Eddy‐mean flow decomposition and eddy‐diffusivity estimates in the tropical Pacific Ocean: 1. Methodology

Abstract

The tropical Pacific Ocean surface current system can be characterized by a strong degree of nonstationarity due to the fast response time of equatorial and near‐equatorial dynamics. The ocean‐atmospheric dynamics create longitudinally coherent zonal flow (zonal length scales lx ∼ 60°) with strong meridional shear (ly ∼ 1° in latitude) in the large‐scale mean and an energetic mesoscale (O(100 km)) component. Parameterization of the effects of the mesoscale field depends on the separation of the large‐scale mean from the observed velocity. In this paper the focus is placed on the key issue: separating the flow into large‐scale mean and mesoscale eddy components in order to compute meaningful eddy diffusivity estimates in flow regimes that demonstrate strong currents and strong shear. Large gradients in the large‐scale mean have precluded diffusivity estimation by traditional binning techniques. In this first of two publications, a method is developed for using Lagrangian data to estimate the diffusivity addressing the inhomogeneity of the mean flow. The spatially dependent estimate of the mean field is computed with a least squares bicubic smoothing spline interpolation scheme with an optimized roughness parameter which guarantees minimum energy in the fluctuation field at low frequencies. Numerical simulations based on a stochastic model of a turbulent shear flow are used to validate our approach in a conceptually simple but realistic scenario. The technique is applied to near‐surface drifter observations obtained from 1979–1996 from two dynamically distinct time‐space regions of the tropical Pacific Ocean. The first region, in the South Equatorial Current, is characterized by a linear zonal shear mean flow and an approximately exponential autocovariance structure in the residuals. The velocity residuals have velocity variance of ŝ2 = 130 cm2 s−2 for both components, and horizontal diffusivities are and . No significant interannual variations of the estimates are detected, but residual trends in the estimators arise from intraseasonal variations in the velocity field during the 3‐month season. The second region, in the North Equatorial Countercurrent and the North Equatorial Current, has a mean flow with a strong zonal shear and a weak northward velocity. The autocovariance is approximately exponential for the zonal component, while the meridional component has a negative lobe at about 10 days, probably due to the presence of instability waves. The variance is 380 cm2 s−2 for the zonal component and 360 cm2 s−2 for the meridional component, while the horizontal diffusivities are and . Strong intraseasonal variability requires a maximum time window of 2 months for approximate stationarity to hold for the covariance calculations.