Multiscale Analysis of Temporal Ocean Turbulence Intermittency

Abstract

AbstractAn analysis of temporal intermittency in ocean turbulent energy transfer is given. Intermittency plays a major role in ocean turbulence, and the level of intermittency strongly depends on different scales of the flow. In this work, in order to understand the temporal aspects of intermittency, we extend our research to the ocean turbulent energy transfer process. Measurements of ocean turbulence are made from a moored turbulence measuring instrument (MTMI) deployed in the South China Sea. Signals related to ocean turbulence have been collected with two orthogonal shear probes at a single level for an extended period, which laid a valid foundation for the understanding of turbulent energy transfer characteristics. Our analysis of ocean turbulence data is based on the combined use of empirical mode decomposition (EMD) and of the wavelet transform method. First, a decomposition of ocean turbulent fields in a limited number of time scales is provided by the EMD method. Then, the wavelet transform method is performed to the decomposed signals to obtain the wavelet coefficients, which enables the detection of energy transfer in the flow. Finally, a novel intermittency measure based on Reynolds shear stress has been proposed to identify intermittent bursts of energy at different time scales. The new intermittency measure is estimated using the obtained wavelet coefficients and can be used as an indication of the turbulent kinetic energy transfer between two different time scales. In order to analyze the turbulent energy transfer process in detail, intermittency between the adjacent and nonadjacent time scales is presented. The results show that, in the locations where the ocean turbulent energy is being transferred between two time scales, the phase synchronization between the modes of field fluctuations occurs. This confirms that phase synchronization observed in ocean turbulence is due to the turbulent energy cascade, which helps improve our understanding of turbulent energy cascade and turbulent mixing processes in ocean dynamic systems.