Sammendrag
Circulation and Water Mass Formation in an Arctic Fjord
Hovedfagsoppgave i Oseanografi, 2003
Kandidat: Elin Darelius, Veileder: Peter M. Haugan
Geofysisk Institutt, UiB


Data from Billefjorden 2002 and 2003 were analysed and the hydrography and circulation in Billefjorden was observed to change substantially throughout the autumn and winter. The seasonal freshwater input and summer heating had created a fresh and "warm" surface layer, causing an estuarine circulation to develop. Stratification was strong in early autumn, and the typical three-layered structure, with a brackish surface layer, an intermediate layer and a basin layer, was observed. Stratification weakened throughout the autumn as the fjord cooled off and the freshwater input subsided. The estuarine circulation deteriorated and stopped, and was not observed during winter.

Tidal currents were observed in Billefjorden and tides are thought to behave similarly to Longyearbyen. A comparison between the results from a harmonic analysis and the tides in Longyearbyen imply a phase-lag of half an hour. Theories on tidal choking suggests that parts of the delay is due to the restricted transport capacity in the sill area and that the tidal amplitude can be expected to be slightly reduced (with a factor of 0.98). Tidal currents in October were observed to be stronger on the eastern side of the fjord and weak or non-existing on the western side.

Wind was observed to cause Ekman-transport and an outcrop of the intermediate layer in the strongly stratified September-fjord while it probably instigated flushing in October when stratification was weaker. Wind is probably just as important as the freshwater forcing in determining the circulation pattern within the fjord.

The sills of Billefjorden effectively shield the deep basin water from advection and influences from Isfjorden. The cold and saline water which is trapped below sill level has been formed locally through convection during winter time and was observed to loose density through vertical diffusion during autumn. The vertical diffusivity was estimated to be on the order of 10-5m2/s. The loss of density is not great enough for deep water renewal through the process observed in lower latitudes, where the bottom water is replaced by dense water flowing over the sill. The basin water of Billefjorden is dependent on ice-freezing and convection to be ventilated. A partial renewal was however observed in mid-October, when a sudden freshwater burst and strong down-fjord winds increased the estuarine circulation. The warm inflow mixed with the basin water and penetrated down to 70 meters depth.

Ice freezing and convection is strongly influenced by the stratification and heat content in the water column. While convection was observed to reach bottom in 2002, the observations from 2003 suggests that convection did not reach deeper than 130 m. The intrusion in October, which effectively transported heat downwards, might be responsible.

A one-dimensional ice/convection model was set up, suggesting that a "warm" renewal
would inhibit convection and ventilation of the basin water and that such a regime could persist for a longer period.