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Water circulation in the Gulf of Carpentaria
Journal of Marine Systems, 4 (1993) 401-420 Elsevier Science Publishers B.V., Amsterdam
401
Water circulation in the Gulf of Carpentaria Eric Wolanski Australian Institute of Marine Science, P.M.B. No. 3, Townsville M.C., Qld. 4810, Australia
(Received March 22, 1993; revised and accepted June 9, 1993)
ABSTRACT The water circulation in the Gulf of Carpentaria was modeled numerically, and the predictions of tides and tidal currents compare very satisfactorily with observations at 9 mooring sites from a five month long field study in 1986-1987. In summer, before the occurrence of tropical cyclones, the Gulf is temperature stratified with a sharp thermocline on which ride internal waves up to 20 m peak to trough. When this thermocline is present, the low-frequency circulation shows both barotropic and baroclinic components. Under tropical cyclones, the water column becomes well-mixed vertically but the water circulation is three-dimensional, with a Gulf-wide gyre in the surface waters with a return flow in the bottom waters. The wind generates seasonal sea level fluctuations of about 0.5 m in the southern region of the Gulf and these create vast salt flats. The salt flats are only tidally inundated in summer when estuarine outwelling occurs. Tidal currents and wind-driven currents interact non-linearly to produce a barotropic coastal boundary layer. There is little mixing between estuarine and offshore waters. Large-scale coastal outwelling occurs in summer when this coastally trapped water, presumably nutrient-enriched by estuarine outwelling all along the coast of the Gulf, is ejected offshore at one point in the southernmost region of the Gulf. Estuarine outwelling, coastal outwelling and coastal trapping may have profound biological implications.
Introduction T h e G u l f of C a r p e n t a r i a is a s e m i - e n c l o s e d b o d y o f w a t e r in n o r t h e r n A u s t r a l i a , covering a n a r e a o f a b o u t 193,000 k m 2 (Fig. 1). T h e G u l f is shallow with m a x i m u m d e p t h o f a b o u t 65 m a n d a w i d e c o a s t a l m a r g i n of w a t e r d e p t h sea. T h e o p e n b o u n d a r y to t h e A r a f u r a S e a occurs b e t w e e n Irian J a y a a n d C a p e W e s s e l a n d is a b o u t 50 m d e e p a n d 200 k m wide. T h e c o n n e c t i o n to t h e C o r a l Sea occurs t h r o u g h T o r r e s S t r a i t which is shallow ( d e p t h --- 10 m), a b o u t 100 k m wide, a n d e n c u m b e r e d by n u m e r o u s reefs, islands a n d shoals which restrict t h e t h r o u g h - S t r a i t c u r r e n t s ( W o l a n ski et al., 1988a). T h e G u l f o f C a r p e n t a r i a is an i m p o r t a n t econ o m i c r e s o u r c e . It s u p p o r t s i m p o r t a n t c o m m e r cial fisheries. It is t h e site o f p o r t activities f r o m m i n e s at W e i p a a n d G r o o t e E y l a n d t , a n d f r o m t h e p r o p o s e d C e n t u r y a n d M c A r t h u r R i v e r mines, a n d f r o m a p o s s i b l e s p a c e p o r t . It m a y a l r e a d y b e locally i m p a c t e d in I r i a n J a y a c o a s t a l w a t e r s by u n t r e a t e d m i n e tailings f r o m t h e F r e e p o r t c o p p e r
m i n e a n d by siltation from d e f o r e s t a t i o n as a result of c o m m e r c i a l logging a n d the t r a n s m i g r a tion p r o g r a m . It m a y b e c o m e the focus of oil e x p l o r a t i o n . It is also an i m p o r t a n t s h i p p i n g r o u t e in t h e n o r t h e r n r e g i o n b e t w e e n C a p e W e s s e l a n d C a p e York, f r o m the A r a f u r a Sea to the east coast of A u s t r a l i a . T h e w a t e r c i r c u l a t i o n in t h e G u l f is p o o r l y k n o w n b e c a u s e of t h e p a u c i t y of o c e a n o g r a p h i c data. N e v e r t h e l e s s t h e r e have b e e n several att e m p t s at m o d e l i n g the tides of the Gulf. W e b b (1981) d e v e l o p e d a linear, d e p t h - a v e r a g e d m o d e l with T o r r e s Strait a s s u m e d closed. C h u r c h a n d F o r b e s (1981) p r e s e n t e d t h e results of a non-linear, d e p t h - a v e r a g e d m o d e l which i n c l u d e d the i n f l u e n c e o f T o r r e s Strait. A s s u m i n g a p r i o r i small n o n - l i n e a r effects in the Gulf, R i e n e c k e r a n d N o y e (1982) o p t e d b a c k to a l i n e a r m o d e l . A l l t h e s e m o d e l s u s e d finite d i f f e r e n c e t e c h n i q u e s a n d p r o d u c e d similar results. T h e y all s u f f e r e d f r o m a lack o f o b s e r v a t i o n a l d a t a for b o t h o p e n b o u n d a r y forcing a n d m o d e l verification. I n d e e d , t h e only d a t a available for o p e n b o u n d a r y forcing
0924-7963/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
402
E. WOLANSK1
by the Arafura Sea were tidal data from Cape Wessel, with no data for Irian Jaya at point Z (see Fig. 1). The tidal harmonic constants for point Z were inferred by extrapolation from tidal data from harbours typically 200 km away. These harbours are located in shallow water where the open water tidal constituents have been significantly altered by shallow water effects. While tidal data were available at a number of coastal stations, there were no offshore tidal data and the only current m e t e r data were those at site R
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(see Fig. 1) of R a d o k (1978), and those at sites O, P and Q of Church and Forbes (1983). The latter sites are located in the southernmost part of the Gulf, in shallow coastal waters, giving little information on the water circulation in the bulk of the Gulf. As part of its contribution to A M E X (Australian Monsoon Experiment) the Australian Institute of Marine Science undertook an oceanographic study of the Gulf. The results of this study are described below.
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403
W A T E R C I R C U L A T I O N IN G U L F O F C A R P E N T A R I A
measured, at 2.5 m above the surface, the wind speed, direction and air and water surface temperature. While all the other instruments yielded 100% data return, the mid-period of the weather station data was lost as a result of a power drainage when the sea surface t e m p e r a t u r e sensor was destroyed by the D e p a r t m e n t of Transport attempting to recover the mooring under the incorrect belief the buoy was drifting. O t h e r current meters were deployed at sites K, L, M and N on a cross-slope transect, in mid-January 1987, during the closed trawling season. It was judged unwise to deploy these moorings in October 1986, because commercial trawling at that time is very intense. The current meters were also A a n d e r a a model RCM4'S' and Inter-Ocean model $4 (see Fig. 2). Data were recorded at 15 min intervals. These moorings were also recovered in March 1987, at the start of the open fishing season. The time series of the data were analyzed for their spectra a n d / o r rotary spectra, for the tidal components using a standard tidal harmonic analysis, and filtered for their tidal and lowfrequency components, following the methods of Wolanski et al. (1988a).
Methods
T h r e e oceanographic cruises were carried out in the period October 1986 to April 1987. Vertical profiles of t e m p e r a t u r e and salinity were measured using the C T D profiler described by Wolanski et al. (1988a). In mid-October 1986 four moorings were deployed at sites A, B, C and D, each with two self-recording current meters, either A a n d e r a a model RCM4's' (i.e. fitted with a " S " kit to avoid wave pumping the rotors) or Inter-Ocean Model $4, as is shown in Fig. 2. At sites A, C and F, an A a n d e r a a water level recorder was bottom-mounted. All instruments recorded data at 0.5 h interval, the current or sea level data being averaged over about 1 min. All the instruments were also fitted with a temperature sensor. The instruments were recovered in March 1987. During the same period a number of current meters and tide gauges were deployed in Torres Strait (Fig. 1). The Torres Strait data have been presented by Wolanski et al. (1988a) and are used in this study as open boundary conditions. Also at the same time an A a n d e r a a weather station was deployed on a large buoy, at site X (see Fig. 1) in the middle of the Gulf. The station
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404
E. WOLANSKI
The water circulation was modeled using the depth-averaged, finite-difference, non-linear, centered, implicit numerical model of Wolanski et al. (1988b) where bottom friction is a quadratic function of the velocity through a Manning's roughness coefficient, n. The model allows for the /3 effect. The model domain is shown in Fig. 3. The grid size was 18.53 km. When presenting wind data the oceanographic convention is used, i.e. the wind direction is that the air is blowing to.
