Eulerian and Lagrangian statistics in the Bluelink numerical model and AVISO altimetry: Validation of model eddy kinetics

Abstract

We compare Eulerian and Lagrangian timescales, velocity autocorrelations, and spectra from 14 years of hindcast ocean model and satellite velocities with direct observations. Our main aim is to determine how well the model (Bluelink, version Bran 2.1) and satellite‐derived currents (Archiving, Validation and Interpretation of Satellite Oceanographic data (AVISO)) simulate mesoscale eddy variability, which is important in mixing heat, salinity, and momentum and in controlling dispersion. Ocean Eulerian observations are from Topex‐Poseidon (T/P) Crossover velocities, and Lagrangian observations are from Global Drifter Program (GDP) surface drifters. Eulerian spectra from Bluelink are similar to Eulerian spectra from Crossover velocities over the range of common frequencies (the ∼10‐day sampling from T/P limits the comparison to low frequencies), and Eulerian timescales from Bluelink are slightly shorter than from the observations. Part of this difference may be an effect of the T/P sampling rate. In contrast, Lagrangian spectra from Bluelink are considerably steeper than spectra from GDP drifters, and consequently, Lagrangian timescales from Bluelink are longer than from the observations. Both Eulerian and Lagrangian spectra from AVISO are steeper than from observations so that both Eulerian and Lagrangian AVISO timescales are longer than observed. The real ocean is an ocean where the Eulerian‐Lagrangian relationship described by Middleton (1985) is followed; the Tasman Sea averaged ratio of Lagrangian to Eulerian timescales is about 0.1 zonally and 0.15 meridionally. However, the Eulerian‐Lagrangian relationship described by Middleton (1985) is not obeyed by Bluelink, and we believe that this behavior in Bluelink reflects a loss of short‐wavelength eddies in the model.