Arctic: Polar Sea is getting sweeter

Strong increase in freshwater content in the Arctic Ocean since the 1990s

Ice landscape in the Arctic © Alfred Wegener Institute
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The salinity in the upper layers of the North Pole has decreased significantly since the 1990s. About 20 percent more fresh water - equivalent to 8, 400 cubic kilometers - have been measured by researchers using more than 5, 000 salt content profiles over the last 20 years. The exact causes are still unclear. Since the amount of fresh water in the upper seawater layers affects the heat transport in the ocean and the currents, this change is also important for the global climate.

About ten percent of the global freshwater flowing into the Arctic Polar Sea reaches the Arctic via the huge Siberian and North American rivers, as well as relatively low-salt water from the Pacific Ocean. This fresh water settles as a light layer on the deeper salty ocean layers and thus also largely decouples their heat from the ice and the atmosphere. Changes in this layer are therefore important parameters for the sensitive heat balance of the Arctic. The amount of fresh water also influences the deep water formation in the Greenland Sea and the Labrador Sea, and thus has an impact on the global circulating circulation of the ocean.

5, 000 salt content profiles evaluated

How the freshwater content of the Arctic Ocean has changed in recent years and decades has now been studied by scientists from the Alfred Wegener Institute for Polar and Marine Research in the Helmholtz Association. For their study, the researchers around Benjamin Rabe evaluated a total of more than 5, 000 measured salinity profiles. These were supplied, inter alia, by probes that were used by ships or attached to large ice floes, so that they recorded data during the ice drift through the Arctic Ocean. In addition, readings from submarines were included in the analyzes. Much of the data comes from expeditions during the International Polar Year 2007/08.

20 percent more fresh water since 1990

The dense network of observations in recent years has for the first time allowed to calculate a comparative balance of freshwater content in the Arctic Ocean between the periods 1992 to 1999 and 2006 to 2008. The result: the freshwater content of the upper Arctic Ocean has increased by about 20 percent compared to the 1990s. This corresponds to an increase of approximately 8, 400 cubic kilometers and is of the same order of magnitude as the amount of fresh water that is annually exported from this marine area in liquid or frozen form.

Changes in the acidity of the Arctic Ocean in 2006-2008 compared to 1992-1999. Negative values ​​of salinity (salinity) are shown in the colors yellow, blue and green and represent a decrease. Benjamin Rabe, Alfred Wegener Institute

Causes still unclear

"The strong changes in the upper water layers consist primarily of a decrease in the salinity, " says Rabe. In addition, the low-salt layers today are significantly more powerful than before. But what is the cause of this "dissolution" of the upper water layers? Possible sources are theoretically an increased inflow from rivers, precipitation or a stronger ice melt. It would also be possible to reduce the transport of the upper, outer layers of water through ocean currents or wind from the Arctic Ocean to southern latitudes. display

AWI scientists believe that reduced livestock exports and increased entries from the coastal areas of Siberia into the central Arctic Ocean are the most likely reasons. What exactly is responsible, further investigations have yet to show.

Since the analyzed data covers only a relatively short period of time, Michael Karcher from the Alfred Wegener Institute, co-author of the study, additionally simulated the observed phenomena using the coupled ocean-sea ice model NAOSIM. The model experiments also allow to map times for which no measured data are available. The results of the simulations indicate that local wind patterns could also play an important role. Measurements and models also show that the changes in Arctic acidity cover much wider areas than previously thought. It is expected that the additional amount of fresh water in the near-surface Arctic Ocean will emanate into the North Atlantic in the coming years. (Deep Sea Research, 2011; DOI: 10.1016 / j.dsr.2010.12.002).

(Alfred Wegener Institute for Polar and Marine Research, 25.03.2011 - NPO)