Mixed layer analysis of a mesoscale eddy in the Antarctic Polar Front Zone

Abstract

Microstructure, acoustic Doppler current profiler, and conductivity‐temperature‐depth (CTD) profiles were taken in a mesoscale eddy in the Antarctic Polar Front Zone at about 2°15′E, 49°15′S during the R/V Polarstern cruise ANT XXI/3 within the scope of the European Iron Fertilization Experiment in January–March 2004. The mixed layer depth (MLD), calculated from the composite of CTD‐ and microstructure sonde (MSS)‐derived data, was 97.6 ± 20.6 m. No significant correlation between the wind work ( E 10 ) and the MLD ( r = 0.02 to 0.22) was found. However, the analysis revealed a negative correlation between the surface buoyancy flux ( B ) and the MLD 1/2 d later. Two approaches were used to estimate the actively mixing layer depth (AMLD). First, the actively mixing layer was determined subjectively by analyzing the MSS‐derived density, energy dissipation, and Thorpe scale profiles, and second, the mixed layer model embedded in a general circulation model was used. The overall mean of the determined depths of the actively mixing layer (AMLD MSS = 66.4 ± 28.8 m) agreed with the model‐predicted boundary layer depths (BLD) (BLD KPP = 69.1 ± 29.5 m), but the individual values sometimes were differing considerably. We deduced estimates of the vertical diffusivity ( K z ) from the MSS‐derived energy dissipation rates and Thorpe scales. Both methods showed that K z decreased with depth from order of magnitude 10 −1 m 2 s −1 in the actively mixed layer to order of 10 −4 m 2 s −1 in the pycnocline.