Baroclinic wave drag from two closely spaced sills in a narrow fjord as inferred from basin water mixing

The dynamics of the entrance to the Ringdalsfjord‐Idefjord at the Swedish‐Norwegian border is described for the first time. A 1‐month‐long high‐resolved data set shows that the mixing rate is extremely high in the basin water between the two sills at Bjällvarpet and Svinesund and that the accompanying horizontal mean buoyancy flux is about 25 mW m−2. During two periods in the observations, the density decrease rate, dρ/dt, below 17 m depth was estimated to be −0.90 and −1.20 kg m−3 d−1, respectively. Our observations support the idea that the tide is subjected to baroclinic wave drag due to generation of internal tides at sills when a densimetric Froude number, based on tidal speed and basin stratification, is subcritical. For supercritical values of the Froude number, basin stratification becomes unimportant, and the tide is subjected to large‐scale topographic drag with jets developing on the lee side of the sills. The total energy loss due to baroclinic wave drag and large‐scale topographic drag during the two periods are 268 and 198 kW, respectively. All power supplied to the internal tides and 1/6 of the power supplied to the jets is assumed to sustain the turbulence in the basin water (beneath the depths of the sills). With a flux Richardson number of 0.06, determined from investigations in many fjord basins, we find that a process‐based mixing model explains the observed mixing between the sills to within 90%.