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Adi Nugraha

Adi NugrahaOpens in new window (Postdoctoral Researcher)

Postdoctoral researcher, Saga University, 2010–2013

Ph.D., Universite Bretagne Occidentale, 2010

M.Sc., Vrije Universiteit Brussel, 2006

B.Sc., Institut Teknologi Bandung, 2003

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Numerical Modeling of Internal Tides in The Oshima Region

Introduction

The existence of internal tides around the Izu Ridge has been recognized from various field observations. The Izu Ridge is known as the region that generates internal tides. These internal tides are generated by an interaction between surface tide and the bottom topography of the Izu Ridge.

Here, we mainly study the possible causes for the generation and propagation of internal tides in the Oshima region. The objective of this study is to gain insight into how Izu Ridge geometry affects the generation, propagation and dissipation of internal tides. We will conduct this study primarily with a three-dimensional non-hydrostatic numerical model as well as a comparison with the observational data taken near Oshima Island.

Method

We employed a parallel, unstructured-grid, non-hydrostatic ocean model, SUNTANS (Fringer et al., 2006) to simulate the generation and propagation of non-linear internal waves around the Oshima region.

We performed simulations over the period 13-16 September 1996. The simulation domain of our model is the southern part of Tokyo Bay.

The bathymetry data of JODC-Expert Grid data for Geography (J-EGG500) from the Japan Oceanographic Data Center (JODC) with 500 m horizontal resolution were used and interpolated to the model’s unstructured grid.

Fig. 1. The location of mooring sites and bathymetry of the Oshima region with a transect used to analyze the results of SUNTANS.

Fig. 2. Observed temperature and salinity profiles used as the initial condition for simulations. The calculated buoyancy frequency is presented here. Data is taken from World Ocean Atlas 2009 (WOA09) Database.

The first eight tidal current components from the OTIS global tidal model (Egbert and Erofeeva, 2002) are forced at the open boundary of the low-resolution grid along the west-east circumference boundary of model domain.

A various triangulation horizontal resolution is used throughout the model domain. The finest horizontal resolution, 100 m, is set along the coastline of Oshima Island in order to resolve the motion in the vicinity of the internal tide generation sites and across Izu Ridge (Fig. 3). A time step of 10 seconds is used to satisfy the Courant condition.

Fig. 3. The model domain used to simulate the nonlinear internal waves in the Oshima Region.

The model domain is parted into 64 subdomains to implement parallel computation of SUNTANS model run by 64 processors on a Linux cluster. (Fig. 4).

Fig. 4. Model domain partition for parallel computing in the model.

Results

A. Model Validation

The diurnal and semi-diurnal tides ranges and spring-neap tidal cycle are well reproduced by the model at this station, indicating that the ocean boundary condition is accurately specified. Yet, the small discrepancy between observed and predicted results from SUNTANS implies that the under-resolved complex bathymetry may be the cause of this discrepancy.

Fig. 5. Comparison of sea surface level variations of SUNTANS and OTIS model results to observations at Okada Station during period of 13 – 19 September 1996.

The model predictability of internal waves in the Oshima region was also validated by current field observations. We compared the model result and the observation of current profiles that were made during the period 13 September to 18 October 1996. In this period, an Acoustic Doppler Current Profiler (ADCP) was moored at the Station O as shown in Fig. 1, located at the top of the northern part of Izu Ridge. Current data were obtained at 5 meter intervals every 5 minutes.

Fig. 6. Comparison of east-west currents at mooring site A during the period 16-18 September 1996. The observations (black line) are obtained by averaging the currents over the depth. The blue line is band-pass filtered observations for 5% around M2 frequency. The red line shows the barotropic velocity for SUNTANS model results.

B. Internal Wave Sources and Generation

The internal wave is likely generated in the region where the bottom slope nearly agrees with the ray slope (Fig. 7).

Fig. 7. East-west baroclinic velocity (in ms-1) along transect A depicted in Fig. 1. The semidiurnal internal tidal ray paths are shown as solid black lines originating from two of the same generation sites.

We also spot a near critical value of log10 (Ri) (-0.5~0.5) near the ray path. This is consistent with a study done by New and Pingree (1990), which found that Richardson numbers of about unity and claimed that mixing was occurring near the ray path (Fig. 8).

Fig. 8. Distribution of Richardson Number, Ri, estimated from the density profiles and the vertical shear for semidiurnal internal tide along the transect A.

C. Internal Wave Propagation

The internal waves propagate in complicated ways. Large amounts of water come through the Oshima region pushed by the tide. The tides come from the west as well as east of the model domain (Fig. 9). The ridges then convert the barotropic tides to internal tides. The internal tidal energy generated at this location propagates northward along the shore into and dissipates in Sagami Bay. It also propagates eastward.

Fig. 9. Elevation of the 20°C isotherm to its depth at rest of 80 m. The insert plot shows the east-west barotropic tidal currents at Mooring O (in ms-1).

The power spectra of the current components indicate that the semidiurnal period is most dominant in both stations (Fig. 10). In a comparison of the two stations, the energy level of the semidiurnal is higher in Station A. In addition, the energy peak of the diurnal period has also been found in both fluctuations in current for both stations. These findings show that both diurnal and semidiurnal tides generated in the northern part of Izu Ridge propagate to Sagami Bay.

Fig. 10. Center Sagami Bay (left) and Station O (right) current spectral analysis at 80 m from the results of SUNTANS.

Conclusions

  1. The northern part of Izu Ridge is known as the region that generates internal tides near Oshima Island.

  2. Both diurnal and semidiurnal tides generated in the northern part of Izu Ridge propagate to Sagami Bay.

  3. The barotropic tides are the major forcing that generates internal waves in the Oshima region.

  4. The semidiurnal tide is more effective than the diurnal tide in generating the large-amplitude internal waves.

Future Plans

Next, our study will focus on understanding the possible interactions among primary production, distribution and internal tides. Supported by a new ecosystem model and high resolution boundary condition, subsequently, we will couple this ecosystem model with the SUNTANS model to provide information on the ecosystem dynamics around Oshima Island.

Acknowledgements

This research is supported by Japan Science and Technology Agency, CREST.

References

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