Open AccessVertical eddy diffusion and nutrient supply to the surface mixed layer of the Antarctic Circumpolar Current
Author(s) -
Law C. S.,
Abraham E. R.,
Watson A. J.,
Liddicoat M. I.
Publication year - 2003
Publication title -
journal of geophysical research: oceans
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.67
H-Index - 298
eISSN - 2156-2202
pISSN - 0148-0227
DOI - 10.1029/2002jc001604
Subject(s) - pycnocline , mixed layer , eddy diffusion , stratification (seeds) , atmospheric sciences , diffusion , surface layer , geology , wind shear , environmental science , wind speed , mechanics , turbulence , climatology , physics , oceanography , chemistry , layer (electronics) , thermodynamics , germination , botany , dormancy , biology , organic chemistry , seed dormancy
Dispersion of the tracer sulphur hexafluoride (SF 6 ) during the Southern Ocean Iron Enrichment Experiment (SOIREE) provided an estimate of vertical exchange at the base of the surface mixed layer (60 m) at 61°S 140°E. Budget analysis confirmed that the SF 6 patch was well constrained by surface mapping, with the decline in total SF 6 showing good agreement with that predicted from wind speed parameterizations. Two approaches were used to calculate the mean effective vertical diffusivity Kz from the diapycnal transfer of SF 6 , with complementary error function and second‐moment fits to the SF 6 depth profiles indicating that Kz was less than 0.3 × 10 −4 m 2 s −1 . This result was examined using a three‐dimensional diffusion model that incorporated lateral dispersion and air‐sea exchange losses, which confirmed that vertical shear and subpycnocline dispersion did not influence the Kz estimate. Current shear at the base of the mixed layer was generated by wind‐driven inertial oscillation, with a decrease in wind speed and increasing stratification in the latter half of the experiment reducing diapycnal transfer of SF 6 . A compilation was used to examine the potential of both N (Brunt‐Väisäla frequency) and Ri (gradient Richardson number) on the basis of parameterizations of Kz in the seasonal pycnocline. Application of Kz to nutrient gradients in the seasonal pycnocline suggests that vertical diffusion represents a significant pathway for silicic acid supply in late summer. Furthermore, use of the mean effective Kz (0.11 ± 0.2 × 10 −4 m 2 s −1 ) indicates that vertical diffusion supplies a large proportion of the iron required for new production in this region.