Deep argo reveals bottom water properties and pathways in the Australian-Antarctic Basin

Changes in properties and quantity of Antarctic Bottom Water (AABW) have major implications for the climate system, through sequestration of heat and carbon into, and ventilation of, the abyssal ocean. Yet, it remains one of the most difficult water masses to observe. An array of 12 Deep Argo floats, capable of profiling from the surface to the seafloor and under sea ice, provides a new perspective on AABW in the Australian-Antarctic Basin. Over 2 years of data from the floats illuminate AABW properties with unprecedented detail, simultaneously sampling AABW at multiple locations, year-round, throughout the basin. Calibrating each float individually with nearby, quasi-simultaneous shipboard profiles ensures the highest quality salinity data, with estimated accuracy of ±0.005 or better. Pathways of Ross Sea and Adélie Land Bottom Water (RSBW and ALBW), defined by their unique temperature and salinity characteristics, are mapped along the continental slope from their respective sources. The main pathway of RSBW, identified by its characteristic deep salinity maximum, is inferred to be inshore of the 3,700 m isobath, where it cools and freshens westward along the slope before interacting with ALBW near 140°E. A pulse of very cold and very fresh (nearly −0.6°C, 34.82 g kg⁻¹) ALBW appears in February 2019, highlighting temporal variability on daily scales near its source. Deep Argo has greatly enhanced our view of AABW in the Australian-Antarctic Basin and will prove to be an essential tool for monitoring future changes in the deep ocean by drastically increasing observations in a cost-effective way.