Inferring upwelling rates in the equatorial Atlantic using Be measurements in the upper ocean

Ocean upwelling rates are difficult to measure because of the relatively small velocities involved, and therefore are typically inferred from indirect methods such as heat budget estimates or tracer observations. Here we present the first results using a novel technique, based on the isotope ⁷Be, to infer rates of upwelling along the equator. Beryllium-7 (half-life=53.3 d) is a cosmic-ray produced radioactive nuclide that is deposited by rainfall upon the ocean surface and subsequently enriched and homogenized within the mixed layer. Previous investigations have utilized the penetration of characteristically high mixed layer concentrations into the upper thermocline to trace ocean ventilation and subduction over seasonal timescales. Here, the tracer is used in a reverse sense; that is, the ⁷Be concentration in the usually ⁷Be-rich surface mixed layer will be diluted from penetration of ⁷Be “dead” water upwelled from below. This dilution provides a means to infer upwelling rates. Furthermore, with knowledge of upwelling rates, ⁷Be profiles can be used to constrain vertical diffusivity within the upper thermocline. These ideas were tested with samples collected during the Tropical Atlantic Climate Experiment (TACE) cruise (May 22-June 27, 2009). The observations indicated a nearly linear relationship between ⁷Be inventory and mixed layer temperature, as with increased upwelling, lower mixed layer temperatures correspond to greater ⁷Be dilution from depth. With this data, upwelling rates were estimated at a number of stations near the equator between 0°E and 30°W within and adjacent to the equatorial cold tongue. The derived upwelling rates ranged from 0 to 2.2 m/d, with maximum values found between the equator and 2°S. The corresponding K_z values derived for the upper thermocline were in the range 1-4×10⁻⁴ m²/s.