Download Currents of the Caribbean and Adjacent Regions

CURRENTS OF THE CARIBBEAN AND ADJACENT
REGIONS AS DEDUCED FROM DRIFT-BOTTLE STUDIES!
JOHN T. BRUCKS
National Marine Fisheries Service, Tropical Atlantic Biological
Laboratory, Miami, Florida 33149
ABSTRACT
Drift bottles were released in the Caribbean Sea and adjacent Atlantic
Ocean in 1967 and 1968 from vessels of the Tropical Atlantic Biological
Laboratory of the Bureau of Commercial Fisheries (now known as the
National Marine Fisheries Service) and the FAO Caribbean Fisheries
Development Project. An analysis of the drift-bottle tracks, supplemented by horizontal sections of thermocline depth and dynamic height,
reveals variations in the surface currents that are not shown in atlases of
average conditions.
INTRODUCTION
Scientists at TABL (the Tropical Atlantic Biological Laboratory) began
a long-term drift-bottle project in 1967, to study the surface currents in the
Caribbean Sea and adjacent regions. The project had two objectives: to
obtain additional information about the general flow patterns throughout
the region, and to obtain detailed information in specific areas of interest.
Before the project began, information on surface currents in the eastern
Caribbean consisted mainly of charts based on reports of ship drift (D. S.
Department of Commerce, 1959). The currents shown on the charts represent mean flow conditions since they were calculated from data averaged
over a number of years and over areas no smaller than I-degree quadrangles.
Short-term currents may differ considerably from the mean currents calculated from historical data. Eddies west of the Windward Islands (Ingham
& Mahnken, 1966) tend to obscure the configuration of mean flow and, because so little is known about the residence time and movement of these
eddies, the spatial and temporal distributions of properties found during an
investigation are valid for only short periods.
The present study shows that data obtained from geostrophic calculations
and thermocline topography may be combined with data from drift bottles to
gain information on local currents of short duration.
DESCRIPTION
OF BOTTLE AND CARD
The drift bottle was a 12-ounce clear glass bottle, ballasted with sand so
that an inch of the neck protruded above the water, and sealed with a waxed
cork (Fig. 1). A card enclosed in the bottle was colored a brilliant fluorescent orange on the exposed side and imprinted with instructions in English,
1 Contribution
No. 189, National Marine Fisheries
75 Virginia Beach Drive, Miami, Florida 33149.
Service,
Tropical
Atlantic
Biological
Laboratory,
456
of Marine
Bulletin
[21 (2)
Science
~fl ri~f
t1l!:
tI'• ~f~
TWt_"_"'_"'_
I
!~
l
I
J
ilE
J
-
t
Cl
-- ---11. •• 1II6IONI
I
I:
U JItf i: "' ..- ....•.....
,...."'--
Co>
til
0
Co>
••••••
-,
~
H~U
Jif~
Ill!
II
.••
,...
I
I
I
I
No _
_,..
•••
••
100
.-
.I'
i·
•........... F_._'
,..- ..•."
, I __
.•....•
--,.
~ ..___•......0.-.
,.....
1
ATINCI6N
_ -
i:
I
i
I
I
__
I
I
I
I
I
I
I
I
I
Mo_
"-
•••
_
•••••
UIl
••••
!
_.-
Nne oI6Ddcr
~~r.:'--
REGARDEZ
Ccae c.n.e .,
-"'-
COW'UtJ
I.
IlUood6a
•• •• •••••••
Soft odd •••••
l3nden<o
l.ocIdoa
fCNOd (0UrtCat
topr ••••••••• _(o pMbIo IDM csraoo)
=
"*
c:u1c
IIIW.a6I
pot,If
••••.••
IDU••.•••••••••.•
IiIlaIIC ct.
•••••..
1* ~
1M
~
'--l>
blDbm'.
Pour oa
lnIc:InMI du
pbtt oiIi •• boutdIla
• '"
KtIlpll
Ien'ice
de )a
CIt
IudL
Ncm
kl'n«,-,>
•••.
j
Dale and time fCMld
F'ec:hI,horlnquefaetllClletnda
Sour et hewt
DIll e han IC.'bada • pmta
'"*'"
••
¥OUt tM:l
ATEN~AO
=(;U~~.pr6I)
-pnla.
de
plaI1,Ia1lt'ttl~d'-'-
" IAalidMIt ••
pmla
t doIk c:attIo
i~at:fttfI&:.l6tn~
rnarftima. Ii. "YOf ~
pot 0'-10
o mWeIo
t
ckpoMt'-io
DOl
COfTeiat
JftlII .,.,.