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Current and wind direction refers to the direction clockwise from north. Except when otherwise noted, the time is expressed in day number in 1986-1987 (1200 h on Dec. 31, 1986 = time 365.5). N O A A 9 raw satellite data were obtained on magnetic tape. After the image was subsetted and the land masked out, a histogram of the brightness values in Channel 1 (580-680 nm) of the water was produced. This was used to identify the relative brightest and darkest water pixels. The
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WATER
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405
IN G U L F O F C A R P E N T A R I A
image was then stretched so that the darkest water pixels were given values of zero (black) and the brightest water pixels were given 239 (yellow). The land was designated the value of 255 which corresponds to green. Results
The tides The contrasting behaviour between the tidal dynamics at diurnal and semi-diurnal periods, is well illustrated by the dominant M2 and K t tides. The observed tidal ellipses are shown in Fig. 4; the observed tidal height amplitude and phase are shown as insets in Fig. 5. In Fig. 4 the tidal ellipses at diurnal frequency along the transect A - D are oriented about n o r t h - s o u t h ; those at semi-diurnal frequency are oriented about e a s t -
west. The K t and M 2 tidal currents each peaked at about 0.4 m s -1. Together with the other tidal components, peak tidal currents are very strong, peaking at 1.2 m s-~. The tidal ellipses show little change with depth, being equal at the two current m e t e r elevations within 0.05 m s - I and 5 ° direction. Figure 5 shows that the semi-diurnal tides that enter the Gulf from the Arafura Sea, propagate little in the Gulf; the diurnal tides also enter the Gulf from the Arafura Sea but propagate readily into the Gulf. These data provided an ideal data set for a detailed calibration, for the first time, of a mathematical model of the tidal dynamics in the Gulf. Preliminary results were presented by Sodusta (1988) who, using an explicit, non-linear, depthaveraged numerical model, showed that the M 2, Kt, S 2 and O I tidal components were dominant and needed to all be included simultaneously in
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406
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the numerical model because of non-linear interactions principally controlled by the bottom friction term. Normal to a solid boundary the flow was specified to vanish. At the Torres Strait open boundary, tidal harmonics derived from the results of Wolanski et al. (1988a) were specified. The Arafura Sea open boundary was specified using linear interpolation between Cape Wessel and point Z in Irian Jaya (see Fig. 1). The tidal harmonic constants at Cape Wessel were taken from the Admiralty tide tables. An initial approximation of the tidal harmonic constants at point Z was taken from the model results of Webb (1981) and Church and Forbes (1981). These latter constants however lead to large discrepancies, up to 45 ° , between the orientation of the observed and predicted tidal ellipses at sites A and B. By trial and error in tuning the constants at site Z, and comparison between observed and predicted tidal el-
lipses at sites A and B, a final selection of tidal harmonic constants was selected for point Z. This set of condition differs markedly from that of Church and Forbes (1981) for the semi-diurnal tides; in particular there is about a 30 ° phase change from Cape Wessel to point Z for the M 2 tide, instead of 130 ° . Figure 4 shows the observed and predicted tidal ellipses; the comparison is very encouraging. A similar good fit is found (not shown) for the S 2 and O I tidal currents. Figure 5 shows the predicted tide amplitude and phase distribution in the Gulf, together with, as insets, my observations at sites A, C and F and the Admiralty values for coastal ports. The comparison is very encouraging at the mooring sites. The predicted values are generally slightly larger (by 0.05 m) than the observed ones in the coastal areas particularly in the southern region of the Gulf. The K 1 tide enters the Gulf with an amplitude
Fig. 5. Predicted co-amplitude and co-phase lines for the M 2 and K 1 tides and, in insets, the observed values.
WATER
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of 0.32 tude of at the similar
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407
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m but arrives at Karumba with an ampli0.91 m. An amphidromic point is located center of the Gulf. This prediction is to the one of Webb (1981) w h o however
located the amphidromic point about 100 km further south. The M 2 tide enters the Gulf at 0.91 m and arrives at Karumba with only 0.17 m. In fact the attenuation is even larger, with the M2
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408
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Low-frequency currents
amplitude less than 0.l m south of the moorings K and N and only increasing near Karumba due the funnel shape of the Gulf near Karumba. Figure 6a shows the predicted Eulerian tidalaverage circulation under the action of the tides only; it shows a gyre encompassing most of the Gulf but the velocities are very small, seldom exceeding 0.02 m s-1. Such small velocities cannot be measured with current meters in the presence of likely greater errors in measuring strong tidal currents. Figure 7 shows the spectra of sea level at site C and the rotary spectra of the currents also at site C. The spectra show dominant peaks at the diurnal and semi-diurnal tidal frequencies. Note also the minor peaks at sub-harmonics. At semidiurnal frequencies the clockwise and anti-clockwise rotating components are practically equal, i.e. the currents are highly rectilinear. At diurnal frequencies, the currents rotation appears to be slightly clockwise.
The energy density of the currents at site C increases with decreasing frequency at the lowfrequency side of the spectra (Fig. 7), indicating that the spectra are energetic at all periods > 3 days; this is the weather band and is due to both the local wind-driven circulation and to events forced by the surrounding oceans, principally the Arafura Sea since there is little low-frequency forcing through the Torres Strait (Wolanski et al., 1988a). Note that in a narrow band near 10 days periods there is a suggestion of anti-clockwise rotation in the current spectra. Figure 8 (lines a and b) shows the time series plot of the wind velocity at site X. There is a gap in the data in December and January. Note the marked diurnal sea breeze at site X located 300 km from the coast. In October and November 1986, the low-frequency wind was variable, with periods of trade winds from the east and south
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T i m e (days) Fig. 8. Time series plot of the wind velocity components along (a) 90 ° (clockwise from north) and (b) along 0°, the low-frequency currents at site B at top and bottom current meters along (c) 90 ° and (d) 0 °, the low-frequency sea level (e) at sites A and F, and (f) the temperature at the three sensors at site C. The wind is shown using the oceanographic convention. Time is expressed in day number in 1986-1987.
410
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east, monsoon winds from the northwest, and a calm weather period. Two tropical cyclones crossed the Gulf in early 1987. Their tracks are shown in Fig. 3. Tropical cyclone I r m a occurred in January 1987, during the malfunction of the weather station at site X; tropical cyclone Jason crossed the Gulf in February 1987, this was recorded as wind speed reaching 25 m s-~ at site X, and rapid changes in wind direction. Figure 8 (lines c and d) show the time series plot of the low-frequency velocities along 90 ° and 0 ° at site B, for both the top and bottom current meters. The currents were highly variable in time and with depth. The currents were frequently < 0.005 m s a at which time the m e a s u r e m e n t s are probably unreliable in view of the likely errors of measuring small nett currents in the presence of
fluctuating tidal currents 20 times stronger. There is no correlation between currents at the mooring point at the two depths; some fluctuations were coherent at the two depths; others were not. In fact between days 380 and 385, under the influence of tropical cyclone Irma, the velocity along 0 ° was of opposite sign at the elevations of the two current m e t e r of site B (Fig. 8, line d). At other times, some of the velocity fluctuations a p p e a r to coincide with fluctuations of lowfrequency sea level at site A (Fig. 8, line e), i.e. were presumably forced by the Arafura Sea. There were also periods when the surface currents were strong ( = 0.1 m s ~) while the bottom currents were weak, such as around day 345 and around day 315. Such currents occurred when the waters were thermally stratified (Fig. 8, line f )
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W A T E R C I R C U L A T I O N IN G U L F O F C A R P E N T A R I A
and may be wind-driven currents restricted to the top layer sliding over the bottom layer (see later). The low-frequency sea level fluctuations at sites A and F, shown in Fig. 8 (line e), were up to 0.2 m peak to trough, and were coherent with each other. Since the wind-driven circulation in the Gulf affects only little the sea level at site F (see later), and since the Torres Strait is largely impervious to low-frequency fluctuations (Wolanski et al., 1988a), these low-frequency fluctuations were thus probably mainly forced by the Arafura Sea. Note also that the sea level at sites A and F increased by about 0.2 m over 5 months; the reason for this increase is unknown; the increase may however be fictitious and be due to the mooring sinking in the sediment. There are no other tidal stations in that area. The velocity fluctuations at site B appear to be correlated with both the wind velocity and the sea level fluctuations because most current fluctuations occurred during a fluctuation of either wind velocity or of sea level. The wind velocity and the sea level fluctuations were not however significantly correlated with each other. This suggests that the circulation was controlled by both the local wind and by events in the Arafura Sea. The currents were also highly variable spatially as is shown in Fig. 9 showing the time series plot of the currents in the surface layer at the moorings A, B, C and D. The largest currents occurred around day 388 following the passage of tropical cyclone Irma when a strong eastward current was measured at all sites, together with a strong northward current at sites A and B and a southward return current at sites C and D (see Fig. 9). A large clockwise gyre with currents reaching 0.4 m s-~ was thus created in the relaxation phase following tropical cyclone Irma. However, this current was strongly sheared vertically, an eastward current prevailing at site B at both current m e t e r elevations (Fig. 8, Line c), but the southward current only existed at the elevation of the top m e t e r while for a few days a northward current prevailed at the elevation of the bottom meter (Fig. 8, Line d). The cyclone-driven circulation was thus three dimensional. Note that tropical cyclone I r m a resulted in vertically homogenising the water column (Fig. 8, Line f ) .
41 I
From Fig. 9a it is apparent that the 0 ° currents were generally correlated and about in phase with each other at sites A and B, and at sites C and D respectively. This indicates that an inflow into the Gulf on the east side of the Gulf was generally compensated by an outflow on the west side. The 90 ° currents (Fig. 9b) showed less coherence between sites about 100 km apart, yet on occasions were strong and readily measurable. T h e r e were also events, several days in duration, where northwestward currents peaking at 0.18 m s -1 were observed at the elevation of the top current m e t e r at Site D while only weak currents prevailed elsewhere. This suggests that the lowfrequency circulation in the deep regions of the Gulf is not a steady gyre-like circulation encompassing the whole Gulf, but is varying in time and space in local events including eddies, generated by, among other things, events in the Arafura Sea and the unsteadiness of the wind field as well as its spatial variability. Along the southern transect K - N the currents were also weak but were spatially much more coherent as can be seen in Fig. 10 showing the time series of the longshore (along 0 °) wind component at site X and the longshore low-frequency currents at site K and N. The currents and wind were highly correlated at site N in shallow coastal waters. U n d e r cyclonic winds between day 400420, (tropical cyclone Jason), the currents at site N responded very quickly to changes in wind, with negligible lag, a characteristic of wind-driven currents in shallow coastal waters. Southward wind prevailed thereafter, as well as before day 395; during these times, a nett southward current was present at site N, of order 0.05 m s 1, while the nett currents in deeper waters, at site K, were too small ( < 0 . 0 2 m s 1) to be reliably measured.
Coastal trapping The term "coastal trapping" is used to describe a sluggish, shore-paralleled, circulation in shallow coastal waters resulting in long-term trapping of that water mass (Wolanski and Ridd, 1990).
