Lateral diffusivity and Lagrangian scales in the Pacific Ocean as derived from drifter data

Abstract

The Global Drifter Program/Surface Velocity Program data set is used to derive maps of the lateral diffusivity and Lagrangian scales of velocity, length and time across the whole Pacific. For the lateral diffusivity, the minor principal component of both the Davis' diffusivity tensor and the half growth rate of the single-particle dispersion tensor were calculated in 5° x 5° bins from drifters for the period 1979-1999. Estimates of the lateral diffusivity, K, obtained by means of the minor principal component approach, are shown to correspond well with other authors' estimates for the Pacific. The highest values of K (2-3 104 m2/s) are found in the eastern equatorial Pacific, while the lowest ones (2-3 103 m2/s) are typical for forty-odd degrees in latitude northward of the subpolar fronts in both hemispheres and for the subtropics in the eastern part of the South Pacific. High values of K (about 1 x 104 m2/s) are also observed in the Kusoshio/Kuroshio Extension area and in the equatorial Pacific along the whole length of the equator. There are two approximately symmetrical tongues of high K value in the western Pacific centered around 21°N and 24°S, respectively. In general, K has a tendency to increase westward (eastward) in the midlatitudes (equatorial region) and toward the equator in both hemispheres. In the midlatitudes, the Lagrangian length scale L is found to be approximately equal to the first-mode baroclinic radius of deformation, Ri, while the Rossby number defined as Ro = Tearth/T sin, where T is the Lagrangian timescale, Tearth is the period of the Earth's rotation, and is the latitude, is less than unity, implying that the quasigeostrophic mesoscale eddies are the main mechanism for lateral mixing. The lateral diffusivity in the midlatitudes is suggested to be parameterized as K = V2Ri, where V2 is the Lagrangian velocity scale inherent to a mesoscale eddy field. In the low latitudes, L becomes less than Ri, and the condition Ro ≤ 1 is no longer valid, which implies the change of the eddy-controlled mechanism of lateral mixing by another mechanism.