Warm Atlantic water wants in, but is prevented from entering Isfjorden
AGF-311/811 course cruise outside Kapp Linné, where they conducted CTD measurements. Photo: Eva Therese Jenssen/UNIS.
Top image: Over years, UNIS has monitored the state of Isfjorden through yearly cruises. Photo: Eva Therese Jenssen/UNIS.
Isfjorden on Svalbard contains less warm and salty Atlantic water than in recent years and is now more like an Arctic fjord. Observations from the UNIS cruise in September show that the “door is closed” for massive intrusion of Atlantic water since the water masses in Isfjorden and in the coastal current have become much fresher than on the shelf outside Isfjorden. A reduction in Atlantic zooplankton in Isfjorden further confirms this. The possibility for sea ice cover in the Isfjorden system this winter is thus good.
14 October 2020
Text: Frank Nilsen, Ragnheid Skogseth, Janne Søreide, Kjersti Kalhagen, Sine-Sara Astad and Anna Vader, UNIS.
Over the last 20 years, we have seen a trend of rising air and sea temperatures in and around Svalbard, especially in winter. This has resulted in a reduced sea ice cover around Svalbard and in the fjords on the west side of Spitsbergen. In the years that there has been sea ice in Isfjorden and its side fjords, such as Tempelfjorden and Billefjorden, the ice has often been thinner than usual and very unsafe. This is due to the fact that warm Atlantic water has displaced the “local” cold and fresh Arctic water, and that temperatures well above the freezing point of seawater (approx. -1.8⁰C) have affected the water at the surface so that in some years the fjords have had open water throughout winter.
The main reason for this is that the low-pressure systems that have their fixed path across the Norwegian Sea have more often passed through the Fram Strait than through the Barents Sea on their way north. This means that southern warm winds towards Svalbard push the sea ice north, and that the warm currents (Vest-Spitsbergen current or the last stage of the Gulf Stream) along the west coast of Spitsbergen are pushed up on the shelf and towards the fjords. While low-pressure systems only last for days or weeks, their effect in the ocean can remain for several years. It has therefore been difficult to reverse the trend of increasing amounts of warm and salty Atlantic water in the fjords along the west side of Svalbard, until 2019.
For this winter (winter 2019/20), most low-pressure systems went directly into the Barents Sea, which was common in the “old days”. This led to winds from the north and cold air masses over Svalbard throughout the winter. In addition, measurements from several UNIS cruises through the autumn of 2019 showed that even though Isfjorden was dominated by warm and salty Atlantic Ocean water from a depth of 80 meters down to the bottom, the surface layer was significantly fresher than in the last 15 years. This is good news, because the fjord will then have a lighter mass of water than the hot and salty Atlantic Ocean water on the shelf. Isfjorden can thus “close the door” for the penetration of the warm water into the surface layer through the winter. Fresher surface water and a cold weather type throughout the winter and spring, provided rapid cooling to the freezing point, and sea ice was formed. This drove a vertical mixing process that led the entire Isfjorden towards a fresher and colder and more Arctic state.
But where does all this fresh water come from? This is something the marine team at UNIS together with partners is working to find out. The candidates are 1) fresh water from melting sea ice locally in Isfjorden as a result of warmer water being mixed up and 2) fresh water introduced with the coastal current, which originates in the Barents Sea. Through UNIS ‘winter field campaigns in Isfjorden, we saw that the surface water remained fresh throughout the winter. In addition, there is no doubt that the summer of 2020 has led to an abnormal amount of fresh water from land to the fjords around Svalbard. With record hot summer months, the glaciers on Svalbard have lost enormous amounts of mass, which has melted into the fjords. Colleagues from the Norwegian Polar Institute can report up to a 3-meter reduction in glacier thickness on some of the glaciers around Kongsfjorden.
The door is closed
The results of all these freezing and melting processes were measured on a UNIS cruise with FF Helmer Hanssen this September. This cruise is an important part of the marine climate monitoring at UNIS and is carried out annually with students. This year, due to the corona, it was an abbreviated version without students with the sole goal of maintaining UNIS ‘long time series in Isfjorden. The salinity section (figure 1) and the temperature section (figure 2) along the entire Isfjorden, from the shelf outside (near Isfjord Radio) all the way to Adolfbukta in Billefjorden (in front of the Nordenskiöld glacier), clearly show that the entire Isfjorden has become much fresher and colder. The fjord is now lighter than the Atlantic water on the shelf and “the door is closed” for large amounts of warm water that wants to enter.
