Formation and Variability of Antarctic Bottom Water off Cape Darnley: the Fourth Antarctic Bottom Water
Kay I Ohshima1, Yoshihiro Nakayama2, Yasushi Fukamachi3, Yoshimasa Matsumura1, Sohey Nihashi4, Takeshi Tamura5, Yujiro Kitade6, Daisuke Hirano7, Daisuke Shimizu7 and Shigeru Aoki1, (1)Hokkaido University, Sapporo, Japan, (2)Jet Propulsion Laboratory, Pasadena, CA, United States, (3)Hokkaido University, Institute of Low Temperature Science, Sapporo, Japan, (4)Tomakomai National College of Technology, Hokkaido 059-1275, Japan, (5)NIPR National Institute of Polar Research, Tachikawa, Tokyo, Japan, (6)Tokyo University of Marine Science and Technology, Tokyo, Japan, (7)National Inst. of Polar Resea., Tachikawa, Tokyo, Japan
Abstract:
It has been recognized that Antarctic Bottom Water (AABW) is formed in the Weddell Sea, the Ross Sea and off the Adélie Coast. Although a fourth variety of AABW has been identified in the eastern sector of the Weddell-Enderby Basin, its production has never been observed. Satellite-derived estimates of sea-ice production suggest that the Cape Darnley Polynya, located northwest of the Amery Ice Shelf, has the second highest ice production. We have conducted mooring observations since 2008 off and in the Cape Darnley Polynya, and revealed that this polynya is the missing source of the AABW. Moored instruments observed overflows of newly formed AABW, bottom-intensified, cascading down the canyon. Cold and dense AABW of about 300m thick flows down at fairly regular periods of 4-5 days. We propose to name this AABW Cape Darnley Bottom Water (CDBW). We estimate that 0.3−0.7 × 106 m3 s-1 of dense shelf water is transformed into CDBW. The CDBW migrates westward and increases its volume, to ultimately constitute part of the AABW in the Weddell Sea, ~15-30% of the Atlantic AABW production. Detailed understanding of descending pathways of dense water, importance of topographic depression and sea-ice formation for the CDBW production, and mechanisms responsible for the periodic downslope flow are difficult to be understood only from observations. Thus, we have also conducted numerical simulations using a nonhydrostatic model. The model is forced by surface salt flux due to sea ice production, estimated from AMSR-E data and a heat budget calculation. We have reproduced the CDBW formation and associated periodic downslope flow of dense water. Intense sea ice formation is the most important factor in the CDBW formation. Together with simplified experiments and analytical interpretations, the mechanism responsible for the periodic downslope flow of dense water is further analyzed. The period of dense water outflow is regulated primarily by the topographic beta effect.
