Baroclinic Instability Induced Mesoscale and Submesoscale Processes in River Plumes: A Laboratory Investigation on a Rotating Tank

Under the influence of buoyancy and the earth rotation, river outflows leaving an estuary typically have a two-part structure, including a recirculating bulge near the river mouth and a coastal current propagating downstream. A continuous river outflow causes the bulge to expand in size and to accumulate freshwater, with the bulge eventually becoming unstable due to baroclinic instability. We conducted a series of laboratory experiments on a rotating tank to simulate an idealized river plume under various Coriolis frequencies, density differences, and shelf slopes. The horizontal velocity structures of the river plume were qutantified using particle imaginary velocimetry (PIV). We designed a velocity-based vortex identification and tracking algorithm to capture anticyclonic eddies (ACEs) at the bulge center, cyclonic eddies (CEs) on the plume front, and coastal cyclonic return flow (CCRF) at the corner between estuary and coastal wall. Our results suggest a linear relationship between bulge instability parameter and bulge wavenumber, which can be used to predict the bulge instability patterns that are classified according to vortex stretching, splitting, and squeezing. Finally, we estimated the eddy kinetic energy contained within ACEs, CEs, and CCRF to explore the cross-scale energy transfer and dissipation. Our results show that the bugle is more unstable in gentle slope cases, and its instability decreases with the inflow Rossby number and increases with the Froude number. The generation of CEs on the plume front may extract the energy from the larger scale anticyclonic core, which plays an important role in mass transport and frontal mixing of river plumes.