Observation of symmetric instability at a bottom attached front

Submesoscale dynamics play a key role in the oceanic energy cycle and drive material transport that shapes marine ecosystems. In this study, we present observational evidence of symmetric instabilities (SI) at the Mississippi River Plume front. The data was collected during the Submesoscales Under Near-Resonant Inertial Shear Experiment (SUNRISE), a project focused on exploring the interactions between wind-driven near-inertial oscillations, internal waves, and submesoscale dynamics in the energetically rich environment of the northern Gulf of Mexico. The observed SI occur during a transition from downwelling to upwelling winds. Downwelling winds initially push the front onshore. These winds introduce negative potential vorticity (PV), destabilizing the front with respect to submesoscale instabilities. Weak stratification and high mixing rates accompany the downwelling winds. Alternating bands of velocity and tracers suggest active SI during this period. As the winds weaken and shift to upwelling conditions, the system restratifies, yet the banded structures persist for about 36 hours. The instabilities are supported by negative PV input from the bottom boundary layer on the shoreward side of the front. The velocity bands associated with SI transport heat and oxygen along the sloping isopycnals, providing a pathway for exchange between surface and bottom waters. After approximately 36 hours, increasing upwelling winds cause the surface front to move offshore, leading to strong upper ocean stratification. These findings highlight SI as a mechanism for ventilating the bottom boundary layer, with potential impact for heat flux and oxygen transport even in the absence of direct wind forcing.