412
E. WOLANSKI
v
o o ._o
Currents
Site K
b) E
-0.1
>
~, 0
b~
10
-.1
>,
-*=o
0
WW"
O 03
>
>
Wind
E
-10
E
-20
380
460
420
440
Time (days)
Fig. 10. Time series plot of the longshore low-frequency currents at sites K and N, and of the longshore wind velocity at site X. The numerical model used for the tidal dynamics, was also used to predict the wind-driven circulation. Typically the model was run for a steady, uniform wind, the wind starting from 0 initially and increasing progressively to reach its steady value in two days. This technique avoids generating free waves from a wind impulse. Figure 6b shows the predicted wind-driven circulation, in the absence of tides, for a trade wind from the south-east at 15 m s 1. This circulation is characterised by strong coastal jets of water at speed of 0.2 m s ~ with a feed-in flow of opposite direction in deeper waters. This predicted circulation is the result of a spatially constant wind on a basin of varying topography, characterised by the current generally directed with the wind in the shallower areas and against the wind in the deeper areas (Csanady, 1973). As suggested by Wolanski and Ridd (1990) however, there exists a strong non-linear interaction, principally in shallow coastal waters through the friction term, between the tidal currents and the wind-driven currents in the Gulf. Under the combined influence of wind and tides, the apparent low-frequency friction coefficient increases and the resulting
wind-driven circulation becomes weaker (Heaps, 1978; Provis and Lennon, 1983). This is shown in Fig. 6c showing the tidally-averaged circulation under the action of all dominant tides and the same trade wind. The coastal jets are still present but are much weaker with velocity of order 0.05 m s-~. Note that the circulation is slightly asymmetric, the width of the coastal jet being larger on the west coast than on the east coast. Figure 6d shows the predicted average circulation under the influence of both tides and a steady, uniform monsoon wind from the northwest at 7.5 m s -1. The coastal jets are now directed southward, the coastally trapped water being ejected offshore as a large scale "coastal outwelling" in the southern region of the Gulf. The "coastal boundary layer" appears to be restricted to shallow coastal waters < 30 m deep on the west coast, and < 15 m deep on the east coast. These results can be explained with the help of the following simple analytical model. Offshore from the coastal boundary layer, the m o m e n t u m balance is at zero order
-gO'q/ay + z/Hg = 0
(1)
where y is the axis oriented longshore (i.e. roughly north-south), g is the acceleration due to gravity, the sea surface elevation, z is the wind stress along the y axis, and Hg ( = 60 m) is the depth in the Gulf away from the coast. At the open boundary (the Arafura Sea), ~7 = 0
(2)
so that at zero-order the only effect of the wind stress is to generate a sea level slope along the y axis. Thus at zero order, there is no wind-driven current in the deeper regions of the Gulf. In shallow coastal waters however, the mom e n t u m balance at zero-order is
r/H~ = C d l V l V / n ~
(3)
where v is the longshore current, H~ (-- 15 m << Hg) is the depth in coastal waters, and c d is the bottom friction coefficient. Since U = U t + Uw
(4)
where v t is the tidal current, and Vw is the wind-driven current, after tidal-averaging the me-
WATER
CIRCULATION
413
IN GULF OF CARPENTARIA
difficult using the present data set because during the 5 month long study period the wind was never steady for long periods and was also presumably variable spatially. During the last 10 days, when the wind at site X was fairly steady, the nett currents at site K were about 0.05 m s - ' , while the nett currents at site N were too small to be measured (Fig. 10), in general agreement with the model predictions assuming a spatially uniform wind. Along the transect A-B, the low-frequency circulation was at times too small to be measured and the rest of the time dominated by events that were generally incoherent from site to site. Only in the tradewind season can one expect the wind to be relatively uniform temporally and spatially; unfortunately this is the open fishing season when current meter moorings would be at risk. Other model runs (not shown) reproduce the observations that transient, low-frequency currents in the
mentum balances in the coastal boundary layer reduces to
T/H~ -- 2cd({
vt I >Vw/HS
(5)
where ( > denotes a tidal average. Because v t >> Vw, the apparent friction coefficient is greatly enhanced by the presence of strong tidal currents. The wind-induced longshore current vw is thus small. By continuity this coastal current generates a return flow in the deeper regions of the the Gulf. The speed of this current in the deeper regions of the Gulf is very small indeed ( < 0.01 m s - ' because the same volume transport in the narrow coastal boundary layer is spread over most of the width and depth of the Gulf. While the predictions of the model for tidal dynamics were successfully compared with observations, a detailed comparison of the observed and predicted wind-driven circulation is more
¢2
Fig. 11. Tidal averaged sea level (in m) in the presence of the dominant diurnal and semi-diurnal tides and (a) of a trade wind from the south-east at 15 m s -I and (b) of a monsoon wind from the north-west at 7.5 m s '.
414
E. WOLANSK1
interior of the Gulf can be quite strong (-- 0.2 m s l) when the wind is spatially non-uniform (i.e. has a significant curl) or changes rapidly (within 1 day) in time. The model predicts that under steady trade winds (Fig. 11a) and in the monsoon season (Fig. l lb) the m e a n sea level fluctuations are largely confined to the southern region of the Gulf where, at Karumba, the sea level respectively rises by 0.3 m and falls by 0.15 m. The total seasonal change is thus 0.45 m. These predictions are in agreement with observations and earlier predictions of Forbes and Church (1983). At site F, the predicted wind-driven sea level fluctuations are about 0.05 m, suggesting that the much larger, observed sea level fluctuations were mostly generated in the Arafura Sea. The wind-driven circulation (Fig. 6) under a steady, uniform wind is very sluggish, with nett velocity seldom exceeding 0.05 m s - i . Cross-slope mixing is presumably small since the tidal currents are oriented longshore. This results in long-term coastal trapping along the coast of the Gulf of Carpentaria. Indeed, the salinity contours at two-monthly interval reveal the presence of a long-lasting (months) mass of brackish water in shallow (depth < 20 m) coastal water several months after cessation of river inflow (Fig. 12). The predicted circulation (Fig. 6d) in the monsoon season also seems realistic in view of the finding of Irian Jaya brackish coastal water in the northwest region of Torres Strait in the summers
/
//
of 1989 and 1990, but not in winter (Wolanski, unpubl, data). Another evidence of coastal trapping comes from remote sensing. A N O A A 9 satellite image, in the visible band and enhanced as described in the Methods, of the east coast on November 23, 1986, is shown in Fig. 13a. At that time, the wind at site X was about 3 m s - ~, and river runoff was negligible and had been so for several months. This image shows a shore-parallel turbidity front in coastal waters extending continuously, without any obvious convolutions, suggesting no largescale cross-slope mixing, along 400 km of the east coast of the Gulf. Other views were obtained for September 17, 18 and 19, 1988, in the dry season. Only on September 17, 1988, was the west coast cloud free, and this image is shown in Fig. 13b. Trade winds from the south-east prevailed at that time as can be seen by the smoke from bush fires; note a band of turbid coastal water on both the east and west coasts; turbidity however was highest on the west coast; the width of that turbid layer was also largest on the west coast, extending offshore from Groote Eylandt to the 40 m depth contour, but only to the 10-15 m depth contour on the east coast. All these observations appear to be qualitatively well reproduced by the model in the sense that the turbid coastal waters appear restricted to the coastal boundary layer predicted by the model (Fig. 6c). The feature " A " on Fig. 13b appears to be a small jet vortex pair system
Surface Salinity
i
"/.
]
L .2(X) . . . .km
Gulf°fCarpentII~~~//) ariai:~
,K
:K
,
,
JJ
" 33 ' i : " ~ : i ~ [28 Aug. -13 S e ~ 2_ " - 1 ~ Fig. 12. Synoptic maps of the surface Salinity at about two-monthly interval in 1976. These data were kindly providedby P. Rothlisberg. Adapted from Wolanski and Ridd (1990).
WATER
CIRCULATION
415
IN G U L F O F C A R P E N T A R I A
which is a flow instability generating cross-slope mixing. Figure 13c shows the east coast around feature " A " on September 17, 18 and 19, 1988. The image e n h a n c e m e n t being sensitive to atmospheric conditions, the shadings are relative so that the same shading can represent somewhat different turbidity on different days. Nevertheless, note in Fig. 13c that the views all show shore-parallel turbidity contrasts in coastal waters. T h e r e is little suggestion of large scale, cross-slope mixing features. Note that feature " A " in deeper waters (also shown in Fig. 13b) moved only a few km over two days, suggesting that mixing proceeded fairly slowly and was a rare occurrence in an otherwise fairly straight turbidity front.
(Fig. 8, line f ) and the elevation of the sensors (Fig. 2a) suggests internal waves up to 20 m peak to trough. However the fluctuations showed no systematic spatial or temporal coherence, as can be seen in Fig. 15, showing the time series of the t e m p e r a t u r e at sites F, B and C, as well as the
ili:
Baroclinic circulation Previous investigators (Rochford, 1966; Forbes and Church, 1983) found that in winter the Gulf waters were were vertically well-mixed in temperature and salinity. In summer however, before the first tropical cyclone, a strong t e m p e r a t u r e stratification was present during my field study, characterised by two well-mixed layers separated by a very sharp thermocline while there was no stratification in salinity (Fig. 14). Note in Figure 14 the fluctuations by 15 m in the depth of the thermocline. These fluctuations were due to the temporal variability as is shown in Fig. 8 (line f ) showing the time series of t e m p e r a t u r e at three elevations at site C. The waters were stratified until day 385 when they became well mixed by tropical cyclone Irma. Thereafter they remained well mixed. Before day 385, the top and bottom sensors experienced t e m p e r a t u r e differences of up to 2°C. At the middle sensor elevation, the t e m p e r a t u r e fluctuated abruptly between that of the top and the bottom sensors, an indication that the thermocline was very sharp and was convoluted by internal waves. Between days 355 and 385 the t e m p e r a t u r e at both the bottom and middlesensors was the same, while it was 2°C higher at the top sensor, an indication that the thermocline was then located always above the middle sensor. A comparison of the t e m p e r a t u r e fluctuations
Fig. 13. NOAA satellite views in the visible band on (a) November 23, 1986 (b) August 17, 1988 and (c) August 17, 18 and 19, 1988 (as is adopted from Wolanski and Ridd, 1990).