Where we previously, and most recently last year, saw warm Atlantic water penetrate all the way up to 80 meters deep, it is now only as far as possible for this water to squeeze deeper than 150 meters. The positive thing about some Atlantic water entering Isfjorden is that there will be good fishing. During the cruise with FF Helmer Hanssen, one could with sonar clearly observe cod in the same depth as the Atlantic Ocean water flowed in. Shallower than 150 meters, the fjord has become noticeably fresher (lower salinity values than before in Figure 1) and this is what prevents the warm water from penetrating. Deeper than 150 meters, the water has become colder (figure 2) due to less intrusion of Atlantic water and mixing with cold winter water. At the very bottom (between stations 36 and 32 in Figures 1 and 2) one sees remnants of heavy winter water from sea ice formation last winter. The relatively high temperature in the surface layer (shallower than 100 meters) is a result of the solar warming this summer, and many leisure boat owners on Svalbard have reported unusually high temperatures in the surface this summer, especially on the north side of Svalbard.
Favourable conditions for sea ice
The biodiversity in Isfjorden can say something about the condition of the fjord, and on the UNIS cruise with FF Helmer Hanssen, the amount of copepods was measured, especially the Calanus glacialis and Calanus finmarchicus, which are numerous in the fjords on Svalbard. If the C. glacialis is dominant of these two species, it reflects a colder fjord climate. If the C. finmarchicus is dominant, the opposite is reflected. In the innermost part of Billefjorden, where the fjord water below a depth of 50 meters is colder than -1⁰C all year round, we found up to 90% C. finmarchicus. Outside Isfjorden, where warm Atlantic water is present throughout the water column, we find up to 90% C. finmarchicus. In the middle parts of Isfjorden we often find a more equal proportion of the two species, but this year the C. glacialis dominates!
There is little doubt that the condition in Isfjorden today is favourable for the coming winter and opportunities for sea ice formation in the fjord system. “The door is closed” and as long as the autumn and winter low pressures systems enter the Barents Sea, the fjord will be able to cool down to the freezing point without much influence from the warm water on the shelf outside. Enormous amounts of heat exist on the shelf along the west coast of Spitsbergen, and there is no doubt that the warming of the oceans around Svalbard continues and that the inflow of hot water to the Arctic is maintained. The wind situation (southerly wind along the coast) and the type of weather we have had in September ensure that hot and salt water from the West Spitsbergen stream is forced towards the fjords. At the same time, more precipitation falls over the fjords and the melting of glaciers continues, so the contrast between a “light” fresh fjord and a “heavy” salty shelf is enhanced. If we get a winter like last winter, we are thus well on our way to an Arctic state in Isfjorden with opportunities for a good sea ice cover. But it is not a stable situation, it is more like a local spasm in an Arctic during warming.
Long marine time series maintained by UNIS are necessary to give us important knowledge about the changes we see in Isfjorden and the processes that control these changes. The students at UNIS are important contributors to the interpretation of the year-round data collection in oceanography, biology, chemistry and marine geology, and they get a first-hand and direct insight into the changes that are taking place in the Arctic through the education program at UNIS. UNIS and its partners have just published all oceanographic data collected around Svalbard since UNIS was founded and even further back and are freely available in the Norwegian Polar Institute’s database. Results on Isfjorden from this data set can be read about in the science article in «Progress in Oceanography». The data is also available through SIOS’ web-based portals; https://sios-svalbard.org/.
UNIS will monitor the situation in Isfjorden throughout the autumn and winter, and report on our findings. We have set up an underwater rig near Bjørndalen that will measure sea temperature, salinity, current speed, tides, wave height and important biological and chemical parameters. Data from the rig will be directly transmitted to UNIS via a surface buoy (with a wave sensor and meteorological sensors) and will be available to everyone. This “online rig”, which is funded through the Research Council’s infrastructure program in the SIOS-InfraNOR project, will be able to tell us if the water in Isfjorden is on its way to freezing, or if warm water penetrates and thus destroys the ice conditions in winter, the time for algae spring blooming and whether the cod is on its way in the summer.
Background material:
Skogseth, R., Olivier, L.L.A., Nilsen, F., Falck, E., Fraser, N., Tverberg, V., Ledang, A.B., Vader, A., Jonassen, M., Søreide, J., Cottier, F., Berge, J., Ivanov, B.V., Falk-Petersen, S., 2020. Variability and decadal trends in the Isfjorden (Svalbard) ocean climate and circulation – An indicator for climate change in the European Arctic. Progress in Oceanography, 187, 102394, https://doi.org/10.1016/j.pocean.2020.102394.