Limnol.
Oceanogr.,
26(6),
1981, 1074-1083
Circulation
in Enewetak
Atoll lagoon1
M. Atkinson,2
S. V. Smith,2 and E. D. Stroup
Hawaii Institute of Marine Biology, University
of Hawaii, Kaneohe 96744, and
Department
of Oceanography,
University
of Hawaii, Honolulu
96822
Abstract
Currents at Enewetak Atoll, Marshall Islands, were measured on the reef margins, in the
channels, and in the lagoon. Lagoon circulation
is dominated by wind-driven
downwind
surface flow and an upwind middepth return flow. This wind-driven
flow has the characteristics of an Ekman spiral in an enclosed sea. Lagoon flushing is accomplished
primarily
by
surf-driven
water input over the windward
(eastern) reefs and southerly drift out the South
Channel. Mean water residence time is 1 month, while water entering the northern portion
of the atoll takes about 4 months to exit.
Shallow lagoons of coral atolls have
current patterns that are largely tidal
(Gallagher et al. 1971; Henderson et al.
1978; Ludington
1979), whereas deep lagoons have circulation patterns that seem
primarily wind-driven
(von Arx 1948).
von Arx’s model, typified
by Bikini
Atoll, Marshall
Islands, conceptualizes
deep-lagoon circulation
using two basic
patterns: a “primary
circulation”
and a
“secondary circulation.”
The primary circulation consists of wind-driven
surface
water moving downwind,
sinking, and
then returning upwind to the windward
(eastern) side of the atoll lagoon as deep
water. The secondary circulation
consists
of a horizontal
recirculation
of deep
water. von Arx reported that the volume
transport
of the upwind-flowing
deep
current is greater than the volume transport of the downwind
surface current and
concluded that some of the deep water is
upwelled
in the eastern part of the lagoon, rejoining the surface current, while
the remaining deep water recirculates in
horizontal gyres. This recirculating
deep
water diverges at the lee edge of the
windward
reef, forming two counter-ro-
t Hawaii Institute
of Marine Biology Contribution 603.
This research was supported by DOE contract
EY-77-5-08-1529.
Parts of the research were done
under the auspices of the Mid-Pacific
Marine Laboratory.
* Present address: Department
of Zoology, University of Western Australia, Nedlands WA 6009,
Australia.
tating bodies of water, the northern one
moving counterclockwise
and the southern clockwise, both appearing to follow
the bathymetric
contour of the lagoon basin. The conspicuous feature of von Arx’s
model is the deep return flow from the
leeward to the windward side of the atoll.
This return flow connects the primary circulation with the secondary circulation.
There has been little further research
on deep atoll lagoon circulation
since the
investigations
by von Arx. We recently
completed
a descriptive
oceanographic
study of the lagoon of Enewetak Atoll,
Marshall Islands, and have arrived at a
circulation
model somewhat
different
from the model presented by von Arx. We
here briefly describe our results and proposed model.
Enewetak Atoll lies about 300 km due
west of Bikini Atoll. Enewetak is somewhat larger than Bikini and has a more
symmetrical,
nearly circular shape (Fig.
1); its physical dimensions are listed in
Table 1. These atolls lie in the North
Equatorial Current, with mean westward
flow in the range 25-50 cm * s-l. The presence of the island group adds much local
complexity
to the westward
flow. The
very shallow, O-2 m deep, atoll-perimeter reefs, with only a small number of
gaps opening to the ocean, effectively
isolate the wide interior lagoons from any
significant
direct influence
of offshore
ocean currents. The offshore surface layer is deep. At a station made by RV Horizon
in October
1952 at 12”14.5’N,
164”50’E (about 100 km NW of Bikini)
1074
Enewetak
I
162=‘E
I
163O
1075
circulation
1
164O
I
I
165”
1660
-120
ENEWETAK
0
RONGELAP
B’K’N’ 0
./
Lc-(
-’
AILINGINAE-
-IIoN
NORTHERN MARSHALL
ISLANDS
WOTHO,
I
I
165O 20’E
NA@J
165”30’E
BIKINI ATOLL
IS.
ENIRIt? IS.
ENYU CHANNEL
AlRtiKllJl
IS.
