Salt‐Fingering Under the Thermal‐Wind and Lateral Shears in the Kuroshio Fronts

We apply a linear normal mode stability analysis to identify instability in fronts stable to symmetric instability (SI). SI stable fronts appear in many of our observations in the Kuroshio current. These fronts consist of strong horizontal and vertical temperature and salinity gradients. Our stability analysis considers salt‐fingering instability in the presence of thermal‐wind and lateral shears along the isopycnal, which we term “geostrophic shear salt fingering (GSF).” When GSF grows faster than SI, the energy source is in the geostrophic and lateral shears, similar to SI. The results of the linear stability analysis are applied to evaluate temperature and salinity observations on the east coast of Japan in the Kuroshio current. The majority of our data do not support the growth of SI, as the measurements were taken in the fall when the fronts were stratified. Instead, our data indicate a preference for GSF growth. The total perturbation growth in GSF is determined by the source and sink terms in the scalar variance dissipation rates. The thermal and saline covariance dissipation rate (codissipation rate) serves as the primary energy source, while the thermal and saline variance dissipation rates function as the energy sink. This suggests that the presence of the GSF is detectable in microstructure measurements as patches of high thermal variance dissipation rate along the isopycnal slope at fronts in the absence of SI. Plain Language Summary We wanted to improve our understanding of the density structure at oceanic fronts, where two distinct bodies of water interact. The density of the ocean is determined by two constituents, namely temperature, and salinity. However, what complicates our understanding is the fact that these two constituents have vastly different diffusion rates. The faster diffusion of heat can leave salt behind when warm, salty water is overlying above cool, fresh water at rest. The salt that is left behind makes the warm, salty water heavier than the cool, fresh water, altering the equilibrium state. This is known as double diffusion. Such conditions are found in the ocean, particularly near the equator and mid‐latitudes, where evaporation and solar heating tend to increase the temperature and salinity of surface water relative to the water below. One of the primary drivers of mixing at an oceanic front is the mechanical energy associated with the ocean current, which is sustained by the horizontal density gradient of the oceanic fr