416
~i. WOLANSKI
water currents at site C. At site F, the temperature fluctuations decreased with increasing tidal amplitude. At site B, large t e m p e r a t u r e fluctuations were also observed, the thermocline generally, but not always, rising at flood tide. At site C the thermocline displacements increased in amplitude with increasing speed of the tidal currents, but they also seem to be related, at times, to the strength of the sea breeze. The nature of these internal waves, whether they are internal tides, internal seiches, solitons etc., remains unresolved.
The t e m p e r a t u r e stratification in summer supported at times a vertical shear of the lowfrequency circulation which was presumably wind-driven. For instance, from days 340 to 345 the upper current meter at site B measured a strong current along 90 ° in the surface layer (Fig. 8, line c), which was not present in the bottom layer, but which was also observed in the surface layer at sites C and D (Fig. 9b). River inflow may also drive a circulation. Church and Forbes (1983) reported the presence of a vertically well-mixed, brackish water mass in
Fig. 13 (continued).
417
W A T E R C I R C U L A T I O N IN G U L F O F C A R P E N T A R I A
Fig. 13 (continued).
coastal waters (see Fig. 12). They assumed that rl = 0 everywhere and, they inferred, by geostroplay from the thermal wind equation, the existence of a steady, longshore current, v, in this
brackish water mass, varying with depth. This depth-averaged current can also be calculated from the depth-averaged m o m e n t u m equation which is a vertical average of the thermal wind
418
V.WOLANSKI
Temperature (°C) Salinity (%o) 26.5 I 27.5 28.5 I I 034.5 1
35.0 I
o/
20E r-
n
where b is the buoyancy, the x axis oriented cross-slope and f is the Coriolis parameter. This current probably ceases soon after cessation of the river inflow though coastal waters are still brackish. Because of friction, a steady longshore current u b in shallow coastal waters generates a longshore sea level gradient r/. When the river inflow has ceased, as in Fig. 12, there is no mechanism to maintain this slope and overcome bottom friction. Once the river inflow ceases, the river-induced level setup r/ disperses at a speed (gH~) 1/2 >1 10 m s - t and will thus all but disappear in 1 day. As a result, as suggested by Csanady (1982), once the river inflow has ceased, the cross-shore salinity gradient raises the sea level at the coast by an amount r/(Csanady, 1982),
4060 I i '-- C-~----~
O~7/ ~ x = - H ( s ~ b / ~ x ) / 2 g
80 Fig. 14. Vertical profiles of temperature and salinity on October 18 and 19, 1986. equation and a balance between cross-slope density gradients and geostrophic currents, (6)
L'b = 0.5~b / O x H J f
27
'7%.6"
BZop
28tCTop 26
CMiddle
27-
CBottom
l
Middle
365
3(}7
3(39
where ~7 = 0 outside the brackish water mass, so that
cb=o
(8)
There is thus no salinity-driven flow in the shallow, friction-dominated, coastal waters of the Gulf once river runoff ceases even though the coastal waters are brackish.
Conclusions
BMiddle
E I--
(7)
311
Time(days) Fig. 15. Time series plot of the temperature at sites F, B and C and stick plot of currents at site C.
A five-month long field study of the circulation in the Gulf of Carpentaria was carried out in the period October 1986 to March 1987. It involved the deployment of 9 moorings with current meters and tide gauges in two transects, one across the mouth of the Gulf, and the other transect mid-way into the Gulf on the east coast. A weather station was also deployed in the middle in the Gulf, 300 km from the coast. A sea breeze was present in summer in the middle of the Gulf. There were also low-frequency events including trade winds from the southeast and monsoonal winds from the north-west, and two tropical cyclones. The depth-averaged barotropic circulation was also modeled numerically and observations and predictions were compared. The tides in the Gulf of Carpentaria are forced by both the Arafura Sea and the Coral Sea; the
WATER CIRCULATION IN GULF OF CARPENTARIA
influence of the former dominates except near Torres Strait. The semi-diurnal tides decrease rapidly as they enter the Gulf. The diurnal tides however propagate readily into the Gulf as Kelvin waves in a rectangular channel. These features were successfully reproduced by a depth-averaged numerical model that was verified against both tidal data and tidal ellipses data at 12 current meter mooring sites in the Gulf. In summer before the first tropical cyclone, the Gulf was temperature-stratified with a sharp thermocline on which ride internal waves up to 20 m peak to trough. These internal waves were incoherent over horizontal distances of 100 km. The temperature stratification also allowed transient, low-frequency currents in the top layer, independently of the bottom layer. The nett circulation near the mouth of the Gulf was weak, frequently incoherent over horizontal distances of 100 km, and dominated by events. This circulation is probably generated by events in the Arafura Sea and by spatial and temporal variations of the local wind field. Under cyclonic conditions, a large clockwise gyre developed encompassing most of the Gulf, but the circulation was at times three-dimensional, though the waters were vertically well-mixed, with a return flow in deeper waters. Large (--0.5 m) seasonal fluctuations in sea level are experienced in the southern region of the Gulf. Model results suggest that these are mostly wind-driven. This study has identified the importance of three coastal oceanographic processes which may have profound biological consequences that deserve investigations. First, the wind appears to be largely responsible for the seasonal fluctuations of sea level which are very large (up to 0.5 m) in the southern region of the Gulf near Karumba, as previously also reported by Forbes and Church (1983). These seasonal fluctuations are very important because they result in vast areas being only inundated by the tides in summer; these areas can support neither mangrove nor freshwater vegetation and are thus salt flats. The first seasonal inundation of these salt flats in summer generates a "estuarine outwelling" of nutrients from these salt flats to the coastal waters of the
419
Gulf (Ridd et al., 1988). This "estuarine outwelling" coincides with the onset of migration between the estuaries and the coastal waters of commercially important resources, such as barramundi and prawn. Second, along the east coast of the Gulf, the non-linear interaction, through bottom friction, between strong tidal currents and weak wind-driven currents generates a sluggish, shore-parallel circulation in shallow waters ( < 20 m) that traps coastal water probably for long period (months). This water is presumably nutrient-enriched by estuarine outwelling, principally in summer, while further offshore there is little nutrient enrichment. Third, in summer this coastally trapped mass is ejected offshore as a large-scale "coastal outwelling" in the southernmost region of the Gulf (Fig. 6d), presumably enriching the offshore waters in this area with nutrients swept from several hundreds kilometres of coastline along both the east and west coasts. Coastal trapping ensure a long residence time of outwelled nutrients and may then enhance productivity in coastal waters. It will also trap pollutants so that particular care should be taken in the disposal of waste from mining and coastal developments.
Acknowledgements It is a pleasure to acknowledge the assistance of Dr. M. Inoue, Dr. P. Ridd, Mr. R. McAllister and Mr. J. Sodusta. Dr. P. Rothlisberg kindly provided the salinity data used to draw Fig. 12. W. Skirving and C. Steinberg produced the satellite images. This study was supported by the Australian Institute of Marine Science and by a small grant from the Commonwealth Bureau of Meteorology.
References Church, J.A. and Forbes, A.M.G., 1981. A non-linear model of the tides in the Gulf of Carpentaria. Aust. J. Mar. Freshwater Res., 32: 685-697. Church, J.A. and Forbes, A.M.G., 1983. Circulation in the Gulf of Carpentaria. I: Direct observations of currents in the South-East corner of the Gulf of Carpentaria. Aust. J. Mar. Freshwater Res., 34: 1-10.
420 Csanady, G.T., 1973. Wind-driven barotropic motions in long lakes. J. Phys. Oceanogr., 3: 429-438. Csanady, G.T., 1982. Circulation in the Coastal Ocean. Reidel, Dordrecht, 279 pp. Forbes, A.M.G. and Church, J.A., 1983. Circulation in the Gulf of Carpentaria. II: Residual currents and mean sea level. Aust. J. Mar. Freshwater Res., 34: 11-22. Heaps, N.S., 1978. Linearized vertically-integrated equations for residual circulation in coastal seas. Dtsch. Hydrogr. Z., 31(5): 147-169. Provis, D.G. and Lennon, G.W., 1983. Eddy viscosity and tidal cycles in a shallow sea. Estuarine Coastal Shelf Sci., 16: 351-361. Radok, R., 1978. Tidal height and stream prediction at Groote Eylandt. BHP Tech. Bull., 22: 46-60. Ridd, P., Sandstrom, M.W. and Wolanski, E., 1988. Outwelling from tropical tidal salt fiats. Estuarine Coastal Shelf Sci., 26: 243-253. Rienecker, M.M. and Noye, J., 1982. Numerical simulation of the tides in the Gulf of Carpentaria. In: J. Noye (Editor), Numerical Solutions of Partial Differential Equations. North Holland, Amsterdam,
E. WOLANSKI Rochford, D., 1966. Some hydrological features the Eastern Arafura Sea and the Gulf of Carpentaria in August 1964. Aust. J. Mar. Freshwater Res., 17: 31-60. Sodusta, J.A., 1988. Non-linear interaction of wind and tide driven currents in the Gulf of Carpentaria. M.Sc. Thesis, Dep. Geol. Geophys. Univ. Sydney, 123 pp. Webb, D., 1981. Numerical model of the tides in the Gulf of Carpentaria and Arafura Sea. Aust. J. Mar. Freshwater Res., 32: 31-44. Wolanski, E. and Ridd, P., 1990. Mixing and trapping in Australian tropical coastal waters. In: R.T. Cheng (Editor), Residual Currents and Long-term Transport. Springer, New York, pp. 167-183 Wolanski, E., Imberger, J. and Heron, M.C.. 1984. Island wakes in shallow coastal waters. J. Geophys. Res., 89: 10,553-10,569. Wolanski, E., Ridd, P. and Inoue, M., 1988a. Currents through Torres Strait. J. Phys. Oceanogr., 18: 1535-1545. Wolanski, E., Drew, E., Abel, K. and O'Brien, J., 1988b. Tidal jets,nutrient upwelling and their influence on the productivity of the alga Halimeda in the Ribbon Reefs, Great Barrier Reef. Estuarine Coastal Shelf Sci., 26: 169-201.
