Observations of mixed layer deepening during an Antarctic gale

Lane-Serff, G.F. and Stansfield, K.L. (2013) Observations of mixed layer deepening during an Antarctic gale. Journal of Geophysical Research: Oceans, 118 (3), 1396-1404. (doi:10.1002/jgrc.20140).

Abstract

Observations of mixed layer deepening made during a gale in February 2005 near an ice shelf, Fimbulisen, Antarctica, are reported. The observations were made from the RRS James Clark Ross in the lee of the ice shelf, using repeated downcasts (“yo-yo”) of a conductivity-temperature-depth package, together with shipboard meteorological and other measurements. The mixed layer deepened from less than 40?m to over 120?m over the course of 27?h, with a very rapid deepening from 80?m to 120?m over a period of under 11?h. The mixed layer became both colder and fresher, with the change in salinity and heat content likely to be caused by melting ice. Oxygen isotope results suggest the source of the fresh water was melting sea ice rather than precipitation or ice shelf melt. The input of melt water at the surface stabilizes the mixed layer, so extra energy is required to deepen the mixed layer. The observations suggest that approximately 1.8% of the available “wind-work” energy was used to mix the upper water column, while the stabilizing surface buoyancy flux inhibits the turbulence in the mixed layer, limiting the mixing length to 1.6?m. The eventual depth of the mixed layer is in line with estimates based on the planetary length scale u*/f. The rate of mixed layer deepening is given by Ue/u*?=?0.035. The apparent peak ice melting rate was approximately 60?mm?hr?1, although this is likely to be exaggerated by convergence and downwelling.

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