Chemistry Reference
In-Depth Information
tance of
x
= 15 km from point
A
and equal to 4.5 km. The residence time
of the spill here was 156 min. Assuming that the current velocity did not
change during this time, we then find
V
c
| 0.5 ms
-1
.
A weak cyclonic circulation associated with a small eddy
15
was super-
imposed on the eastward current in area
C
(Figure 1b). The size of the
eddy between the boundaries, where the direction of the band changed,
was about 8 km. The current in the southern part of the eddy shifted the
spill south-eastward and the current in its northern part shifted it north-
westward. The magnitude of these oppositely directed displacements was
about r 700-800 m. This corresponds to an average velocity of about 0.07-
0.08 ms
-1
, taking into account that the age of the spill in this region was
170 min.
The second large displacement of the spill occurred in area
D
at a dis-
tance of 1325 km behind the ship. The ship had crossed this area 3060
min earlier. An enlarged fragment of the SAR image covering area
D
is
shown in Figure 5. As opposed to area
C
, sharp changes in the bottom to-
pography are absent in this area (see Figure 3). The cause, which initiated
the appearance of the flow
9
normal to the Kuroshio front and formation of
cyclonic circulation
10,
is not clear at this time. The maximum displace-
ment from line
AB
occurred in the central part of area
D
, where flow
9,
bounded by lines
7
and
8,
crossed the spill. The maximum average normal
component of the flow velocity
V
c
, as calculated by equation (2), reached
1.2 ms
-1
. The magnitude of the velocity vector should be still higher. The
unclear boundaries and decreased darkness of the oil band in the central
part of area
D
(Figures 1a and 5) were, probably, due to the fact that the
spill was partly broken by strong surface currents.
As evident from Figure
5
, the profile
V
c
(
x
) is not a smooth curve: sever-
al step-like changes in the current velocity have to be noted, corresponding
to step-like shifts in the spill. Such features were observed in the places
where the narrow light lines
7
,
8
and
11
crossed the spill, which may be in-
terpreted as current shift zones (Johannessen et al. 1996, Marmorino and
Trump 1994, Marmorino et al. 1994). The shift zones are often character-
ized by intensive wave breaking, and an increased concentration of foam,
wastes, algae, etc. (Monin and Krasitskii 1985, Marmorino et al. 1994).
Their width, as a rule, is small, typically ranging from several meters to
several hundred meters. For example, the mean width of the shear zones
7
and
8
comprised values around 200 r 25 m. The displacement of the band,
where it was crossed by line
7,
was about 300 r 25 m after about 34 min.
This points to the existence of a differential current of about 0.15 r 0.01
ms
-1
across the band and an average shear of roughly (0.70.8)x10
-3
s
-1
.
These values are in good agreement with the estimates obtained by Ochad-
