Inverse energy cascade in ocean macroscopic turbulence: Energy transfer rate ε and Richardson-Obhukov constant g from an surface drifter experiment in the Benguela upwelling system
<p>We derive the energy transfer rate &#949; from the 3<sup>rd</sup> order relative (longitudinal) &#160;velocity structure function <&#916;u<sub>l</sub><sup>3</sup>>=(3/2)&#949;s from ocean surface drifter trajectories in the turbulent mixed layer of the Benguela upwelling region off the coast of Namibia.&#160; Combination with the &#160;mean squared pair separation<s<sup>2</sup>(t)> =g&#949;t<sup>3 </sup>reveals the Richardson-Obhukov constant g&#8773;0.5, which is remarkably close to the one measured in &#160;controlled two-dimensional turbulent flows in laboratory. We verify the &#160;two coupled &#160;cascades of energy (upscale/inverse) and enstrophy (downwscale) by&#160; the &#160;theoretically predicted &#160;slope 1 &#160;for <&#916;u<sub>l</sub><sup>3</sup>> for inertial scales (above the injection scale) and slope 2 for &#160;the 2<sup>nd</sup> order structure function <&#916;u<sub>l</sub><sup>2</sup>> for non-local scales (below the injection scale) respectively. We detect&#160; additional 'ballistic contributions' in the central regime of the corresponding probability distribution P(st) of relative separations s for fixed time t, leading to an additional &#160;power law factor s<sup>-&#945;</sup> with&#160; &#945; &#8773; 5/3. The algebraic decay with 1<&#945; <2 revives &#160;to the relevance of Levy distributions in the stochastic description of the turbulent transport process in contrast to former claims. Our findings &#160;of a positively skewed &#160; probability distribution P(&#916;u<sub>l</sub>s) of relative longitudinal velocity &#916;u<sub>l</sub>&#160; for inertial scales s renews the question of intermittency in the &#160;inverse energy cascade.</p>