Abstract
AbstractSinking dense plumes are important in many oceanographic settings, notably the polar formation of deep and bottom waters. The dense water sources feeding such plumes are commonly affected by tidal modulation, leading to plume variability on short time scales. In a simple unsteady theory of one-dimensional plumes (based on conservation equations for volume, momentum, and buoyancy), this plume variability is manifested as waves that travel down the resulting current. Using numerical techniques applied to the hyperbolic conservation equations, this study investigates the novel concept that these waves may break as they travel down the plumes, triggering intense local mixing between the dense fluid and surrounding ocean. The results demonstrate that the waves break at geophysically relevant distances from the plume source. The location of wave breaking is very sensitive to plume drag from the seabed, the properties of the dense source, and the amplitude and period of the source modulation. To the extent that the simple model represents the real world, these results suggest that wave breaking originating from the tidal modulation of dense plumes could lead to a strong and previously unexplored source of local deep-ocean mixing.
Highlights
The global oceans are filled with dense deep and bottom waters formed at the poles
In an effort to simplify the effects of tidal modulation of source conditions on the evolution of dense plumes, Holland (2011) formulated an unsteady, onedimensional model subjected to unsteady ambient flow and/or modulation of the source. ‘‘Top hat’’ profiles were assumed for the plume quantities
We compare the point of wave breaking for l 5 1/4, 1⁄2, 1, 3/2, where l 5 1 corresponds to the results of Fig. 4a. This variation in the source conditions leads to differences of less than 20% in the predicted point of wave breaking; a variation that is comparable with that found for modified values of E and u
Summary
The global oceans are filled with dense deep and bottom waters formed at the poles. Around Antarctica, dense shelf waters, formed by sea ice growth over the continental shelf seas, cascade down the continental slopes into the deep ocean, mixing with ambient waters to form the Antarctic Bottom Water that underlies the majority of the world’s oceans (Baines and Condie 1998; Orsi et al 1999). A common feature of these cascades and overflows is that they occur in relatively shallow shelf-edge regions where tidal amplitudes are large (Padman and Erofeeva 2004; Padman et al 2009) This implies that a full understanding of the formation of globally important dense water masses requires knowledge of the effects of shortperiod modulation of dense water sinking down seabed slopes. A pulsed source produces waves that travel down the plume, and simple analytical investigation of a reduced system suggests that these waves should break within 50 km of the source (Holland 2011) If such wave breaking does occur on geophysically relevant scales, it will lead to rapid, localized mixing between dense water and its ambient fluid, with important consequences for the depth and characteristics of the deep water masses produced. Establishing the well-posedness of the conservative model of a nondiffusive line plume descending a slope is a necessary prerequisite for this study
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