Analysis of the ERTS data in portions of the eastern Caribbean are presented for October 1972 showing features which are, as yet, not explained. Ground truth data obtained in that area during November 1972 are presented. These include vertical temperature structure in the mixed layer and thermocline, and surface measurements of salinity, temperature, and chlorophyll

Cram, R.

Hanson, K. J.

Hebard, F.

English

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Paper M 1 2 
OCEANOGRAPHIC FEATURES IN THE LEE OF THE WINDWARD AND LEEWARD 
ISLANDS: ERTS AND SHIP DATA 
Kirby J. Hanson, Frank Hebard,l and Richard Cram,* Atlantic Oceanographic and 
Meteorological Laboratories, National Oceanic and Atmospheric Administration, 
Miami, Florida 
ABSTRACT 
Analysis of the ERTS data in portions of the eastern Caribbean 
are presented for October 1972 showing features which are, as yet, 
not explained. 
Ground truth data obtained in that area during Nobember 1972 
are presented. These include vertical temperature structure in the 
mixed layer and thermocline, and surface measurements of salinity, 
temperature, and chlorophyll. 
1. INTRODUCTION 
3 In previous work in this area, Ingham and Mahnken found that tuna schools 
and bird flocks were concentrated in an area west of St. Vincent Island, Lesser 
Antilles. They also noted that plankton and primary productivity in the mixed 
layer were higher in this area than in the Atlantic, and that horizontal tempera- 
ture differences existed in the upper thermocline in this area which suggested 
the presence of eddy-like features in the ocean. They suggested the possibility 
that the increased productivity of this area is a consequence of downstream 
turbulence on the Caribbean side of St. Vincent Island. 
, 
2. ERTS-1 DATA 
NASA has collected ERTS-1 MSS Channel 4-7 data over portions of the 
eastern Caribbean on four cycles in the September-November, 1972 period. For 
the two ERTS cycles of best photographic coverage, the ERTS photos have been 
National Marine Fisheries Service, NOM, Washington, D. C. 
Rosensteil School of Marine and Atmospheric Science, University of Miami, Coral Gables, Florida 
Turbulence and Productivity near St. Vincent Island, B.W.I., A preliminary Report, Contribution 
No. 42, Tropical Atlantic Biological Laboratory, Bureau of Commercial Fisheries, Miami, Florida 33149 
1357 
figlnal photography may be 
WOS Data Center 
0th m d  Dakota Avenue 
SIOUX Falls, SD ~~~~ 
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https://ntrs.nasa.gov/search.jsp?R=19730019619 2020-03-17T08:51:34+00:00Z
mosaicked and are shown in Figures 1 and 2. The island arc is visible in many 
of these ERTS photos. 
In comparing Figure 1 (October 13-16, 1972) and Figure 2 (Nov. 17-21,1972), 
it is clear that there are horizontal differences in the upward spectral radiance 
from the ocean surface during the October 13-16 period, but such differences are 
only slightly apparent in the November 17-21 period. These horizontal differ- 
ences appear in the lee (west side) of each of the major islands of the Lesser 
Antilles. Figures 1 and 2 have been presented as negative prints of MSS Channel 4. 
Figure 1. ERTS MSS Channel 4, 13-14, 1972 
Figure 2. ERTS MSS Channel 4, Nov. 17-21, 1972 
The ocean area west of Guadeloupe and Dominica is shown in more detail 
in Figure 3 and the ocean area west of Grenada and St. Vincent is shown in 
Figure 4 from the October 14, 1972, MSS Channel 5 data, These same ocean 
features are visible in MSS Channels 4-7. 
3. GROUND TRUTH DATA (Nov. 15-16, 1972) 
Ground truth information was obtained in the eastern Caribbean on the 
NOAA/CARIB cruise of the NO& ship DISCOVERER, The ship track and times 
are shown in Figure 5; the hours are GMT. The ocean variables measured were: 
1359 
Figure 3. ERTS MSS Channel 5, 
October 14, 1972. Shows 
Guadeloupe and Dominica 
and features in the lee of 
