Arnold L. Gordon
Lamont-Doherty Earth Observatory
Palisades, New York, 10964-8000
Telephone (914) 365 8325
fax (914) 365 8157
e-mail: agordon@ldeo.columbia.edu
I Objective: To produce an improved description and understanding of Tsushima Current. Specific objectives are: 1) to provide a detailed description of Tsushima Current through data analysis; and 2) to elucidate essential physics governing the current behavior through analytical and numerical modeling. Two prominent features of the current representing its main evolution stages will be investigated. They are the branching of Tsushima Current after it enters the Japan Sea, and the generation of Tsushima eddies farther downstream.
II Approach: An interactive approach of data analysis and modeling will be pursued. The data analysis will provide a detailed description of the current, and which would guide the model development; the model on the other hand is intended to isolate and elucidate essential physics governing the flow behavior, thus aiding the interpretation of the observational data. The data source include the archived and ongoing data collection activities of JES-DRI, CREAMS, and Japanese marine agencies' in situ and satellite data. The numerical model to be used is Lamont Ocean Circulation Model (LOAM).
III Short Work Statement:
[A] OBJECTIVE 1: . the branching of the current after it leaves the Tsushima Strait: The objective is to investigate the dynamics of a buoyant flow through a shallow sill, with specific application to the Tsushima Current. The questions to be addressed include: How is such problem closed dynamically? How does it constrain the volume flux and flow structure? Can it explain the branching of Tsushima Current?
Observational data will be used to describe the form and variability of Tsushima Current from its initiation in the channel east of the Tsushima Island to its flow path along the Japanese coast. The data sources include the archived and data to be collected by Japan and US JES participants. The current monitoring array to be deployed in TPKS by NRL and Kordi (Perkins) will provide the initial conditions of the Tsushima Current on entering the Japan Sea. Various Japanese agencies are planning to obtain an extensive array of CTD, hull ADCP down to 500-m in the Tsushima Current every month of the year (except January) at least for the next 4 years.
The modeling task is to construct an analytical model to explore the consequence of above hypothesis, and to compare the model results with observation. For simplicity, we shall consider a two-layer fluid entering a rectangular gap. The interface in the upstream reservoir lies below the sill, and the reservoir is sufficiently vast that it can be considered motionless. For the downstream boundary, it is taken first to be vertical, but a sloping bottom will be incorporated later. When non-dimensionalized, the problem contains only two dimensionless parameters: the (unperturbed) thermocline depth scaled by the sill depth; and the width of the gap scaled by baroclinic deformation radius(sometimes called Kelvin Number). The task is simply to derive the solution and examine how it depends on these two parameters. After investigating the basic physics for branching, we want to incorporate a sloping coastal boundary to examine how the flow structure evolves downstream. Since the offshore edge is a free streamline, one expects the flow overlying the sloping bottom to eventually dissipate by frictional torque. To conserve the volume flux, the flow would then migrate slightly offshore, characterized by a sharper inshore maximum.
[B] OBJECTIVE 2. the generation of Tsushima eddies farther downstream: The objectives are thus to investigate: a) the interaction of a buoyant current with curved boundary; and b) the movement of eddies that are detached from the boundary. The questions we hope to explore include: How does the flow structure evolve when encountering boundary curvature? Can there be singular development that facilitates formation of eddies? If so, what are eddy properties (size, spacing, translation speed, generation frequency, etc.) based on conservation laws? What is the condition when eddies are shed from land boundary? And what is their subsequent movement?
For numerical calculations, we will conduct following experiments:
1) buoyant flow over a uniform slope. The numerical solution will be
compared with the analytical solution to assess the basic closure used in
the analytical model.
2) buoyant flow along a curved vertical wall. The purpose is to demonstrate
the eddy generation and shedding process as postulated above. A series of
runs will be conducted using different external and upstream conditions,
and the results will be compared with scale derivation to assess its
validity.
3) simulation of Tsushima Current using realistic topography and geometry
of Japan Sea. Such simulation would allow a more detailed comparison with
observation, thus providing a more definitive test of the importance of the
hypothesized physics.
The observational component of this part of the study will focus on current variability associated with the Oki Island Spur and the Noto Peninsula regions, where eddies are shed. The eddies initially migrate seaward, forming a thick winter mixed layer, often merging with other warm eddies, to eventually move towards the east to re-join the Tsushima Current east of Noto Peninsula. The observational data to be used to describe the evolution of the Tsushima Current eddies are the archived and ongoing data collection of the Japanese agencies, as mentioned above. The TOPEX POSEIDON data will be used to describe the eddy form and evolution.
IV The ONR and Foreign Sea of Japan research with most direct relevance to this proposal are:
Hank Perkins (and Kordi) "Measurements in the Korean/Tsushima Strait" The ADCP instruments will provide vertical and horizontal shear structure of the Tsushima Current while the cable will monitoring total transport. Both will allow us to apply real initial conditions.
Amy Bower: "The upper-layer circulation of the Japan/East Sea: Historical data analysis" will help in resolving the Tsushima Current eddy development and evolution.
Craig Lee (with Brink, WHOI; and Jones, USC) "Physical and optical structures in the upper ocean of the Japan/East Sea" will identify the ventilation products derived from the Japan Sea subpolar front. This may be helpful in differentiating between those polar front processes and ventilation stemming from the warm water regime eddies.
Chris Mooers "Modeling support for CREAMS II: Oceanic and atmospheric mesoscale circulation and marine ecosystem simulations for the Japan/East Sea" will provide a basin-wide context for interpreting the Tsushima Current variability.
Steve Riser "Observations of upper ocean hydrography and currents in the Japan Sea using PALACE floats". Provides basin scale water mass stratification information
Lynne Talley "Hydrographic measurements in support of Japan/East Sea circulation, ventilation, and frontal process studies." Provides basin scale water mass stratification information
Randy Watts and Mark Wimbush "Shallow and deep current variability in the southwestern Japan/East Sea." Can be used to relate the observed Tsushima Current structure and variability to the western boundary current regime.
Japan: Various Japanese agencies are planning to obtain an extensive array of CTD, hull ADCP down to 500-m in the Tsushima Current every month of the year (except January) at least for the next 4 years. Japanese collaborators include: Prof. Yoon, Prof. Isoda, Dr. Hirai, Dr. Yoritaka, Dr. Michida and Prof. Awaji.
Korea: Kordi's PORES program (Suk and Chang of Kordi; integrated with NRL Korean/Tsushima Strait array).