A Numerical Study of Submesoscale Processes Generated by Mixed Transitional Layer Instability

Abstract Mixed transitional layer instability (MTI) was proposed as a mechanism to generate deep‐penetrating submesoscale processes (submesoscales) in the upper ocean. This paper conducts a numerical validation of the MTI theory and provides a comprehensive characterization of its evolution and energetics, which is a necessary step toward identifying or parameterizing this mechanism in the real ocean. Specifically, we employ the Regional Ocean Modeling System with a 1‐km horizontal grid to simulate the evolution of a typical mesoscale front in a zonally reentrant channel. We design a 3‐layer model including mixed, transitional, and deep layers and compare its results with those from a realistically continuous model and a 2‐layer model without the transitional layer. Submesoscales occur in all three models due to the development of baroclinic instability. In the 3‐layer model, submesoscales are more energetic, with larger horizontal and vertical scales than those in the 2‐layer model, and these features are consistent with those in the continuous model and with the MTI theory. The difference between the 3‐ and 2‐layer models arises because the MTI occurring in the former can release available potential energy from both the mixed and transitional layers, leading to a larger energy injection scale and deeper influence depth than the mixed layer instability (MLI) in the latter. These differences allow MTI to transport nutrients and other tracers more effectively than MLI, with implications for biogeochemical cycles and air‐sea interaction. Overall, this study establishes a numerical basis for evaluating MTI's role in real ocean submesoscale dynamics.