Lagrangian versus Eulerian spectral estimates of surface kinetic energy over the global ocean

In this study, we carried out a novel massive Lagrangian simulation experiment derived from a global 1/48° tide-resolving numerical simulation of the ocean circulation. This first-time twin experiment enables a comparison between Eulerian (fixed-point) and Lagrangian (along-flow) estimates of kinetic energy (KE), and the quantification of systematic differences between both types of estimations. This comparison represents an important step forward for the mapping of upper ocean high-frequency variability from drifter database. Eulerian KE rotary frequency spectra and band-integrated energy levels (e.g., tidal and near-inertial) are considered as references, and compared to Lagrangian estimates. Our analysis reveals that, apart from the near-inertial band, Lagrangian spectra are systematically smoother, e.g., with wider and lower spectral peaks compared to Eulerian counterparts. Consequently, Lagrangian KE levels obtained from spectra band integrations tend to underestimate Eulerian levels on average at low-frequency and tidal bands. This underestimation is more significant in regions characterized by large low-frequency KE. In contrast, Lagrangian and Eulerian near-inertial spectra and energy levels are comparable. Further, better agreements between Lagrangian and Eulerian KE levels are generally found in regions of convergent surface circulation, where Lagrangian particles tend to accumulate. Our results demonstrate that Lagrangian estimates may provide a distorted view of high-frequency variance. To accurately map near-surface velocity climatology at high frequencies (e.g., tidal and near-inertial) from Lagrangian observations of the Global Drifter Program, conversion methods accounting for the Lagrangian bias need to be developed.