Upscale and downscale energy transfer over the tropical Pacific revealed by scatterometer winds

According to two-dimensional turbulence theory, the sign of the third-order structure function identifies the direction of energy transfer, with implying downscale transfer and upscale transfer. Using near-surface winds inferred from radar backscatter measurements by SeaWinds-on-QuikSCAT and ASCAT-on-MetOp-A scatterometers, third-order structure functions (where the subscript indicates the along-track direction) were calculated for both rainy and dry regions in the tropical Pacific. The skewness was found to asymptote to an approximate constant value when the separation variable exceeded 200 - 300 km. The time evolution of was followed using its value at 300 km, and was found to vary in regionally and seasonally in magnitude and sign. Fluxes were calculated using the third-order structure function law and split into upscale (where velocity differences ) and downscale (where ) components. The variability in magnitude and sign was shown to be due to the changing relative strength of convergence and divergence within a region. Thus our main result may be expressed as follows. Energy fluxes (i) downscale where and when surface convergence (deep convection) dominates, (ii) upscale where and when surface divergence dominates, and (iii) have both large upscale and downscale components in regions frequented by mesoscale convective systems. The link with surface convergence and divergence challenges the usual picture of mesoscale turbulence as either a 2D or 3D energy cascade.