river plumes, coral reefs and mixing in the gulf of papua and the northern great barrier reef

Estuarine, Coastal and Shelf Science (1984) 18, 291-314

River Plumes, Coral Reefs and Mixing in the Gulf of Papua and the Northern Great Barrier Reef

E. Wolanskia, G. L. Pickarda,b and D. L. B. Juppc aAustralian Institute of Marine Science, P.M.B. No. 3, Townsville M. C., Queensland 4810, Australia; b Department of Oceanography, University of British Columbia, Vancouver, B. C., Canada V6T 1 WS and cCSIRO Division of Water and Land Resources, I? 0. Box 1666, Canberra City, ACT 2601, Australia

Received 22 February 1983 and in revised form 13 June I983

Keywords: river plumes; mixing processes; wakes; Great Barrier Reef; Papua; Coral Sea

The mean annual freshwater discharge in the Gulf of Papua, principally from the Fly, Kikori and Purari rivers, is estimated to be 13 000 m3 s-1. Water from these rivers forms a low salinity surface layer in the Gulf, where it has a residence time of about two months and flows generally eastward. A small fraction of this water intrudes through Bligh Entrance on the Great Barrier Reef continental shelf. Horizontal patchiness and mixing of these intruding waters with shelf waters are considerably enhanced by secondary circulation (wakes) around coral reefs. The wakes shapes, visible in enhanced LANDSAT imagery, are similar to those around cylinders and plates in laboratory experiments at low Reynolds numbers. Topographically enhanced mixing may explain why the cross-shelf gradients of temperature and of the structure of fish communities are smaller in the northern region than in the central region of the Great Barrier Reef.

Introduction

The area studied, shown in Figure 1, includes the waters of the northern Great Barrier Reef (GBR) continental shelf and the Gulf of Papua between about 1 lo40 ‘S and 8%. The ribbon of reefs forming the outer GBR separates the deep waters ( > 1500 m) of the Coral Sea from the shallow waters (20-40 m) of the continental shelf. The bathymetry of the shelf is highly complex, with a large number of coral reefs and submerged sand banks scattered (often very densely) throughout the shelf. The waters of the Gulf of Papua are deeper and are unencumbered by reefs. The GBR continental shelf is open by Torres Strait to the Gulf of Carpentaria, and by Bligh Entrance to the Gulf of Papua. The Torres Strait is a shallow water body, 1+15 m deep ( < 10 m in the northern half), 100 km long (N-S) and 20-60 km wide (E-W) with extensive reef formations. Bligh Entrance, with a number of coral reefs scattered throughout (see Figure 13), is roughly 2&30 m deep and 30 km wide (E-W) at its narrowest point. To the east of Bligh Entrance, there exists an extremely dense network of reefs covering the remaining width ( = 50 km) of the shelf

This northernmost area is of particular scientific and socio-economic interest. Its scientific interest is due to the interaction of large recent tectonic movements with coral reefs

291

0272-7714/84/030291 + 24 $03.00/O 0 1984 Academic Press Inc. (London) Limited

292 E. Wolanski, G. L. Pickard & D. L. B. Jupp

42’E 143“E

/

I Gulf of Papua

Figure 1. Map of the study area. Bottom contours (in m) are approximate and smoothed. (No attempt was made to draw the contours around reefs and islands.) Deep ocean casts are marked 0.79, 0.81 and 0.82. The framed areas A and B are those shown in the LAND- SAT pictures of Figures 13 and 19 respectively. The other symbols refer to three station numbers (a, S, y) and to current meters mooring sites near Bird Island (BI), Middle Banks (MB), Turtle Cay (TC), Rains Island (RI), Shortland Reef (SR), Thursday Island (TI), Yorke Island (YI), Keats Island (KI), Pearce Cay (PC), Murray Island (MI) and Yule En- trance (YE).

and the substantial freshwater discharges in the Gulf of Papua (Tanner, 1969; Manser, 1973). It is crossed by two important shipping lanes (Whiteman, 1978). It has been identified as a major breeding ground for rock lobsters and prawns (see MacFarlane, 1980). Also, the major Ok Tedi mining developments may result in significant increases in the concentration of suspended sediment and soluble metals (principally Cu and Fe, see Boyden et uZ., 1974) in the Fly River waters, a fraction of which intrude on the GBR.

