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 Kappa (rho) 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. Plain Language Summary High-resolution observations of solitary waves in the ocean interior show the instability induced by the roll-up of the water layer and the waveform breaking around the wave core. The occurrence of roll-up depends on the ratio between the magnitude of stratification and velocity shear, with a critical value of 1/4. If the magnitude of the shear is 4 times larger than that of the stratification, roll-up of the water layer occurs. Our observations reveal a balanced status within the wave core; that is, statistically, the value of the above ratio oscillates around 1/4. This is caused by the shear-induced roll-up of the water layer when the ratio is <1/4, which can eventually evolve into turbulence. The turbulence then weakens the shear and stratification and consequently brings the ratio back to >1/4. This cyclic process could preserve the interior waves against breakdowns resulting from uninterrupted roll-ups. Key Points Marginal instability is evident in the cores of internal solitary waves The observations suggest that the critical value for the occurrence of shear instability in Internal solitary waves (ISWs) is 0.25 Marginal instability is a cyclic process that preserves ISWs against breakdowns resulting from uninterrupted Kelvin-Helmholtz (KH) instability growth