Analysis of Near Surface Circulation of the Japan East Sea
Pearn P. Niiler, Scripps Institution of Oceanography
Dong-Kyu Lee, Department of Marine Sciences, Busan National University
The project goal is to study the variability of the upper ocean current systems and their relationship to meteorological forcing. To this end, spatially coherent field observations are required of the ocean response and the atmospheric forcing. A new instrument, the MINIMET, a water following drifter, was designed as a tool to obtain ocean and atmospheric data. Arrays of MINIMETS will be utilized in a coordinated field experiment in marginal seas to study wind-driven ocean currents.
The scientific objectives are to obtain direct measurements of ocean circulation on various time and space scales that are of sufficient spatial density and time duration so both seasonal means and intra-seasonal variations can be determined. These data are used to test model dynamics, both in the processes that determine the mean motions, as well as the exchange of energy and momentum between the mean and the eddies.
In marginal seas, circulation patterns are spatially complex and seasonably variable. Lagrangian techniques are well suited for developing quantitative estimates of the circulation in the areas where the general pattern of currents are not well known. Wind fields are complex, often affected by mountains or other land boundaries. Large numbers of drifters were deployed from volunteer ships in the Japan-East Sea to map the circulation patterns. In the period of 1998-2001 we released 56 MINIMETs and 33 SVP drifters from VOS and research vessels. In addition to drifters deployed by this project, 137 drifter data are used for analysis. Data from the QuikSCAT was used for verification of the wind observations.
Data processing has been completed from a uniform format from a suite of 131 SVP, 44 MINIMET and 72 NAVY drifters. Data from QuikSCAT have been interpolated on to the drifter tracks and the co-located data have been analyzed. The analyses and dynamical interpretations of drifter tracks with satellite altimeter data have been completed.
From drifters and the historical file of temperature at a depth of 100 m, 15-m depth circulation and its variances in relationship to the seasonal mean warm and cold water regions were constructed on 0.5 resolutions. The strongest seasonal currents were: the East Korean Warm Current (EKWC), the Tsushima Current (TC), the North Korean Cold Current (NKCC) and local eastwards intensifications along the sub-polar frontal patterns. The TC and NKCC were observed during summer and drifters passed through the Soya Strait only in summer. The sub-polar front occurred along 40 N in the northeastern JES; a separate front occurred in winter that was associated with the flow of warm water restricted to the southern JES.
Small eddy energy was found in the cold water regime and large eddy energy in the warm water regime. Satellite altimeter and drifter tracks were used to mark the Wonsan Eddy found in the Eastern Korean Bay, a yearly late summer occurrence. Mechanical energy flux from the mean circulation to mesoscale eddies occurred in EKWC, TC and the sub-polar front in the eastern JES with eddy kinetic energy doubling time of 15-60 days. The yearly mean circulation east of the Korean coast is organized into a broad, eddy-filled East Sea Current that contains weak anticyclonic gyres in the Ulleung and Yamato Basins. It accelerates toward the Yamato Rise and against the eastern coast of the JES.