Little was known of the physical oceanography of this area until recently (Pickard et al., 1977; Amin, 1978). From 1979 to 1982, one of us (EW) deployed self-recording current meters and tide gauges at a number of sites shown in Figure 1. It was found from

River plumes and mixing 293

these data (Wolanski & Ruddick, 1981; Wolanski, 1983; Wolanski & Thomson, 1984) that a mean current through Torres Strait does not exist contrary to the belief of Wyrtki (1960). Instead, the GBR shelf waters north of 11”s can be viewed as a backwater of the Gulf of Papua. The local wind drives strong reversing (alternately northward and southward) low-frequency currents (typically 10 days duration) through Bligh Entrance. The tides on the GBR shelf are forced by the Coral Sea and the Gulf of Papua, the tides and the low- frequency sea level perturbations in the Gulf of Carpentaria being unable to propagate eastward across Torres Strait.

In what follows, the oceanographic data collected in these cruises are used to identify the various water masses in this area from their temperatures, salinity and silicate concentration. From river discharge data, the mean annual freshwater discharge in the Gulf of Papua is estimated to be roughly 13 000 m3 s-l, and some of that water intrudes on the GBR continental shelf. The areas of intrusion are mapped from ship and satellite data. The latter reveal the existence of wakes in the lee of coral reefs. Topographically enhanced mixing is estimated to be the dominant mixing process in the GBR waters.

Methods

Oceanographic data Oceanographic cruises were completed in the area in November-December 1979, November 1981, April-May 1982 and October 1982. Salinity, temperature and silicate concentration were measured from water samples (Mitchell et al., 1982) and, for salinity and temperature, also with a profiler (Colman et al., 1981) and a thermosalinograph.

Remote-sensing data Few cloud-free satellite images are available. The only data are first-generation negatives from the Coastal Zone Colour Scanner (CZCS) for September 1979, and the tapes from the LANDSAT satellite from 1972 to 1981.

LANDSAT, on water, can delineate (but not distinguish between) the patterns of bathymetry (e.g.. Bina et al., 1978; Doak et al., 1980) and turbidity (e.g. Jerlov, 1976; Munday & Alfoldi, 1979). Experience by one of us (Jupp et al., 1983) with the use of LANDSAT on the GBR shelf, has shown that when the water is less than 5 m in depth, or turbid with concentrations higher than 5 p.p.m. of the fine particles typically found in the GBR region (Wolanski et al., 1981), LANDSAT bands 4 and 5 have significant signal levels above the background noise, while in the regions of deeper water and (or) lesser turbidity, only small variations in band 4 signals can be used to delineate depth and water masses, but repeated views are necessary (e.g. Moore, 1980; Jupp et al., 1983). As a result, each LANDSAT image was enhanced using the water depth algorithm of Jupp et al. (1983). This algorithm relies on finding a deep water area with ‘average’ optical properties. The histogram for this area is then used to define a level above which the signal in each band must rise to respond to bathymetry or turbidity.

Results

Water properties The spatial average of the STD data is somewhat limited by the remoteness of the area and severe navigation difficulties in these reef-studded, mostly uncharted, waters. The Fly River discharges at Kuambit (500 km upstream from the river mouth) during the four


Temperature PC 1

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25 30

Sallnlty Woo) 15 20 253031 32 33 3435

1 Shallow well-mIxed Llttle Influenced by runoff

15 20 25 30 31 32 33 34 35

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Influence of

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ruver runoff

15 20 25 30 31 32 33 34 35

Oceanic type

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Figure 2. Types of temperature and salimty profiles-Torres Strait to Gulf of Papua. The station locations are shown in Figure 1.

cruises can be read from Figure 10(a). It can be seen that the discharges were of comparable magnitude except in October 1982 when a brief drought prevailed in the upper Fly River catchment.

