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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).

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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.