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JOURNAL OF GEOPHYSICAL RESEARCH,
VOL. 110,
C11001,
doi:10.1029/2004JC002417,
2005
A comparison of hydrographically and optically derived mixed layer depths
David G. Zawada
WET Labs, Inc., Philomath, Oregon, USA
J. Ronald V. Zaneveld
WET Labs, Inc., Philomath, Oregon, USA
Emmanuel Boss
School of Marine Sciences, University of Maine, Orono, Maine, USA
Wilford D. Gardner
Department of Oceanography, Texas A&M University, College Station, Texas, USA
Mary Jo Richardson
Department of Oceanography, Texas A&M University, College Station, Texas, USA
Alexey V. Mishonov
Department of Oceanography, Texas A&M University, College Station, Texas, USA
Abstract
Efforts to understand and model the dynamics of the upper ocean would be significantly advanced given the ability to rapidly
determine mixed layer depths (MLDs) over large regions. Remote sensing technologies are an ideal choice for achieving this
goal. This study addresses the feasibility of estimating MLDs from optical properties. These properties are strongly influenced
by suspended particle concentrations, which generally reach a maximum at pycnoclines. The premise therefore is to use a gradient
in beam attenuation at 660 nm (c660) as a proxy for the depth of a particle-scattering layer. Using a global data set collected during World Ocean Circulation
Experiment cruises from 1988–1997, six algorithms were employed to compute MLDs from either density or temperature profiles.
Given the absence of published optically based MLD algorithms, two new methods were developed that use c660 profiles to estimate the MLD. Intercomparison of the six hydrographically based algorithms revealed some significant disparities
among the resulting MLD values. Comparisons between the hydrographical and optical approaches indicated a first-order agreement
between the MLDs based on the depths of gradient maxima for density and c660. When comparing various hydrographically based algorithms, other investigators reported that inherent fluctuations of
the mixed layer depth limit the accuracy of its determination to 20 m. Using this benchmark, we found a ∼70% agreement between
the best hydrographical-optical algorithm pairings.
Received 5
April
2004;
accepted 7
July
2005;
published 1
November
2005.
Keywords: optical mixed layer depth algorithms;
optical scattering.
Index Terms: 4572 Oceanography: Physical: Upper ocean and mixed layer processes; 4264 Oceanography: General: Ocean optics (0649); 4568 Oceanography: Physical: Turbulence, diffusion, and mixing processes (4490); 4594 Oceanography: Physical: Instruments and techniques; 4275 Oceanography: General: Remote sensing and electromagnetic processes (0689, 2487, 3285, 4455, 6934).
Read Full Article (file size: 568930 bytes) Cited by
Citation: Zawada, D. G., J. R. V. Zaneveld, E. Boss, W. D. Gardner, M. J. Richardson, and A. V. Mishonov
(2005),
A comparison of hydrographically and optically derived mixed layer depths,
J. Geophys. Res.,
110,
C11001,
doi:10.1029/2004JC002417.
Copyright 2005 by the American Geophysical Union.
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