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The zonal structure and time variability of the abyssal flow (below 3,000 dbar) in the South Atlantic western boundary is investigated using a combination of moored observations and simultaneous hydrographic/velocity sections at 34.5 degrees S during 2009-2018. Moored direct velocity measurements near the bottom show strong variability with a peak-to-peak range exceeding 80 cm/s and dominant signals at times scales of 1-2 months. Daily time series of the meridional absolute geostrophic volume transport computed from the moorings reveals a highly energetic record with a temporal standard deviation of 8.3 Sv and peak-to-peak variations of 49 Sv, suggesting a significant contribution of the abyssal layer flows to the Deep Western Boundary Current time variability. The absolute transport is mostly driven by barotropic changes that are dominated by variations in the bottom pressure similar to 650 km away from the continental slope.

Plain Language Summary The coldest, densest waters of the world's oceans sink near Antarctica to depths greater than 3,000 m and flow into other basins along the complex seafloor of the abyss. These waters play a key role in redistributing heat and carbon throughout the globe on time scales of centuries to millennia and therefore have a profound impact on the Earth's climate. The gateway northward into the Atlantic Ocean is along the western edge of the South Atlantic. This study presents 9 years of unprecedented, continuous observations of the abyssal flow across 34.5 degrees S off the South American coast together with deep current velocity measurements collected during eight oceanographic cruises in the region since 2009. By combining these observations, we provide a general picture of the abyssal circulation in the region and show that, rather than being slowly changing, it rapidly and strongly varies on time scales as short as 1-2 months.