Date of Award
Doctor of Philosophy in Oceanography
D. Randolph Watts
The observed water motions in the Japan/East Sea (JES) and the associated sea surface height and bottom pressure fields are heavily influenced by the semi-enclosed nature and the specific configuration of this marginal sea. As part of the United States Office of Naval Research JES program, a two-dimensional array of pressure-gauge-equipped inverted echo sounders (PIESs) was deployed in the southwestern JES for two years, from June 1999 to July 2001. The PIESs recorded hourly vertical acoustic travel time and pressure, which are respectively good proxies of baroclinic and barotropic sea level variability. Three topics are investigated based on the PIES data sets: (1) implications of the in-situ measurements for improving interpretation of satellite altimetric data in the JES; (2) basin-mode oscillations in the JES; (3) vertical coupling between upper circulation and abyssal eddy fields in the JES.
All the pressure records exhibit a strong remarkably similar signal (common mode). The common mode is driven by sea level changes outside the JES, atmospheric pressure, and along strait wind stress in the straits connecting the JES to the outside ocean. The common mode has a barotropic wavelength much larger than the size of the JES, so it is in phase throughout the basin. The rms of the common mode is about 5 cm, and is energetic at time scales of 2-70 days, which are shorter than the ERS-2 satellite altimetry Nyquist period of 70 days. Our results show the common mode produces a substantial alias when sampled by satellite altimeter; furthermore, the combined aliasing effects on multi-tracks can mimic mesoscale eddies and may qualitatively alter the synoptic mapping. The alias can be suppressed by removing the common mode from satellite SSH. For time periods other than 1999-2001, 78% of the common mode variance can still be removed in the Japan/East Sea by using coastal tide gauge data to infer the common mode.
High frequency oscillations with period around 7 hours are shown to be organized in a fundamental basin mode in the JES. The semi-enclosed nature of the JES not only accounts for the existence of the common mode signal but also limits energy propagation away from the basin and constrains its free oscillations to discrete modes. The fundamental oscillation in the JES is determined by its boundaries and topography. Influenced by the rotation of the earth, the oscillation consists of a single amphidromic point around which the high water propagates counter-clockwise. It has a period of 7 hours with along-coast wavelength equal to the circumference of the basin. The basin oscillations have largest amplitude at the narrow northeast region of the JES. The time series of basin oscillations is modulated in packets with time scales of 2-16 days, and they also exhibit a seasonal modulation. Wind in the direction 60°/240°T is optimal to generate basin oscillations in the Japan/East Sea.
Vertical-coupled patterns between upper sea surface height and abyssal pressure field are investigated. The coupled mode analysis confirms that bottom topography has a significant influence on the vertical coupling in the Japan/East Sea. In the first coupled mode, the deep response flows are largely expressed on closed potential vorticity regions created by the Ulleung Basin depression or the Korea Plateau; the upper layer exhibits a migration of the Ulleung Warm Eddy and downstream shift of a meander trough of the Subpolar Front. In the second mode, the upper and deep layer have similar spatial patterns peaked at a site near the southwestern corner of the Korea plateau. The second mode appears to arise from time-varying strength of a mean deep anticyclone, which had been revealed by intermediate °oat data, °owing around the contours of the Korea Plateau.
Xu, Yongsheng, "ANALYSES OF SEA SURFACE HEIGHT, BOTTOM PRESSURE AND ACOUSTIC TRAVEL TIME IN THE JAPAN/EAST SEA" (2006). Open Access Dissertations. Paper 1601.