What's a density/gravity current anyway?

Density or gravity currents are simply currents set in motion by density differences, and (broadly speaking) where the flow is directed down the density gradient. The penetration of cold air through an open door (in the absence of external wind forcing), and the semi-diurnal shifts in sea-breeze direction are commonly experienced examples of density currents in the atmosphere. In the oceans, density currents manifest in many ways as well, including in the initial downslope flow of dense water formed in polar regions (hence playing a large role in the oceans' meridional overturning circulation);  in turbidity currents; and in the outflow of relatively fresh and buoyant river plumes. 

The SPLASH campaign in the Gulf of Mexico

As part of my work in the UCLA Marine Operations program, I participated in the 2017 "Submesoscale Processes and Lagrangian Analysis on the Shelf" (SPLASH) campaign in the Gulf of Mexico, which targeted observations of processes which may contribute to transport and fate of pollutants in the sea.  The observational campaign was part of a multi-year observational program funded in response to the 2010  Deepwater Horizon oil spill in the gulf. 

Density current propagation in a stratified environment, and interaction with internal waves

As part of the SPLASH campaign, we observed the Mississippi river outflow into the Louisiana Bight, from an array of different observational platforms and instruments.  The river outflow plume behaves as a buoyant density current, propagating offshore over the denser ambient water on the shelf. We used the observations to investigate recent theoretical predictions for the propagation speed of density currents in stratified environments (Solodoch et al, 2020). These predictions were not tested in field conditions previously.

Furthermore, we observed that internal gravity waves co-occurred with the density current front.  Detailed examination has shown that the waves initially propagated faster than the density current and bypassed it (this is called a sub-critical Froude number state) ; but that later on, waves converged to the density current front, as the velocities became more similar and eventually the wave lagged behind the front (super-critical Froude number state). To our knowledge, such a transition was not observed before. Previously, density currents transitioning from supercritical to subcritical states were observed, as the current slowed down due to spreading and dissipation. However, in the present case we have shown based on theoretical analysis, that the opposite transition occurs, most likely due to the slow down of the waves due to the shoaling of the density current in its approach to the opposite shore of the Bight region. Another finding is that the observations are consistent with a role of the internal waves in slowing down the density current in the initial phase, a phenomenon which was documented previously in several idealized numerical simulations and laboratory experiments. 

Towed instrument array transects across the Mississippi plume front. The variable shown is density. Internal gravity waves are estimated to have been faster than the frontal speed (i.e., a subcritucal state) during "period 1" (left panel). Isopycnals downwards portrusions exist at the front and ahead of it (to its right). During period 2 (center panel) waves and front had similar speeds (critical state), and hence the wave is locked to the front. During period 3 (right panel) waves were slower than the front (subcritical state). Therefore localized downward density portrusions at the front and ahead of do not appear at period 3.

Broader implications

These findings may be of dynamical relevance in other manifestations of density currents in the ocean and atmosphere (see 1st paragraph). Both mediums are in general stratified, and hence the testing of stratified density current theory, and of theoretical results on its interaction with internal waves (possible only in a stratified medium) are of relevance in many other atmosphere and ocean density-current phenomena.

Broader implications for the coastal region  may include the influence of river plume propagation speed and structure on the transport of pollutants (e.g., from oil spills or from land-sourced river-transported pollution), and of biological species and nutrients. For example, the transport mechanisms of fish larvae into estuaries are still only partially understood. 

Evolution and structure of convergence in a density current

In followup work to the analyses described above (Solodoch et al 2020, JPO), we are characterizing the convergence field and associated mixing within the buoyant plume (manuscript in preparation). Convergence estimates were constructed based on the motions of an array of 25 tracked drifters that we deployed near the density front. The rate and structure of convergence have implications for trapping and transport of matter by the density current, and to its evolution.