Oceanographic Conditions of the Comau Fjord

Written by Leif Djurfeldt

The Comau Fjord is characterized by very large vertical density differences; the vertical gradients are among the strongest found in the world. Three different layers can be distinguished:

1. a surface layer, usually less than 10 m deep, with variable internal stratification that depends on recent fresh water input, local heating, cooling and wind mixing.  Salinity, which is at minimum at the head of the fjord, grows gradually towards the mouth in this layer. Time variation of the local wind forces cause variation up to several meters in the depth of the surface layer at the inner end. These disturbances propagate as internal waves towards the mouth.
2. a very sharp pycnocline below the surface layer.  In winter it is primarily caused by salinity differences, and in summer by both temperature and salinity. Because of this strong stratification, the surface water can be very warm during summer – above 20º C.
3. the deep water has a temperature around 11º C and salinity over 32 ppt – similar to the water outside the fjord. Considering the depth along the whole fjord exceeds 400 m, this is the main part of the water volume of the fjord. Longitudinal variations and currents within this water mass are small.

Circulation and water exchange within the area outside the fjord are governed by estuarine processes, i.e. they are driven by fresh water influx from rivers and mixing across the pycnocline. The brackish surface water that leaves at the mouth is replaced by a slow compensating flow below the pycnocline and fresh water.

Even if the tidal range is over 7 m at spring tides, the tidal currents are weak everywhere because of the wide mouth and the large depth throughout the fjord. Only in the narrow entrance to the small Quintupeu fjord is the tidal current noticeable.