer. The sun returns in February and March, and by May it is providing energy to the ice 24 hour per day, helping to first melt the snow, and then the ice. But as the melting progresses, the surface becomes very variable, with white, bare ice mixed with ponds of melt water that range in color from light blue to nearly black. This variable surface causes the melting to proceed in complicated ways as some areas absorb a lot of sunlight, while others reflect most of it back to space. Understanding how all of these variations, and the processes involved, affect the melt in different ways is the main goal of the ongoing NPI work in Barrow.
Using a system of instruments mounted on a sled, three researchers from NPI, Mats Granskog, Dmitry Divine, and Stephen Hudson, are measuring the amount of sunlight reaching the surface, how much of that is reflected, and also how much infrared light is warming and cooling the surface. At the same time, the sled photographs the surface and sky and measures the location and weather conditions. Using this sled to make many measurements along a line, they can see the effect of the spatial variability, such as ponds and areas of deeper or thinner snow cover. Then, by repeating the measurements over many days during the melt season, they also observe the seasonal changes.
To capture the full extent of the seasonal changes, the researchers made two trips to Barrow in 2012. From the end of March to the beginning of April, Granskog and Hudson made measurements on the ice before any melt had started, while temperatures were still around -30°C. Then they returned in mid-May to begin observing the progression of the melt. Divine replaced Granskog at the end of May, and he and Hudson will remain in Barrow until mid-June, observing the ice melt and pond development. The measurements and photographs of the ponding are also being used to improve methods for interpreting satellite data, in collaboration with colleagues at the University of Tromsø.
Light under the ice
Not all of the sunlight is reflected back to space or absorbed by the ice; some passes through to the ocean beneath, where it can heat the water and drive biological activity. To find out how much light is making it through the melting ice, Divine and Hudson are working with sea ice researcher Chris Polashenski from the Cold Regions Research and Engineering Laboratory in Hanover, New Hampshire, USA, to pull light sensors along beneath a portion of the observation line. The ponded meltwater is especially effective at allowing sunlight to penetrate to the ocean, an important factor for the Arctic Ocean energy balance and biological productivity.
Heat from the atmosphere
Another source of warming for the sea ice can be the transportation of warm air down to the ice surface. To measure this, the researchers from NPI deployed an eddy covariance system, which measures the three-dimensional wind direction and the air temperature 20 times each second. From these data they can see if air motions are warming or cooling the surface, and therefore how it’s contributing to the ice melt.
Mapping the surface
When there is significant variation in the topography of the ice surface, water from the melting ice and snow accumulates in the lower areas of the ice. It’s important therefore to observe what variations are found in the ice topography in order to understand where and how the ponds will form. Polashenski is using a LIDAR (Light Detection and Ranging) system to map the surface height variations at scales down to less than one centimeter. The system fires thousands of laser pulses at the ice, measuring the time it takes for each to return to the sensor, and calculates the distance to the ice from the time traveled. By combining his measurements with the NPI measurements made over the same area, they will be able to better understand the processes taking place during the melt season, ultimately leading to improvements to climate models and satellite observation algorithms.
Melting is well under way
Hudson and Divine have recently been making daily measurements to observe the rapid changes happening during this critical phase of the melt, when ponds and flooding form and eventually drain. The ponds initially formed below a layer of intact ice, but between 7 and 8 June they started opening up, revealing a largely flooded ice surface. Much of the ice is now covered by 5 to 30 cm of fresh melt water. Over the coming days, small holes in the ice, which is still 1.8 m thick, will open up, and much of this melt water will drain to the ocean, leaving smaller, distinct ponds that will deepen with time, changing color as they do. Their work will continue during these changes, observing the effects they have on the changing ice conditions.
This work is being funded by the Research Council of Norway, under the FRINAT and POLRES programs, along with additional funding from the Centre for Ice, Climate and Ecosystems (ICE) at NPI and the Fram Centre project Polhavet. It includes active collaborations with researchers at the University of Tromsø, University of Bergen, Cold Regions Research and Engineering Laboratory in New Hampshire, USA, and University of Alaska Fairbanks.