NIo
Ie
pteCb;I
IdoL
En\III,,*Mnf'diatcmtal1l.V.s..~
In~
I6bn: 0 kx:aI GlIde c • data
\luanda
lohlld.
• ptnfa.
Muko
"*
'''''pdo
FIGURE
1.
Drift bottle and card.
Spanish, French, and Portuguese;
a detachable,
self-addressed
postcard
contained an identification number, blank spaces for the name and address
of the finder and the date and place of recovery, and a frank that exempted
the retriever from postage fees.
PROCEDURES FOR RELEASE AND RECOVERY
From February 1967 to December 1968, 3,494 drift bottles were released
from T ABL's research vessel UNDAUNTED and from the vessels CALAMAR
and FREGATA,which were operated monthly (with some exceptions) by the
F AO Caribbean Fisheries Development Project in Bridgetown, Barbados
1971]
Brucks:
Currents of the Caribbean
80'
75'
and Adjacent
'0'
.,.
Regions
60'
".
457
50'
•....\,
<.Ori;:;n~;,
co""'('"-~'!'
.•. ----.,.~
Cl'IfI,tI
"
>
¥
10
"
"
FIGURE
2,
Drift-bottle
release areas,
(Fig. 2), Twelve bottles were usually deployed at each release location,
By July 1969, the number of reported recoveries was 338, or 9,6 per cent.
This recovery rate is reasonably consistent with the 7.4 per cent return reported by Luedemann (1967) for bottles released off the north coast of
Brazil.
Returned cards were separated into groups according to the month of
release, and trajectories were drawn for each recovery. When several trajectories were closely spaced, a single line was used to represent their midpoint. If more than one bottIe was recovered at a particular location, the
number of returns was indicated beside the arrowhead (Figs. 3,5, 7, 9).
GENERAL
SURFACE
CIRCULATION
The release and recovery data are consistent with the well documented
westward drift throughout the Caribbean and the eastward current in the
Straits of Florida (Fig. 3). A northward extension of the Yucatan Current
traversed the Gulf of Mexico and veered, when it approached the north
central coastal area, to form a westward current extending from the Mississippi River Delta to the Texas-Mexico border. The east current reported
in the northeast Gulf of Mexico was not evident during the present study.
The lack of an eastward flow is not in contradiction with other data, however, because drift bottles deployed within the gulf have shown that the CUfrent between Louisiana and Florida apparently varies from a westward to
an eastward drift (Chew et al., 1962; Gaul et at., 1966).
458
Bulletin
of Marine
Science
[21 (2)
O'
FIGURE 3.
Current
pattern
derived from drift
March 1967.
bottles
released
in February-
Velocities derived from drift-bottle data must be considered minimal because of the necessity to use the total time elapsed between release and recovery and the inferred, minimum distance traversed. Although the individual, minimum velocities in this study range from less than 0.1 knot to
2.0 knots, the monthly averages of these values fall into distinct groups that
suggest a seasonal periodicity in the speed of the surface current of the
Caribbean Sea and Gulf of Mexico (Table 1). Speeds were greater during
the summer and midwinter than during the spring and fall. A comparison
of historical fluctuations in wind and current speeds revealed similar results
(Fuglister,1951).
SPECIFIC
REGIONAL
STUDIES
Currents were studied in detail in three regions of the Caribbean: the
Windward Islands, the Gulf of Honduras, and the southwest Caribbean.
TABLE 1
SPEED
1967
1968
(KNOTS)
Jan.
Feb.
0.66
0.55
0.38
COMPUTED FROM DRIFT BOTTLES
MONTH ANDYEAR OF RELEASE
Mar.
June
July
Aug.
RELATIVE
Sept.
Oct.
TO
Nov.
THE
Apr.
May
Dec.
0.22
0.30 0.76 0.84 0.74 0.54 0.48 0.37 0.55
0.65 0.61 0.38 0.40 0.44
0.40
1971]
IS
Brucks:
Currents of the Caribbean and Adjacent Regions
60
61W
62
459
IS
~""""'
14
14
N
Sf lUCIA
13
13
-
o
,
62
FIGURE
~
4.
--
--
---~,
Current
Drrcclion
Geoslrophlc
VclocilY
in Knots
12
----.-00
Dynamic topography of the sea surface relative to the 400 decibar
surface, February 1967. Contour interval is two dynamic centimeters.
(Taken
from Brucks, in press.)
