Influence of offshore transportation across continental shelf in East China Sea onthe Kuroshio south of Japan and its extension
Ridong Zhang
Ocean University of China, Qingdao, 266100, China

1 AimsMany studies have shown that the Kuroshio carries out multi-scale exchange with
the East China Sea (ECS) in the process of heading north, and the substances such as
nutrients are of great significance to support the biogeochemical process in the ECS. On
the other hand, the influence of the Kuroshio current and the existence of the Tsushima
Warm Current (TWC) make the exchange affect not only the ECS, but also thedownstream of it. Up to now, however, there is still a lack of quantitative understandingof the temporal and spatial variation of material flux in this process as well the mechanism
behind it. In this study we will reveal the distribution characteristics of the ECS Shelf
Water in the downstream area, which refers to the Kuroshio Current (including Kuroshio
in the ECS and in south of Japan) and its extension. Besides, the flux of ECS Shelf Waterin the typical sections and the percentage that the ECS Shelf Water take in these sections
in different months will be calculated. The works mentioned above are helpful tounderstanding of the interaction between the Kuroshio and the coastal ocean and establish
a good background for further research on the spatial and temporal variations of nutrientsand primary production in the northwestern Pacific.

2 Procedure
To simulate the water exchange between the ECS and the Kuroshio more accurately
and meet the needs of high-precision 3D ecological dynamics model in a further study,
the horizontal resolution of the Princeton Ocean Model (POM) is improved to 1/18°, andthe sigma coordinate is increased to 65 layers. Combined with historical data and1 / 7

numerical experiments, our project expounds the temporal and spatial characteristics ofthe distribution of the ECS Shelf Water in the downstream area after the cross-shelftransportation in different seasons. Then compares the influence of the ECS Shelf Water
at the typical section in different months and reveals the role of the underlying mechanism.
It should be noted that the “downstream area” in this study refers to the area locateddownstream of the continental shelf of the ECS on the concept of geographical location
Namely the Kuroshio in the ECS, the Kuroshio in south of Japan and Kuroshio Extension.

3. Results
3.1 Offshore transportation of ECS Shelf Water in 2006

Fig.1 Model domain. Here the red area is set with tracer in all depth, and the yellow solid line
represents the taken sections: KC1 is the beginning of Kuroshio in ECS, KC2 is located at
Osumi-kaikyo, KC3 is at the middle of the Kuroshio in south of Japan, KC4 is the location toentering the Kuroshio Extension

Firstly, the cross-shelf transportation in the ECS in 2006 is simulated. In the model,tracer with a value of 1 is set in the continental shelf area of the ECS (within 200 m
isobath and in all depth, Fig. 1) and remain unchanged for one year, and the initial valueof tracer in other area is set to 0. The distribution in each season during the period isshown in Fig.2. It is obvious that the offshore transportation of ECS Shelf Water exits allyear and mainly includes two branches: one is to pass through the Tsushima Strait andentering Japan Sea, the other is entering the Kuroshio through the Osumi-kaikyo. The
2 / 7former has been studied in many researches and will not be repeated here. The high valueof tracer passing through the Osumi-kaikyo exist along the Kuroshio and extends northand south to 45°N and 25°N under the effect of the high velocity of the Kuroshio andeddies. Moreover, it can be seen from the time that the cross shelf transportation is weakin winter (February), the strongest in summer (August), and in the transitional stage inspring and autumn.

Fig.2 Surface distribution of tracer in different seasons

3.2 Distribution characteristics of ECS Shelf Water after in the next months
The temporal and spatial distribution characteristics of ECS Shelf Water transport in
the next year are compared through numerical experiments. It consists of 12 cases: Thetracer with a value of 1 on the ECS continental shelf area is given from January toDecember, respectively, and its value remains unchanged in this month. Then the tracer
on the ECS continental shelf area is cleared at the beginning of the next month. At thistime, the tracer contained in the model represents the offshore transportation of the ECS
Shelf Water in this month. The temporal and spatial characteristics of distribution
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beginning in different seasons in next months are compared (Fig. 3).
It is obvious that the offshore transport of the ECS Shelf Water is stronger in springand summer. Besides, eddies or the high velocity of the Kuroshio make it easier for tracer
to expand or maintain a high concentration and enter the Kuroshio Extension, especiallyin spring; On the contrary, the offshore transport in autumn and winter is weak. All thetracer decreased to a lower level in the fourth month.
In addition, taking the Osumi-kaikyo section (KC2) as an example (Fig.4), it can beclearly seen that the tracer started to pass through the Osumi-kaikyo (with a core insideand outside the strait respectively) into the Kuroshio in south of Japan in the beginningmonth. Then its flux increased with the effect of the current field, and the transportation
cores mentioned above were at about 50 m. The high flux can be maintained until the next
month (i.e. the first month after emptying the tracer on the ECS continental shelf), andthen began to decrease. At the beginning of the third month, the tracer has basically passed
through the Osumi-kaikyo, when the flux is very low. In terms of seasonal variation, thetracer transport in Osumi-kaikyo when the tracer is first released is the strongest in winter,but the overall transportation is stronger in spring and summer than in autumn and winter.

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Fig.3 Distribution characteristics of ECS Shelf Water after in the next months. The blue, green
and yellow solid lines in the figure are 50m, 100m and 200m isobaths respectively. The whitearrow indicates the surface current field. As for the name of sub-fig, F/M/A/N indicate that
tracer was released in February (winter)/May (spring)/August (summer)/November (Autumn),
The number after the point represents the next month (n means next year).(e.g. “N.n1” means
tracer was released at November, the picture shows the distribution in January of the next year)

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Fig.4 Distribution characteristics of ECS Shelf Water after in the next months in KC2. White
solid line represents the current velocity. Naming method is consistent with Fig.3

3.3 Percentage of ECS Shelf Water in typical sections in different seasons.
Take Osumi-kaikyo section (KC2) as an example as above, to calculate thepercentage of ECS Shelf Water in the past year at the same typical section in different
seasons. The result shows that the percentage of ECS Shelf Water at KC2 in all monthsof the previous year is 4.08% (winter), 5.04% (spring), 7.00% (summer) and 4.19%(Autumn) respectively. It can be seen from Fig.5 that the law of affection of ECS Shelf
Water in different seasons is generally the same. The most significant one is the latest
month, its percentage of ECS Shelf Water is much higher than that in other months; And
the percentage of the last three months reached 84.5% (winter), 78.2% (spring), 76.1%(summer) and 89.9% (Autumn) of all shelf water in KC2. Combined with the above
calculation, it can also be found that the impact of shelf water in the last few months issignificantly inversely proportional to the total amount of all shelf water in the section.

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Fig.5 Percentage of ECS Shelf Water in typical sections in different seasons.

4 Publication/conference presentation
Currently, the results have not been published.

5 Perspectives in future
In the future, we will pay more attention to the temporal variation of nutrient flux
and primary production in the northwestern Pacific.

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