both data. At St.c2 and St.c5, the rms difference is large over 20 cm/s. The rms 
differences of speed and direction plotted against cross angle of two beams of 
HFOSR(Figure 2) and water depth of each station (Figure 3). As to speed, the rms 
difference seems to have relation rather to water depth at measuring point than to cross 
angle. In shallow ocean, the shear stress of horizontal velocities in vertical direction 
become larger than in deep ocean in the cause of surface wind (see Figure 10) and bottom 
friction effect. In the case such as St.c2, St.c5 and St.c8, the difference of measuring 
depth between HFOSR(about 1 m) and current meters(2 m) may cause discrepancy of 
velocity speed. Neglect of time interpolation in the process of synthesizing current may 
also cause this descrepancy in these cases. At the other stations with their water depth 
larger than 20 m, however, the rms differences of speed are almost constant about 8 cm/s. 
On the other hand, the relation between rms difference of direction and cross angle or 
water depth seems not to be apparent. The rms difference, however, increases gradually 
in proportion to cross angle. Except for St.c5, the rms differences of direction are in the 
range from 30° to 60°. Figure 4 to Figure 8 show the result of correlation analysis of both 
data as to speed and direction at St.cl, St.c3, St.c4, St.c6 and St.c8. Totally the 
applicability of HFOSR to the monitoring of ocean surface currents are confirmed. 
5. Characteristics of the current in the sea off Fukushima 
5.1 Typical pattern of the current 
Many researchers reported the characteristics of the currents in the sea off Fukushima 
that the currents flow with irregulaly changing the direction mainly south or north in one 
to three days. Figure 9 shows the time series of the stick vector diagram of the surface 
current velocities measured by the current meter at St.c3. In this observation period, the 
same phenomena of the ocean currents, irregulaly changing the direction south or north in 
two to three days, are seen. However, the mechanism of these ocean currents’ pattern 
have not been investigated sufficiently yet. Periodical surface currents observation by 
HFOSR can be expected to investigate such mechanism of currents. 
Figure 1 l(a)-(c) show a time series of the velocity vectors, the horizontal divergence 
and the vertical component of rotation of the ocean surface currents by ORO/CRL 
HFOSR in the area from the coasts to 40 km off the coast at an interval of 4 hours on 
March 30 when the coastal currents flowed with changing the direction from south to 
north. Figure 11(b) indicates there is the boundary about 10 kilometer off the coast that 
the convergence (divergence) is dominant in the coastal side when the currents flow south 
(north). The phase variation of current vectors on the coastal side is the same as wind, 
reversely on the offshore side. But on March 25 to 26 when the coastal currents were 
similarly changing the direction from south to north, the change of current pattern was 
more complicated. In the other current patterns, the same complicated features were also 
shown. The variation of current pattern may indicate that the currents in this area are 
affected by several different current motions in time and space such as wind driven 
currents, the Kuroshio Extension, the Oyashio First branch and etc. 
5.2 Wind forcing 
Figure 10 shows the time series of the stick vectors diagram of wind with 2 hours 
running mean in 10 m above the surface measured at the weather station(see Figure 1). 
The data of wind are well correlated with the ocean surface currents indicated in Figure 9. 
Figure 12 shows the spatial distribution of correlation coefficients between wind and 
ocean surface currents derived by HFOSR from the coasts to 40 km off the coasts. Both 
data are highly correlated in the area between 2 km and 15 km off the coast. In the area 
over 15 km off the coast, the correlation coefficients are between 0.2 and 0.3. Figure 13