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 distribution is almost unknown. 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 entire water column, with 1/4° horizontal resolution at annual frequency during the 1993 – 2019 period, available on both depth and isopycnal levels. Since the geostrophic LVB-derived vertical velocities only capture the geostrophic component of the vertical velocity, their performance is tested using an OGCM numerical model data against the total vertical flow. In the upper ocean, the LVB accurately reproduces the annual variability and captures the climatology of the large-scale total vertical flow (for scales larger than 5° horizontal resolution) with errors below 50 % across the major ocean gyres. OLIV3 capability to estimate vertical velocities in different ocean circulation regimes is assessed against three reference datasets: two reanalysis-based and one observation-based product. A strong spatial and temporal correlation is evidenced between OLIV3 and reanalysis datasets, in contrast to the observation-based product, demonstrating even higher correlation than within themselves and proving that while the geostrophic components of the reanalyses are highly correlated, the ageostrophic part is not. OLIV3 also reconstructs a baroclinic vertical velocity field, consistent with the basin oceanographic concept of Sverdrup balance theory. Regarding one of the most common applications of vertical velocities, the transfers between the surface and thermocline, results from an OGCM simulation show that the baroclinic geostrophic vertical velocity is a better estimator of the temporal variability of the vertical flow in the ocean interior than Ekman pumping, and it is essential to consider the variability of the horizontal transport. The OLIV3 dataset developed in this study is available on 50 vertical levels (https://doi.org/10.5281/zenodo.16981061; Cortés-Morales and Lazar, 2025a) and 71 isopycnal levels (https://doi.org/10.5281/zenodo.16962780; Cortés-Morales and Lazar, 2025b).