Deep western boundary current variability off northeastern Brazil

Observations from a year-long deep current meter mooring at 8°N, 52°W are used to describe the structure and variability of the Deep Western Boundary Current (DWBC) in the tropical Atlantic. The DWBC has a deep core near 4300 m depth, is extremely swift and narrow (35 cm s^-1 core speed, 60 km width), and shows a total transport of 22 Sv between 2500 m and the bottom. Approximately 3 Sv of the DWBC transport are carried in near-bottom layers below θ=1.8°C, which appears to be recirculating in the western tropical Atlantic north of about 4°N. Variations in the DWBC are dominated by large-amplitude vertical displacements of the deep temperature field at 60-70 day periodicities that cause significant changes in the deep stratification and vertical transport structure of the DWBC. These fluctuaions appear to be related to periodic surges of cold bottom waters up onto the base of the continental rise that become entrained in the DWBC. While the forcing mechanism for these deep fluctuations remains unexplained, the picture emerging from these data is that of a highly active abyssal layer in the tropical Atlantic.