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