Vertical distributions of water properties: There are three basic structures (Figure 2), which we will refer to as Types 1, 2 and 3. The Type 1 structures are found in the shallow regions in the south where tidal currents are strong and the vertically well-mixed structures result from mixing generated at the bottom. The Type 2 structures are of estuarine character and are found chiefly along the north-west and north sides of the study area, resulting from the river runoff from Papua New Guinea. Sometimes a stepped salinity structure is found. This is common in estuarine regions where runoff varies with time or when periods of strong wind cause mixing of the upper layer followed by calm periods when runoff generates a lower salinity layer over the previously mixed layer. The Type 3 structures were found in the Gulf of Papua in deep waters and are similar to those of the NW Coral Sea (Pickard et al., 1977).

Horizontal distribution of properties: The horizontal distributions, near the surface and at 20 m, of salinity and temperatures are shown in Figures 3-6. At the surface, the 1979 and 1981 spring cruises show similar salinity distributions with the isohalines running north- east-south-west with low salinities along the north-west side and the lowest values at the north end close to the Gulf of Papua coast. This pattern suggests that the bulk of the river runoff in the Gulf of Papua moves east along the coast of Papua New Guinea. At 20 m depth, the general trend of the isohalines is the same as that at the surface but values are


142’E 144OE

100s

120s

1 4 2OE 144OE

, .

Figure 3. Salinity distributions (UO) at the surface and 20 m depth for the 1979, 1981 and April-May 1982 cruises. Note that (e) and (fi are one degree latitude further south than (a)-(d). 0 indicates a vertical profile station.

generally slightly higher. A review of the vertical profiles of salinity shows that 80-90% of the freshwater in the Gulf of Papua is in the top 10 m.

In the 1979, 1981 and October 1982 cruises, high salinity water was present at the extreme south-west in the Thursday Island area. This is characteristic of the northern GBR shelf where salinity reaches a maximum of over 36%0 in November (Pickard et al., 1977). In April-May 1982, the salinity values were less than 34%0 in the same region following the passage of a cyclone.

The shape of the salinity distribution in Bligh Entrance was similar in all four cruises and shows a tongue of Gulf of Papua brackish water intruding on the GBR shelf. This


r Y-k Il42”E 14j+ ‘;=sv . =hl440E

Figure 4. Surface salinity distribution for the October 1982 cruise.

intruding lens is probably a nearly permanent feature in the area, because the four cruises covered a wide range (300-1300 m3 s-i) of Fly River discharges at Kuambit (unfortunately never a major flood) and weather conditions. The October 1982 salinities in Bligh Entrance were higher than for the three previous cruises, as a consequence of the Fly River runoff (at Kuambit) being by far the smallest.

The Warrior Reefs are not a western boundary (a wall; Figures 4 and 6) for the Gulf of Papua brackish water intruding on the GBR shelf during steady river runoff conditions, and this finding can be attributed to the (slow) leakage over the reef and through Missionary Passage. This finding may not necessarily be true in the case of large unsteady Fly River discharges (i.e. in flood). Indeed, during a transient flood event (April-May 1982 cruise), large differences of salinity were observed on both sides of the Warrior Reefs which, in such conditions, may thus be a western boundary for the intruding brackish water lenses.

The distribution of temperature (Figures 5 and 6) are less systematic than the salinity ones. At the surface, in 1979 and in 1981, the highest temperatures (> 29 “C) were at the south-west and north-east ends of the study area. In the north-east, this observation may result from the stability due to the turbid, low salinity surface layer trapping the heat in this layer. The reason for the warmer water at the south-west is less obvious. Previous observations (Pickard et al., 1977) had indicated that the temperature in the northern part

River plumes and mixing

142’E 144OE 1 S’SI

142OE 144OE

B .i4 27.5

I -- -.

1 “c’ :28.5” { i’i

Figure 5. Same as Figure 3 for temperature (“0

of the GBR Lagoon is typically less than 28 “C in November. At 20 m depth, this warm water is still evident in the south-west. The slightly cooler water at the extreme north-west at 20 m may be evidence of estuarine inflow to replace the saline water entrained and moved eastward along the Papua New Guinea coast in the upper layer. The 1982 temperature values in the south-western area were I-2.5 “C lower than in 1979 and 1981, as a consequence of winter cooling.

