Oscillating dense plumes

The flow of dense polar shelf waters down continental slopes is a critical component of the global ocean circulation. Recent observations suggest that such plumes can be heavily impacted by tidal variability, and many of the world's important dense-water sources are located in tidally active areas. Tides affect the source of dense water (by modulating the location of hydrographic gradients) and control the subsequent plume mixing and flow path. In an effort to separate these effects, dense plumes are modeled here by extending a classical one-dimensional plume model to two unsteady scenarios in which the plume path is fixed. The first case features a pulsed release of dense water into a stagnant ambient, and the model predicts that gravity waves propagate down the plume. Advective waves in plume density travel with the mean velocity of the current (U) over bar and thus have a wavelength of (U) over barP, the product of plume velocity and the oscillation period P. The second case is of a steady-sourced plume flowing through an ambient that has uniformly oscillating flow. This drives fluctuating shear at the plume-ambient interface (and/or seabed) that leads to variable entrainment of ambient fluid into the plume. Perturbed properties are subsequently advected by the plume, leading to standing "entrainment waves" that also have a wavelength of (U) over barP. Pulsed-source effects may be distinguished from variable-entrainment effects by the phase difference between waves in the different state variables of each plume. Both effects are maximized when the ratio (U) over barP/L approximate to 1, where L is the plume length. This condition is satisfied in the Ross Sea, Antarctica, where observations show dense plumes that are strongly affected by tides. Modeled pulsed-source effects qualitatively agree with the observations, implying that hydrographic variability in Ross Sea plumes is associated with variability in their dense-water source rather than unsteady plume mixing. These results might help inform the gathering and interpretation of oceanographic data in tidally active dense-water source regions

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