Flow structures of the benthic ocean

Abstract

Three‐dimensional structure of the near‐bottom density field was observed with a towed yo‐yoing profiler and a fixed current/temperature measuring array on the Hatteras abyssal plain. A great variety of structures were seen. Immediately above the bottom a well‐mixed bottom layer extends vertically 5–60 m, with less than 1 m°C potential temperature change. This mixed layer is often capped by a region of strong vertical potential temperature gradient, with up to 100‐m°C potential temperature change in ∼10 m. The boundary layer may be uniform for 10 km or exhibit a bottom temperature gradient of up to 20 m°C/km. Interior layers of nearly constant potential temperature and horizontal extent of 2–100 km are seen ∼25% of the time above the bottom mixed layer. When an interior layer is present, the bottom mixed layer is thinner. On many occasions an interior layer was seen to be horizontally continuous with the bottom mixed layer, suggesting formation of interior layers by detachment of the bottom mixed layer. A benthic front was observed. Differential horizontal advection is required to explain the observed structures. Velocity fluctuations above 1 cph increase in energy near the bottom, presumably a signature of turbulence in the mixed layer; these fluctuations are modulated by the passage of structures observed in the moored record.