Evolution of a Meander in the Kuroshio Extension Region from A Large-Scale Three-Dimensional Acoustic Tomography Experiment During Summer 1997
Gang Yuan*1, Iwao Nakano*2, Hidetoshi Fujimori*2, Toshiaki Nakamura*2, Takashi Kamoshida*3 and Akio Kaya*3
*1 Institute for Global Change Research, Frontier Research System for Global Change
*2 Japan Marine Science and Technology Center
*3 Oki Electric Industry Co., Ltd.
A 1000-km scale three-dimensional acoustic tomography experiment has been conducted with five 200-Hz ransceiver moorings in the Kuroshio Extension region during July to September 1997. The objectives of the experiment are to (1) demonstrate the capability of the tomographic techniques in monitoring the variability of the 1000km-scale three-dimensional oceanic structures (both the temperature and current fields) in the Kuroshio Extension region, (2) confirm the reliability of transceiver moorings in preparation for the next two long-term, 1000-km scale tomography experiments, in (a) the Central Equatorial Pacific Tomography Experiment (CEPTE) from December 1998 to December 2000, and (b) the Kuroshio Extension Tomography Experiment (KETE) in the Kuroshio Extension region from 2001 to 2005. In this study, we report the preliminary results of the tomography experiment. The five transceiver moorings, each equipped with a sound source and a 5-element vertical hydrophone array were deployed at 1100-m depth over a 1000-km-scale domain centered at 33N, 149E; rhe five-mooring array produced 10 horizontal connecting paths. The sound transmissions occurred. every 3 or 6 hours within a 2 hour interval relaying around the five moorings. Large-scale environmental surveys (XCTD, CTD, XBT and ship-boarded ADCP) were conducted along several acoustic transmission sections during the deployment and recovery cruises. Linear stochastic inverse techniques are used to invert the time data for the sound speed field. A range-average sound speed profile was determined from NODC data and used as a reference in the inversion analysis. For sound speed perturbations dC, empirical orthogonal functions (EOFs) and a truncated Fouier series were chosen to represent vertical and horizontal structure, respectively. The a priori variance of dC was determined from NODC data. The 2-D temperature field along the 1000-km slice derived from acoustic data is consistent with that the determined from XCTD measurements during the deployment cruise, by which the Kuroshio front is shown at 35N, 145E. The four-dimensional temperature fields determined from acoustic data dynamically show the large variabilities (path fluctuations, and cold and warm eddies formation) associated with the meander of the Kuroshio Extension front. These results are well consistent with other measurements such as ADCP, XCTD and TOPEX / POSEIDON satellite altimetry. It is, thus, concluded that four-dimensional oceanic structures of 1000-km scale, which are difficult targets for the conventional point measurement techniques, can be measured by the ocean acoustic tomography with a real-time data telemetry.
