Press Releases
JAMSTEC
Atmosphere and Ocean Research Institute,
The University of Tokyo
Understanding Tidal control on Flow through the Seto Inland Sea
—Possible determination of eastward or westward flow—
1. Key Points
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- A simulation focusing on the tidal effects on flow through the Seto Inland Sea has revealed that tides suppress throughflow by enhancing vertical mixing and forming complex eddies.
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- The direction of flow through the Seto Inland Sea in this simulation (primarily eastward) is opposite to that estimated in a previous observational study (primarily westward), and this discrepancy may be due to the fact that the complex eddies caused by tidal currents were not taken into account in the previous study.
2. Overview
Using an ocean simulation of the Seto Inland Sea with a 500-m horizontal mesh, Masao Kurogi of the Research Institute for Global Change (RIGC), Japan Agency for Marine-Earth Science and Technology (JAMSTEC) and Hiroyasu Hasumi of the Atmosphere and Ocean Research Institute (AORI), University of Tokyo, have revealed that tides suppress flow through the Seto Inland Sea (i.e., throughflow). The results of their simulation have also suggested that the long-term flow direction may be eastward.
The Seto Island Sea is characterized by strong and complex tidal currents; however, it is primarily enclosed, with the exchange of seawater restricted to the Kanmon Strait, the Bungo Channel, and the Kii Channel. Environmental problems, such as red tides and pollution from oil spills, have occurred due to the long residential time of contaminants and nutrient (salts) originated from human activities. To understand these problems and preserve the environment, knowledge of the throughflow that causes water exchange over long timescales is fundamental. Though eastward throughflow is expected based upon differences in sea levels (Figure 1), a westward throughflow was estimated in the observational study. Thus, it remains unclear whether the throughflow is eastward or westward, and the effects of strong, complex tidal currents on the throughflow have not been sufficiently investigated.
In the simulation-based study reported here, the effects of tides on throughflow were investigated using an ocean model covering the Seto Inland Sea with an approximately 500-m horizontal mesh. In both simulations with tides (hereafter referred to as the “tidal experiment”) and without tides (hereafter referred to as the “control experiment”), the throughflow was primarily eastward, but the volume transported was considerably lower in the tidal experiment than in the control experiment (Figure 2). It is found that tides enhance vertical mixing and induce complex eddies (Figure 3), and both works to reduce the throughflow (Figure 4). Moreover, the throughflow estimated in the observational study may be in the opposite (westward) direction because the complex eddies caused by tidal currents were not taken into account (Figure 5).
These results could be applicable to throughflow in other straits around the world, and they may contribute to improving the accuracy of both ocean and climate simulations, including global warming projections. The researchers intend to investigate throughflow in other straits in the future.
This study was supported by FLAGSHIP2020 project of the Ministry of Education, Culture, Sports, Science and Technology (hp160230, and hp170234). The above results were published in Scientific Reports on August 29, 2019 (JST).
Title: Tidal control of the flow through long, narrow straits: a modeling study for the Seto Inland Sea
Authors: Masao Kurogi1 & Hiroyasu Hasumi2
1. Research Center for Environmental Modeling and Application, Research Institute for Global Change, JAMSTEC 2. Atmosphere and Ocean Research Institute, The University of Tokyo
*1 Reanalysis data: data reproducing the state of the ocean by using observational data and numerical models. This study utilized the dataset ‘Four-dimensional Variational Ocean Reanalysis for the western North Pacific’ (FORA-WNP30), which was produced by JAMSTEC and Meteorological Research Institute of Japan Meteorological Agency.
Figure 1. (a) Bottom topography of the ocean model with an approximately 500-m horizontal mesh. The red line is the section used to calculate volume transport in the previous observational study. The green line is the section used in this simulation. (b) Monthly average east-west difference in sea level (Bungo Channel–Kii Channel) calculated from the latest reanalysis data*1. Averaged values on the orange lines in (a) were used to estimate sea levels in the Bungo and Kii Channels.
Figure 2. Monthly eastward volume transport across the green line in Figure 1 (black) and the east-west difference in sea level (red). Solid and dotted lines are the results of the control and tidal experiments, respectively.
The eastward volume transport in both experiments varied in the same manner as the sea level difference, suggesting that throughflow is driven by the sea level difference. Volume transport was considerably lower in the tidal experiment than in the control experiment.
Figure 3. (a) Vertical average of velocity in the tidal experiment (black arrows) and control experiment (red arrows). Time averaged values from February to December of 2012 are shown. The red line is along the observational section (red line in Figure 1). (b) Schematic of time-averaged currents caused by tidal currents. The red and blue arrows indicate the tidal currents when they are nearly opposite; as they are not exactly opposite due to interactions with the topography, the currents indicated by the black arrows appear as a result of time-average calculation.
Figure 4. Monthly eastward volume transport across the green line in Figure 1 (black). The blue line indicates the values when vertical mixing obtained from the tidal experiment was used in the control experiment. The red line indicates the values when the forces to reproduce time-averaged eddies were further applied.
When vertical mixing enhanced by tidal currents was used in the control experiment, the eastward volume transport decreased (blue). When the forces to reproduce time-averaged eddies caused by tidal currents were further applied, the eastward volume transport was reduced to a similar level as in the tidal experiment (red). These two tidal effects (enhancement of vertical mixing and time-averaged eddies) were found to suppress the eastward volume transport of the throughflow.
Figure 5. Vertical average of velocity in the tidal experiment (arrows). Time averaged values from February to December of 2012 are shown. The line PQ is along the observational section (red line in Figure 1). The red arrows schematically show the relationship between the time-averaged velocity in the observational study(
)and the tidal experiment(
).
In the observational study, the velocity component along PQ (
)was obtained, and this was southward in time-average, the same as in the tidal experiment. The flow direction was necessary to calculate volume transport from . In the observational study, the direction of the tidal current was estimated to be along the dotted line, and by assuming that the time-averaged currents were in the same direction, the southwestward currents were () evaluated. However, in the tidal experiment, time-averaged currents averaged along PQ were southeastward () due to time-averaged eddies. Since such eddies were not taken into account, the throughflow estimated in the observational study may be in the opposite (westward) direction.
Contacts:
- (For this study)
- Masao Kurogi, Project Technical Scientist, Research Center for Environmental Modeling and Application, Research Institute for Global Change, JAMSTEC
- Hiroyasu Hasumi, Professor, Atmosphere and Ocean Research Institute, The University of Tokyo
- (For press release)
- Public Relations Section, Marine Science and Technology Strategy Department, JAMSTEC
- Public Relations Office, Atmosphere and Ocean Research Institute, The University of Tokyo