Within these areas, drift-bottle data were supplemented by data from an
electronic salinity-temperature-depth system and a bathythermograph. The
information obtained from the hydrographic observations was used to study
the distribution of the field of mass, which is one of the most reliable
methods for determining mean currents.
The construction of drift-bottle trajectories was facilitated by using the
currents derived from the hydrographic data as guidelines. The present
study shows that when drift-bottle trajectories are combined with the circulation described by the dynamic and thermocline topographies, the dimensions of the hydrographic survey field are effectively increased.
Bulletin of Marine Science
460
61'
62'
W
60'
[21 (2)
16'
15'
MARTINIQUE
lA'
ST. LUCIA
N
HONDURAS
FLORIDA
5T. VINCENT
13'
GRENADA
f1
12'
FIGURE 5. Current pattern derived from the combination of drift-bottle recoveries and the circulation portrayed by the dynamic topography (Fig. 4).
Drift bottles were released in February 1967.
Windward lslands.-The surface current immediately east of the Windward
Islands is generally represented as flowing west to northwest (Wlist, 1964),
but during portions of three fishery-oceanography cruises by the UNDAUNTED,
the current varied considerably in speed, direction, and depth (Brucks, in
press) .
The recovery of drift bottles released in February 1967 revealed that the
northward current portrayed by the dynamic topography (Fig. 4) flowed
along the island chain as far north as Dominica (Fig. 5) instead of to the
west, as suggested in the literature. A comparison of average current velocities revealed that the minimum speed of the drift bottles (0.70 knot) was
in close agreement with the computed, geostrophic velocity (0.81 knot).
1971] Brucks:
Currents of the Caribbean and Adjacent Regions
0-
461
BT POSITION
O-DRIFT
BOTTLE RELEASE
BRITISH
HONDURAS
FIGURE
6.
Thermocline depth, in meters, May 1967.
Additional drift-bottle recoveries suggested that the northward current also
was present in the summer and early fall.
Gulf of Honduras.-Although
the surface current in the Gulf of Honduras in
May 1967 was in the form of an inverse S meander (Fig. 6), the flow was
generally not at variance with that described by the U. S. Department of
Commerce (1959) as a northward flow in the northern gulf and a cyclonic
gyre in the southern gulf. Southward intensification of the current is suggested by the greater isopleth relief within the eastward flow in the vicinity
of Utila and Roatan islands. The dominance of the circulation in the
southern Gulf of Honduras is apparent when drift-bottle recoveries are
incorporated (Fig. 7). The major portion of a cyclonic gyre is evident;
88 per cent of all recoveries were from releases in the southern half of the
survey field, and 50 per cent of all returns were from the southwestern gulf.
The northwest to west current, which terminates near Ambergris Cay in
the north portion of the Gulf of Honduras, apparently is a relatively permanent feature. Monthly plots of drift-bottle trajectories show that this current
was present during nine of the 11 months in which bottles were released in
1967, and during four (July through October) of the eight months of release
[21 (2)
Bulletin of Marine Science
462
FIGURE 7. Current pattern derived from the combination of drift-bottle recoveries and the circulation portrayed by the thermocline depth (Fig. 6). Drift
bottles were released in May 1967.
• - BT POSITION
0- DRIfT
BOTTLE RELEASE
o
2.
-<!>-
COSTA
FIGURE 8.
Thermocline depth, in meters, April 1967.
1971] Brucks:
Currents of the Caribbean and Adjacent Regions
463
100
FIGURE9. Current pattern derived from the combination of drift-bottle recoveries and the circulation portrayed by thermocline depth (Fig. 8). Drift
bottles were released in April 1967.
in 1968. The same current was found by geomagnetic electrokinetograph
measurements in May 1968 (Cochrane, 1968).
Southwest Caribbean.-The circulation in the southwest Caribbean appears
to be considerably more complex than that indicated by ship drift, as compiled by the U. S. Department of Commerce (1959). Although a portion
of the generally accepted cyclonic circulation was apparent, the current was
obviously displaced offshore along the western half of Panama (Fig. 8).
According to drift-bottle returns, the current-which veered in a southerly
(onshore) direction immediately east of central Panama-eventually
doubled back to form an inshore, westward flow along the north central coast
of Panama (Fig. 9). To my knowledge, this westward flow has not been
reported heretofore in the literature for the area.
464
Bulletin
of Marine
Science
[21 (2)
ACKNOWLEDGMENTS
I thank the Miller Brewing Company, Milwaukee, Wisconsin, for donating 20,000 bottles in support of the, TABL project; the FAO Caribbean
Fisheries Development Project, Bridgetown, Barbados, for releasing most of
the drift bottles; and the student members of the Coral Gables High School
Marine Biology Club, Coral Gables, Florida, for preparing the drift bottles.