401
Water circulation in the Gulf of Carpentaria Eric Wolanski Australian Institute of Marine Science, P.M.B. No. 3, Townsville M.C., Qld. 4810, Australia
(Received March 22, 1993; revised and accepted June 9, 1993)
ABSTRACT The water circulation in the Gulf of Carpentaria was modeled numerically, and the predictions of tides and tidal currents compare very satisfactorily with observations at 9 mooring sites from a five month long field study in 1986-1987. In summer, before the occurrence of tropical cyclones, the Gulf is temperature stratified with a sharp thermocline on which ride internal waves up to 20 m peak to trough. When this thermocline is present, the low-frequency circulation shows both barotropic and baroclinic components. Under tropical cyclones, the water column becomes well-mixed vertically but the water circulation is three-dimensional, with a Gulf-wide gyre in the surface waters with a return flow in the bottom waters. The wind generates seasonal sea level fluctuations of about 0.5 m in the southern region of the Gulf and these create vast salt flats. The salt flats are only tidally inundated in summer when estuarine outwelling occurs. Tidal currents and wind-driven currents interact non-linearly to produce a barotropic coastal boundary layer. There is little mixing between estuarine and offshore waters. Large-scale coastal outwelling occurs in summer when this coastally trapped water, presumably nutrient-enriched by estuarine outwelling all along the coast of the Gulf, is ejected offshore at one point in the southernmost region of the Gulf. Estuarine outwelling, coastal outwelling and coastal trapping may have profound biological implications.
Introduction T h e G u l f of C a r p e n t a r i a is a s e m i - e n c l o s e d b o d y o f w a t e r in n o r t h e r n A u s t r a l i a , covering a n a r e a o f a b o u t 193,000 k m 2 (Fig. 1). T h e G u l f is shallow with m a x i m u m d e p t h o f a b o u t 65 m a n d a w i d e c o a s t a l m a r g i n of w a t e r d e p t h sea. T h e o p e n b o u n d a r y to t h e A r a f u r a S e a occurs b e t w e e n Irian J a y a a n d C a p e W e s s e l a n d is a b o u t 50 m d e e p a n d 200 k m wide. T h e c o n n e c t i o n to t h e C o r a l Sea occurs t h r o u g h T o r r e s S t r a i t which is shallow ( d e p t h --- 10 m), a b o u t 100 k m wide, a n d e n c u m b e r e d by n u m e r o u s reefs, islands a n d shoals which restrict t h e t h r o u g h - S t r a i t c u r r e n t s ( W o l a n ski et al., 1988a). T h e G u l f o f C a r p e n t a r i a is an i m p o r t a n t econ o m i c r e s o u r c e . It s u p p o r t s i m p o r t a n t c o m m e r cial fisheries. It is t h e site o f p o r t activities f r o m m i n e s at W e i p a a n d G r o o t e E y l a n d t , a n d f r o m t h e p r o p o s e d C e n t u r y a n d M c A r t h u r R i v e r mines, a n d f r o m a p o s s i b l e s p a c e p o r t . It m a y a l r e a d y b e locally i m p a c t e d in I r i a n J a y a c o a s t a l w a t e r s by u n t r e a t e d m i n e tailings f r o m t h e F r e e p o r t c o p p e r
m i n e a n d by siltation from d e f o r e s t a t i o n as a result of c o m m e r c i a l logging a n d the t r a n s m i g r a tion p r o g r a m . It m a y b e c o m e the focus of oil e x p l o r a t i o n . It is also an i m p o r t a n t s h i p p i n g r o u t e in t h e n o r t h e r n r e g i o n b e t w e e n C a p e W e s s e l a n d C a p e York, f r o m the A r a f u r a Sea to the east coast of A u s t r a l i a . T h e w a t e r c i r c u l a t i o n in t h e G u l f is p o o r l y k n o w n b e c a u s e of t h e p a u c i t y of o c e a n o g r a p h i c data. N e v e r t h e l e s s t h e r e have b e e n several att e m p t s at m o d e l i n g the tides of the Gulf. W e b b (1981) d e v e l o p e d a linear, d e p t h - a v e r a g e d m o d e l with T o r r e s Strait a s s u m e d closed. C h u r c h a n d F o r b e s (1981) p r e s e n t e d t h e results of a non-linear, d e p t h - a v e r a g e d m o d e l which i n c l u d e d the i n f l u e n c e o f T o r r e s Strait. A s s u m i n g a p r i o r i small n o n - l i n e a r effects in the Gulf, R i e n e c k e r a n d N o y e (1982) o p t e d b a c k to a l i n e a r m o d e l . A l l t h e s e m o d e l s u s e d finite d i f f e r e n c e t e c h n i q u e s a n d p r o d u c e d similar results. T h e y all s u f f e r e d f r o m a lack o f o b s e r v a t i o n a l d a t a for b o t h o p e n b o u n d a r y forcing a n d m o d e l verification. I n d e e d , t h e only d a t a available for o p e n b o u n d a r y forcing
0924-7963/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
402
E. WOLANSK1
by the Arafura Sea were tidal data from Cape Wessel, with no data for Irian Jaya at point Z (see Fig. 1). The tidal harmonic constants for point Z were inferred by extrapolation from tidal data from harbours typically 200 km away. These harbours are located in shallow water where the open water tidal constituents have been significantly altered by shallow water effects. While tidal data were available at a number of coastal stations, there were no offshore tidal data and the only current m e t e r data were those at site R
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(see Fig. 1) of R a d o k (1978), and those at sites O, P and Q of Church and Forbes (1983). The latter sites are located in the southernmost part of the Gulf, in shallow coastal waters, giving little information on the water circulation in the bulk of the Gulf. As part of its contribution to A M E X (Australian Monsoon Experiment) the Australian Institute of Marine Science undertook an oceanographic study of the Gulf. The results of this study are described below.
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403
W A T E R C I R C U L A T I O N IN G U L F O F C A R P E N T A R I A
measured, at 2.5 m above the surface, the wind speed, direction and air and water surface temperature. While all the other instruments yielded 100% data return, the mid-period of the weather station data was lost as a result of a power drainage when the sea surface t e m p e r a t u r e sensor was destroyed by the D e p a r t m e n t of Transport attempting to recover the mooring under the incorrect belief the buoy was drifting. O t h e r current meters were deployed at sites K, L, M and N on a cross-slope transect, in mid-January 1987, during the closed trawling season. It was judged unwise to deploy these moorings in October 1986, because commercial trawling at that time is very intense. The current meters were also A a n d e r a a model RCM4'S' and Inter-Ocean model $4 (see Fig. 2). Data were recorded at 15 min intervals. These moorings were also recovered in March 1987, at the start of the open fishing season. The time series of the data were analyzed for their spectra a n d / o r rotary spectra, for the tidal components using a standard tidal harmonic analysis, and filtered for their tidal and lowfrequency components, following the methods of Wolanski et al. (1988a).
Methods
T h r e e oceanographic cruises were carried out in the period October 1986 to April 1987. Vertical profiles of t e m p e r a t u r e and salinity were measured using the C T D profiler described by Wolanski et al. (1988a). In mid-October 1986 four moorings were deployed at sites A, B, C and D, each with two self-recording current meters, either A a n d e r a a model RCM4's' (i.e. fitted with a " S " kit to avoid wave pumping the rotors) or Inter-Ocean Model $4, as is shown in Fig. 2. At sites A, C and F, an A a n d e r a a water level recorder was bottom-mounted. All instruments recorded data at 0.5 h interval, the current or sea level data being averaged over about 1 min. All the instruments were also fitted with a temperature sensor. The instruments were recovered in March 1987. During the same period a number of current meters and tide gauges were deployed in Torres Strait (Fig. 1). The Torres Strait data have been presented by Wolanski et al. (1988a) and are used in this study as open boundary conditions. Also at the same time an A a n d e r a a weather station was deployed on a large buoy, at site X (see Fig. 1) in the middle of the Gulf. The station
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404
E. WOLANSKI
The water circulation was modeled using the depth-averaged, finite-difference, non-linear, centered, implicit numerical model of Wolanski et al. (1988b) where bottom friction is a quadratic function of the velocity through a Manning's roughness coefficient, n. The model allows for the /3 effect. The model domain is shown in Fig. 3. The grid size was 18.53 km. When presenting wind data the oceanographic convention is used, i.e. the wind direction is that the air is blowing to.
I
Current and wind direction refers to the direction clockwise from north. Except when otherwise noted, the time is expressed in day number in 1986-1987 (1200 h on Dec. 31, 1986 = time 365.5). N O A A 9 raw satellite data were obtained on magnetic tape. After the image was subsetted and the land masked out, a histogram of the brightness values in Channel 1 (580-680 nm) of the water was produced. This was used to identify the relative brightest and darkest water pixels. The
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410 Fig. 3. Model domain and resolution grid, and approximate track of tropical cyclones Irma and Jason with positions of their eye at 0 h on days expressed as day number in 1986/87.
