Abstract

Spectral analysis of observed large-scale horizontal velocity fields in the atmosphere show a power law dependence of kinetic energy on spatial wavenumber k. A recent analysis of near-surface winds over the tropical western Pacific [Wikle, Milliff and Large (1999)] indicates a k ^-5/3 power law over the entire mesoscale range (10-1000 km). Both upscale quasi-2D and downscale 3D turbulence cascades can have k ^-5/3 energy spectra, but since the k ^-5/3 power law was found in the tropics where rotation and stratification effects are too weak to suppress three dimensional motions, the temptation is to assume that the near-surface mesoscale energy cascade must be 3D-downscale. On the other hand, it is possible that the strong and sweeping horizontal winds arising from the large-scale subsidence into the marine boundary layer in the tropics could give rise to a dimensionality constraint resulting in a quasi-2D upscale cascade. To investigate this issue we have calculated second- and third-order velocity structure functions from QuikSCAT measured near-surface winds over the Pacific Ocean (40N to 40S, and 150E to 110W), covering the period January 2000 to December 2004. Latitude variations, regional variations (east vs west Pacific), as well as variability in time have been investigated. Calculations of the third-order structure function yield results consistent with a down-scale energy transfer in the tropical western Pacific and the possibility of an up-scale energy transfer in the tropical eastern Pacific. In addition, the third- order structure function shows an interesting spatial oscillation arising from events along the ITCZ in early autumn.

This work was carried out in collaboration with R M Kerr and K C Pien.