Cycle
Date
1
16
Nov
86
2
3
Dec
86
3
20
Dec
86
4
6
Jan
87
5
23
Jan
87
6
9
Feb
87
7
26
Feb
87
8
15
Mar
87
9
1
Apr
87
10
18
Apr
87
11
5
May
87
12
22
May
87
13
8
Jun
87
14
25
Jun
87
15
12
Jul
87
16
29
Jul
87
17
15
Aug
87
18
1
Sep
87
19
18
Sep
87
20
5
Oct
87
21
22
Oct
87
22
8
Nov
87
Feb 88
18 Feb 88
6 Mar 88
23 Mar 88
Apr 88
26 Apr 88
13 May 88
30 May 88
14010
Figure 5. Tracks of mean positions of height anomalies S, H, A, B and C, deduced from the
image sequence of which Figure 3 is a part. Numbers indicate 17 day cycles
whose mean dates are listed in the table at the left. Dashed lines indicate periods
where a feature was not detectable.
3 EDDY DISTRIBUTION AND MOVEMENT
In Figures 3 and 4 interpolated height anomaly data for
single 17 day periods are shown as grey scale images for
different mean dates. The brightness scale is similar to that
used in Figure 2. Data is interpolated between tracks using a
gaussian weighted average with an e-folding distance of
60km as described by Gower (1989b). This distance was
chosen as being the minimum needed to smoothly fill
between tracks.
Figure 3 shows height anomalies in the north-east Pacific
near the start of the Geosat ERM. The most striking,
consistently observed height anomaly is indicated by the
white arrow off the coast of California. This is a negative
height anomaly (cyclonic eddy), moving westwards at about
2.5 cm/s over the period November 1986 to August 1987
from 126° 20’W to 133° 20’W at a latitude of 37°N. (Mean
speed over 273 days = 2.51 cm/s towards 268°). A list of
positions for this eddy is given in Table 1.
Arrows in the Gulf of Alaska indicate eddy motion there,
shown on a larger scale in Figure 4. This Figure shows the
height anomalies during cycles 13, 15 and 17, corresponding
to mean dates June 7, July 12 and August 15 1987. Several
anticyclonic eddy-like features (positive height anomalies)
are evident, especially in the north-west half of the study
area, and a westward movement can be seen in most cases.
Mesoscale features are less evident in the region to the
south-east, which is dominated by larger scale anomalies
with an annual period, as noted above.
The data on the tracks plotted in Figure 4 (n = 166 to 173)
are collected on days 13, 16, 2, 5, 8, 11, 14 and 1 respectively
of the 17 day cycle. Displacements of the order of the track
separation, occurring within this time could therefore be
aliased on these plots. This would affect features with
speeds greater than about lOcm/s. Slower moving features
should be adequately sampled in time.
Figure 5 (from Gower, 1989b) shows a plot of the mean
centroid positions of eddies identified from the data
sequence of repeat cycles of Geosat data, covering
November 1986 to May 1988. Mean positions and velocities
are listed in Table 2. Those shown represent the main
features that formed between cycles 1 and 20. Some eddies
that formed later, whose tracks would have overlapped
those shown, are omitted.
The eddy "S" is identified with the Sitka eddy previously
reported in ship observations by Tabata (1982). The name
"Haida" has been suggested for eddy "H". Other eddies are
here labeled "A", "B" and "C".
The positions in Figure 5 appear to cluster on the satellite
tracks, suggesting that the eddies are under-sampled by the
satellite tracks, that is, that the width of a typical height
anomaly is less than the track separation. In this case
attempts to track peaks in a noisy background will tend to
give locations nearly centered on a track.
Table 1 lists the results of eddy tracking in the Gulf of
Alaska. Speeds are deduced from positions selected to
reduce the effect of the clustering noted above. They are
mostly in the range 1.0 to 1.4 cm/s, with headings between
240 and 280° (through east from north), that is, westwards.
Major differences from these mean values occur at the end
of the data sequence (B 34 and H 34) or at the edge of the
spatial coverage (C 13). Mean and standard deviations for
all the values shown are 1.3 ± 0.4 cm/s in speed, and 252°
±_ 28° in direction.
100