In 1979, silicate was also sampled. It is interesting to note (Figure 7(a)) the inverse relationship between salinity and silicate, the latter presumably brought in by the river water. Figure 7(b) shows the distribution of silicate in the surface water. It is seen to be the inverse of the salinity distribution (Figure 3(a)).


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Figure 6. Surface temperature (“C) distributions in the October 1982 cruise.

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Figure 7. (a) Correlation between surface concentrations of silicates and salinity. (b) Silicate concentration distribution at the surface for the 1979 cruise.


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Figure 8. Depth profiles of (a) q, (b) temperature and (c) salinity, and (d) T-S curves for the ocean stations.

T-S characteristics: The characteristics of Coral Sea water, the source of ocean water for most of the study area, are described from deep casts made on each of the cruises (stations 0.79, 0.81 and 0.82, Figure 1). In Figure 8 is shown: (a) o, density/depth profiles; (b) temperature/depth profiles; (c) salinity/depth profiles and (d) temperature/salinity (T-S) characteristic curves. o, is almost completely determined by temperature. The temperature profiles show the usual mixed layer (most marked in April-May 1982, following the passage of a cyclone, to 130 m) and a strong thermocline below this. The salinity profiles show the weak salinity maximum of the subtropical lower water at 100-180 m depth. The lower surface value in May than in November is typical (Pickard et al., 1977) but not as low (salinity(s) = 34.7%0) as the previous observations had suggested. The surface temperature of 27.5-28.5 “C for 1979 and 1981 is typical for that time of the year. The value of 25-26 “C in April-May and October, 1982, is lower than usual.

T-S curves were drawn for all stations. As they are too numerous to show individually, envelopes of the T-S curves are shown in Figure 9 (a)-(d), where the main areas within the envelope have been designated A-F. On the accompanying chartlet, the study area for 1979 is outlined and the types of T-S curves indicated for parts of the area as follows. For example, in the north-east, ‘ AF ’ indicates that the T-S curves extend from envelope area A (which represents the upper layer) to envelope area F (deeper water). ‘ BF ’ indicates curves for the east side of the study area, while the curves for the shallow water to the south-west are entirely within the envelope areas B or C respectively. The T-S curve for the ocean station is indicated by ‘ 0.79 ‘. The symbolism is similar for the other cruises, The envelope areas D and E have been added to the study areas, for these latter cruises, shown on the chartlets (Figure 9).


Solinlty f”&)

22 24 26 28 30 32 301

34 36 I 1 I 1

Salinity (?A)

22 24 26 28 30 32 34 36

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1962 ‘0.82 I(l)

(b)

Nov. 1981

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Figure 9. Envelope of T-S curves for (a) 1979, (b) 1981, (c) April-May 1982 and (d) October 1982, with relevant ocean station T-S curves. See text for explanation of symbols.

These T-S envelopes describe the water characteristics fairly completely. The influence of river runoff is indicated by the envelope area A for the north side of the study area. The absence of freshwater (in quantity) to the south is clear from the T-S curves lying within the higher salinity envelope areas RF. It should be noted that the T-S curves for stations at the south-west end in 1979 and 1981 are entirely within the envelope areas C and do not extend into the T-S areas B or F. For 1982 the stations were mainly in the shallow south-west part of the study area and the T-S curves for the stations were entirely within the envelope areas B, D and E respectively.

Rainfall-runof in Papua New Guinea: Rainfall in the New Guinea highlands is amongst the highest in the world, reaching as high as 13 m year-l (SMEC, 1978). The available river discharge data for the Fly River at Kuambit, about 500 km from the Gulf of Papua, are shown in Figure IO(a). Some Fly River discharge data, of lesser accuracy, at Ogwa, about 250 km from the Gulf of Papua, are available and show discharge values up to three times the mean Kuambit figure, i.e. about 5500 m3 s-1, with only a relatively small annual fluctuation with a maximum in June/August and a minimum during October/November. Some data for 1979 are available (Figure 10(b) and (c)) for the Kikori River (eight complete months and four part months) and for the Purari River (six part months). These data also suggest relatively little seasonal variation for either river and give a total (for the gauged area only) for all three rivers of about 8400 m3 s-1. Hence, the freshwater discharge in the Gulf of Papua is about 13 000 m3 s- 1, taking into account the ungauged areas and the uneven rainfall distribution (Brookfield & Hart 1966; Aitken et al., 1972). This discharge is significant, of comparable magnitude to that of the Mississippi River.

