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 acoustic Doppler current profiler (ADCP) and thermosalinograph (TSG) datasets, these small-scale dynamics are characterised in the range of scales of 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 is 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.

Bi‐Directional Energy Cascades in the Pacific Ocean From Equator to Subarctic Gyre

“…With research vessels sailing at a speed of ∼5 m/s, the ADCP velocity signals have a spatial resolution of 1 ∼ 2 km. Many investigators have in the past utilized the ADCP data to separate balanced flows from unbalanced motions and explored their relative contributions to the KE wavenumber spectra at different oceanic regions (Callies et al., 2015; Callies & Ferrari, 2013; Chereskin et al., 2019; Qiu et al., 2017; Rocha et al., 2016; Sérazin et al., 2020). A consensus resulting from these studies is that the balanced flows in the meso‐submesoscale range has a KE spectral slope of $${k}^{-2}\sim {k}^{-3}$$ as a result of interior and surface‐intensified baroclinic instability, where $$k$$ is horizontal wavenumber.…”

Regional modeling of internal tide dynamics around New Caledonia: energetics and sea surface height signature

“…Callies et al, 2015;Rocha et al, 2016). This was explicitly shown for New Caledonia by Sérazin et al (2020) where the authors attributed the increasing importance of mixed layer instabilities and frontogenesis to more available potential energy in winter months. Superinertial processes dominate subinertial motions in both seasons at scales below 160-180 km, but are more pronounced in summer.…”

“…This is partly in accordance with the global analysis from Qiu et al (2018), where the SSH-derived transition scale may be lowered by up to 60-80 km in the Southwestern Tropical Pacific when removing the coherent tide. The in-situ analysis from Sérazin et al (2020) reveals a general transition scale as low as 10 km, where divergent motions take over rotational dynamics.…”

Internal tides vertical structure and steric sea surface height signature south of New Caledonia revealed by glider observations

“…Insight into fine-scale dynamics around New Caledonia is given by a unique set of glider surveys undertaken in the period from 2011 to 2014 (Durand et al, 2017). One of these glider missions surveyed the region of high internal tide activity south of New Caledonia, which is also a region of high mesoscale eddy activity (Keppler et al, 2018) and submesoscale activity (Sérazin et al, 2020). Disentangling balanced from unbalanced motions (mesoscale and submesoscale features from internal waves) in this area is a challenge of particular interest in the context of the Surface Water Ocean Topography (SWOT) satellite altimetry mission and the SWOT Adopt-A-Crossover (AdAC) initiative (d 'Ovidio et al, 2019;Morrow et al, 2019).…”