0
6
km
Fig. 1. Location
channels. Smoothed
of Encwetak
and Bikini atolls. Individual
bathymetry
in meters; coral knolls omitted.
the temperature
at 73 m was only 0.3”C
lower than that at 9 m. The deepest pass
through the reef at Enewetak has maximum depth of 56.7 m, so normally only
relatively
uniform ocean surface water
will enter as tidal inflow. The strong tidal
current in this restricted pass also tends
to mix away any slight vertical gradients
that may be present. The major input to
the lagoon is the surf-driven
inflow over
the shallow perimeter reef; this normally
occurs over the eastern half of the atoll,
which faces the prevailing
tradewinds.
This inflow consists of water from the upper few meters of the ocean. These considerations
make it seem unlikely
that
the lagoon can ever contain anything cxcept uniform water, and we measured no
perceptible
stratification
within
the lagoon.
We thank the various agencies and individuals on Enewetak Atoll for cooperation and assistance, the many technicians and volunteers for assistance in the
maps show
distribution
of islands
and
field operation; V. Noshkin and R. Buddemeier for assistance, comments, and
cooperation
throughout
the study; and
several women for the use of sewing machines to make drogues. We particularly
thank B. Gallagher and J. Capcron for
comments on the manuscript.
Methods
Most of our current measurements
in
Encwetak
lagoon were made during a
smnmer period (July-August
1978) and
a winter period (February-March
1979).
Our data and observations,
however,
span the period from 1971 to 1979, during
all seasons of the year. Measurements
of
current speed and direction were calculated from positions of current drogues
(surface area, 5.2 m2; for design see Montgomery and Stroup 1962). Drogue positions were determined with a Del Nortc
microwave trisponder system, accurate to
_t3 m over a range of 20-25 km. Surface
drogues were tracked for 2-6 h, with fixes
1076
Atkinson
Table 1. Physical dimensions
(from Emery et al. 1954).
of Enewetak
Atoll
Area of atoll
Total island area
Height of islands
Length of windward
reef
including islands
Length of windward
reef
not including islands
Length of total leeward reef
et al.
Atoll
1,015 km*
6.5 km2
4m
49 km
27 km
47 km
Lagoon
Area
Volume
Mean depth
Max depth
Sill depth
Max length
Max width
Mean tide range
Spring tide range
420.5
932
lo8
47.9
64.0
47.5
37
37
0.85
1.46
x
km*
m3
m
m
m
km
km
m
m
o CURRENT METERS
t DRIFT BOTTLES
SOUTHCHANNEL
Fig. 2. Approximate
initial positions
measurements
over entire study period.
of current
Channels
Deep Entrance
Width
Max depth
Cross-sectional
area
1.2 km
56.7 m
34,000 m*
area
9.7 km
11-22 m
145,000 m*
Southwest Passage
Width
Depth
Cross-sectional
area
4.2 km
1.8 m
9,000 m*
South Channel
Width
Depth
Cross-sectional
every 30-45 min. Deep drogues were
tracked for 2-10 days, with fixes l-2
times per day.
The speed of water flow over the reef
was measured with drift bottles and current meters. Fluorescein
dye was also
used to estimate the direction of flow on
the perimeter of the atoll during ebbing
and flooding
tides. Since dye-bombs
were deployed and observed from a helicopter, these perimeter measurements
were virtually
simultaneous;
they provide estimates of flow direction, but not
of speed. Currents in the Deep Entrance
and the South Channel were measured
with current drogues, a Bendix geomagnetic bidirectional
ducted current meter,
and fluorescein dye.
The thickness of the surface layer and
changes in the direction of water movement with depth were determined
with
fluorescein dye dispensers hung on a vertical line. The dispensers
were 20-ml
plastic vials with a pinhole in the side.
Figure 2 shows the approximate
positions of each type of current measurement during our investigation.
Currents
Cross-reef currents-Shallow
currents
across the reef margins of the atoll constitute significant exchange between the
ocean and lagoon.
