Scale-dependent analysis of in situ observations in the mesoscale to submesoscale range around New Caledonia

Abstract

Abstract. Small-scale ocean dynamics around New Caledonia (22° S) in the Southwest Pacific Ocean occur in regions with substantial mesoscale eddies, complex bathymetry, complex intertwined currents, islands, and strong internal tides. Using second order structure functions applied to observational ADCP and TSG dataset, these small-scale dynamics are characterised in the range of scales 3–100 km in order to determine the turbulent regime at work. A Helmholtz decomposition is used to analyse the contribution of rotational and divergent motions. A surface intensified regime is shown to be at work south and east of New Caledonia, involving substantial rotational motions such as submesoscale structures generated by mixed layer instabilities and frontogenesis. This regime is however absent north of New Caledonia, where mesoscale eddies are weaker and surface available potential energy is smaller at small scales. North of New Caledonia and below 200 m in the regions south and east of New Caledonia, the dynamical regime at work could be explained by stratified turbulence as divergent and rotational motions have similar contribution, but weakly nonlinear interaction between inertia gravity waves are also possible as structure functions get close to the empirical spectrum model for inertia gravity waves. Seasonal variations of the available potential energy reservoir, associated with a change in the vertical profile rather than in horizontal density variance, suggest that submesoscale motions would also seasonally vary around New Caledonia. Overall, a loss of geostrophic balance is likely to occur at scales smaller than 10 km, where the contribution of divergent motions become significant.

Deep Eddy Kinetic Energy in the Tropical Pacific From Lagrangian Floats

“…This metric has been largely inspired by structure functions (Babiano et al., 1985; Sérazin et al., 2020), developed to compute the scale dependence of kinetic energy from irregular data sampling (Morel & Larcheveque, 1974). They have been successfully applied to Lagrangian float measurements to study turbulent energy cascades (Balwada et al., 2016; McCaffrey et al., 2015).…”