Predicting upwelling due to down-fjord winds

Down-fjord winds have been implicated in upwelling and exchange of water in fjords. In one temperate fjord, strong winds oxygenated and cooled water deeper than 100 m. Deep temperature minima accompanied by oxygen maxima are a common feature in fjords, yet do not have a ready prediction. Here it is shown that the dominant process driving the upwelling is wind-driven transport divergence at the head of the fjord, as opposed to local convection due to cooling or mechanical mixing. Previous work has focused on two-layer approximations, but those fail to differentiate the depth of upwelling if the bottom layer is continuously stratified. Simulations with a constant stratification shows that the depth that the densest water comes from is proportional to , where L is the length of the fjord or the horizontal scale of the wind, is the wind stress, and N 2 is the buoyancy profile. The time scale of the upwelling is similarly scaled as . These scalings apply over a wide range of forcings and geometry, so long as the depth scale does not approach the depth of the fjord. A simple procedure can be used to get a similar scaling if N ( z ) is not constant. If the wind is allowed to relax, the simulations come back to rest after a vigorous seiche, subducting the upwelled water. Despite strong turbulence, the net exchange with water outside the fjord is found to be small, and oxygen concentrations are mostly modified by air-sea gas exchange rather than diapycnal mixing.