Horizontal and vertical exchanges and diffusion in the water masses of the oslo fjord

Kurzfassung

Der Wasseraustausch des inneren Oslofjords wird von einer Zirkulation innerhalb des Ästuars beherrscht. Die Advektion von Brackwasser erreicht Maxima im Frühjahr und Herbst, während sie im Winter und Sommer praktisch fehlt. In tieferen Schichten setzt die Zirkulation periodisch aus; hier ist ein Jahreszyklus feststellbar. Dieser Vorgang scheint in Beziehung zu den winterlichen Nordwinden zu stehen. Ein horizontaler Austausch erfolgt durch Gezeitenströme. Diese Form des Wasseraustausches ist vornehmlich während des Sommers von Bedeutung.

Summary

1. Both advective and diffusive processes are agents in the water exchange of the inner Oslo Fjord. The estuarine circulation is the most important form of water exchange. Analysis on a monthly basis has revealed great seasonal variations with peaks in spring and fall, and slack periods in winter and summer. The estuarine circulation is predominantly limited to the upper 20 m, comprising the zone of brackish water.

2. To a lesser extent also the entire body of sea-water in the fjord is involved through vertical eddy diffusion. Seasonally varying density conditions in the outer fjord prevent a continuous replenishment of the deeper sea-water layers. This water is renewed intermittently, and to a varying extent, by heavier water flowing in over the sill. The process takes place about every winter and appears to be controlled by the strength and duration of the seasonal northerly winds.

3. Vertical eddy diffusion coefficients are computed in the sea-water phase on the basis of the salt budget. In the upper layers the vertical diffusion is determined from harmonic analysis of the heat wave resulting from surface heating and cooling.

4. Horizontal exchange in the upper layers also rises from diffusive processes. Tidal currents, however weak, are the main generating agent of turbulence, but also wind drift has significant effects when present. Horizontal eddy diffusion is determined from the budget of orthophosphate in winter. Diffusion coefficients are of the order 10⁶ cm²/sec, but reach 10⁷ cm²/sec in Drøbak Sound.

5. Drastic forms of water exchange take place under influence of fresh winds prevailing for more than four days. Both pure wind drift and secondary density currents occur.