Global thermocline vertical velocities: a novel observation based estimate

Abstract

Vertical velocities at large scales are crucial for understanding ocean dynamics, influencing large-scale circulation and associated biochemical processes, yet their rationale is poorly understood, and their three-dimensional mean distribution and temporal variability are mainly known by models. This paper introduces OLIV3 (Observation-based LInear Vorticity Vertical Velocities), a novel observation-based estimation product of vertical velocities over the global thermocline. This product relies on the geostrophic linear vorticity balance (LVB) applied to ARMOR3D observation-based meridional velocities with ERA5 Ekman pumping vertical velocity as surface boundary condition. It covers the water column over 71 isopycnal levels, with 1/4° horizontal resolution at annual frequency during the 1993–2019 period. Since the geostrophic LVB-derived vertical velocities only capture the geostrophic component of the vertical velocity, their performance is tested using ocean general circulation model (OGCM) data against the total vertical flow. In the thermocline, the LVB accurately reproduces the interannual variability and captures the climatology of the large-scale total vertical flow (horizontal scales larger than 5°) with errors below 50 % across the major ocean gyres. Focusing on surface-thermocline exchanges, one of the most common applications that needs vertical velocities, OGCM results indicates that baroclinic geostrophic vertical velocities are largely more accurate than the classic Ekman pumping proxy at estimating the interannual variability of the total vertical flow in the ocean interior. OLIV3 capability to estimate real ocean vertical velocities is assessed against three reference datasets: two reanalyses and the observation-based product OMEGA3D. A strong spatial and temporal correlation is evidenced between OLIV3 and reanalysis datasets, in contrast to the OMEGA3D, demonstrating even higher correlation than within themselves and supporting the dominance of the geostrophic component of interannual variability of vertical movements. OLIV3 also reconstructs a baroclinic vertical velocity field, consistent with the basin oceanographic concept of Sverdrup balance theory. By building on theoretical advances made since the introduction of Sverdrup and Ekman transport theories, OLIV3 provides a simplified yet physically consistent estimate of large-scale vertical transport. The OLIV3 dataset developed in this study is available at https://doi.org/10.5281/zenodo.16962780.