the islands. 
Figure 4. ERTS MSS Channel 5, 
October 14, 1972. Shows 
features in the lee of St. 
Vincent (top) and Grenada 
(bottom). 
1. 
2. surface salinity, 
3. surface chlorophyll, 
4. surface nutrients, 
5. 
temperature profile in the mixed layer and thermocline, 
upward spectral radiance at the sea surface. 
The first three variables have been analyzed and are  presented in Figures 6 
through 9. 
4. DISCUSSION 
At the time of collection of ground truth data in the Eastern Caribbean in 
mid-November, 1972, ERTS data were not yet available to give an indication of 
1360 
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+ +QANn=fi+ + I +  0 
OSOO,*.Z' + + I6.N 
[Now. 15. 1972) , '." '200--"%~ + 
1400--\, DOMINICA 
0400--. 
'. + 
+ 
+ 
(No". 16.1972) 0 0 0 0 '  0 ST. 
c + + f  ti+ + 
0200-: , + : d' 
0400-. 
l2.N 
60' W 
+ ioGRfNA? 
62.W 
4- + 
64.W 
Figure 5. Cruise of DISCOVERER, 
NOV. 15-16, 1972 
LATITUDE (deg. No~hl 
16 1 5 O  14 13 O 
Figure 6. Temperature cross-section along ship track of Figure 5. 
1361 
SURFACE . . . . . . . . . . . . . .  
TEMPEZATURE . . . .  ." ..... 
( d e s  C) 
LATITUDE (dug. North) 
Figure 7. Surface temperature along ship track of Figure 5. 
36 
SURFACE 
SALINITY 35 
(P$) 
I I I I 
34 
16" 15" 14" 13 " 1.2 " 
LATITUDE (deg. North) 
Figure 8. Surface salinity along ship track of Figure 5. 
I 
- ' 0  c: 1 1 
15-16 NOV 7 2  CSL@ROPHYLL-a 
Figure 9. Surface chlorophyll along ship track of Figure 5. 
1362 
the size and spatialedistribution of the features which were apparent in ERTS 
data, occurring in the lee of each major island in the Antilles arc (Figure 1). 
Because of this lack of information when the ground truth were collected, only 
a single north-south cross section was obtained which parallelled the Antilles 
Island arc. 
In the November 19-20, 1972 ERTS data (Figure 2), there are only very 
small horizontal gradients in image density in the lee of Martinique and Grenada 
compared to the October 13-14 period. The ground truth data (Figures 6 - 9) 
obtained four days earlier in that area do not reveal a pattern similar to that of 
the ERTS data of November 19-20, 1972. 
Unfortunately, there were no ground truth taken during the October 13-14, 
1972 period which is the only time these ocean features have been clearly 
apparent in the ERTS data. The features visible in the October 13-14, 1972 
ERTS data (Figure 1) may result from one or a combination of causes associated 
with the atmosphere or ocean. Some possibilities are: 
1. 
2. 
The sea state and/or swell is modified in a wind shadow of the island, 
The run off of island rainfall carries high turbidity water in the mixed 
layer downstream of the island, 
The phytoplankton in the mixed layer of the ocean has increased due to 
increased nutrients associated with upwelling. 
3. 
We have compared the surface wind observations on the islands during the 
October 13-14 period with the November 17-21 period and found that the east- 
erly winds were lighter during the October 13-14 period than the November 
19-20 period. Thus, the wind observations at the island stations do not support 
the possible explanation in (1) above, that the sea state is modified due to wind 
shadow of the island. 
In examining the ocean features which are visible in Figure 1, we find that 
the features are most apparent in channels 5 and 6 ,  and are somewhat less 
apparent in channels 4 and 7. 
Based on the present information, it is not possible to reach a firm con- 
clusion about the cause of the features visible in Figure 1. It is hoped that in 
the future ground truth in coincidence with the ERTS data, and adequate ERTS 
data coverage, will resolve the question of the cause of these features. 
1363 


Oceanographic features in the lee of the windward and leeward islands: ERTS and ship data

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