Estimating mixed layer depth from oceanic profile data
JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY, 20 (2):
319--329 (2003)Abstract
Estimates of mixed layer depth are important to a wide variety of
oceanic investigations including upper-ocean productivity, air-sea
exchange processes, and long-term climate change. In the absence
of direct turbulent dissipation measurements, mixed layer depth is
commonly derived from oceanic profile data using threshold, integral,
least squares regression, or other proxy variables. The different
methodologies often yield different values for mixed layer depth.
In this paper, a new method-the split-and-merge (SM) method-is introduced
for determining the depth of the surface mixed layer and associated
upper-ocean structure from digital conductivity-temperature-depth
(CTD) profiles. Two decades of CTD observations for the continental
margin of British Columbia are used to validate the SM method and
to examine differences in mixed layer depth estimates for the various
computational techniques. On a profile-by-profile basis, close agreement
is found between the SM and de facto standard threshold methods.
However, depth estimates from these two methods can differ significantly
from those obtained using the integral and least squares regression
methods. The SM and threshold methods are found to approximate the
"true'' mixed layer depth whereas the integral and regression methods
typically compute the depth of the underlying pycnocline. On a statistical
basis, the marginally smaller standard errors for spatially averaged
mixed layer depths for the SM method suggest a slight improvement
in depth determination over threshold methods. This improvement,
combined with the added ability of the SM method to delineate simultaneously
ancillary features of the upper ocean (such as the depth and gradient
of the permanent pycnocline), make it a valuable computational tool
for characterizing the structure of the upper ocean.