Acoustic mapping of mixed layer depth

Christian Stranne,Martin Jakobsson,Larry Mayer,Katarina Gårdfeldt,Thomas C Weber,Göran Björk,Elizabeth Weidner,Johan Nilsson,Leif G Anderson,Kevin Jerram

doi:10.5194/os-14-503-2018

Abstract

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.

Highlights

The surface mixed layer is an important and nearly universal feature of the world oceans
Acoustic water column data were collected throughout the Arctic Ocean during two expeditions with Swedish Icebreaker (IB) Oden: Leg 2 of the Swedish–Russian–US Arctic Ocean Investigation of Climate–Cryosphere–Carbon Interactions 2014 Expedition (SWERUS-C3) and the Arctic Ocean 2016 Expedition (AO2016)
We categorize CTD stations where EK80 data are available into three classes (Fig. 1): black indicates that a mixed layer is present in the CTD data and the MLD is visible in the EK80 data; red indicates that a mixed layer is present in the CTD data but the MLD is not visible in the EK80 data; and yellow indicates that a mixed layer is not present in the CTD data

Summary

Introduction

The surface mixed layer is an important and nearly universal feature of the world oceans. It is defined as a quasihomogeneous layer that extends from the surface down to the penetration depth of turbulent mixing, generated by wind stress and buoyancy fluxes at the air–sea interface (Kraus and Turner, 1967; Price et al, 1986). The MLD is an important parameter within several atmospheric and oceanographic research disciplines as the transfer of mass, momentum, and buoyancy across the mixed layer provides the source of almost all oceanic motions (de Boyer Montégut et al, 2004).

Methods

Results

Discussion

Conclusion