Lobe‐cleft instability in the buoyant gravity current generated by estuarine outflow

Gravity currents represent a broad class of geophysical flows including turbidity currents, powder avalanches, pyroclastic flows, sea breeze fronts, haboobs, and river plumes. A defining feature in many gravity currents is the formation of three‐dimensional lobes and clefts along the front and researchers have sought to understand these ubiquitous geophysical structures for decades. The prevailing explanation is based largely on early laboratory and numerical model experiments at much smaller scales, which concluded that lobes and clefts are generated due to hydrostatic instability exclusively in currents propagating over a nonslip boundary. Recent studies suggest that frontal dynamics change as the flow scale increases, but no measurements have been made that sufficiently resolve the flow structure in full‐scale geophysical flows. Here we use thermal infrared and acoustic imaging of a river plume to reveal the three‐dimensional structure of lobes and clefts formed in a geophysical gravity current front. The observed lobes and clefts are generated at the front in the absence of a nonslip boundary, contradicting the prevailing explanation. The observed flow structure is consistent with an alternative formation mechanism, which predicts that the lobe scale is inherited from subsurface vortex structures. Key Points Lobe‐cleft instabilities occur on a river plume front in the absence of a surface stress The mechanism classically invoked for the generation of lobe‐cleft instabilities cannot apply We observe a connection between lobe vorticity and the subsurface structure