Abstract
The basin water of a sill fjord is renewed intermittently, when the density of the water at sill level in the ocean outside the fjord is high enough for it to sink down to the bottom of the fjord. How often the basin water is renewed, the renewal frequency, depends on how fast diffusion and mixing cause the density in the basin to decrease and on the variability of the density in the ambient ocean. Here, we suggest a statistical approach to investigate how a trend – e.g. imposed by global warming – in the ambient ocean density will affect the renewal frequency of a sill fjord. Negative ambient trends that are large compared to the rate at which density decreases the fjord will have large impact on the renewal frequency. It is shown that the observed negative trend in the annual maximum density at a hydrographical station near the Norwegian fjord Masfjorden, is very likely to have reduced the renewal frequency and increased the length of the stagnation period compared to pre-trend conditions. Negative trends in the annual maximum density since 1990 are observed at six out of eight longterm hydrographical stations along the Atlantic Norwegian coast, suggesting that the deep water renewal frequency in many Norwegian fjords has been reduced during the last 30 years.
Highlights
Fjords are long and narrow, steep-sided and glacially carved inlets that are found at high latitudes in both hemispheres
Following Gade (1973) we have considered deep water renewal in a sill fjord to be a stochastic process depending solely on the density variability at sill level outside of the sill (σ) and the rate at which vertical mixing cause the density at the bottom of the fjord to decrease (−D kg m−3 yr−1)
As expected, imposed trends that are small compared to the density decrease within the fjord (|α| ≪ D) have little effect, positive trends (α > 0) reduce the length of the stagnation periods and negative trends (α < 0) increase the length of the stagnation periods
Summary
Fjords are long and narrow, steep-sided and glacially carved inlets that are found at high latitudes in both hemispheres. Aksnes et al (2019) suggest that the changes are caused by a reduction in the density of the coastal waters, which has led to reduced ventilation rates (i.e. longer periods of stagnation) and decreased oxygen concentrations in the basin waters of the fjord They apply a simplistic empirical model to infer the evolution of the oxygen concentration in the fjord, in which the annual renewal rate of the fjord, i.e. the percentage of the basin water that is renewed with high oxygen open ocean water in a year, increases linearly as a function of what they call the ‘‘high density frequency’’, or the percentage of times that the density at sill depth at a nearby hydrographic station (observed quasi bi-weekly) is above a certain threshold. The framework does not allow for partial renewals (see Section 4) and it cannot be applied to e.g. fjords on Svalbard where deep water renewal is caused by convection during winter (Cottier et al, 2010), or to fjords where deep water renewal is a continuous process, such as e.g. in the Ilulissat Icefjord in western Greenland (Gladish et al, 2015)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
