The Deep‐Water Plume in the Northwestern Weddell Sea, Antarctica: Mean State, Seasonal Cycle and Interannual Variability Influenced by Climate Modes

We provide an updated estimate of the annual‐mean, seasonal cycle and interannual variability of the transports and properties of the Weddell Sea Bottom Water (WSBW) plume in the northwestern Weddell Sea. For this we used a densely instrumented mooring array deployed across the continental slope between January 2017 and January 2019. We found that the annual‐mean WSBW transport is 3.4 ± 1.5 Sv, corresponding to a cross‐section area of 35 km2 and a maximum thickness of 203 m. The annual mean transport‐weighted properties of WSBW are −0.99°C (Θ), 34.803 g/kg (SA) and 28.44 kg/m3 (γn). The WSBW is characterized by 3 bottom‐intensified velocity cores, which display seasonal variations in flow speed and transport different varieties of WSBW. The seasonal peak of WSBW transport and density is reached in May (4.7 Sv, 28.443 kg m−3) while the minimum values are observed in February (2.8 Sv, 28.435 kg m−3). The coldest WSBW is found between March and May, and the warmest between August and October. The density decrease of WSBW observed in the austral autumn of 2018 can be explained by warmer ambient waters being entrained during the formation of WSBW. This was enabled by the weakening of the along‐shore winds associated with a positive Southern Annular Mode index, reinforced by a La Niña event in early 2018. The synchronous decrease of total WSBW transport and volume between September 2018 and February 2019 indicates a reduction in the export of the dense precursors of WSBW from the Weddell Sea continental shelf.Plain Language Summary: The Meridional Overturning Circulation (MOC) redistributes heat and carbon dioxide in the world ocean. Thus, it plays an important role in the regulation of our planet's climate. The Weddell Sea is the main contributor to the deep branch of the MOC in the Southern Hemisphere. Despite the importance of this contribution, uncertainties still remain associated to the plume of dense waters transported along the continental slope of the Weddell Sea. To reduce these uncertainties, we analyzed the most densely instrumented mooring array deployed across the continental slope in the northwestern Weddell Sea. We found that this plume flows faster close to the seafloor and that it presents important seasonal and interannual variability. The Weddell Sea Bottom Water interannual variability is influenced by changes in the along‐shore winds driven by the phase of two important climate modes, the Southern Annular Mode and the El Niño‐Southern Oscillation, but also by changes in the export of the dense precursors of WSBW in its formation areas. Increasing our knowledge on the along‐slope plume variability and properties is important to better understand the causes behind the variability of the MOC observed further downstream.Key Points: The Weddell Sea Bottom Water (WSBW) plume presents 3 velocity cores and a clear seasonal cycle, with maximum transports and densities in May and minimum in February. A +SAM, reinforced by a ‐ENSO, favors the warming of WSBW via a wind‐driven warming of the ambient waters entrained during its formation. We observed a marked decrease in WSBW density and transports between September 2018 and February 2019 compared to the previous year.EU Horizon 2020 Research and Innovation ProgramGerman Research FoundationAlfred Wegener Institute Helmholtz‐Centerhttps://doi.org/10.5281/zenodo.750016