Satellite observation of mixing processes In this section, we show satellite pictures as evidence of river plumes in the Gulf of Papua,


4000 = 1979 * 1980

3000 t l 1981

n 1982 b

2000 . ” * ,*-•-

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Figure 10. Monthly mean discharge, a in m3s-1, of (a) the Fly River at Kuambit, (b) the Kikori River at Kaiam, (c) the Purari River at Wabo. < > indicates that the data are incomplete for the month.

of brackish water intrusion from the Gulf of Papua on the GBR shelf, and of wakes in the lee of coral reefs. It is argued that secondary circulation around reefs considerably enhances mixing of shelf waters. This mechanism is used to explain the small cross- sectional gradient of temperature, salinity and the structure of reef fish populations in the northern region of the GBR.

CZCS data: The only two cloud-free views are in the eastern Gulf of Papua and show river plumes extending straight offshore (i.e. weak ambient currents prevailed) for 30 km on 13 September 1979 (Figure 1 l(a)). There is some evidence of a south-eastward current, with mixing, on 24 September 1979 (Figure 1 l(b)). The pictures are similar in all bands (some of which are in the region of high or low chlorophyll a (Chla) absorption) suggesting that the contrast is not due to biological conditions but to actual differences in colour due to the confluence of sediment-laden rivers and the sea (e.g. Hovis et al., 1980). The large CZCS pixel size (=850 m) accounts for the ‘blurry’ figure at these small scales and precludes the use of this sensor for small scale mixing studies.

LANDSAT data: Cloud-free LANDSAT views are available on 18 August 1972, 27 October 1972, 1 November 1980, 22 June 1981 and 13 November 1981. The August 1972 view was taken in calm weather and during an exceptional drought when the Fly River discharge did not exceed 150m3 s-r (SMEC, 1973). Only the area around Cape York is cloud-free in the LANDSAT views on 27 October 1972, 22 June 1981 and 13 November 1981. The average Fly River discharge at Kuambit for the two weeks previous to the 1 November 1980 view was 1300 m3 s-l.


P c

Gulf of Papua ,

Figure 11. (a) Band 3 raw Coastal Zone Colour Scanner view of a coastal stretch, roughly 100 km long, of the central-eastern region of the Gulf of Papua on 13 September 1979, showing the southernmost mouth of the Purari River (left) and other river plumes further east. (b) Same for 24 September 1979. In (b), the arrow points to Cape Suckling.

The band 4 view for 1 November 1980 shows a considerable range of contrast (i.e. a wide spectrum) east of the Warrior Reefs, while the band 5 view has much less variation, indicating that over much of the area the water is deeper than about 5 m and/or has low turbidity. In addition, there is a sun glint effect (Figure 13). The enhanced image is shown in Figure 13.

To separate some of these compounded effects, the enhanced image of 18 August 1972, at 1004 h was also analysed (Figure 12). The data are of lesser quality with additional scatter due to extensive patches of Trichodesmium algae which were delineated using the near-infra-red bands.

Training sets were taken on both the 1972 and 1980 images, and the results are tabulated in Table 1. It can be seen that the band zones around the reefs in the two images are spatially very similar in the two images. One existing bathymetric feature that is well reproduced in both views is the sediment erosion fan (marked A in Figure 12) west of Basilisk Passage. Also, it is clear from Figure 12 (and bathymetric data confirm this) that there exists no extensive shoals around the coral reefs and islands in Bligh Entrance. Figure 12 is thus an approximate bathymetric map.

