When mixed layers are not mixed. Storm-driven mixing and bio-optical vertical gradients in mixed layers of the Southern Ocean
Mixed layers are defined to have homogeneous density, temperature, and salinity. However, bio-optical profiles may not always be fully homogenized within the mixed layer. The relative timescales of mixing and biological processes determine whether bio-optical gradients can form within a uniform dens...
Published in: | Journal of Geophysical Research: Oceans |
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Format: | Article in Journal/Newspaper |
Language: | English |
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2018
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Online Access: | https://doi.org/10.1029/2018JC014416 |
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ftncar:oai:drupal-site.org:articles_22182 2023-09-05T13:19:12+02:00 When mixed layers are not mixed. Storm-driven mixing and bio-optical vertical gradients in mixed layers of the Southern Ocean Carranza, Magdalena M. (author) Gille, Sarah T. (author) Franks, Peter J. S. (author) Johnson, Kenneth S. (author) Pinkel, Robert (author) Girton, James B. (author) 2018-10-01 https://doi.org/10.1029/2018JC014416 en eng Journal of Geophysical Research: Oceans--J. Geophys. Res. Oceans--21699275 articles:22182 ark:/85065/d7xs5zcv doi:10.1029/2018JC014416 Copyright 2018 American Geophysical Union. article Text 2018 ftncar https://doi.org/10.1029/2018JC014416 2023-08-14T18:49:08Z Mixed layers are defined to have homogeneous density, temperature, and salinity. However, bio-optical profiles may not always be fully homogenized within the mixed layer. The relative timescales of mixing and biological processes determine whether bio-optical gradients can form within a uniform density mixed layer. Vertical profiles of bio-optical measurements from biogeochemical Argo floats and elephant seal tags in the Southern Ocean are used to assess biological structure in the upper ocean. Within the hydrographically defined mixed layer, the profiles show significant vertical variance in chlorophyll-a (Chl-a) fluorescence and particle optical backscatter. Biological structure is assessed by fitting Chl-a fluorescence and particle backscatter profiles to functional forms (i.e., Gaussian, sigmoid, exponential, and their combinations). In the Southern Ocean, which characteristically has deep mixed layers, only 40% of nighttime bio-optical profiles were characterized by a sigmoid, indicating a well-mixed surface layer. Of the remaining 60% that showed structure, approximate to 40% had a deep fluorescence maximum below 20-m depth that correlated with particle backscatter. Furthermore, a significant fraction of these deep fluorescence maxima were found within the mixed layer (20-80%, depending on mixed-layer depth definition and season). Results suggest that the timescale between mixing events that homogenize the surface layer is often longer than biological timescales of restratification. We hypothesize that periods of quiescence between synoptic storms, which we estimate to be approximate to 3-5 days (depending on season), allow bio-optical gradients to develop within mixed layers that remain homogeneous in density. Article in Journal/Newspaper Elephant Seal Southern Ocean OpenSky (NCAR/UCAR - National Center for Atmospheric Research/University Corporation for Atmospheric Research) Southern Ocean Journal of Geophysical Research: Oceans 123 10 7264 7289 |
institution |
Open Polar |
collection |
OpenSky (NCAR/UCAR - National Center for Atmospheric Research/University Corporation for Atmospheric Research) |
op_collection_id |
ftncar |
language |
English |
description |
Mixed layers are defined to have homogeneous density, temperature, and salinity. However, bio-optical profiles may not always be fully homogenized within the mixed layer. The relative timescales of mixing and biological processes determine whether bio-optical gradients can form within a uniform density mixed layer. Vertical profiles of bio-optical measurements from biogeochemical Argo floats and elephant seal tags in the Southern Ocean are used to assess biological structure in the upper ocean. Within the hydrographically defined mixed layer, the profiles show significant vertical variance in chlorophyll-a (Chl-a) fluorescence and particle optical backscatter. Biological structure is assessed by fitting Chl-a fluorescence and particle backscatter profiles to functional forms (i.e., Gaussian, sigmoid, exponential, and their combinations). In the Southern Ocean, which characteristically has deep mixed layers, only 40% of nighttime bio-optical profiles were characterized by a sigmoid, indicating a well-mixed surface layer. Of the remaining 60% that showed structure, approximate to 40% had a deep fluorescence maximum below 20-m depth that correlated with particle backscatter. Furthermore, a significant fraction of these deep fluorescence maxima were found within the mixed layer (20-80%, depending on mixed-layer depth definition and season). Results suggest that the timescale between mixing events that homogenize the surface layer is often longer than biological timescales of restratification. We hypothesize that periods of quiescence between synoptic storms, which we estimate to be approximate to 3-5 days (depending on season), allow bio-optical gradients to develop within mixed layers that remain homogeneous in density. |
author2 |
Carranza, Magdalena M. (author) Gille, Sarah T. (author) Franks, Peter J. S. (author) Johnson, Kenneth S. (author) Pinkel, Robert (author) Girton, James B. (author) |
format |
Article in Journal/Newspaper |
title |
When mixed layers are not mixed. Storm-driven mixing and bio-optical vertical gradients in mixed layers of the Southern Ocean |
spellingShingle |
When mixed layers are not mixed. Storm-driven mixing and bio-optical vertical gradients in mixed layers of the Southern Ocean |
title_short |
When mixed layers are not mixed. Storm-driven mixing and bio-optical vertical gradients in mixed layers of the Southern Ocean |
title_full |
When mixed layers are not mixed. Storm-driven mixing and bio-optical vertical gradients in mixed layers of the Southern Ocean |
title_fullStr |
When mixed layers are not mixed. Storm-driven mixing and bio-optical vertical gradients in mixed layers of the Southern Ocean |
title_full_unstemmed |
When mixed layers are not mixed. Storm-driven mixing and bio-optical vertical gradients in mixed layers of the Southern Ocean |
title_sort |
when mixed layers are not mixed. storm-driven mixing and bio-optical vertical gradients in mixed layers of the southern ocean |
publishDate |
2018 |
url |
https://doi.org/10.1029/2018JC014416 |
geographic |
Southern Ocean |
geographic_facet |
Southern Ocean |
genre |
Elephant Seal Southern Ocean |
genre_facet |
Elephant Seal Southern Ocean |
op_relation |
Journal of Geophysical Research: Oceans--J. Geophys. Res. Oceans--21699275 articles:22182 ark:/85065/d7xs5zcv doi:10.1029/2018JC014416 |
op_rights |
Copyright 2018 American Geophysical Union. |
op_doi |
https://doi.org/10.1029/2018JC014416 |
container_title |
Journal of Geophysical Research: Oceans |
container_volume |
123 |
container_issue |
10 |
container_start_page |
7264 |
op_container_end_page |
7289 |
_version_ |
1776200018279006208 |