Eddy-mean flow decomposition and eddy-diffusivity estimates in the tropical Pacific Ocean. I - Methodology

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 with strong meridional shear in the large-scale mean and an energetic mesoscale 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. 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. The first region is in the South Equatorial Current; second is in the North Equatorial Countercurrent and the North Equatorial Current. Strong intraseasonal variability requires a maximum time window of two months for approximate stationarity to hold for the covariance calculations. (Author)