Identification

Title

When mixed layers are not mixed. Storm-driven mixing and bio-optical vertical gradients in mixed layers of the Southern Ocean

Abstract

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.

Resource type

document

Resource locator

Unique resource identifier

code

http://n2t.net/ark:/85065/d7xs5zcv

codeSpace

Dataset language

eng

Spatial reference system

code identifying the spatial reference system

Classification of spatial data and services

Topic category

geoscientificInformation

Keywords

Keyword set

keyword value

Text

originating controlled vocabulary

title

Resource Type

reference date

date type

publication

effective date

2016-01-01T00:00:00Z

Geographic location

West bounding longitude

East bounding longitude

North bounding latitude

South bounding latitude

Temporal reference

Temporal extent

Begin position

End position

Dataset reference date

date type

publication

effective date

2018-10-01T00:00:00Z

Frequency of update

Quality and validity

Lineage

Conformity

Data format

name of format

version of format

Constraints related to access and use

Constraint set

Use constraints

Limitations on public access

None

Responsible organisations

Responsible party

contact position

OpenSky Support

organisation name

UCAR/NCAR - Library

full postal address

PO Box 3000

Boulder

80307-3000

email address

opensky@ucar.edu

web address

http://opensky.ucar.edu/

name: homepage

responsible party role

pointOfContact

Metadata on metadata

Metadata point of contact