Recent changes and distribution of the newly-formed Cape Darnley Bottom Water, East Antarctica

Hydrographic top-to-bottom CTDs and profiling float profiles were collected during 2003-2020 periods in the south-west Indian Ocean sector of the Antarctic margin. Those calibrated dissolved oxygen, temperature and salinity records were used to document the distribution, pathway and changes of the newly-formed Cape Darnley Bottom Water (CDBW). We found that the newly-formed CDBW, with high fractions (50-90%) in the bottom 300 m, primarily appeared on the continental slope (64.5 degrees-70 degrees E) and the abyssal ocean (57 degrees-64.5 degrees E) in the eastern Cooperation Sea. Those distributions of CDBW fractions suggested a pathway originating from the Cape Darnley Polynya (CDP) and descending down Wild and Daly Canyons, and finally accumulating in the northwest abyssal ocean. The newly-formed CDBW presented decadal changes in potential temperature, salinity, dissolved oxygen, and neutral density between 2003-2006 and 2013-2020. It showed significant cooling (similar to 0.098 degrees C/decade), freshening (similar to 0.013/decade), increasing dissolved oxygen (similar to 11.8 mu mol/kg/decade) rates on the continental slope just off CDP, while warming (0.018-0.038 degrees C/decade), freshening (0.0044-0.0055/decade), decreasing density (0.009-0.014 kg/m(3)/decade) and increasing dissolved oxygen (4-12 mu mol/kg/decade) in the Daly Canyon. The most likely explanation of those high change rates is the enhanced CDBW formation caused by increased cascading Dense Shelf Water (DSW) plumes in Daly Canyon, and subsequently enhanced entrainment of warmer mid-depth Modified Circumpolar Deep Water (MCDW). In the northwest abyssal plain, the potential temperature and dissolved oxygen show little change while the freshening and more buoyant signals remain considerable, which might be influenced by the changes of AABWs from different sources. Finally, we relate the recent changes of newly-formed CDBW to the Sea Ice Production (SIP) change in the CDP. The SIP in the CDP shows significant increasing trends (0.52 m/decade) during 2002-2020. It increased by 12.5% during this period, which indicates an increased formation of regional DSW driven by the sea-ice formation and associated brine rejection, thus increasing formation of CDBW. Those processes were likely responsible for the decadal changes of the recently-formed CDBW.

Automated summary

This study analyzes hydrographic CTD and float data collected from the southwest Indian Ocean sector of the Antarctic margin between 2003 and 2020 to investigate the distribution, pathway, and changes of newly-formed Cape Darnley Bottom Water (CDBW). The results show that CDBW appears primarily on the continental slope and abyssal ocean in the eastern Cooperation Sea, with high fractions in the bottom 300 m. The formation of CDBW is influenced by factors such as sea ice production and cascading Dense Shelf Water plumes.