The area of windward
cross-reef currents is shown in Fig. 3 by horizontal
lines along the eastern boundary of the
atoll. Waves breaking on the windward
reef create a raised sea surface on the
ocean side of the reef, a phenomenon
first described
by Munk and Sargent
(1948) for Bikini Atoll. The windward
cross-reef currents flow downhill
from
this raised sea surface, across the reef flat
to the lagoon. The strength of the flow
therefore responds to surf height and tide
height and not to local wind conditions.
Water enters the lagoon approximately
perpendicular
to the reef front; we have
not observed the windward
cross-reef
currents
to reverse direction
and dis-
Enewetak
charge water from the lagoon. The speed
ranges from 10 to 150 cm * s-l, and the v01ume transport from 0.05 m3 * s-l per meter
of reef front during low tide and low surf
to about 1.5 I113* s-l *m-l during high tide
and high surf. Over a l-week period in
June 1971, an average value of 0.556
m3,
s-‘.
m-l
was derived for a site immcdiately north of Japtan. This flow is equivalent to 6.6 x 10s m3 per tidal cycle across
the entire windward
reef. (This time interval, 12 h 25 min, is used throughout to
facilitate
comparisons
of volume transports .)
The area of leeward cross-reef currents
is represented by vertical lines on Fig. 3.
These currents are weak and do not flow
in any well developed pattern. They flow
to and from the lagoon and along the reef,
but generally have a slow net oceanward
drift. WC have estimated them to have a
net outflow of 0.4 x lo* m3 per tidal cycle
(i.e. about 6% of the inflow across the
windward
reefs).
Channel currents-The
current in the
Deep Entrance (Fig. 3) has a maximum
speed of about 80 cm. s-l and reverses
with the tide. The slack period in the
channel is no more than a few minutes.
Velocity
is nearly constant throughout
the water column. Near maximum spring
tide, using current meter data, we calculated a transport of 3.0 x 108 m3 per half
tidal cycle each way through the channel.
Surface and deep drogues placed in the
channel on a flooding tide traveled several kilometers into the lagoon and then
reversed direction
on the ebbing tide.
From our limited data we suggest that the
net volume transport through the Deep
Entrance is about zero over a tidal cycle.
The South Channel (Fig. 3) has nearly
continuous
outflow. The surface water
moves westward
as wind drift, while
water below 5 m moves southwest
to
south, depending
on the tide condition.
Current
speeds range from 8 to 30
cm-s-l. The average outflow is 6.9 x lo*
m3 per tidal cycle (105% of the calculated
inflow over the windward reef).
The Southwest Passage (Fig. 3) has a
reversing current similar to that in the
Deep Entrance, but that passage is much
1077
circulntion
Fig. 3. Cross-reef currents
See text for discussion.
an d channel
currents.
smaller than the Deep Entrance (Table
1). Net flow through the channels of this
passage has been incorporated
into the
net flow over the leeward reefs (0.4 x 10’
m” per tidal cycle).
Lagoon currents-The
central lagoon
has surface, middepth,
and deep flows,
distinguishable
by their characteristic
speed and direction, We discovered the
detailed vertical flow structure not from
the drogue studies, but late in our investigation
when we deployed
vertical
strings of dye dispensers. Consequently
the characteristics
of these flows are not
well quantified.
Although the central lagoon flow is vertically
structured,
the
water column within the lagoon may bc
considered to be of uniform density because of nearly isohaline and isothermal
conditions. At-any time salinity varies by
no more than about 0.2%0 (mean near
varies by no
34.5%0), and temperature
more than 0.5”C (annual range 27”-29°C).
The general surface drift is downwind
(Fig. 4). In the central lagoon drogues
moved south, west, and north, appearing
to respond to the wind direction of the
previous
6-12 h. (Wind direction
and
speed were measured three times daily
from Enewetak Island.) The speed of surf ace flow is about 2% of wind speed. The
surface current moves in a layer that var-
1078
Atkinson
Fig. 4. Lagoon surface currents from drogue
data. Arrows represent smoothed drogue trajectories over varying lengths of time; they are not vectors. Some drogue runs were made during calm or
variable wind.
ies from 5 to 15 m thick. Downwind
volume transport
of the surface layer is
about 9.2 x 108 m3 per tidal cycle.