WATER
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405
IN G U L F O F C A R P E N T A R I A
image was then stretched so that the darkest water pixels were given values of zero (black) and the brightest water pixels were given 239 (yellow). The land was designated the value of 255 which corresponds to green. Results
The tides The contrasting behaviour between the tidal dynamics at diurnal and semi-diurnal periods, is well illustrated by the dominant M2 and K t tides. The observed tidal ellipses are shown in Fig. 4; the observed tidal height amplitude and phase are shown as insets in Fig. 5. In Fig. 4 the tidal ellipses at diurnal frequency along the transect A - D are oriented about n o r t h - s o u t h ; those at semi-diurnal frequency are oriented about e a s t -
west. The K t and M 2 tidal currents each peaked at about 0.4 m s -1. Together with the other tidal components, peak tidal currents are very strong, peaking at 1.2 m s-~. The tidal ellipses show little change with depth, being equal at the two current m e t e r elevations within 0.05 m s - I and 5 ° direction. Figure 5 shows that the semi-diurnal tides that enter the Gulf from the Arafura Sea, propagate little in the Gulf; the diurnal tides also enter the Gulf from the Arafura Sea but propagate readily into the Gulf. These data provided an ideal data set for a detailed calibration, for the first time, of a mathematical model of the tidal dynamics in the Gulf. Preliminary results were presented by Sodusta (1988) who, using an explicit, non-linear, depthaveraged numerical model, showed that the M 2, Kt, S 2 and O I tidal components were dominant and needed to all be included simultaneously in
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406
E. WOLANSKI
the numerical model because of non-linear interactions principally controlled by the bottom friction term. Normal to a solid boundary the flow was specified to vanish. At the Torres Strait open boundary, tidal harmonics derived from the results of Wolanski et al. (1988a) were specified. The Arafura Sea open boundary was specified using linear interpolation between Cape Wessel and point Z in Irian Jaya (see Fig. 1). The tidal harmonic constants at Cape Wessel were taken from the Admiralty tide tables. An initial approximation of the tidal harmonic constants at point Z was taken from the model results of Webb (1981) and Church and Forbes (1981). These latter constants however lead to large discrepancies, up to 45 ° , between the orientation of the observed and predicted tidal ellipses at sites A and B. By trial and error in tuning the constants at site Z, and comparison between observed and predicted tidal el-
lipses at sites A and B, a final selection of tidal harmonic constants was selected for point Z. This set of condition differs markedly from that of Church and Forbes (1981) for the semi-diurnal tides; in particular there is about a 30 ° phase change from Cape Wessel to point Z for the M 2 tide, instead of 130 ° . Figure 4 shows the observed and predicted tidal ellipses; the comparison is very encouraging. A similar good fit is found (not shown) for the S 2 and O I tidal currents. Figure 5 shows the predicted tide amplitude and phase distribution in the Gulf, together with, as insets, my observations at sites A, C and F and the Admiralty values for coastal ports. The comparison is very encouraging at the mooring sites. The predicted values are generally slightly larger (by 0.05 m) than the observed ones in the coastal areas particularly in the southern region of the Gulf. The K 1 tide enters the Gulf with an amplitude
Fig. 5. Predicted co-amplitude and co-phase lines for the M 2 and K 1 tides and, in insets, the observed values.
WATER
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of 0.32 tude of at the similar
IN GULF
407
OF CARPENTARIA
m but arrives at Karumba with an ampli0.91 m. An amphidromic point is located center of the Gulf. This prediction is to the one of Webb (1981) w h o however
located the amphidromic point about 100 km further south. The M 2 tide enters the Gulf at 0.91 m and arrives at Karumba with only 0.17 m. In fact the attenuation is even larger, with the M2
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408
E. WOLANSKI
Low-frequency currents
amplitude less than 0.l m south of the moorings K and N and only increasing near Karumba due the funnel shape of the Gulf near Karumba. Figure 6a shows the predicted Eulerian tidalaverage circulation under the action of the tides only; it shows a gyre encompassing most of the Gulf but the velocities are very small, seldom exceeding 0.02 m s-1. Such small velocities cannot be measured with current meters in the presence of likely greater errors in measuring strong tidal currents. Figure 7 shows the spectra of sea level at site C and the rotary spectra of the currents also at site C. The spectra show dominant peaks at the diurnal and semi-diurnal tidal frequencies. Note also the minor peaks at sub-harmonics. At semidiurnal frequencies the clockwise and anti-clockwise rotating components are practically equal, i.e. the currents are highly rectilinear. At diurnal frequencies, the currents rotation appears to be slightly clockwise.
The energy density of the currents at site C increases with decreasing frequency at the lowfrequency side of the spectra (Fig. 7), indicating that the spectra are energetic at all periods > 3 days; this is the weather band and is due to both the local wind-driven circulation and to events forced by the surrounding oceans, principally the Arafura Sea since there is little low-frequency forcing through the Torres Strait (Wolanski et al., 1988a). Note that in a narrow band near 10 days periods there is a suggestion of anti-clockwise rotation in the current spectra. Figure 8 (lines a and b) shows the time series plot of the wind velocity at site X. There is a gap in the data in December and January. Note the marked diurnal sea breeze at site X located 300 km from the coast. In October and November 1986, the low-frequency wind was variable, with periods of trade winds from the east and south
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WATER
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T i m e (days) Fig. 8. Time series plot of the wind velocity components along (a) 90 ° (clockwise from north) and (b) along 0°, the low-frequency currents at site B at top and bottom current meters along (c) 90 ° and (d) 0 °, the low-frequency sea level (e) at sites A and F, and (f) the temperature at the three sensors at site C. The wind is shown using the oceanographic convention. Time is expressed in day number in 1986-1987.
410
E. WOI.ANSKI
east, monsoon winds from the northwest, and a calm weather period. Two tropical cyclones crossed the Gulf in early 1987. Their tracks are shown in Fig. 3. Tropical cyclone I r m a occurred in January 1987, during the malfunction of the weather station at site X; tropical cyclone Jason crossed the Gulf in February 1987, this was recorded as wind speed reaching 25 m s-~ at site X, and rapid changes in wind direction. Figure 8 (lines c and d) show the time series plot of the low-frequency velocities along 90 ° and 0 ° at site B, for both the top and bottom current meters. The currents were highly variable in time and with depth. The currents were frequently < 0.005 m s a at which time the m e a s u r e m e n t s are probably unreliable in view of the likely errors of measuring small nett currents in the presence of
fluctuating tidal currents 20 times stronger. There is no correlation between currents at the mooring point at the two depths; some fluctuations were coherent at the two depths; others were not. In fact between days 380 and 385, under the influence of tropical cyclone Irma, the velocity along 0 ° was of opposite sign at the elevations of the two current m e t e r of site B (Fig. 8, line d). At other times, some of the velocity fluctuations a p p e a r to coincide with fluctuations of lowfrequency sea level at site A (Fig. 8, line e), i.e. were presumably forced by the Arafura Sea. There were also periods when the surface currents were strong ( = 0.1 m s ~) while the bottom currents were weak, such as around day 345 and around day 315. Such currents occurred when the waters were thermally stratified (Fig. 8, line f )
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W A T E R C I R C U L A T I O N IN G U L F O F C A R P E N T A R I A
and may be wind-driven currents restricted to the top layer sliding over the bottom layer (see later). The low-frequency sea level fluctuations at sites A and F, shown in Fig. 8 (line e), were up to 0.2 m peak to trough, and were coherent with each other. Since the wind-driven circulation in the Gulf affects only little the sea level at site F (see later), and since the Torres Strait is largely impervious to low-frequency fluctuations (Wolanski et al., 1988a), these low-frequency fluctuations were thus probably mainly forced by the Arafura Sea. Note also that the sea level at sites A and F increased by about 0.2 m over 5 months; the reason for this increase is unknown; the increase may however be fictitious and be due to the mooring sinking in the sediment. There are no other tidal stations in that area. The velocity fluctuations at site B appear to be correlated with both the wind velocity and the sea level fluctuations because most current fluctuations occurred during a fluctuation of either wind velocity or of sea level. The wind velocity and the sea level fluctuations were not however significantly correlated with each other. This suggests that the circulation was controlled by both the local wind and by events in the Arafura Sea. The currents were also highly variable spatially as is shown in Fig. 9 showing the time series plot of the currents in the surface layer at the moorings A, B, C and D. The largest currents occurred around day 388 following the passage of tropical cyclone Irma when a strong eastward current was measured at all sites, together with a strong northward current at sites A and B and a southward return current at sites C and D (see Fig. 9). A large clockwise gyre with currents reaching 0.4 m s-~ was thus created in the relaxation phase following tropical cyclone Irma. However, this current was strongly sheared vertically, an eastward current prevailing at site B at both current m e t e r elevations (Fig. 8, Line c), but the southward current only existed at the elevation of the top m e t e r while for a few days a northward current prevailed at the elevation of the bottom meter (Fig. 8, Line d). The cyclone-driven circulation was thus three dimensional. Note that tropical cyclone I r m a resulted in vertically homogenising the water column (Fig. 8, Line f ) .
41 I
From Fig. 9a it is apparent that the 0 ° currents were generally correlated and about in phase with each other at sites A and B, and at sites C and D respectively. This indicates that an inflow into the Gulf on the east side of the Gulf was generally compensated by an outflow on the west side. The 90 ° currents (Fig. 9b) showed less coherence between sites about 100 km apart, yet on occasions were strong and readily measurable. T h e r e were also events, several days in duration, where northwestward currents peaking at 0.18 m s -1 were observed at the elevation of the top current m e t e r at Site D while only weak currents prevailed elsewhere. This suggests that the lowfrequency circulation in the deep regions of the Gulf is not a steady gyre-like circulation encompassing the whole Gulf, but is varying in time and space in local events including eddies, generated by, among other things, events in the Arafura Sea and the unsteadiness of the wind field as well as its spatial variability. Along the southern transect K - N the currents were also weak but were spatially much more coherent as can be seen in Fig. 10 showing the time series of the longshore (along 0 °) wind component at site X and the longshore low-frequency currents at site K and N. The currents and wind were highly correlated at site N in shallow coastal waters. U n d e r cyclonic winds between day 400420, (tropical cyclone Jason), the currents at site N responded very quickly to changes in wind, with negligible lag, a characteristic of wind-driven currents in shallow coastal waters. Southward wind prevailed thereafter, as well as before day 395; during these times, a nett southward current was present at site N, of order 0.05 m s 1, while the nett currents in deeper waters, at site K, were too small ( < 0 . 0 2 m s 1) to be reliably measured.
Coastal trapping The term "coastal trapping" is used to describe a sluggish, shore-paralleled, circulation in shallow coastal waters resulting in long-term trapping of that water mass (Wolanski and Ridd, 1990).
