Emmy T. Y. Chang

Seafloor secrets revealed

Satellite data reveal formerly unknown tectonic structures [Also see Report by Sandwell et al.] The trenches and ridges on Earths seafloor are shaped by tectonic processes such as seafloor spreading and plate subduction. Detailed knowledge of seafloor tectonics is lacking in many areas. The most comprehensive data come from satellite altimeters, which use the strength and waveform of the radar signal returned from the sea surface to determine the tectonic properties of the underlying seafloor. On page 65 of this issue, Sandwell et al. (1) present the latest global marine gravity and depth data from altimeter missions CryoSat-2 and Jason-1. The data reveal buried tectonic structures, for example, in the Gulf of Mexico and the South Atlantic Ocean, that help to elucidate past tectonic processes.

Internal tides recorded at ocean bottom off the coast of Southeast Taiwan

An ocean-bottom experiment consisting of an array of four ocean-bottom seismometers (OBS) was conducted off the coast of southeast Taiwan during May–July 2011. We develop comprehensive analyses of the space-time kinematics of the tidal signals recorded in the compact high-sensitivity temperature loggers (CHTL) and the OBS geophones at the ocean bottom with depths ranging from 1254 to 1610 m. The evidence suggests that internal tides are responsible for the recorded signals: baroclinic internal waves (mainly the M2 tide) are generated by barotropic tidal currents in the Luzon Strait. The internal tides exhibit gradual phase changing and irregularly fluctuating strength, leaving signatures in the CHTL as ambient temperature variations, signifying low-mode wave motions within the stratified water layers; and in OBS geophones as intermittent “tremor” agitations, signifying high-mode turbulent flows on the seafloor. The M2 internal tides across our array are found to propagate in the northeast direction at speeds ranging from 1 to 2+ m s−1. Furthermore, the internal tides are identified at the ocean-bottom based on an operational hydrodynamic hindcast/forecast model. The simulations show good agreement with the observed temperature variation on the seafloor and substantiate the vertical velocity and displacement of the water parcel driven by the internal tides. The joint detection of the temperature and tremor signals provides further information about the interactions of internal tides with the seafloor topography and the associated energy dissipation. Our results elucidate the space-time ubiquity of the internal tides at the ocean bottom, which is an important interface of dynamic oceanography.