| dc.description.abstract | This study is aimed to investigate water exchanges in the inner Oslofjord, between the Bunnefjord and the Vestfjord. A 46-year historical dataset of salinity and temperature shows that the surface-mixed layer is about 20 m deep and has high seasonal variations. Summer heating at the surface diffuses to the lower layer which results in relatively high temperature at depth in winter. The deepwater below the Bunnefjord-Vestfjord sill depth, which of 55 m, is homogenized in both temperature and salinity. Two significant events related to deepwater exchanges were observed from the salinity time series. First, the stagnant periods of deepwater, the periods of progressively decrease of salinity due to the vertical eddy diffusivities. Vertical eddy diffusivities were estimated in the periods of stagnation using the Budget method. The results confirm earlier claims that the Vestfjord has higher diffusivities than the Bunnefjord. This is a result of breaking internal waves that are generated at the Drøbak sill. Second, actual deepwater renewal events, periods of rapidly increase of salinity, require availability of dense water outside the basin in order to replace old basin water. In addition, northerly winds are needed to push the surface water out of the basins and induce the deepwater influx. New observations were conducted by the F/F Trygve Braarud research vessel during 7 August-11 December 2020 in order to study details of water exchanges in the inner fjord. The FjordOs numerical ocean model was compared to the observations. In return, the model was used to help interpret the observations. Both model and observations agree on the time-average inflows at a station, a station between Nesoddtangen and Bygdøy, and time-average outflows at another station near the Nesoddtangen. Local river discharge and tides play minor roles on the exchange flows. Winds and pressure (SSH) gradient are important factors influencing the water exchanges. The circulations in the surface layer are mainly influenced by the winds, whereas the pressure gradients relate to the exchanges of intermediary water. The model reveals recirculation cells in the Bunnefjord and the Vestfjord, particularly in the surface layer. This may suggest that most of the water exchanges between the Vestfjord and the Bunnefjord take place in intermediate and bottom layers. | eng |