Symmetric instability drives exchange between surface and bottom waters in a coastal front.

The coastal ocean in the northern Gulf of Mexico is highly productive and socioeconomically important. Here, stratification can inhibit vertical exchange, with consequences for ecosystem health and hypoxia. Previous work emphasized the role of wind-driven turbulent mixing as a mechanism to overcome the stratification barrier. We identify symmetric instability (SI) as an additional and more energy-efficient pathway linking surface and bottom waters. In high-resolution observations, diagonal bands of overturning motions, a telltale sign of SI, connect the sea surface to the bottom and produce intrusions of temperature and oxygen anomalies. Overturning persists for 2 days after instability-favorable wind ceases. During this time, vertical advective fluxes exceed turbulent fluxes by an order of magnitude, ventilating low-oxygen bottom waters and transporting surface heat downward. Our results show that SI facilitates vertical exchange that can outlive direct wind forcing, highlighting an instability-driven mechanism that may be important in coastal oceans more generally.