Subsurface imbalance stimulated in a mesoscale eddy. Part I: Observations

Abstract

This study presents observational evidence of a stimulated imbalance below the mixed layer from a fast-tow cruise survey with approximately 4 km resolution that traverses a cyclonic mesoscale eddy in the central South China Sea. The observed physical and biochemical properties reveal both geostrophic and ageostrophic physics in the mesoscale eddy. In addition to the well-known eddy-induced features, banded structures of temperature and chlorophyll anomaly across the section indicate unbalanced motions. These features are notably active near the eddy's edge and show larger amplitudes of variance than those induced by the mesoscale eddy. The results also show remarkable unbalanced variances of temperature and salinity, as well as lateral temperature-salinity compensation on isopycnals, likely due to stirring associated with internal waves. Spectral analysis suggests that there exist significant unbalanced variances of temperature and chlorophyll at ∼13 km. Our further analysis suggests that symmetric instability may have been stimulated due to strong lateral buoyancy gradients and vertical shear of velocity, which may account for the imbalance signature in the pycnocline. The observed imbalances point to the instability due to the interaction between mesoscale, submesoscale, and internal wave dynamics, which may play an important role in catalyzing the forward energy cascade in the ocean.