## 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 l_{x} ∼ 60°) with strong meridional shear (l_{y} ∼ 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 ŝ^{2} = 130 cm^{2} s^{-2} for both components, and horizontal diffusivities are κ̂_{u} ≈ 7 × 10^{7} cm^{2} s^{-1} and κ̂_{v} ≈ 3 × 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 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 cm^{2} s^{-2} for the zonal component and 360 cm^{2} s^{-2} for the meridional component, while the horizontal diffusivities are κ̂_{u} ≈ 15 × 10^{7} cm^{2} s^{-1} and κ̂_{v} ≈ 4 × 10^{7} cm^{2} s^{-1}. Strong intraseasonal variability requires a maximum time window of 2 months for approximate stationarity to hold for the covariance calculations.

Original language | English (US) |
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Article number | 1998JC900009 |

Pages (from-to) | 30855-30871 |

Number of pages | 17 |

Journal | Journal of Geophysical Research: Oceans |

Volume | 103 |

Issue number | C13 |

DOIs | |

State | Published - Dec 15 1998 |

## ASJC Scopus subject areas

- Geochemistry and Petrology
- Geophysics
- Earth and Planetary Sciences (miscellaneous)
- Space and Planetary Science
- Atmospheric Science
- Astronomy and Astrophysics
- Oceanography