Temporal variability of measured vertical velocity over a canyon-incised slope in the Coral Sea

Abstract. Oceanic vertical velocity (w) is often neglected due to its small magnitude compared to horizontal currents. However, it plays a fundamental role in coupling the ocean surface with its interior, with major implications for a wide range of physical and biogeochemical processes. In this study, four ADCPs were deployed on fixed Eulerian moorings and collected w measurements over slightly less than one year, covering the water column from 50 to 410 m depth. Using a combined approach based on Fourier spectral analysis and Ensemble Empirical Mode Decomposition, the variance distribution was characterized. The spectral analysis highlights the fundamental anisotropy of the flow, as vertical and horizontal velocities exhibit distinctly different spectral properties. While horizontal currents are mainly constrained by topography, vertical motions are primarily driven by internal gravity waves and intermittent small-scale processes. Furthermore, a persistent w signal at synoptic scales shows that the velocity is not purely geostrophic at all temporal scales. The results show that the variability of w time series is primarily dominated by short timescales, on the order of a few days or less. The data are also strongly influenced by biologically driven diel vertical migrations, which impose a pronounced periodic signal. The overall dynamics of the study area, strongly influenced by a canyon-incised slope topography, is characterized by a prevailing weak downwelling regime (3−4 mm s^-1), intermittently disrupted by short-lived (few days) strong upwelling events (exceeding the 90^th percentile of 1-day moving averaged w data). Finally, our results suggest that the canyons could act as an efficient tracer sink (e.g., carbon), with vertical velocities reaching several mm s^-1.