The middepth flow is generally northeastward, opposite to the surface flow, as
shown by dye traces (Figs. 5,6). The middepth flow is observed between 10 and
30 m at speeds of 24 cm. s-l (about half
the surface speed). The volume transport
is about 8.6 x 108 m3 per tidal cycleabout 93% of the surface-layer
volume
transport.
Below
30 m the deep flow moves
southward at l-2 cm. s-l (Figs. 6, 7). We
followed drogues in this flow for up to 10
days. While the southerly movement of
the drogues was consistent and predictable, there seemed to be a 6-12-h eastwest movement in apparent response to
flow around
lagoon pinnacles,
tidal
pumping, or both. The volume transport
of the deep flow through a cross-section
near the middle of the lagoon is about
2.2 x lo8 m3 per tidal cycle.
The deeper flows are offset to the right
(clockwise)
from the shallower
flows
(Fig. 5a, b). From Runit Island to West
Spit (Fig. 6), across the lagoon center, the
current spirals to the right, forming the
et al.
eastward motion which we have called
the middepth flow.
The vertical spiral of Fig. 7 reveals the
basic system: the surface flow (O-10 m)
is westerly, and the middepth flow (lo30 m) is easterly during a period of winds
from the east. The transition to the weak
southerly deep flow varies with location
(see below) and thus is not clearly shown
in this summary figure.
This vertical flow structure can be distorted by cross-reef currents, tidal currents near the channels (Figs. 5c, 6), and
the boundaries
of the lagoon. The lefthand spiral in Fig. 5c is an example of
tidal current
overwhelming
the righthand open lagoon vertical flow structure.
Along the central lagoonward margin of
the windward
reef, the current direction
shifts with varying tide and surf conditions.
The area near the northwestern
leeward reef tends to be one of convergence.
Little lagoon water escapes over the leeward reef, particularly
when large surf,
resulting
from a northern swell, drives
ocean water over this reef into the lagoon. We have often observed large aggregations of jellyfish
as well as strong
southwesterly
flow along the lagoonward
margin of the reef in this region,
Water budget and residence time
Table 2 summarizes the volume transports for the important components of the
water budget. Transports do not sum to
zero over a tidal cycle because the data
were collected
during
different
tide
stages and were not forced to balance.
Water flows into the lagoon from the
windward
reef, the Deep Entrance, and
the Southwest Passage. The windward
cross-reef current transports about twice
as much water as the Deep Entrance current. It does not reverse as do the currents in the Deep Entrance
and the
Southwest Passage. Therefore,
the flow
over the windward
reef represents the
only significant
net input of water into
the lagoon (Table 2).
The water flows out of the lagoon from
the leeward reef, the Deep Entrance, the
Atkinson
1080
et al.
NE
TRADES
N
A
0
km
DEEP- WATER
CURRENT
(30-50
m)
6
Fig. 6. Vertical flow profiles using dye dispensers suspended on a vertical line. Arrow shows direction of flow and number at end of arrow gives
depth in meters. Circled number is bottom depth
in meters. Winds blew from the east when these
measurements
were made.
the cumulative
input over the windward
reef increases from north to south, the
southerly flow also increases.
In the most elementary
analysis, the
average residence time of water in the
lagoon is given by dividing
the lagoon
volume by the rate of water input. If approximately
30% of the ocean water entering the lagoon through the Deep Entrance exchanges with lagoon water-the
percentage suggested by von Arx (1948)
for exchange of passages and channels at
Bikini-then
the total rate of input is the
influx along the windward reef plus 30%
of the inflow through the Deep Entrance.
When we use this total, the average residence time of water in the lagoon is 28
days. The contribution
from the windward reef comprises 85% of the total inflow. Clearly there is a variation of actual
residence time from one part of the lagoon to another, because water is introduced all along the windward
reef but
exits primarily
through the South Channel, and there is no major north-south recirculation
The residence time will be
relatively
long for water entering
the
0
LA-J
cm.s-I
A
D
0
0
l
O-IO
IO-20
20-30
30-40
40-50
m
m
m
m
m
Fig. 7. Summary of drogue results. Shaded spiral represents approximate
end points of flow vectors from the origin.
north end of the lagoon and short for
water entering the south end of the lagoon.
