Tidal Enhancement of Antarctic Dense Shelf Water Export via Suppression of Continental Slope Mixing

Abstract Mixing and transformation of dense shelf water (DSW) descending the Antarctic continental slope play a key role in the formation of Antarctic Bottom Water (AABW) that fills the global ocean abyss. This transformation may be anticipated to be enhanced by tides via increased near-bottom shear, yet some studies indicate that tides suppress the transformation of DSW. This study investigates the mechanisms of tidal influences on AABW formation using in situ observations, process-oriented simulations, and a conceptual model. It is shown that tidally induced near-bottom mixing is confined within DSW, thereby contributing little to the transformation of DSW. In particular, the ebb tide plays a key role in suppressing DSW transformation, mainly through two mechanisms: First, the tidal flow advects DSW downslope quickly, shortening the duration of DSW mixing on the continental slope; second, the shorter cross-slope transit time results in weaker vertical shear, and thus less mixing entrainment at the upper interface of the DSW layer. These mechanisms explain the suppressed DSW transformation and the formation of denser AABW during the spring tide yet are not currently included in overflow parameterizations in global ocean models.