Abstract. The ocean surface mixed layer is a nearly universal feature of the world
oceans. Variations in the depth of the mixed layer (MLD) influences the
exchange of heat, fresh water (through evaporation), and gases between the
atmosphere and the ocean and constitutes one of the major factors controlling
ocean primary production as it affects the vertical distribution of
biological and chemical components in near-surface waters. Direct
observations of the MLD are traditionally made by means of conductivity,
temperature, and depth (CTD) casts. However, CTD instrument deployment limits
the observation of temporal and spatial variability in the MLD. Here, we
present an alternative method in which acoustic mapping of the MLD is done
remotely by means of commercially available ship-mounted echo sounders. The
method is shown to be highly accurate when the MLD is well defined and
biological scattering does not dominate the acoustic returns. These
prerequisites are often met in the open ocean and it is shown that the method
is successful in 95 % of data collected in the central Arctic Ocean. The
primary advantages of acoustically mapping the MLD over CTD measurements are
(1) considerably higher temporal and horizontal resolutions and
(2) potentially larger spatial coverage.
