Is Interleaving in the Agulhas Current Driven by Near-Inertial Velocity Perturbations?

Abstract

Recent observations taken at a number of latitudes in the Agulhas Current reveal that the water mass structure on either side of its dynamical core is distinctly different. Moreover, interleaving of these distinct water masses is observed at over 80% of the stations occupied in the current, particularly within the subsurface density layer between tropical surface water and subtropical surface water masses, and within the intermediate layer between the Antarctic Intermediate Water and Red Sea water masses. Direct velocity measurements allow for a comparison between the characteristic vertical length scales of the Agulhas intrusions and those of velocity perturbations found throughout the current. It is found that the interleaving scales match those of the velocity perturbations, which are manifest as high-wavenumber vertical shear layers and are identified as near-inertial oscillations. Furthermore, the properties of the intrusions indicate that double diffusion is not an important process in their development: they are generally not associated with a density anomaly, their slope and thickness fall outside the predicted maxima for instability, and a strong horizontal shear field acts to separate water parcels more quickly than intrusions would be able to grow by double-diffusive processes. Instead, the position, thickness, and slope of Agulhas intrusions relative to the background salinity and density field suggest that they are forced by rotating inertial velocities, with subsequent growth possibly driven by small-scale baroclinic instabilities. However, not all the evidence points conclusively toward advectively driven intrusions. For instance, there is a discrepancy between the observed salinity anomaly amplitude and the predicted inertial displacement given the background salinity gradient, which deserves further examination. Hence, there is a future need for more pointed observations and perhaps the development of an analytical or numerical model to understand the exact nature of Agulhas intrusions.

Characterizing Thermohaline Intrusions in the North Pacific Subtropical Frontal Zone

“…The lack of linear correlation highlights the integrating nature of the interleaving, whereby the intrusion amplitude is determined by its advective history. Beal (2007) reports a similar lack of correlation between intrusions and the velocity field in the Agulhas Current.…”

“…Various modifications of DD theories have dominated interpretations of intrusion observations for the past 30 years (e.g., Kuzmina and Rodionov 1992;Ruddick 1992;MK97;Smyth 2008). A number of field observations, however, have reported slopes and scales for the intrusive features that are inconsistent with the ''active'' theories (Kuzmina et al 2005;Beal 2007). Moreover, it is uncertain whether the classical DD instabilities exist in the presence of typical oceanic levels of turbulent mixing (Zhurbas and Oh 2001;Edwards and Richards 2004).…”

Wind-driven submesoscale subduction at the north Pacific subtropical front

“…Following a strong storm, during which wind stress and oceanic heat loss were especially large, intrusions appeared both within the surface mixed layer and beneath the seasonal pycnocline (Figures c and d). Similar frontal features have been previously observed and attributed to double diffusive interleaving [ Gregg , ], submesoscale turbulence [ Shcherbina et al ., ], and advection by near‐inertial waves [ Beal , ]. To our knowledge, no previous observations have shown intrusions to occur both above and below the seasonal pycnocline.…”

The Production and Dissipation of Compensated Thermohaline Variance by Mesoscale Stirring

“…Double-diffusive instabilities can be substantially damped by background internal wave or geostrophic shears (Kunze 1994;Beal 2007). Similarly, it is unclear whether mesoscale strain modulation of the internal wave filed can trigger substantial wave breaking.…”