Marginal Instability Within Internal Solitary Waves

High‐resolution observations of the currents, hydrography, and turbulence within internal solitary waves (ISWs) reveal Kelvin‐Helmholtz (KH) billows and breaking waveforms occurring in the lower periphery and in the trailing edges, respectively, of the wave cores. Marginal instability (MI) is evident in wave cores, where the probability distribution of the Richardson number Ri , fluctuates around 0.25. The critical value for the occurrence of KH instability in ISWs is evidently 0.25, the canonical criterion of a parallel stratified shear flow. When Ri  > 0.25, the turbulent diffusivity Κ ρ remains O (10 −3 ) m 2 s −1 with a standard deviation of one order. When Ri crosses the critical value, the logarithmic turbulent diffusivity linearly increases with a logarithmic reduction in Ri to smooth the velocity and density differences so that Ri is ultimately restored to >0.25. Therefore, the processes of MI cycling preserve ISWs against breakdowns resulting from the uninterrupted growth of KH instability.