Chan-Su Yang1, Hitoshi Tanaka2, Masaki Sawamoto2 and
Kimio Hanawa3
1Frontier Observational Research System for Global Change, Climate Variations Observational Research Program, 2-15 Natsushima-cho, Yokosuka-shi, Kanagawa 237-0061,Japan
2Dept. of Civil Eng., Tohoku University, Aoba-yama 06, Sendai, 980-8579,Japan
3Dept. of Geophysics, Tohoku University, Aoba-yama , Sendai, 980-8579,Japan
Tel: +81-468-67-3774, Fax: +81-468-66-1085, E-mail: yangcs@jamstec.go.jp
Abstract: A biological and physical study of a western boundary current flow over variable topography is presented, with the investigation of abnormal catch of fish around sea area of Soma in June 1999. In order to analysis the changes of water mass due to the offshore current, time series of vertical distribution of chlorophyll-a, dissolved oxygen and nutrient concentration (nitrogen, phosphorus), wind vector and satellite images (AVHRR/SST, NASA/SeaWiFS, LANDSAT/TM) are used from 1999 to 2000. The results indicate that the year-to-year variation of the offshore is very high and the 1999’s condition is distinct with the 2000. Chlorophyll-a imagery and wind data reveal that the coastal region around the bay seems apparently to be not affected directly by offshore condition. Nevertheless, it is found that the bottom Ekman transport could cause the abnormal catch phenomenon of fish through enhanced advection over deep depths since the Kuroshio exceptionally exists off Soma.
Keywords: sea
temperature, upwelling, chlorophyll a, kuroshio, abnormal
catch of fish
Surrounded by mountainous land and open to the Pacific Ocean at the eastern side of its 55-km extent, the Sendai Bay locates at confluence zone of the Kuroshio, Oyashio and Tsugaru Warm Current (Fig. 1). In the bay, the influence of the inflowing water mass in the bay is detectable by strong anomalies of water mass properties and the sea surface temperature (SST) environment differs markedly with place to place around the bay.
Fig. 1 Map of the Sendai Bay. Gray color indicates a continuous chain of mountains
Fig.
2 Catch of flatfish at Fisheries Co-operatives of Soma during
the period from June to early July (a gill net)
At the end of June 1999, abnormal fish-catch event was occurred at the near- and off- shore zone of Soma, northeast Japan. As shown in peak value of Fig. 2, the quantity amounts double what it did before and fresh fish is over twice as much as live fish on June 29. However, since fresh fish diminishes in value, it is no happy news to fishermen. Great numbers of them indicate a change of water mass. In general, scientists use flatfishes as their reliable barometer of developing low oxygen. If it surfaces it is an infallible sign of decreasing oxygen. The more the number of it get caught, the worse the oxygen condition.
Two considerations, here, are given to investigate the mechanism. One factor contributing to migration of these fish is the upwelling of warm water due to the Kuroshio. Second, the increasing sea surface temperature (SST) is accompanied by a migration of the thermocline and subsequent deepening of the phytoplankton layer. In the latter, from the surface the heating does not reach the bottom and encourage the fish in bottom to migrate north and south in search of cooler waters and food. Many fish, not able to migrate fully, die from lack of food or unbearable temperature elevation. This factor has been shown to have some effect on the fish populations, but does not seem to be as important as upwelling of the Kuroshio.
This paper is organized with a description of ocean configuration and data analyses using satellite and in-situ datasets; AVHRR, SeaWiFS, LANDSAT TM, dissolved oxygen (DO), nutrients (N and P), and chlorophyll-a concentrations.
Usually upwelling is induced by the wind acting under suitable conditions although the occurrence of upwelling in some areas does not appear to be the local wind (Bowden, 1983; Alaee et al., 1998). Upwelling is the term used to describe the processes which cause the upward movement of water from depths of the order of 100 to 300 m into the surface layer. Since the temperature in the sea usually decreases with depth, the upwelled water will be colder that the surface water which it displaces. Very often it has higher concentrations of nutrient salts (nitrate, phosphate and silicate) than the surface water, which may have been depleted of nutrients by the growth of phytoplankton. Regions of upwelling are usually, therefore, regions of high biological productivity. The enhanced growth of phytoplankton can support a greater zooplankton concentration which in its turn can maintain fish populations.
In general, wind at Soma is weak compared to the near stations and at June its velocity becomes the lowest throughout the year. Wind variable in this region represents low year-to-
Fig.
3 Wind vector at Matsukawaura station in June of 1999
year variation. Figure 3 shows time series of wind data for June 1999 at Soma. The wind velocity is about 1.6 m/s on the monthly average at a height of 9m above sea level and does not represent a specific direction. The calm ratio for total wind frequency is almost 18%. The mean wind energy density is calculated to be 8.1 W/m2 by the equation,
(1)
where Vw is the wind speed in m/s, N is the number of wind
data, and
is the air density. Here, the
density of dry air at standard atmospheric pressure at sea level at the mean air
temperature of June (22.5 ℃)
is used as an air density, 1.194 kg/m3.
