Remote Wind Connections to Strait Transports

G.A. Jacobs, H.T. Perkins, R.H. Preller, H.E. Ngodock, W.J. Teague, S.K. Reidlinger, D. Ko, and J.W. Book
Oceanography Division

Introduction: NRL's Oceanography Division research is providing new understanding of the mechanisms controlling flow through the Korea Strait. Located between Korea and Japan, the strait is the critical juncture between the East China Sea, the Yellow Sea, and the Sea of Japan. The Korea Strait transport provides the largest portion of horizontal heat and salt flux into the Sea of Japan. The strait transport forms a relatively warm fresh surface layer in the summer and strengthens the anticyclonic circulation south of the subpolar front. Understanding the fundamental dynamics controlling transport through the strait is crucial to developing environmental monitoring and prediction systems for Navy applications. The area is challenging for making direct observations as well as for numerical modeling. The successful deployment of 12 acoustic Doppler current profilers (ADCP) (Fig. 6), the reproduction of observed transport variations by the Navy Coastal Ocean Model (NCOM) (Fig. 7), and sensitivity provided by the numerical adjoint (Fig. 8) are significant achievements. They are leading to an improved understanding of the dynamics controlling the area, and this understanding guides the continued development of Navy environmental prediction systems.

Trawl-resistant Bottom Mounts: In situ current measurements have been limited in this area because of the high regional fishing activity. Extensive bottom trawling endangers conventional moorings deployed for more than just a few days. To counter this problem, NRL in conjunction with NATO's SACLANT Research Centre developed trawl-resistant bottom mounts (TRBM) to contain the ADCP instrument (Fig 6). Instrument sensors that record orientation indicate encounters with fishing activities by many of the 12 ADCPs deployed in the strait from May 1999 through March 2000. In spite of the interference, the instruments provided excellent coverage. Such an extensive array over such a long time period has never been achieved in this area. These data are providing extensive new understanding of the dynamics controlling exchanges between the Asian marginal seas.

FIGURE 6
Twelve bottom-mounted ADCPs were deployed in trawl-resistant bottom mounts between Korea and Japan. Red arrows represent the mean velocities at different depth levels for each mooring. Colored sections represent the mean transport across each section.

Numerical Modeling: NRL has been developing NCOM, which is an evolution of previous ocean models. NCOM is designed to represent circulation on the continental shelf and the deep ocean more accurately by combining the unique methods previous ocean models used to represent features in these different regions. The model used in this project covers the Asian marginal seas from the South China Sea through the Sea of Japan so that the connections between the seas can be examined. The horizontal resolution used in the examples here is 1/8 . The eastern boundary conditions are provided by a larger North Pacific model that assimilates satellite sea level and temperature measurements. Wind forcing is provided by the Navy Global Atmospheric Prediction System (NOGAPS), and the model is run from 1997 through 2000. Wind forcing in the synoptic band (2 to 20) days is expected to produce a deterministic transport response through the strait. The model and observed synoptic transports compare well (Fig. 7).

FIGURE 7
The NRL Coastal Ocean Model (NCOM) is set up for this experiment to cover all the Asian marginal seas at 1/8 resolution. The general circulation of the region may be viewed by the sea level (flow generally following lines of constant height, which is represented by the color contours). The model also reproduces the local features of circulation such as the transport through the Korea Strait. The synoptic transport (bottom) observed by the instruments (red line) and reconstructed by the numerical model (blue line) are in good agreement.

Sensitivity through Adjoints: The numerical model dynamical equations represent the physics governing ocean circulation. The good comparison between the model and observed synoptic transports indicates that the wind forcing and the model dynamics connecting the wind forcing to the transport are both good. However, the numerical model does not provide an immediate indication of the area over which wind stress is most important to forcing the strait transport. While carefully designed numerical model experiments may be performed to provide this insight, the adjoint of the model gives a more direct answer. The adjoint is a method that provides the derivative of a model output (such as the transport through the strait) with respect to the model inputs (such as the wind forcing). NRL is presently constructing numerical model adjoints to understand ocean dynamics and assimilate measurements into models.

The strait transport sensitivity to wind stress at a time lag of 3 h (Fig. 8) indicates that the southerly wind stress off the east Korean coast is most influential. A slightly less sensitive area for transport through the strait lies directly south of the strait. Counterintuitively, wind stress across the relatively shallow Yellow Sea shelf is not a large contributor to the strait transport. The physical mechanism connecting the regions of wind influence to the strait transport is the propagation of oceanic Kelvin waves. An observer moving with the wave would have the coast on their right-hand side when facing the direction of propagation. Wind-generated Kelvin waves in the Yellow Sea propagate away from the Korea Strait, taking the wind information to the Taiwan Strait; Kelvin waves generated in the Sea of Japan propagate along the Korea coast to the Korea Strait.

FIGURE 8
The adjoint model provides an estimate of the sensitivity of the strait transport to the wind stress (displayed here at a 3-h lag). The color indicates the amplitude and vectors indicate direction. The transport through the Korea Strait is most sensitive to the wind stress across the area east of the Korea peninsula and less sensitive to the area south of the strait.

Using Knowledge to Build Systems: Knowledge of the dynamics controlling exchanges between the interconnected seas is needed to build accurate monitoring and prediction systems. Evaluation through in situ measurements is crucial for confidence in any system. Understanding the model sensitivity is required to know where efforts must be concentrated to provide the maximum payoff. All these research issues are leading to improved operational capability. NCOM is presently being implemented in a global 1/8° system to provide surface currents and temperatures throughout the world.

Acknowledgments: This work is supported by the NRL 6.1 Dynamical Linkage of the Asian Marginal Seas (LINKS) and the NRL 6.1 Error Propagation on the Continental Shelf (EPIC) projects.

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