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Seasonal variability of Eulerian tracer dispersal
in an estuary and a continental shelf margin
Yota Suzue1* and Yusuke Uchiyama1
1

Department of Civil Engineering, Kobe University, Japan*ys10121213kzkb@gmail.com

Abstract. Coastal marginal seas and estuaries are generally dumpsites for wastewater fromsewage and power plants (e.g., Uchiyama et al., 2014). Accidental leakage of toxic materials
may result in serious hazardous incidents that should be predicted and assessed urgently upon
the occurrence. We thus develop an offline passive tracer model by exploiting oceanic reanalysis outcomes and analyze differences in spatiotemporal variability of the leaked tracers inan estuary and a continental shelf margin on the Pacific side of Japan. In the estuary, tidal oscillations enhance short-term dilution locally, while the month-long transport is dominatedrather by inter-seasonal variability of the clockwise circulation of the estuarine through flow.
On the marginal coast, the Kuroshio and associated secondary lateral circulation readily traps
the tracer, leading to immediate transport mostly in the alongshore direction. The seasonal
difference in the tracer dispersal is apparent such as sporadic offshore tracer eruption episodes, depending on the locations of the Kuroshio axis.
Keywords: ROMS, coastal dispersal, offline tracer model, Eulerian passive tracer

1. Introduction
It is generally true in Japan that nuclear power plants and sewage treatment plants are located mostly in the coastal areas. If accidental leakage of toxic materials occurs, it may cause
hazardous influences on marine environment. We therefore develop an offline passive tracer
model that computes 3-D dispersal of arbitrary Eulerian tracers with a point source capability
by exploiting 3-D oceanic model reanalysis and prediction such as JCOPE2 (Miyazawa et al.,2009) as an urgent oceanic assessment tool. An advantage of the offline tracer model oversimultaneous (viz., online) simulations of hydrodynamics and tracer transport is its computational efficiency that is requisite for immediate forecasts. The offline model is applied to investigate seasonal variability of tracer dispersal released from two particular sites as test cases.
We choose 1) Ikata in Ehime Prefecture located on the Iyo Sea coastline in the western partof the Seto Inland Sea (hereinafter SIS), and 2) Hamaoka in Shizuoka Prefecture is on thePacific coast which is an open coast affected by the transient Kuroshio. Mimicking the oceanic leakage upon the accident occurred at the Fukushima Dai-ichi Nuclear Power Plant in314

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Fig.1 Surface tracer concentrations in Bq/m3 for the online (left) and the offline (right) cases on the31st days since May 1st, 2013. The release site (red circle mark) corresponding to the Ikata NPP inthe Iyo Sea. The black line denotes the transect used in Fig.2.

the spring 2011, the tracer is discharged for 11 days in 4 different seasons since the 1st daysin Feb., May, Aug., and Nov., 2013, and is tracked for 31 days after each release.

2. Reproducibility of the offline tracer model
The offline computation is carried out for each site by using the pre-computed 3-D flow
field with a double-nested JCOPE2-ROMS oceanic downscaling model of the SIS. Figure 1
shows comparison of the surface tracer concentration in Bq/m3 on the 31st model day in thespring (May) condition in the Iyo Sea by the online tracer model (left) and that by the offline
tracer model (right). Both the cases exhibit a similar dispersal pattern where the northeastward transport is dominant, demonstrating that the offline model agrees well with the onlinemodel. The offline computational cost is significantly reduced by about 1/20 compared to theonline model. The offline model is suitably run on a single quad-core CPU workstation,
whereas the online model needs a LINUX cluster for a reasonably short computational time.

3. Seasonal tracer variability in the SIS
In May (spring) and August (summer), the tracer is mainly transported northeastward while
the southwestward transport is predominant in November (fall) and February (winter). In
general, the subtidal mean circulation in the SIS is persistently clockwise (e.g., Uchiyama etal., 2012) that leads to the northeastward transport. The monthly mean temperature along thetransect off Ikata (the black line in Fig.1) represents that a cold core water mass exists at
depth within 5-60 m below the surface in May (Fig. 2). This structure corresponds to theso-called “cold dome” (Chang et al., 2009) which is the remainder from the cold water
formed in winter and has regularly observed in the Iyo Sea (as well as the Harima Sea and theSuo Sea where the basins are semi-isolated by shallow straights) during spring and summer.
A counter-clockwise lateral circulation near the surface is thus developed geostrophically by
this density structure (Fig.2), promoting the northeastward tracer transport. The cold dome315

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Fig.2 Cross-sectional plots of the monthly-meantemperature (upper) and northeastward velocity(lower) along the transect (the black line inFig.1) in May, 2013.

Fig.3 Monthly-averaged wind stress curl
(N/m3) in the western part of the SIS in Nov.,
2013.

Fig.4 Sequential plots of the 10 days-averaged results from the offline case in Nov. (fall), 2013.
Surface velocity vectors are plotted on surface tracer concentration in color.

keeps staying in the Iyo Sea to maintain the counter-clockwise circulation until summer(Aug.). In contrast, in fall and winter, seasonal southeastward wind is predominant andsouthwestward Ekman transport is developed accordingly. In November (Fig. 4), a divergent
pattern in the surface wind is formed near the shore characterized by a partial northeastward
dispersal, while it is not that obvious in February when the southwestward transport is evident. The tracer bifurcation occurred in the middle of the Iyo Sea is affected by the bifurcating wind stress curl (Fig. 3). Change of the sign occurs in the Iyo Sea that promotes the separating dispersal pattern in November, whereas the weak and positive stress curl is widely distributed in the entire Iyo Sea in February (not shown).

4. Seasonal tracer variability in the Pacific coast316

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The comparative offline tracer computation is conducted for the open coast condition affected significantly by the Kuroshio. The tracer is released from Hamaoka near Cape Omaezaki in the same way as the SIS cases. The location of the transient Kuroshio axis near thecoast lines plays a key role in the tracer dispersal (Fig. 5). The secondary circulation develops
between the Kuroshio and the coastline readily traps the released tracer, promoting the eastward transport with sticking to the shore around the release site in May. In the late May, acyclonic gyre is enhanced off Hamaoka, resulting in the westward transport with inflowing
flux into the Ise and Mikawa Bays. However, once the Kuroshio largely meanders in November, the secondary circulation grows seaward to form a large gyre and standing
mesoscale eddies, inducing predominant eastward transport along with sporadic diluted
southward transport far to the offshore by about 32oN. The shore-attached transport is foundalso in February, although the offshore episode is dominant in August, depending on the locations of the Kuroshio axis relative to the shore.

Fig.5 Same as Fig. 4 but for the Pacific offline case. May (upper row) and November (lower row)
are presented.

References[1] Uchiyama, Y., Idica, E., McWilliams, J.C. and Stolzenbach, K.D. : Wastewater effluent dispersal in Southern California Bays, Cont. Shelf Res., No. 76:(2014), pp. 36-52.
[2] Chang, P.-H., Guo, X., and Takeoka, H. : A numerical study on the seasonal circulation in the Seto Inland Sea, Japan. Journal of Oceanography, 65:(2009), 721-736.

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