For the prediction of upwelling, the phenomenon has to be quantified. A common way is to use an upwelling index, as it reflects the intensity of the water transports As the vertical transport during upwelling compensates for the horizontal water flow, the Ekman transport can be taken as a quantitative characteristic for the upwelling intensity. According to Bowden (1983), the integral Ekman transport in the surface layer U can be roughly estimated by
(2)
where
is the coastline parallel wind
stress on the water surface,
is the density of water, and
is the Coriolis acceleration. Here,
it is supposed that the coastline is in north-south direction and axes are taken
with the y-axis along the coast and the x-axis perpendicular to it. This
upwelling index, called the Ekman index, is directly proportional to the local
wind stress and normalized by the local Coriolis acceleration and the density of
water. At Soma station, perpendicular components have the value of daily mean
–0.12 m/s, while parallel components have 0.06 m/s. Furthermore, the values
are similar to those of climatology. Since the wind did not also continue long
in existence, it can be said that wind has nothing to do with upwelling.
Comparison between 1999 and 2000 in distribution of temperature (satellite images)
According to satellite AVHRR images in the months of June in 1999 and 2000, all the derived images clearly display the feature and its Kuroshio or Oyashio roots. The frontal boundaries is apparently defined and in 1999 the Kuroshio Extension is linked to Sendai Bay but in 2000 the bay water is isolated by the Oyashio invasion flowing southward in front of the bay.
As an alternative, the SST index is commonly used to quantify upwelling, for instance, Nykjaer and van Camp (1994), which is defined as
(3)
where the SSTindex is the difference of the onshore SST, SSTon and the offshore SST, SSToff. A negative SSTindex, respectively, a decrease of the SSTindex, represents upwelling. However, the SSTindex cannot be applied to this area because it is difficult to a reference point as the result of the strong currents.
Vertical profiles of temperature and salinity
The cross sections of temperature and salinity
in May and June 1999 are illustrated in Fig. 4 at line
of off Soma. Temperature and salinity are plotted in 1 degree and 0.2 psu
intervals. A doming of isotherms is observed in the near distance from the
coastline. For example, the 13 ℃
isotherm rose near the surface about 30-40 km off and other isotherms are in the
vertical directions. The upwelled water is separated centering around the peak
of 13 ℃.
Salinities are also distributed vertically in the distance such as the doming
isotherms. It is thought that warmer water has been brought from below the
surface, bringing with it low DO, nutrients, and chlorophyll-a concentration.
Fig. 4 Observed isotherms and salinity profiles in the months of May and June1999
Time series of chlorophyll-a, do and nutrients
Figure 5 shows the temporal variation of chlorophyll a averaged for all points below 30 m in depth (at depths less than 30m). One reason of abnormal fish-catch may be the red tide resulted from algal bloom. However, chlorophyll-a concentration in the months of May and June 1999 is lower than that of 2000. Therefore, it can be said that a red tide does not seem to be related with that phenomenon, because chlorophyll-a represents no sharp peaks in concentration. There is a possibility that the chlorophyll-a decrease in spring of 1999 was caused by oceanic advection due to the Kuroshio. A drastic change of its concentration occurs because of a sudden change in weather or oceanographic phenomena such as wave, current, wind, or discharge from a river (Adachi et al., 1999).
Fig. 5 In situ chlorophyll-a concentration averaged from surface to 30 m in depth for the coastal region (U1 to 4)
In the research area, most nutrients tend to be supplied from the Oyashio (Kusaka et al., 1998; Suzuki, 1998). However, a high value in May is a phenomenon that cannot be expected during the spring because the nutrient concentration will be decreased due to uptake by phytoplankton. Furthermore, over the months of May and June, concentration variation varies with depth; at surface concentration of May is higher than at June except No 1, and at 20m depths it is oppositely decreased from May to June. From the figures 6.4 to 6.8, it can be suggested that water mass near Soma is influenced slowly by the upwelling of bottom water. In the dissolved oxygen concentration in June 1999, extremely low values of DO, below 4.5 mg/l, are observed and do not seem to result from the release of nutrients.
In many cases the Kuroshio does not appear to extend across the adjacent shelf areas and there is little obvious connection between the oceanic and coastal circulations. Therefore, when the Kuroshio Extension flows northward in the near distance from the coastline, SST images obtained from LANDSAT TM (Band 6) are investigated to the interaction between the major ocean currents and the circulation in the coastal waters. The observation is very helpful us to understand what factors effect the formation of coastal water mass; if there is a coastal count current, the coastal upwelling will occur. That’s why the water, which rises into the surface layer, has been brought from the ocean by the offshore currents.
The six images are obtained from October 28 1993 to May 22 1999. On the whole, in the inner bay plumes of rivers have a formation of stable plume in the offshore direction but in other regions the plumes are bended to left or right side. It suggests that the coastal count-current exists around Soma station if there is the Kuroshio.
Fig. 6 Schematic picture of upwelling
5 CONCLUDING REMARKS
Such distributions of chlorophyll-a, DO, and nutrient are also suggestive of coastal upwelling. Therefore, schematic picture of coastal upwelling off Soma is shown in Fig.6. Strong upwelling of the Kuroshio occurs and extends shoreward on the bottom beneath the warm surface. An upward water transport from subsurface layers into the surface layer is induced by the western intensification of the Kuroshio at northeast coasts of Japan. Since the vertical transport of subsurface water needs some time (typical upwelling velocity is around 10m/s; Kriebel et al., 1998), it is estimated that the upwelling began from early May as indicated by the low chlorophyll-a and high nutrient concentrations at all stations.
References
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