The residence
time of lagoon water
based on flushing rates of transuranic radionuclides
is about 4 months (V. Noshkin pers. comm.). This does not contradict our result of 1 month, since the
sediment radionuclide
activities are about
two orders of magnitude
higher in the
north end of the lagoon than in the south
(Nelson and Noshkin 1973). On the assumption that diffusion of these isotopes
into the water is proportional
to their concentration in the sediments, the northern
lagoon water will be labeled preferentially, resulting
in long apparent residence times for lagoon water. On the basis of our finding that most of the water
entering
over northern
reefs (about a
quarter of the total inflow) must transit
the entire lagoon to exit from the South
Channel, this water should have a residence time about four times longer than
the residence time for water in the lagoon
as a whole.
Enewetak
1081
circulation
Table 2. Water budget. Estimates of mean and range. “+” represents water flowing into lagoon, “-”
represents water flowing out of lagoon. Cross-reef current estimates are based on average transport per
meter of open reef front. Channel current estimates arc based on average speeds from all data, observations,
and cross-sectional
area. Lagoon wind-driven
flows are based on averages of drogue data and maximum
cross-sectional
area at that depth.
Volume
Current
Windward
Leeward
cross-reef
cross-reef
transport
mean and (mngc)
1oR m* per 12.4 h
+6.6 (+2.2 to t19.8)
Continuous
-0.4
Variable
(slightly
plus to -0.8)
Deep Entrance
Net = 0 (-1.0
m3 transport
South Channel
-6.9
Southwest
Net = 0 (-0.2
m3 transport
Passage
Comments
(-4.5
to +l.O) (3.0x1@
each way)
to -8.5)
Surface
9.2 (3 to 30)
Middcpth
8.6 (unknown, but probably
the same as surface)
Deep
2.2 (unknown)
Circulation
The circulation
of Enewetak Atoll lagoon can be explained as a response to
three driving mechanisms:
surf on the
windward
ocean reef, wind stress on the
lagoon surface, and tides.
Surf-The
breaking
waves on the
windward
reef drive water over the
windward reef and into the lagoon on the
eastern (prevailing
windward)
side of the
atoll. The cross-reef currents and the currents behind the reef are therefore dependent
on surf height and depth of
water on the reef (O-2 m). This oceanic
water spreads into the lagoon, moving
downwind
and mixing both vertically
and horizontally.
Since the South Channel is the only significant region of outflow, the water has a net transport to the
south, increasing from north to south to
accommodate water inflowing
across the
windward
reef.
Wind stress -The
wind creates the
downwind
drift of surface water and the
upwind drift of middepth water.
The downwind
drift of surface water
and the shallow vertical current spiral
(Fig. 7) resemble the pattern predicted
by Ekman (1905) for an enclosed sea in
Reversing; typical
80 cm *s-l
Continuous
tide
to +0.2) (0.8x lOa
each way)
about
inflow
speed and direction
Reversing;
outflow;
typical
tidal currents
pulsing
O-
with
tidal currents
Variable;
function
of wind
speed
Variable;
function
of wind
speed
Variable; function of wind
and windward
cross-reef
speed
input
which the following
conditions
apply:
impermeable,
closed boundary; constant,
unidirectional
wind stress over the entire
surface; homogeneous
water; uniform
depth; constant eddy viscosity. At Enewetak these conditions are only partially
met. In particular,
the lagoon rim is
closed neither to leeward nor to windward (Table 2).
In a fully enclosed sea, the Ekman flow
integrated over depth is zero at every location. In a lagoon such as Enewetak this
will not rigorously be the case, but the
detailed
effects of “leaky”
boundaries
and irregular bathymetry cannot be evaluated from our present data. We have observed similar current spirals in Kaneohe
Bay, Hawaii, another “leaky” and irregular basin, and we suggest that these conditions do not impose severe constraints
on Ekman’s model.
These wind-driven
currents at Enewetak are superimposed
on the net flow of
water toward the South Channel.
The
southerly flow can be directly observed
only in the deep water, below the layers
affected by the wind, and in the South
Channel itself.
Our deepest drogue deployments were
Atkinson
1082
et al.
riods of low inflow across the windward
reef relative to downwind
surface drift,
a divergence may develop on the eastern
side of the lagoon and force intermittent
upwelling.