412
E. WOLANSKI
v
o o ._o
Currents
Site K
b) E
-0.1
>
~, 0
b~
10
-.1
>,
-*=o
0
WW"
O 03
>
>
Wind
E
-10
E
-20
380
460
420
440
Time (days)
Fig. 10. Time series plot of the longshore low-frequency currents at sites K and N, and of the longshore wind velocity at site X. The numerical model used for the tidal dynamics, was also used to predict the wind-driven circulation. Typically the model was run for a steady, uniform wind, the wind starting from 0 initially and increasing progressively to reach its steady value in two days. This technique avoids generating free waves from a wind impulse. Figure 6b shows the predicted wind-driven circulation, in the absence of tides, for a trade wind from the south-east at 15 m s 1. This circulation is characterised by strong coastal jets of water at speed of 0.2 m s ~ with a feed-in flow of opposite direction in deeper waters. This predicted circulation is the result of a spatially constant wind on a basin of varying topography, characterised by the current generally directed with the wind in the shallower areas and against the wind in the deeper areas (Csanady, 1973). As suggested by Wolanski and Ridd (1990) however, there exists a strong non-linear interaction, principally in shallow coastal waters through the friction term, between the tidal currents and the wind-driven currents in the Gulf. Under the combined influence of wind and tides, the apparent low-frequency friction coefficient increases and the resulting
wind-driven circulation becomes weaker (Heaps, 1978; Provis and Lennon, 1983). This is shown in Fig. 6c showing the tidally-averaged circulation under the action of all dominant tides and the same trade wind. The coastal jets are still present but are much weaker with velocity of order 0.05 m s-~. Note that the circulation is slightly asymmetric, the width of the coastal jet being larger on the west coast than on the east coast. Figure 6d shows the predicted average circulation under the influence of both tides and a steady, uniform monsoon wind from the northwest at 7.5 m s -1. The coastal jets are now directed southward, the coastally trapped water being ejected offshore as a large scale "coastal outwelling" in the southern region of the Gulf. The "coastal boundary layer" appears to be restricted to shallow coastal waters < 30 m deep on the west coast, and < 15 m deep on the east coast. These results can be explained with the help of the following simple analytical model. Offshore from the coastal boundary layer, the m o m e n t u m balance is at zero order
-gO'q/ay + z/Hg = 0
(1)
where y is the axis oriented longshore (i.e. roughly north-south), g is the acceleration due to gravity, the sea surface elevation, z is the wind stress along the y axis, and Hg ( = 60 m) is the depth in the Gulf away from the coast. At the open boundary (the Arafura Sea), ~7 = 0
(2)
so that at zero-order the only effect of the wind stress is to generate a sea level slope along the y axis. Thus at zero order, there is no wind-driven current in the deeper regions of the Gulf. In shallow coastal waters however, the mom e n t u m balance at zero-order is
r/H~ = C d l V l V / n ~
(3)
where v is the longshore current, H~ (-- 15 m << Hg) is the depth in coastal waters, and c d is the bottom friction coefficient. Since U = U t + Uw
(4)
where v t is the tidal current, and Vw is the wind-driven current, after tidal-averaging the me-
WATER
CIRCULATION
413
IN GULF OF CARPENTARIA
difficult using the present data set because during the 5 month long study period the wind was never steady for long periods and was also presumably variable spatially. During the last 10 days, when the wind at site X was fairly steady, the nett currents at site K were about 0.05 m s - ' , while the nett currents at site N were too small to be measured (Fig. 10), in general agreement with the model predictions assuming a spatially uniform wind. Along the transect A-B, the low-frequency circulation was at times too small to be measured and the rest of the time dominated by events that were generally incoherent from site to site. Only in the tradewind season can one expect the wind to be relatively uniform temporally and spatially; unfortunately this is the open fishing season when current meter moorings would be at risk. Other model runs (not shown) reproduce the observations that transient, low-frequency currents in the
mentum balances in the coastal boundary layer reduces to
T/H~ -- 2cd({
vt I >Vw/HS
(5)
where ( > denotes a tidal average. Because v t >> Vw, the apparent friction coefficient is greatly enhanced by the presence of strong tidal currents. The wind-induced longshore current vw is thus small. By continuity this coastal current generates a return flow in the deeper regions of the the Gulf. The speed of this current in the deeper regions of the Gulf is very small indeed ( < 0.01 m s - ' because the same volume transport in the narrow coastal boundary layer is spread over most of the width and depth of the Gulf. While the predictions of the model for tidal dynamics were successfully compared with observations, a detailed comparison of the observed and predicted wind-driven circulation is more
¢2
Fig. 11. Tidal averaged sea level (in m) in the presence of the dominant diurnal and semi-diurnal tides and (a) of a trade wind from the south-east at 15 m s -I and (b) of a monsoon wind from the north-west at 7.5 m s '.
414
E. WOLANSK1
interior of the Gulf can be quite strong (-- 0.2 m s l) when the wind is spatially non-uniform (i.e. has a significant curl) or changes rapidly (within 1 day) in time. The model predicts that under steady trade winds (Fig. 11a) and in the monsoon season (Fig. l lb) the m e a n sea level fluctuations are largely confined to the southern region of the Gulf where, at Karumba, the sea level respectively rises by 0.3 m and falls by 0.15 m. The total seasonal change is thus 0.45 m. These predictions are in agreement with observations and earlier predictions of Forbes and Church (1983). At site F, the predicted wind-driven sea level fluctuations are about 0.05 m, suggesting that the much larger, observed sea level fluctuations were mostly generated in the Arafura Sea. The wind-driven circulation (Fig. 6) under a steady, uniform wind is very sluggish, with nett velocity seldom exceeding 0.05 m s - i . Cross-slope mixing is presumably small since the tidal currents are oriented longshore. This results in long-term coastal trapping along the coast of the Gulf of Carpentaria. Indeed, the salinity contours at two-monthly interval reveal the presence of a long-lasting (months) mass of brackish water in shallow (depth < 20 m) coastal water several months after cessation of river inflow (Fig. 12). The predicted circulation (Fig. 6d) in the monsoon season also seems realistic in view of the finding of Irian Jaya brackish coastal water in the northwest region of Torres Strait in the summers
/
//
of 1989 and 1990, but not in winter (Wolanski, unpubl, data). Another evidence of coastal trapping comes from remote sensing. A N O A A 9 satellite image, in the visible band and enhanced as described in the Methods, of the east coast on November 23, 1986, is shown in Fig. 13a. At that time, the wind at site X was about 3 m s - ~, and river runoff was negligible and had been so for several months. This image shows a shore-parallel turbidity front in coastal waters extending continuously, without any obvious convolutions, suggesting no largescale cross-slope mixing, along 400 km of the east coast of the Gulf. Other views were obtained for September 17, 18 and 19, 1988, in the dry season. Only on September 17, 1988, was the west coast cloud free, and this image is shown in Fig. 13b. Trade winds from the south-east prevailed at that time as can be seen by the smoke from bush fires; note a band of turbid coastal water on both the east and west coasts; turbidity however was highest on the west coast; the width of that turbid layer was also largest on the west coast, extending offshore from Groote Eylandt to the 40 m depth contour, but only to the 10-15 m depth contour on the east coast. All these observations appear to be qualitatively well reproduced by the model in the sense that the turbid coastal waters appear restricted to the coastal boundary layer predicted by the model (Fig. 6c). The feature " A " on Fig. 13b appears to be a small jet vortex pair system
Surface Salinity
i
"/.
]
L .2(X) . . . .km
Gulf°fCarpentII~~~//) ariai:~
,K
:K
,
,
JJ
" 33 ' i : " ~ : i ~ [28 Aug. -13 S e ~ 2_ " - 1 ~ Fig. 12. Synoptic maps of the surface Salinity at about two-monthly interval in 1976. These data were kindly providedby P. Rothlisberg. Adapted from Wolanski and Ridd (1990).
WATER
CIRCULATION
415
IN G U L F O F C A R P E N T A R I A
which is a flow instability generating cross-slope mixing. Figure 13c shows the east coast around feature " A " on September 17, 18 and 19, 1988. The image e n h a n c e m e n t being sensitive to atmospheric conditions, the shadings are relative so that the same shading can represent somewhat different turbidity on different days. Nevertheless, note in Fig. 13c that the views all show shore-parallel turbidity contrasts in coastal waters. T h e r e is little suggestion of large scale, cross-slope mixing features. Note that feature " A " in deeper waters (also shown in Fig. 13b) moved only a few km over two days, suggesting that mixing proceeded fairly slowly and was a rare occurrence in an otherwise fairly straight turbidity front.
(Fig. 8, line f ) and the elevation of the sensors (Fig. 2a) suggests internal waves up to 20 m peak to trough. However the fluctuations showed no systematic spatial or temporal coherence, as can be seen in Fig. 15, showing the time series of the t e m p e r a t u r e at sites F, B and C, as well as the
ili:
Baroclinic circulation Previous investigators (Rochford, 1966; Forbes and Church, 1983) found that in winter the Gulf waters were were vertically well-mixed in temperature and salinity. In summer however, before the first tropical cyclone, a strong t e m p e r a t u r e stratification was present during my field study, characterised by two well-mixed layers separated by a very sharp thermocline while there was no stratification in salinity (Fig. 14). Note in Figure 14 the fluctuations by 15 m in the depth of the thermocline. These fluctuations were due to the temporal variability as is shown in Fig. 8 (line f ) showing the time series of t e m p e r a t u r e at three elevations at site C. The waters were stratified until day 385 when they became well mixed by tropical cyclone Irma. Thereafter they remained well mixed. Before day 385, the top and bottom sensors experienced t e m p e r a t u r e differences of up to 2°C. At the middle sensor elevation, the t e m p e r a t u r e fluctuated abruptly between that of the top and the bottom sensors, an indication that the thermocline was very sharp and was convoluted by internal waves. Between days 355 and 385 the t e m p e r a t u r e at both the bottom and middlesensors was the same, while it was 2°C higher at the top sensor, an indication that the thermocline was then located always above the middle sensor. A comparison of the t e m p e r a t u r e fluctuations
Fig. 13. NOAA satellite views in the visible band on (a) November 23, 1986 (b) August 17, 1988 and (c) August 17, 18 and 19, 1988 (as is adopted from Wolanski and Ridd, 1990).
