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Abstract:
The subpolar Southern Ocean is characterized by the strong thermohaline gradient separating the onshore-upwelling warm/salty Lower Circumpolar Deep Water (LCDW) and the offshore-descending cold/fresh Dense Shelf Water (DSW). Although mixing of these distinct water masses is thought to play a significant role in transforming water masses ultimately into the Antarctic Bottom Water, it remains unclear what mixing processes are actually involved in this region. As one potential process of lateral mixing, here I will present anomalous double-diffusive interleaving observed off Vincennes Bay, the moderate DSW formation site in East Antarctica. Vertical CTD profiles obtained in the subpolar region exhibit O(100)-m thick density-compensated inversions below 2000-m depth such that warm/salty LCDW-like water intrudes into ambient cold/fresh DSW-like water. Behavior of the density stability ratio as well as microstructure data suggests that diffusive and salt-finger instabilities are quite active at the upper and lower interfaces of the warm/salty intrusions, respectively, in contrast to weak salt-finger activity for the ambient stratification. Interestingly, the O(100)-m thickness of the intrusive layers are well represented by the so-called Chen scale, the height through which an intrusive buoyant fluid element can rise in a given density stratification, which is consistent with double-diffusive intrusions in a “developed” stage obtained from previous side-wall heating laboratory experiments. Similarities to previous studies and implications for the observed interleaving will be discussed in more details.
