Internal solitary waves on the NW African shelf: A heuristic approach to localize diapycnal mixing hotspots

Highlights

• Internal solitary waves (ISWs) are ubiquitous over the Moroccan shelf.

• ISWs increase Thorpe displacements and dissipation rates in the water column.

• ISWs contribute to the overall mixing processes.

• An echosounder is used as a proxy of stratification to derive the Richardson number.

Abstract

Turbulent mixing on continental shelves plays roles in the structure and dynamics of marine ecosystems, nutrient cycling, primary production and dispersion of pollutants. Describing and understanding internal wave dynamics enables improved mapping of mixing over continental shelves, especially in complex environments where many processes may interact, such as in upwelling systems. This paper describes internal wave propagation and dissipation in the Moroccan upwelling system using a comprehensive set of hydrographic observations made during two in situ surveys. The acoustic backscatter of the echosounder is shown to be a rapid and convenient survey tool for detecting internal solitary waves (ISWs) on large spatial scales, through the high-frequency oscillation of the zooplankton layer during nighttime conditions. Along ISW trains, enhanced diapycnal mixing episodes are observed with increased dissipation coefficients in the thermocline (O(10^-7 W kg^-1)), associated with overturning up to 6 m. Mixing due to internal wave soliton packets shows large spatial and temporal variability, but contributes to the overall mixing processes and is responsible for large intermittent variations in the thermocline position. The joint use of a multifrequency echosounder and a current profiler ADCP allows mixing to be quantified (via the Richardson number) on large spatial scales. Validation and use of this method in other coastal regions could be useful to determine regional mixing parameterization in numerical models.

Introduction

Estimation of turbulent mixing on the continental shelf is important for understanding the structure and dynamics of marine ecosystems, nutrient cycling, primary production and dispersion of pollutants (Nash et al., 2004; Pineda et al., 2020; Rippeth et al., 2005; Schafstall et al., 2010). Diapycnal mixing in the ocean is mostly driven by intermittent patches of small-scale turbulence (Müller and Briscoe, 2000). Internal waves (IWs) act as an intermediate process that transfers energy from large-scale forcing (wind and tide) to fine-scale mixing at vertical scales of a few meters.

A number of observational studies have shown that IWs are ubiquitous in the stratified ocean (Chang et al., 2021; Garrett and Munk, 1975, 1979; Helfrich and Melville, 2006; Holloway, 1987), and cover a wide spectrum, from the inertial frequency to the Brünt-Väïsälä frequency, with a vertical displacements of 1–100 m. Internal tides are generated through the interaction of the barotropic tide with the topography over the continental slope and shelf (Baines, 1973; Garrett and Kunze, 2007; Llewellyn Smith and Young, 2002; St. Laurent and Garrett, 2002; Wunsch, 1975). They are highly sensitive to the ambient stratification, which controls their amplitude. Gerkema (1996) showed that internal tides generated at the shelf break tend to evolve nonlinearly and give rise to shorter-scale internal solitary waves (ISWs) as they propagate nearshore. They are associated with intense vertical shear potentially leading to super-critical Richardson numbers (a measure of the relative importance of the stabilizing effect of stratification and destabilizing effect of vertical shear). Hence, they can trigger shear instability and intense diapycnal mixing.

The signature of internal waves consists in a vertical displacement of the thermocline and pycnocline and these waves are commonly observed using mooring temperature time series. However, the intermittency of this process can make it difficult to detect them using occasional one-off measurements. Several studies have shown that methods based on acoustic data, commonly used in fisheries studies, make it possible to track ISWs (Apel, 2003; Moum et al., 2003; Sandstrom et al., 1989). Sandstrom et al. (1989) showed that a significant increase in acoustic backscatter is observed in the layers, generated by the passage of ISWs.

Over the past four decades, a number of studies have focused on quantifying the mixing induced by IWs (Jones et al., 2020; Lamb, 2014; MacKinnon et al., 2017; Moum et al., 2003; Palmer et al., 2015; Sandstrom and Oakey, 1995; Vic et al., 2019). However, the complex interactions of internal waves with topography mean that understanding the energy dissipation processes and quantifying IW mixing rates on the continental shelf is not an easy task and remains an active area of research (Grados et al., 2016; Hamann et al., 2018; Nash et al., 2012; Zulberti et al., 2020). As pointed out by Schafstall et al. (2010), a general parameterization of mixing for such areas is still lacking. Understanding and mapping mixing processes induced by ISWs over continental shelves is of prime importance for the parameterization of their effects in regional numerical models.

Here we present a study combining hydrological and hydrodynamical datasets, including those collected with an Acoustic Doppler Current Profiler (ADCP), multifrequency echosounder (EK60) and conductivity/temperature/depth probe (CTD), to detect ISWs and assess the increased diapycnal mixing they trigger on the NW African shelf.

Modeling studies of IW activity and dissipation emphasized that the Moroccan shelf is a global hotspot for IWs (de Lavergne et al., 2019; Garrett and Kunze, 2007). The Moroccan continental margin is characterized by: 1/the presence of a slope break between the continental shelf and the abyssal plane with a bathymetry rising from 2000 m to 180 m over a length of 12.8 km; 2/a well-marked thermocline; 3/a semi-diurnal tide (micro-tidal regime), with an average tidal range of 90 cm; 4/a Coriolis parameter f = 6.83 x 10^-6 s^-1; and 5/a maximum buoyancy frequency N around 5 cycle.h^-1 close to the value observed in the north of the system near Gibraltar (Vlasenko et al., 1996). These conditions are favorable to the generation of internal tides and their evolution into solitons (Gerkema, 1996; Yuan et al., 2020). A persistent annual upwelling subject to seasonal fluctuations is observed over the Moroccan shelf. Upwelling of cold water can be clearly seen in the sea surface temperature map in Fig. 1a (MODIS-Aqua; monthly composite). This upwelling is stronger in summer, associated with the trade-wind migration (Cropper et al., 2014). Like all upwelling regions, the Moroccan upwelling system is subject to intense mesoscale and submesoscale activity. Capet et al. (2017) suggested that the intense mixing driven by ISWs might play a key role in the upwelling dynamics. Observational evidence of ISWs on the NW African shelf was reported by Capet et al. (2017), Schafstall et al. (2010) and Arístegui et al. (2009). They found average turbulent dissipation rates of 4.9 x 10^-8 W.kg^-1 on the Mauritanian continental slope (Schafstall et al., 2010) and values occasionally reaching O(10^-7 W kg^-1) on the Senegal shelf (Capet et al., 2017). However, the origin of mixing is difficult to identify over the Moroccan shelf, as many processes may contribute and interact: upwelling fronts, submesoscale filaments, large-scale canary current, tide and ISWs.

The present work describes a comprehensive set of hydrological and hydrodynamical observations collected on the Moroccan shelf. To our knowledge, the impact of ISWs has not been documented in this area. The methods and instruments used to detect ISWs are described in section 2. Section 3 then presents actual ISW activity and its effects on mixing. Finally, we discuss the validity of these observations in section 4.