416
~i. WOLANSKI
water currents at site C. At site F, the temperature fluctuations decreased with increasing tidal amplitude. At site B, large t e m p e r a t u r e fluctuations were also observed, the thermocline generally, but not always, rising at flood tide. At site C the thermocline displacements increased in amplitude with increasing speed of the tidal currents, but they also seem to be related, at times, to the strength of the sea breeze. The nature of these internal waves, whether they are internal tides, internal seiches, solitons etc., remains unresolved.
The t e m p e r a t u r e stratification in summer supported at times a vertical shear of the lowfrequency circulation which was presumably wind-driven. For instance, from days 340 to 345 the upper current meter at site B measured a strong current along 90 ° in the surface layer (Fig. 8, line c), which was not present in the bottom layer, but which was also observed in the surface layer at sites C and D (Fig. 9b). River inflow may also drive a circulation. Church and Forbes (1983) reported the presence of a vertically well-mixed, brackish water mass in
Fig. 13 (continued).
417
W A T E R C I R C U L A T I O N IN G U L F O F C A R P E N T A R I A
Fig. 13 (continued).
coastal waters (see Fig. 12). They assumed that rl = 0 everywhere and, they inferred, by geostroplay from the thermal wind equation, the existence of a steady, longshore current, v, in this
brackish water mass, varying with depth. This depth-averaged current can also be calculated from the depth-averaged m o m e n t u m equation which is a vertical average of the thermal wind
418
V.WOLANSKI
Temperature (°C) Salinity (%o) 26.5 I 27.5 28.5 I I 034.5 1
35.0 I
o/
20E r-
n
where b is the buoyancy, the x axis oriented cross-slope and f is the Coriolis parameter. This current probably ceases soon after cessation of the river inflow though coastal waters are still brackish. Because of friction, a steady longshore current u b in shallow coastal waters generates a longshore sea level gradient r/. When the river inflow has ceased, as in Fig. 12, there is no mechanism to maintain this slope and overcome bottom friction. Once the river inflow ceases, the river-induced level setup r/ disperses at a speed (gH~) 1/2 >1 10 m s - t and will thus all but disappear in 1 day. As a result, as suggested by Csanady (1982), once the river inflow has ceased, the cross-shore salinity gradient raises the sea level at the coast by an amount r/(Csanady, 1982),
4060 I i '-- C-~----~
O~7/ ~ x = - H ( s ~ b / ~ x ) / 2 g
80 Fig. 14. Vertical profiles of temperature and salinity on October 18 and 19, 1986. equation and a balance between cross-slope density gradients and geostrophic currents, (6)
L'b = 0.5~b / O x H J f
27
'7%.6"
BZop
28tCTop 26
CMiddle
27-
CBottom
l
Middle
365
3(}7
3(39
where ~7 = 0 outside the brackish water mass, so that
cb=o
(8)
There is thus no salinity-driven flow in the shallow, friction-dominated, coastal waters of the Gulf once river runoff ceases even though the coastal waters are brackish.
Conclusions
BMiddle
E I--
(7)
311
Time(days) Fig. 15. Time series plot of the temperature at sites F, B and C and stick plot of currents at site C.
A five-month long field study of the circulation in the Gulf of Carpentaria was carried out in the period October 1986 to March 1987. It involved the deployment of 9 moorings with current meters and tide gauges in two transects, one across the mouth of the Gulf, and the other transect mid-way into the Gulf on the east coast. A weather station was also deployed in the middle in the Gulf, 300 km from the coast. A sea breeze was present in summer in the middle of the Gulf. There were also low-frequency events including trade winds from the southeast and monsoonal winds from the north-west, and two tropical cyclones. The depth-averaged barotropic circulation was also modeled numerically and observations and predictions were compared. The tides in the Gulf of Carpentaria are forced by both the Arafura Sea and the Coral Sea; the
WATER CIRCULATION IN GULF OF CARPENTARIA
influence of the former dominates except near Torres Strait. The semi-diurnal tides decrease rapidly as they enter the Gulf. The diurnal tides however propagate readily into the Gulf as Kelvin waves in a rectangular channel. These features were successfully reproduced by a depth-averaged numerical model that was verified against both tidal data and tidal ellipses data at 12 current meter mooring sites in the Gulf. In summer before the first tropical cyclone, the Gulf was temperature-stratified with a sharp thermocline on which ride internal waves up to 20 m peak to trough. These internal waves were incoherent over horizontal distances of 100 km. The temperature stratification also allowed transient, low-frequency currents in the top layer, independently of the bottom layer. The nett circulation near the mouth of the Gulf was weak, frequently incoherent over horizontal distances of 100 km, and dominated by events. This circulation is probably generated by events in the Arafura Sea and by spatial and temporal variations of the local wind field. Under cyclonic conditions, a large clockwise gyre developed encompassing most of the Gulf, but the circulation was at times three-dimensional, though the waters were vertically well-mixed, with a return flow in deeper waters. Large (--0.5 m) seasonal fluctuations in sea level are experienced in the southern region of the Gulf. Model results suggest that these are mostly wind-driven. This study has identified the importance of three coastal oceanographic processes which may have profound biological consequences that deserve investigations. First, the wind appears to be largely responsible for the seasonal fluctuations of sea level which are very large (up to 0.5 m) in the southern region of the Gulf near Karumba, as previously also reported by Forbes and Church (1983). These seasonal fluctuations are very important because they result in vast areas being only inundated by the tides in summer; these areas can support neither mangrove nor freshwater vegetation and are thus salt flats. The first seasonal inundation of these salt flats in summer generates a "estuarine outwelling" of nutrients from these salt flats to the coastal waters of the
419
Gulf (Ridd et al., 1988). This "estuarine outwelling" coincides with the onset of migration between the estuaries and the coastal waters of commercially important resources, such as barramundi and prawn. Second, along the east coast of the Gulf, the non-linear interaction, through bottom friction, between strong tidal currents and weak wind-driven currents generates a sluggish, shore-parallel circulation in shallow waters ( < 20 m) that traps coastal water probably for long period (months). This water is presumably nutrient-enriched by estuarine outwelling, principally in summer, while further offshore there is little nutrient enrichment. Third, in summer this coastally trapped mass is ejected offshore as a large-scale "coastal outwelling" in the southernmost region of the Gulf (Fig. 6d), presumably enriching the offshore waters in this area with nutrients swept from several hundreds kilometres of coastline along both the east and west coasts. Coastal trapping ensure a long residence time of outwelled nutrients and may then enhance productivity in coastal waters. It will also trap pollutants so that particular care should be taken in the disposal of waste from mining and coastal developments.
Acknowledgements It is a pleasure to acknowledge the assistance of Dr. M. Inoue, Dr. P. Ridd, Mr. R. McAllister and Mr. J. Sodusta. Dr. P. Rothlisberg kindly provided the salinity data used to draw Fig. 12. W. Skirving and C. Steinberg produced the satellite images. This study was supported by the Australian Institute of Marine Science and by a small grant from the Commonwealth Bureau of Meteorology.
References Church, J.A. and Forbes, A.M.G., 1981. A non-linear model of the tides in the Gulf of Carpentaria. Aust. J. Mar. Freshwater Res., 32: 685-697. Church, J.A. and Forbes, A.M.G., 1983. Circulation in the Gulf of Carpentaria. I: Direct observations of currents in the South-East corner of the Gulf of Carpentaria. Aust. J. Mar. Freshwater Res., 34: 1-10.
420 Csanady, G.T., 1973. Wind-driven barotropic motions in long lakes. J. Phys. Oceanogr., 3: 429-438. Csanady, G.T., 1982. Circulation in the Coastal Ocean. Reidel, Dordrecht, 279 pp. Forbes, A.M.G. and Church, J.A., 1983. Circulation in the Gulf of Carpentaria. II: Residual currents and mean sea level. Aust. J. Mar. Freshwater Res., 34: 11-22. Heaps, N.S., 1978. Linearized vertically-integrated equations for residual circulation in coastal seas. Dtsch. Hydrogr. Z., 31(5): 147-169. Provis, D.G. and Lennon, G.W., 1983. Eddy viscosity and tidal cycles in a shallow sea. Estuarine Coastal Shelf Sci., 16: 351-361. Radok, R., 1978. Tidal height and stream prediction at Groote Eylandt. BHP Tech. Bull., 22: 46-60. Ridd, P., Sandstrom, M.W. and Wolanski, E., 1988. Outwelling from tropical tidal salt fiats. Estuarine Coastal Shelf Sci., 26: 243-253. Rienecker, M.M. and Noye, J., 1982. Numerical simulation of the tides in the Gulf of Carpentaria. In: J. Noye (Editor), Numerical Solutions of Partial Differential Equations. North Holland, Amsterdam,
E. WOLANSKI Rochford, D., 1966. Some hydrological features the Eastern Arafura Sea and the Gulf of Carpentaria in August 1964. Aust. J. Mar. Freshwater Res., 17: 31-60. Sodusta, J.A., 1988. Non-linear interaction of wind and tide driven currents in the Gulf of Carpentaria. M.Sc. Thesis, Dep. Geol. Geophys. Univ. Sydney, 123 pp. Webb, D., 1981. Numerical model of the tides in the Gulf of Carpentaria and Arafura Sea. Aust. J. Mar. Freshwater Res., 32: 31-44. Wolanski, E. and Ridd, P., 1990. Mixing and trapping in Australian tropical coastal waters. In: R.T. Cheng (Editor), Residual Currents and Long-term Transport. Springer, New York, pp. 167-183 Wolanski, E., Imberger, J. and Heron, M.C.. 1984. Island wakes in shallow coastal waters. J. Geophys. Res., 89: 10,553-10,569. Wolanski, E., Ridd, P. and Inoue, M., 1988a. Currents through Torres Strait. J. Phys. Oceanogr., 18: 1535-1545. Wolanski, E., Drew, E., Abel, K. and O'Brien, J., 1988b. Tidal jets,nutrient upwelling and their influence on the productivity of the alga Halimeda in the Ribbon Reefs, Great Barrier Reef. Estuarine Coastal Shelf Sci., 26: 169-201.