Upwelling Rates and Vertical Diffusivities Determined During the 2013 US GEOTRACES Eastern Tropical Pacific Zonal Transect: Results from 7Be Analysis

Abstract:

Upwelling is an important physical process affecting biogeochemical cycling within the global ocean. Direct measurements are difficult because of the relatively small velocities involved, and must therefore be inferred by indirect methods such as those provided by tracer observations. Measurements of the cosmogenic radioisotope ⁷Be (half-life = 53.3 d) were used to derive upwelling rates and upper thermocline vertical diffusivities during the 2013 US GEOTRACES Eastern Tropical Pacific Zonal Transect. In the eastern, upwelling zone of this transect, the ⁷Be activity in the mixed layer varied between low values of 63 dpm/m³ in areas with the coldest sea surface temperatures (SSTs) to values of 190 dpm/m³ associated with warm SSTs. The ⁷Be inventory in the coldest water was only 5,150 dpm/m² while that in the warmer water was 19,000 dpm/m². The decrease in mixed layer ⁷Be with decrease in temperature occurs as ⁷Be ‘‘dead’’, cold water is upwelled from below. The deficit of the ⁷Be inventory relative to the non-upwelling stations provides a measure of the upwelling rate. For the eastern-most station with the lowest SST (17.6 deg C) an upwelling rate of 2.56 m/d was derived. Towards the west, as SST increased, derived upwelling rates decreased. At SST of 20.9 deg C, upwelling was zero. With knowledge of upwelling rates, ⁷Be and temperature profiles were used to constrain vertical diffusivity within the upper thermocline. Diffusivities derived from ⁷Be profiles were approximately twice that derived from temperature, likely reflecting the difference in vertical resolution of these measurements. Diffusivities derived from temperature were on the order of 1-4 x 10^-4 m²/s. These parameters will be applied to profiles of nutrients and trace elements to derive fluxes of these species into the mixed layer.