Sea Ice Station Umeà, Winter 2005 
31 
The system started to work well, and ice started to form at about December 10th. The growth 
was rather slow, and in the end of December the ice thickness was 10 cm. Then a heavy storm 
arose with southerly winds. As a consequence, the fast ice at the float broke and experienced a 
significant displacement. The float instrumentation was harmed and stopped functioning, and 
also part of the data was spoiled by penetration of seawater into the data logger box. 
Therefore, the data collected covers just a 40-day period; however, it is an interesting period 
since the surface water then cooled from 3°C to zero and ice grew from zero to 10 cm 
thickness. 
At 5-m depth the water temperature fell from 4°C to 0.5°C in by the end of November (Fig. 2). 
Then the temperature was constant for two weeks. The water salinity was about 4.4%o. There 
was an inflow of warm water with temperatures of 1-2°C for one week, and then toward the 
end of December the water cooled down again. The salinity of the water varied between 4.0 
and 4.6%o and was not so clearly connected with the temperature evolution. 
TS evolution 
O 5.00 
f- 4.00 
a> 
H 3.00 
Z 2.00 
S 1 - 00 
« 0.00 
O) 
LO 
CO 
T— 
O) 
LO 
CO 
T— 
■3- 
O) 
O) 
00 
CO 
00 
CO 
00 
LO 
o 
CO 
T— 
CM 
00 
CO 
O) 
I— 
1— 
C\J 
C\J 
CO 
CO 
■3- 
■3- 
LO 
Time (18 Nov - 29 Dec) 
■Salinity 
Temperature 
Fig. 2. Temperature and salinity of the water (5-m depth) at the Umea float. 
The breakage of fast ice was due to a combination of thin ice and strong wind. Since normally 
the ice grows faster than in the study winter, the period of potentially unstable ice cover was 
long. Landfast ice behaves as a plastic medium, with stationary state for 
x a L < P*h 
where x a is wind stress, L is basin size, P* is ice compressive/tensile strength, and h is ice 
thickness (Lepparanta, 2004). Empirical data show that in the present case the thickness of 
15-20 cm should have been enough for the ice sheet to resist the wind forcing. 
At the end of March a short field trip was made to the landfast ice site. Ice sample was taken, 
and the springtime light transfer through ice and snow was determined. The fast ice thickness