Tides-The
tidal currents directly influence the flow of water within several
kilometers of the passes, especially in the
southern part of the lagoon. These tidal
currents can locally overwhelm the winddriven circulation,
leading to such effects
as the “left-handed”
spiral observed
within the lagoon 1 km north of Enewetak Island (Fig. 5~).
8-------___
--L
0
\B
0
0
0
0
0
27Oc
0
N
’
L
IO
1
0
0
00
1
20
DISTANCE (km)
1
0
1
30
I
S
Fig. 8. Overall direction of drogues suspended
at 38 m as a function of distance from north end of
the atoll.
at 38 m. At that depth in the north end of
the lagoon, where the net southward
transport of water is small (see above),
the water moves northeastward
as the
middepth
flow. In the south end of the
lagoon, where the net southward transport of water is large (from the accumulated water input from the windward
reef), a southerly drift is observed at the
same depth. Figure 8 shows the change
in direction
of the 38-m flow with distance from the north end of the lagoon.
Surface speeds are 5-20 cm- s-l, about
2% of wind speed. The surface drift is
generally downwind
and seems responsive to the wind direction of the previous
6-12 h. The speed of upwind flow at middepth is about half that of the surface layer.
currents
would
The se wind-driven
cause the lagoon surface water to turnover in 5-lo-days
if there were no vertical mixing. von Arx (1948) estimated approximately the same time for turnover at
Bikini. von Arx (1948), Munk et al. (1949),
and Ford (1949) suggested that the surface water sinks in the western portion of
the lagoon and upwells in a small band
in the-eastern portion of the lagoon. We
co uld find no direct evidence of this process at Enewetak. However, during pe-
Discussion
Our model
for the circulation
of
Enewetak lagoon has similarities
to the
model proposed by von Arx for Bikini. In
particular, both models show downwind
surface flow and subsurface return flow.
The deep circulation
of our model
seems different
from that proposed by
von Arx. We found deep flow moving
southward and apparently escaping out
the South Channel. He concluded
that
there was an upwind
(eastward) deep
flow (equivalent
to both middepth
and
deep flows in our terminology)
with
transport greater than that of the downwind surface flow, and hence that there
must be upwelling
and deep water recirculation.
In spite of the apparent discrepancy,
we feel that both lagoons may be responding
in fundamentally
the same
way. At Bikini,
besides
net outflow
through the southwestern
channels (von
Arx 1948), we believe there is significant
net outflow through the Enyu Channel
near the eastern end of the atoll (recalculation of information
given by von Arx
1948). This is also seen in surface radionuclide distribution
(Noshkin et al. 1974)
and distribution
of indigenous zooplankton (Johnson 1949). In order to reach this
exit (Enyu Channel) the deep water in
Bikini Lagoon must move eastward. In
Enewetak the only effective exit is at the
southernmost
part of the atoll, so the
deep water must move southward.
Note especially in our circulation
model that the deep motion is primarily con-
Enewetak
trolled by the location of the major exit
points
from the lagoon.
Water flow
through other atoll lagoons stems to hc
dominated
by morphology
and local
wave and tide climate (Milliman
1967;
Gallagher et al. 1971; Henderson
et al.
1978; Ludington
1979). Studies of currents in other deep atoll lagoons would
be valuable in testing our interpretation.
Conclusions
Windward
and leeward cross-reef currents, channel currents, and tidal flow arc
the major factors influencing
the exchange of water between atoll lagoons
and the surrounding
ocean. Because
these factors are specific to local wave
climate, tidal conditions,
and atoll morphology, atoll lagoons have widely vary.ing flushing characteristics.
Wind-driven
circulation,
a pervasive
feature of lagoons, contributes
primarily
to internal
circulation
rather than to flushing.
Upwelling on the windward
side of the lagoon may occur as a summation of the
above phenomena but does not seem to
be a generalizable
feature of lagoon circulation. Deep water flow appears to orient itself toward the channels
of net
water output.
References
EKMAN, V. W. 1905. On the influence of the earth’s
rotation on ocean currents. Arkiv Mat. Astron.
Fysik 2(11): l-53.
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Submitted:
7 March
Accepted: 20 February
1980
1981