SUMMARY
Investigations of currents based on geostrophic calculations, thermocline
topography, and drift-bottle data are complementary. When drift-bottle data
are combined with the detailed circulation described by the dynamic height
and thermocline topographies, the dimensions of the survey field are effectively increased. Subjectivity, in turn, is reduced by the use of dynamic
height and thermocline topographies as guidelines for the construction of
drift-bottle trajectories.
Two features were found in the surface circulation which are not evident
in atlases of average conditions:
1. A predominantly north current-in
lieu of a westward drift-was
evident along the island chain north of St. Vincent Island.
2. The cyclonic circulation in the southwest Caribbean was found to be
adjacent to Costa Rica, but offshore along western Panama. An inshore,
westward current was evident along the north central coast of Panama.
The variation in surface circulation found during this somewhat incomplete study in the Panama area suggests a need for additional investigation. To describe the circulation and oceanic conditions in the region, a
detailed oceanographic surveillance of the entire southwest Caribbean
should be conducted on a seasonally synoptic basis from the coastline to no
less than 11 degrees north latitude.
SUMARIO
CORRlENTES
DEL CARIBE Y REGIONES ADYACENTES SEGUN SE DEDUCE
DEL ESTUDIO DE BOTELLAS DE DERIVA
En 1967 y 1968 se lanzaron botellas de deriva en el Mar Caribe y el
Oceano Atlantico adyacente desde dos barcos pertenecientes al Tropical
Atlantic Biological Laboratory (National Marine Fisheries Service) y al
Proyecto de la FAO de Desarrollo de Pesca del Caribe. Un amllisis de la
trayectoria de las botellas de deriva, suplementado con secciones horizontales de la profundidad y altura dinamica de la termoc1ina, revelaron dos
aspectos en la circulaci6n de la superficie que no son evidentes en atlas de
condiciones promedio.
1. Una corriente predominantemente norte-en
lugar de una deriva
1971] Brucks:
Currents of the Caribbean and Adjacent Regions
465
bacia el oeste-fue
evidente a 10 largo de la cadena de islas al norte de la
Isla San Vincente.
2. La circulaci6n cicl6nica en el suroeste del Caribe se encontr6 ser
adyacente a Costa Rica, pero lejos de la costa a 10 largo de Panama occidental. Una corriente bacia el oeste, cerca de la costa, fue evidente a 10
largo de la costa central septentrional de Panama.
REFERENCES
T.
In press. The current east of the Windward Islands, West Indies. Proc.
Symp. Invest. and Resour. Carib. Sea and Adj. Reg.
CHEW, FRANK, K. L. DRENNAN, AND W. J. DEMORAN
1962. Some results of drift bottle studies off the Mississippi delta. Limno!.
Oceanogr., 7(2): 252-257.
BRUCKS, JOHN
COCHRANE, J. D.
1968.
Current and waters of the eastern Gulf of Mexico and western Caribbean, of the western tropical Atlantic Ocean, and of the eastern tropical Pacific Ocean. Pp. 19-38, in Richard A. Geyer, Oceanography
of the Gulf of Mexico. Progress report. Tex. Agric. Mech. Univ.,
Ref. No. 68-8T, June, ii + 74 pp.
FUGLISTER, FREDERICK C.
1951. Annual variations in current speeds in the Gulf Stream system. J.
mar. Res., 10(1):
119-127.
GAUL, R. D., R. E. BOYKIN, AND D. E. LETZRING
1966. Northeast Gulf of Mexico hydrographic survey data collected in 1965.
Tex. Agric. Mech. Univ., Ref. No. 66-8T, 202 pp.
INGHAM, MERTON C., AND CONRAD V. W. MAHNKEN
1966. Turbulence and productivity near St. Vincent Island, B.W.I. A preliminary report. Carib. J. Sci., 6(3-4): 83-87.
LUEDEMANN, E. F.
1967. Preliminary results of drift-bottle releases and recoveries in the western tropical Atlantic. Bolm Inst. Oceanogr., S Paulo, 16: 13-22.
U. S. DEPARTMENT OF COMMERCE
1959. Climatological and oceanographic atlas for mariners. Vo!. 1. U. S.
Government Printing Office, Washington, D. C.
WUST,
GEORG
1964.
Stratification and circulation in the Antillean-Caribbean basins. Part
I. Spreading and mixing of the water types, with an oceanographic
atlas. Columbia University Press, New York, 201 pp.