DriftStories – 01: Detective work on ice that’s far too thin
How AWI sea-ice physicist Thomas Krumpen is tracing the fate of the MOSAiC ice floes back to their roots
AWI sea-ice physicist Thomas Krumpen is the ‘profiler’ in the MOSAiC Sea-Ice Group. Even if the largest-scale Arctic expedition in history isn’t a whodunit, where the goal is to bring the wrongdoer to justice, nevertheless most of the researchers on board the icebreaker RV Polarstern, which serves as the base of operations for the expedition, are preoccupied with two fundamental questions: where did the sea ice that we’re living and working on actually come from? And just what type of ice is it composed of? Finding the answers as early in the expedition as possible is a key priority, since this information is essential for nearly all model-based studies and satellite validations, not to mention the investigations into the material flows and food webs that will be carried out in the course of the year-long drift experiment.
Accordingly, in September 2019 Krumpen was one of the first members of the AWI’s Sea-Ice Group to depart for the Central Arctic, and was tasked with profiling the ‘scene of the crime’ – in other words: describing the sea ice in MOSAiC’s starting region in painstaking detail, and determining its origins. The 41-year-old conducted his detective work on board the Russian research icebreaker Akademik Federow, as it engaged in advance scouting of the region, located ca. 950 kilometres north of the New Siberian Islands.
To gain a first impression both ships went to coordinated search regions and followed the same procedure. From Akademik Federow Thomas Krumpen and two other sea-ice observers began by documenting from the ship’s bridge how many ice floes were in the target region, roughly how old and thick the ice was, at which points channels were forming in the pack ice, and whether the ice was covered with meltwater pools, or whether the floes had collided, forming pack ice hummocks. In the next step, the researchers used the ship’s on-board helicopter to fly to five larger floes within a 40-km radius, surveying the ice thickness and amount of snow cover on each. Krumpen compared the team’s on-site readings with extensive weather satellite and ice satellite data on the Russian Arctic, which he had gathered from a variety of sources prior to the expedition. In this regard, the weather data came from a eteorological monitoring station on Kotelny Island, the largest of the New Siberian Islands.
A history of extremes
The results of the initial analysis in both search areas were sobering: the ice in the starting region was less than a year old, had a mean thickness of only 30 centimetres, and had undergone substantial melting during the summer, as a result of which it showed heavy weathering and was littered with meltwater pools. The ice’s life story read like a string of negative records. “The summer of 2019 was the warmest in the Russian Arctic since the beginning of weather observations on Kotelny Island, back in 1935. Air temperatures over the Laptev Sea and East Siberian Sea beat the previous record high by two to four degrees Celsius,” Krumpen reports.
Since the previous winter had been one to three degrees Celsius warmer than the average in the reference period 1981 to 2010, the ice that formed in the ‘nursery’ for Arctic sea ice – the Laptev Sea and adjacent East Siberian Sea – was far thinner than in the past. Strong offshore winds then rapidly blew it out to open sea. As Krumpen recalls, “When the air temperature quickly rose in the spring of 2019, this extremely thin ice melted so rapidly and extensively that we not only saw the earliest break-up of the ice cover since 1992, but also the rapid and unexpected northward retreat of the ice edge.”
Consequently, in the autumn of 2019 it took longer than ever before for the surface water, warmed by the summer sun, to grow cold enough for new ice to form. According to the sea-ice physicist: “At the beginning of the expedition, roughly 80 percent of the sea ice in MOSAiC’s starting region was only a few days old. Floes that had survived the summer, and were therefore thick enough for us to work on, were definitely the exception, and hard to come by.” Finally, the scientists were successful in the RV Polarstern's search region. The expedition leader chose one of the most stable floes in this sector of the Arctic as home for the expedition.
Tracking with ice satellites
Once these initial conditions had been established, Thomas Krumpen’s real detective work began. The goal was to trace the course of the pack ice in the starting region back to its point of origin. To do so, the remote-sensing expert used a time series of high-resolution satellite data, which allowed him to identify the MOSAiC floes and therefore reconstruct their journey from the marginal seas of the Arctic Ocean to the Central Arctic – down to the exact day. “The ice floes that we set up the MOSAiC monitoring network on were formed off the northern coast of the New Siberian Islands on 5 December 2018, and in a shallow region with a depth of less than ten metres. When RV Polarstern dropped anchor at one of the floes, on 4 October 2019, the ice was exactly 318 days old and had travelled a total distance of 2240 kilometres, on a zigzagging course determined by the wind,” says Krumpen.
These new insights into the ice’s provenance are supported e.g. by sediment and particle deposits that the researchers found in the sea ice. These trapped deposits, referred to as inclusions, can only be found in sea ice that forms in coastal waters less than 30 metres deep: in shallow waters, the intense winter winds stir up large amounts of sediment from the seafloor, which are subsequently locked into the newly formed ice. Alternatively, the particles can be acquired when the young ice comes into contact with the seafloor in the surf zone. Chemical tests, which will likely tell us exactly which section of coastline the deposits hail from, aren’t yet complete.
One last look at the old Arctic
The unexpectedly high number of inclusions has given the expedition participants the chance to thoroughly analyse the role of Arctic sea ice as a means of transport for sediments, nutrients, climate-relevant gases and toxins – and an opportunity that, in Krumpen’s opinion, is very unlikely to come around again in the future: “Due to climate change, the majority of the sea ice formed in the marginal seas now melts before it can reach the Central Arctic. As a result, essential transport processes are now faltering, producing changes in the material flows of the Arctic Ocean. In MOSAiC we’re now taking one last look at the Arctic as we know it, and as we’ve investigated it over the past several decades. At the same time, we’re getting a first impression of what things will look like in the future.” For the sea-ice profiler, one thing is certain: the old Arctic’s days are numbered.
Author: Sina Löschke
Translation: Matthew Fentem (gonative.de)
Contact and information:
Dr. Thomas Krumpen
Corresponding scientific paper:
The MOSAiC ice floe: sediment-laden survivor from the Siberian shelf
Krumpen, T., Birrien, F., Kauker, F., Rackow, T., von Albedyll, L., Angelopoulos, M., Belter, H. J., Bessonov, V., Damm, E., Dethloff, K., Haapala, J., Haas, C., Hendricks, S., Hoelemann, J., Hoppmann, M., Kaleschke, L., Karcher, M., Kolabutin, N., Lenz, J., Morgenstern, A., Nicolaus, M., Nixdorf, U., Petrovsky, T., Rabe, B., Rabenstein, L., Rex, M., Ricker, R., Rohde, J., Shimanchuk, E., Singha, S., Smolyanitsky, V., Sokolov, V., Stanton, T., Timofeeva, A., and Tsamados, M.: The MOSAiC ice floe: sediment-laden survivor from the Siberian shelf, The Cryosphere Discuss., doi.org/10.5194/tc-2020-64, in review, 2020.