CD 3178
CORRELATION (LONGITUDE)
with time
ents with
ays there
he earlier
' from a
orrelation
ider then
3 days in
st drifter
his report
eries are
trajectory
Figure ?
direction
> towards
ates that
ent meter
t speed
or LCDs
ter time
shown in
ories and
nplicated
requently
present or passing through the study area (Fig.
2). The mean surface circulation in the eastern
half of the study area (2200m water depth) was
found to flow to be toward the SW. Inshore of
the C200 mooring, the surface currents were
sometimes to the NE, particularly during the
winter months.
S
1
m
Ch
T
8
c
=
g
&
latitude
1 | l
—-70 -—65 —60 -55 z: 5 —45 —40 — 356 zu
^
longitude (W)
Figure 3. Displacement trajectories for satellite
tracked drifters.
Average current velocity for the southbound
flow at CM32 was 43 cm/s toward 201°. The
corresponding mean current velocity measured
by the 5 drifters was 31cm/s toward 219°. The
two forms of measurement shows the drifter
data to be about 8 cm/s slower than the current
meter measurements. Current directions were
also different, with the current meter data
showing the currents to be about 17° to the left
of the drifter currents. The variability in the
measurements is best seen from their standard
deviations. The overall standard deviations for
the data are almost equal: +21cm/s for current
speed and about +72° for the current directions.
For the individual northbound and southbound
grouping of data, the standard deviations for the
CM data are slightly smaller than for the drifter
data sets. The analysis shows a good
correspondence for the two velocity fields as
determined from the drifter trajectories and the
current menter, especially when considering the
presence of eddies in the study area.
657
During 8 March (Julian Day 67.23-.99) the
LCD 3178 passed in very close proximity to the
200m current meter mooring (Fig. 4). Positional
data from the LCD3178 and the current meter
for the time interval 67.23-68.0 were used to
determine the mean velocity for both the drifter
and current meter (see Table 1). The mean
speeds and directions for the LCD and current
meter were 66 cm/s and 60 cm/s toward 200?
and 204°, respectively, considered to be very
similar to each other.
CONCLUSIONS
Both satellite tracked drifters and in situ moored
current meters were used to measure currents on
the continental platform off SE Brazil for
Project COROAS. In order to determine
whether these different forms of measurement
could provide similar results, an autocorrelation
series of a 140 day long drifter series was
computed and found to determine a
decorrelation time scale of 23 days for the
locale. Since most of the time series were less
than 15 days, the comparisons were considered
to be reliable indicators of the similarity of the
two data sets.
The mean current speed and direction for the
southbound flow indicated by the 5 drifters (set
for 15 m depth) used in the study was 31 cm/s
toward 219° and 43 cm/s toward 201° for the
32m deep current meter located at the 200 m
depth mooring. Analysis of standard deviations
for both sets of data show the variation to be
essentially the same for speed and direction
indicating that the two data sets are of about the
same quality. Mean current meter velocities
were on the average 17° to the left of the drifter
velocities and 8 cm/s faster. A combination of
small scale non-uniformities in the velocity
field and the fact that the currents were
measured at 15m depth by the drifters and at
32m depth by the current meter are considered
the principal reasons for the observed
differences between the two forms of
measurements. The results of this study strongly
support the view that WOCE standard, drifters
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B7. Vienna 1996
