[M-IS15] グローバル南極学

[MIS15-P07] Oceanic river-like flow of dense shelf water off the Cape Darnley Polynya revealed by a realistic model

*Vigan Mensah¹、Yoshihiro Nakayama^1,2、Masakazu Fujii⁴、Kay I. Ohshima^1,3 (1.Institute of Low Temperature Science, Hokkaido University、2.Engineering Faculty, Dartmouth College, NH, U.S.A、3.Arctic Research Center, Hokkaido University、4.National Institute of Polar Research)

キーワード：Dense water flow, Realistic model, Bottom Water, East antarctica

The formation of Antarctic bottom water (AABW) in the Southern Ocean generally starts by the generation of dense water on the shelf due to sea ice production in winter, followed by the overflowing of this dense shelf water (DSW) over the shelf break, and its downward flow along the continental slope to the oceanic bottom. The downslope flow typically occurs in areas with rough topography that can favor the rapid descent of DSW. Due to the dynamic properties of such flows, occurring in a time scale of a few days, and the difficulty in obtaining observation data in winter in the Southern Ocean, the details about the flow structure, its changes along the slope, and the transformation of water from DSW to AABW are not fully resolved. In a recent study, Morrison et al. (2020) demonstrated that a counter flow of modified circumpolar deep water (mCDW) occurs together with the downslope flow of Dense Shelf Water (DSW) across canyons. However, this study focused on single large-scale canyons (~100 km width). Mizuta et al. (2024), using observations from three moorings set across a narrow (~ 15 km width) canyon off the Cape Darnley Polynya region, provided some details about the flow structure and water mass transformation at that location. However, the spatial variability of the flow could not be described in that study due to the limited extent of the mooring array. Here, we develop a high-resolution model (500 m horizontal and 20 m vertical grid spacing) with a realistic configuration in the Cape Darnley region in East Antarctica, where DSW is formed and flows down the continental slope -via several canyons- to form Cape Darnley Bottom Water. We first proceeded to make a thorough comparison of the model results with the mooring observations of Mizuta et al. (2024). The modelled cross-canyon structure of hydrographic properties and current velocity as well as the time series of salinity, density, layer thickness, and the volume transport through the canyon exhibit remarkable similarities with the observations, which allows us to validate the model results. Following this, we describe the downslope flow structure from the top of the shelf break to the bottom of the continental slope, and across the whole Wild Canyon system, which includes dozens of sub-canyons with width varying from ~1 km near the shelf break to ~100 km at the bottom of the slope. The main feature of the dense water flow along the slope can be described as the way small tributaries (the narrow canyons) merge into rivers of increasingly large width (the larger canyons) before forming the main river (the largest downslope flow at the bottom of the canyon). As the flow becomes wider, the thickness of the newly formed bottom water increases as well, from about 150 m near the shelf break to more than 500 m at the bottom of the slope. One feature that distinguishes clearly the dense water downslope flow from a land-based river is the presence of counter-currents on the right-hand side of the canyons, including some of the narrowest ones. Thus, the results of Morrison et al. (2020)’s mechanism may be extended to a whole canyon system, in addition to individual, wide canyons.