Warm water reappears in Spitsbergen fjords

Warm water reappears in Spitsbergen fjords

Top image: CTD measurements taken in Isfjorden. Photo: Eva Therese Jenssen/UNIS.

Oceanographic measurements along western Spitsbergen show that the warm water is back in the fjords. This is due to inflow of warm Atlantic water from the shelf outside the fjords, as a result of large production of sea ice in the fjords and along the shelf area last winter. So the prospect of solid fjord ice cover in winter 2012 is bleak, however, the wind climate over Svalbard will be the deciding factor for whether there will be ice or not.

4 November 2011
Text and images: UNIS Associate Professor Frank Nilsen; UNIS adjunct professor Tor Gammelsrød, and UNIS Research Associate Ragnheid Skogseth.

Every September there is a scientific cruise along the western and northern coast of Svalbard as part of the UNIS course AGF-214 ”Polar Ocean Climate”. The main aim is to monitor the inflow of warm Atlantic water into the Arctic Ocean and the spreading of this water around Svalbard.

The data material collected is also used in the Polish-Norwegian project AWAKE (Arctic Climate and Environment of the Nordic Seas and the Svalbard – Greenland Area). The AWAKE project funds moorings in Isfjorden and Hornsund that observe the exchange of water masses between the shelf area and the western fjords of Svalbard.

Warm water in Isfjorden
Measurements of temperature and salinity in the water column between Bjørndalen (outside Longyearbyen) and across the fjord to Bohemanneset reflect the situation in the Isfjorden system as a whole (figure 1). The measurements are repeated every September and April and the temperature data for September 2010 and 2011 are shown in figures 2 and 3. The warm, salty water intruding into the fjord is illustrated in red. There is always warm and more saline Atlantic water at the shelf area outside the western fjords of Svalbard. The important difference between every year is the volume of warm water that flow into the fjords and how close to the surface this water penetrates. The warm water comes in along the southern side of the fjord and is illustrated by a red arrow on the map and with a current cross into the fjord in the temperature section.

Isfjorden temperature section

Figure 1: Map of ocean floor topography in Isfjorden. The temperature section across the fjord between Bjørndalen (south) and Bohemanneset (north) is shown. The long red arrow illustrates where the warm water is flowing into the Isfjorden system. The warm current follows the southern slope all the way to Sassenfjorden, where some of the water continues towards Tempelfjorden and Billefjorden, while most of the water turns and flows outwards along the northern side of Isfjorden with a reduced temperature.

In the Bjørndalen-Bohemanneset section (Bjørndalen to the right) one clearly sees that the volume of warm water was significantly lower in 2010 compared to this year. In 2010, the warm water could only flow into the fjord along the bottom and the slope on the southern side of the fjord. This was the situation for all the western fjords where the water masses were dominated by “local” water which is colder than the warm Atlantic water in the shelf area. The local cold water therefore acted as a lid above the inflowing warm water and shielded the surface water and sea ice from this heat source.

But why didn’t we get a good “snowmobile year” even though the conditions for solid sea ice were perfect and the air temperatures were low last winter? And how did the warm water fill up the fjords again?

Isfjorden temperature section September 2010

Figure 2: Temperature section across Isfjorden (September 2010) between Bjørndalen (right) and Bohemanneset (left), where the logging stations are indicated at the top. The red area marks the amount of warm water from the shelf and the warm current flowing along the southern side of the fjord is marked by a cross and a circle.

The explanation is the same for both questions. The answer to question number one is that it was a good sea ice year; however it was not possible to drive snowmobiles on it.
The volume of sea ice produced was above average, but it was in the form of frazil ice (floating ice crystals) and thin ice which was immediately blown out of the fjord and made room for production of more ice. In open water close to the freezing point and under thin sea ice there is produced ten times as much sea ice as under thicker ice covers. Thus, in years with good snowmobile conditions there is relatively low ice production because there is a solid fast ice lid above the fjord which grows more slowly than when the fjord is open and close to the freezing point. This was the situation last winter where there were low air temperatures, but where the wind frequently blew the ice out of the fjords in January and February, the period vital for the production of solid ice.

In years with high ice production (i.e. bad snowmobile conditions) the local water in the fjords becomes denser than the water in the shelf area. The warm Atlantic water will thus flow into a higher level in the water column, while the denser local water will be at the bottom. The protective water layer for the sea ice is thus gone and the door is literally open for an increased inflow of warm water. That is why we see a larger red area with warm water in September 2011 (figure 3) compared to September 2010 (figure 2).

Isfjorden temperature section September 2011

Figure 3: Temperature section across Isfjorden (September 2011) between Bjørndalen (right) and Bohemanneset (left), where the logging stations are indicated at the top. The red area marks the amount of warm water from the shelf.

Will there be ice?
Low air temperatures combined with the wind climate is the most important factor for production of a solid ice cover in the fjords. Observations made by Svalbard Airport show that 1998 and 2004 were years with cold winters. In the period 1 December – 1 May the average temperature in 1998 was -15.0 °C and -12.9 °C in 2004. The 2011 winter (-11.6 °C average temperature) is between these cold winters and the relatively warm winters of 2007 (-8.0 °C) and 2010 (-9.2 °C), but is still considered a cold winter. Until mid-January this year’s winter was colder than 1998. The winter of 2010, on the other hand, was “postponed” till February, because of rain and mild temperatures in January.

The decisive factor for a solid ice lid in the cold winters is the wind force in the fjord.

In our model where we predict the rate of sea ice production in Isfjorden, the wind force in and out of the fjord is of special importance. Normally the wind blows out of the fjord in winter. If there is wind blowing into the fjord, as in mid-January 2010, it normally coincides with southerly winds bringing warmer air and sometimes rain over Svalbard.

If one compares the winter of 2011 with the two cold winters of 1998 and 2004, the big difference is in the wind force from December to March. Even though 2011 was colder than 1998 until mid-January, the wind direction was more frequently out of the fjord in 2011 so that the sea ice could not establish itself properly like it could in the “ice year” of 1998. The winter of 2004 is also a so-called good ice year, with calm winds and low air temperatures in January and February, a critical period for formation of a solid ice cover in the fjords.

Even though the outlook for the coming winter is bleaker compared to 2011, there is still a chance for a solid ice cover in 2012. It will be interesting to see how fast the temperature in the surface water will decrease as soon as there is minus degrees over Svalbard. When the surface water has reached freezing point (-1.9 °C), the wind climate in January and February will be the decisive factor for the establishment of solid sea ice.

 

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