The Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean (DIMES)


The Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean (DIMES) was initiated in 2009, with an aim to quantify the role of small- and large-scale turbulent mixing in the Antarctic Circumpolar Current (ACC). The DIMES field program has focused on the part of the ACC that flows from the abyssal plain in the southeastern Pacific into the topographically rough Drake Passage region and beyond. This special collection brings together analyses of new in-situ data, theory, and numerical simulations all carried out as part of DIMES. DIMES was funded by the US National Science Foundation and by the UK Natural Environment Research Council.

Collection organizers:
Sarah T. Gille, Scripps Institution of Oceanography, University of California San Diego
Raffaele Ferrari, Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology
James R. Ledwell, Woods Hole Oceanographic Institution
Alberto C. Naveira Garabato, National Oceanography Centre, University of Southampton

The Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean (DIMES)

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Marina Frants
Gillian M. Damerell
Sarah T. Gille
Karen J. Heywood
Jennifer MacKinnon
, and
Janet Sprintall


Finescale estimates of diapycnal diffusivity κ are computed from CTD and expendable CTD (XCTD) data sampled in Drake Passage and in the eastern Pacific sector of the Southern Ocean and are compared against microstructure measurements from the same times and locations. The microstructure data show vertical diffusivities that are one-third to one-fifth as large over the smooth abyssal plain in the southeastern Pacific as they are in Drake Passage, where diffusivities are thought to be enhanced by the flow of the Antarctic Circumpolar Current over rough topography. Finescale methods based on vertical strain estimates are successful at capturing the spatial variability between the low-mixing regime in the southeastern Pacific and the high-mixing regime of Drake Passage. Thorpe-scale estimates for the same dataset fail to capture the differences between Drake Passage and eastern Pacific estimates. XCTD profiles have lower vertical resolution and higher noise levels after filtering than CTD profiles, resulting in XCTD κ estimates that are, on average, an order of magnitude higher than CTD estimates. Overall, microstructure diffusivity estimates are better matched by strain-based estimates than by estimates based on Thorpe scales, and CTD data appear to perform better than XCTD data. However, even the CTD-based strain diffusivity estimates can differ from microstructure diffusivities by nearly an order of magnitude, suggesting that density-based fine-structure methods of estimating mixing from CTD or XCTD data have real limitations in low-stratification regimes such as the Southern Ocean.

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