Oceanographic variability and change in two fjords in northern Norway

Abstract

Long-term hydrographic time series data from two fixed stations in the northern Norwegian fjords Malangen and Balsfjorden from the period 1980 - 2022 have been examined. The data have been supplemented with model results from the ocean model NorFjords160 over the period April 1st 2017 to December 31st 2022. To gain a deeper understanding of the oceanographic variability and change, the environmental drivers wind, precipitation and air temperature from the nearby weather stations Hekkingen fyr (Malangen) and Tromsø (Balsfjorden) and river runoff from the major rivers discharging in the fjords, Målselva (Malangen) and Nordkjoselva (Balsfjorden), have been examined for the same period. Additionally, for the years 2017 - 2022, particle tracking simulations have been used to investigate surface currents and the spread of particles from Målselva and Nordkjoselva.

The findings reveal statistically significant, positive air temperature trends at both Hekkingen fyr and Tromsø (up to 2.2 °𝐶 in autumn over the period 1980 - 2022). There is found seasonal variations in precipitation, however there are no discernible trends in river runoff at either locations. For the hydrographic conditions, the findings reveal statistically significant, positive temperature trends for both the surface layer (0.03 - 0.06 °C yr−1 in Malangen and 0.04 - 0.07 °C yr−1 in Balsfjorden) and deep layers (0.03 - 0.04 °C yr−1 in Malangen and 0.01 - 0.04 °C yr−1 in Balsfjorden) throughout all four seasons, in both fjords. There are no statistically significant trends regarding deep layer salinity in Malangen, whereas in Balsfjorden there is a statistically significant, negative trend in the spring and summer surface layers, as well as the summer deep layer. I hypothesize that the changes in temperature are linked to warming of the coastal water masses observed over the same period (Albretsen et al.,2011a; IMR, 2023), and the observed increase in air temperature. During the study period the surface CW has gotten fresher, while the deep CW has gotten more saline (Albretsen et al., 2011a; IMR, 2023). However, the changes in salinity in the fjords are less pronounced. The negative slopes of the estimated surface salinity trends may indicate an impact of freshening CW combined with a "normal" runoff pattern. For the deep layer, effective downward mixing of freshwater, changes in the frequency of AW inflow and the closeness of the stations to the river mouth versus the fjord mouth are presented as possible reasons as to why the signal from the coast is not visible in the fjords.

The particle tracking simulations reveal that particles suspended from Målselva are transported far along the coast into the fjords and sounds north of Tromsø, while particles suspended from Nordkjoselva mainly stay in Balsfjorden. Further it is found that runoff from Målselva, which has a far higher water transport compared to Nordkjoselva, is not a source of (relatively) fresh water to Balsfjorden, but supplies relatively fresh water to the coastal area.