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

The tropical Pacific Ocean surface current system can be characterized by a strong degree of nonstationarity due to the fast response time of equatori al and near-equatorial dynamics. The ocean-atmospheric dynamics create long itudinally coherent zonal flow (zonal length scales l(x) similar to 60 degr ees) with strong meridional shear (l(y) similar to 1 degrees in latitude) i n the large-scale mean and an energetic mesoscale (O(100 km)) component. Pa rameterization of the effects of the mesoscale field depends on the separat ion of the large-scale mean from the observed velocity. In this paper the f ocus is placed on the key issue: separating the flow into large-scale mean and mesoscale eddy components in order to compute meaningful eddy diffusivi ty estimates in flow regimes that demonstrate strong currents and strong sh ear. Large gradients in the large-scale mean have precluded diffusivity est imation by traditional binning techniques. In this first of two publication s, a method is developed for using Lagrangian data to estimate the diffusiv ity addressing the inhomogeneity of the mean flow. The spatially dependent estimate of the mean field is computed with a least squares bicubic smoothi ng spline interpolation scheme with an optimized roughness parameter which guarantees minimum energy in the fluctuation field at low frequencies. Nume rical simulations based on a stochastic model of a turbulent shear flow are used to validate our approach in a conceptually simple but realistic scena rio. The technique is applied to near-surface drifter observations obtained from 1979-1946 from two dynamically distinct time-space regions of the tro pical Pacific Ocean. The first region, in the South Equatorial Current, is characterized by a linear zonal shear mean flow and an approximately expone ntial autocovariance structure in the residuals. The velocity residuals hav e velocity variance of (s) over cap(2) = 130 cm(2) s(-2) for both component s, and horizontal diffusivities are <(kappa)over cap>(u) approximate to 7 X 10(7) cm(2) s(-1) and <(kappa)over cap>(v) approximate to 3 X 10(7) cm(2) s(-1). 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 mea n flow with a strong zonal shear and a weak northward velocity. The autocov ariance is approximately exponential for the zonal component, while the mer idional component has a negative lobe at about 10 days, probably due to the presence of instability waves. The variance is 380 cm(2) s(-2) for the zon al component and 360 cm(2) s(-2) for the meridional component, while the ho rizontal diffusivities are <(kappa)over cap>(u) approximate to 15 X 10(7) c m(2) s(-1) and <(kappa)over cap>(v) approximate to 4 X 10(7) cm(2) s(-1). S trong intraseasonal variability requires a maximum time window of 2 months for approximate stationarity to hold for the covariance calculations.