|Schematic of the Arctic surface circulation including the Transpolar Driftal.|
The Siberian shelves are vast and shallow, seasonally ice-covered, and receive more than 80% of the Arctic freshwater input, in particular from the Ob, Yenisei, and Lena, three of the largest rivers on earth. These rivers shape the hydrographic and biogeochemical environment in the region. Nevertheless, the Siberian shelves are among the least studied shelf seas on the planet and host a complex series of processes impacted by freshwater, warm Atlantic-derived waters in the Arctic basin, considerable tidal currents and sea ice. The Laptev Sea, as a representative of the Siberian shelves, features vast areas that are covered by immobile (landfast) sea ice in winter. Along the landfast ice edge, offshore winds during winter and spring frequently move the pack ice cover to open up leads (so-called polynyas), where new sea ice is formed when open water is exposed to extremely cold air temperatures. Owed to the large ice formation rates observed in the polynyas, the Laptev Sea is also referred to as the “sea ice factory” of the Arctic and the beginning of the Transpolar Drift system, which transports sea ice from Siberia across the North Pole to exit the Arctic through the Fram Strait, East of Greenland.
Laptev Sea project
The Laptev Sea has been a focus of Russian-German partnerships since the 1990’s, under a variety of projects with science questions focusing on sediment transport, sea ice formation and the role polynyas for the ocean circulation and ecosystem. “The Transpolar Drift System of the Arctic Ocean,” a new multi-disciplinary project was launched in spring 2013, with the goal to assess the processes that dominate the Laptev Sea shelf at present in order to predict the fate of the Transpolar Drift (TPD) System under a changing climate. Scientists from different Russian and German Research Institutes participate to investigate sea ice and ocean circulation, biological and chemical properties of the water column and the sea floor as well as aim to improve the understanding of the region’s geological past. Additionally, the central Arctic as well as the Fram Strait region are investigated as well for a comprehensive picture of the TPD.
The Russian research vessel Viktor Buynitskiy. Image courtesy of the captain of the icebreaker 50 Years of Victory.
Laptev Sea Expedition Transdrift 21
During 20+ years of Russian-German Laptev Sea research, a total of 21 (Transdrift) expeditions were carried out, most of them shipboard summer expeditions, but also some helicopter-based winter expeditions. The recentmost expedition, Transdrift 21, was carried out during August/September 2013 aboard the ~50m-long Russian-flagged vessel Viktor Buinitsky. The science party boarded the ship in Arkhangelsk and sailed along the Northern Sea Route across the Barents and Kara Seas into the Laptev Sea. As was often the case in the recent years, the passage was largely ice-free, except a small region in Vilkitsky Strait, a narrowing passage between the Kara and Laptev Seas. The expedition was guided into the study region through the Strait in a convoy of four vessels, lead by the Russian nuclear icebreaker Yamal.
The Laptev Sea shelf features extreme fronts and gradients between the different water masses that are characterized by the Lena River outflow and the warm and saline water masses that are found offshore along the continental slope. The different water masses are not only characterized by differences in temperature and salinity, but also carry their own chemical signatures in nutrients and isotopes, and have a strong impact on the organisms that inhabit the water column and dwell on the sea floor.
One aim of the expedition was to map the major physical and biogeochemical parameters, which can then be compared with conditions found during previous expeditions in order to determine the variability in the region. The sampling strategy is such that the vessel occupies selected stations throughout the region. Each station then follows a clear plan of action: First, temperature and salinity of the water column are measured with a CTD (conductivity-temperature-depth) device, attached to a water sampling rosette, which allows researchers to take water samples from selected depths for subsequent analysis of the basic geochemical parameters. Biologists take net samples to determine the amount and species of organisms that are found in the water column, as well as sediment cores of the seafloor with a heavy multicoring device. The full procedure takes several hours to complete.
|Sea ice concentration in the Arctic Ocean 8/27/2013. Courtesy of the UNiversity of Illinois.|
Besides the usual station work carried out during the expedition, the project takes measurements from year-round oceanographic moorings (seafloor observatories). The procedure is to anchor oceanographic instruments on the seafloor and in the water column, which record parameters such as temperature, salinity or ocean currents in half-hourly intervals until the following year, when instruments are recovered and data are downloaded for analysis. Mooring work in difficult environments such as the seasonally ice-covered Laptev Sea is quite risky and loss of instrumentation is not unusual, but previous successful deployments in combination with shipboard expeditions lead to significant advancements in the understanding of how the ocean interacts with sea ice and the atmosphere.
The reduction in Arctic Sea ice opens up new opportunities for shipping along the Northern Sea Route and other offshore activities, but can certainly complicate the research from mid-sized vessels. Storms that periodically move over the region generate large waves and swell, as the ice edge is far to the north, and may interrupt the sampling or at the least make life on board less comfortable.
A novelty applied during the Transdrift 21 expedition was the use of an UnderwayCTD system, which allowed to profile temperature and salinity of the water column in rougher seas and while the ship is en route. The underway sampling complemented the station work to provide a more comprehensive and high-resolution picture of the region’s water masses. The Lena River water, for instance, was found in the north-east of the Lena Delta, several hundred kilometers away from the source. The extent is surprising, but it follows a general trend in the recent past which is characterized by longer open water seasons; the ocean’s temperature was up to 6°C throughout much of the region. This can be considered quite warm for an Arctic shelf sea, and the amount of heat that is stored in the water column likely has consequences on the organisms and may further delay the onset of sea ice in the fall.
|Russian nuclear Icebreaker Yamal|
Along those lines, some of the scientific questions that the project pursues is to identify the processes that help to vertically distribute water properties in order to understand, for instance, 1) the rate at which the Lena River water is mixed downwards, which has implications for the stratification, i.e. the layering of the ocean; 2) the vertical redistribution of warm surface waters may store heat in the water column or near the seafloor, which may have implications for subsea permafrost, methane and seafloor organisms; or 3) the upward mixing of nutrients, which are often enhanced near the seafloor, which generally enhances biological productivity. Some of the important mechanisms that help the mixing and vertical redistribution of properties include mixing due to tidal currents and the effect of strong winds, and in particular the wind mixing may play an increasing role in the Laptev Sea System, if the trend towards longer open water season continues.
The Transdrift 22 expedition is scheduled for late summer 2014, during which the scientists plan the recovery of the seafloor observatories and hope to identify more pieces of the Laptev Sea puzzle for a clearer understanding of the fate of the Laptev Sea and the Transpolar Drift System under a changing climate.
Markus Janout is a physical oceanographer at the Alfred Wegener Institute (AWI) for Polar and Marine Research, Germany, specialized in oceanographic processes on high latitude continental shelves and slopes. Janout completed a PhD in Physical Oceanography at the University of Alaska, Fairbanks in 2010, where he focused on heat and freshwater controlling processes in the Gulf of Alaska and the Bering Sea, before transitioning to AWI to join the Laptev Sea group.