The dark tongue of water in the 1980 image (Figure 13 and 14) has significant lower band-4 radiance than the ‘old’ shelf water into which it seems to be intruding (Table 1). This indicates it is less turbid than the surrounding water (Moore, 1980). The tongue is


Figure 12. Enhanced LANDSAT view of the study area on 18 August 1972. The field of view is slightly narrower than that marked A in Figure 1. Lightest colour indicates shallow water (maximum depth of penetration = 20 m). The contrast across the vertical line one third of the width from the left is due to hardware problem. ‘ A ’ refers to a sediment erosion fan, and ’ B ’ to Basilisk Passage.

a feature only of the 1980 image. Further, there is some evidence (Table 1) that the water body in the 1972 image and the ’ old ’ shelf water in the 1980 image are of similar optical types. Of the two, the 1980 water is possibly slightly more turbid since the ratio of bands 4 and 5 radiances (LANDSAT colour index) decreases from 2.6176 in the 1972 image to 2.1771 in the 1980 image.

The spectral characteristics suggest (Table 1) that the wakes to the south-west of many reefs, within and on the edge of the tongue in the 1980 image which have no counterparts in the 1972 image, are remnant shelf water surrounded by the tongue water.

The overall features of the tongue of water intruding on the GBR shelf through Bligh Entrance (Figures 13 and 14), are only marginally influenced by water depth for the area east of the Warrior Reefs and, further, bear some resemblance to those of the surface distribution of salinity (in all the cruises) and silicate concentration (Figures 3, 4 and 7(b)).

One difference between the salinity data and the LANDSAT imagery is the western boundary of the intruding tongue, located respectively west and east of the Warrior Reefs.


TABLE 1. Spectral characteristics of the 1972 and 1980 scenes. DN stands for Digital Number and is the value recorded on the LANDSAT tape scaled to a range of O-255. The ‘rad’ column is the radiance in W cm-2 sr-1 (Thomas, 1975; Robinove, 1982). Despite known problems relating LANDSAT data to such physical measures, the ‘rad’ column does provide a better comparison between the scenes.

‘Old’ shelf water (mean)

‘Old’ shelf water

(threshold)

Intruding tongue (mean)

Intruding tongue

(threshold)

DN rad DN rad DN rad DN rad

1972 38 0.3696 42 0.4085 - - - - 18 0.1412 26 0.2039 - - - - 8 0.0552 14 0.0966 - - - - 0 (<0.0317) 8 0.1255 - - - -

1980 37 0.4500 42 0.5000 33 0.4100 35 0 4300 32 0.2067 28 0.2467 22 0.2067 28 0.2467 12 0.1287 18 0.1631 12 0.1287 18 0.1631 7 0.2143 14 0.3186 8 0’2292 14 0.3186

The transient floods in the Fly River, which occurred in October 1980, may have created a river plume that may not have had enough time to diffuse westward of the Warrior Reefs before the November 1980 LANDSAT photograph was taken. The interpretation of the satellite data in this area west of the Warrior Reefs is further complicated by the high turbidity (secchi disc visibility < 2 m, compared to about 5-10 m in Bligh Entrance).

Island wakes and eddies: Considerable mixing by the complex circulation through the reef matrix is apparent along the plume boundaries as made evident by patches of plume water being entrained into the ‘old’ shelf waters (e.g. the features marked A-E in Figure 14). Another striking feature of this imagery is the existence of wakes in the lee of coral reefs or low-lying islands and coral cays (Figures 15 and 16). The axis of the wakes is roughly parallel to the main channel in Bligh Passage. The size of the visible wake (Figures 15 and 16) diminishes with distance northward. This finding may indicate that, for similar currents (as the data show), the wake is always present but that, in the northern area, the waters have had more time to mix than in the southern area. Indeed, at Dalrymple Island (Figure 15), the wake is barely visible, while at Rennel, Arden Islands, Shoal Reef and Roberts Island the visible wake is much longer.

These processes result in patchiness in shelf waters (Figures 15 and 16), the mixing being least complete (hence a high degree of patchiness prevails) near the plume boundaries and near islands or reefs.

Eddies shed by obstacles in a flowing stream have been extensively studied in laboratory experiments (e.g. Batchelor, 1967; Gerrard, 1978). There has been, however, no field study giving a detailed synoptic map of the currents in the wake of an island a few kilometres in diameter on a shallow continental shelf, although there is evidence that wakes do exist (Uda & Ishino, 1958; Patzert, 1969; Barkley, 1972; Emery, 1972; Hogg et al., 1978; Maddock & Pingree, 1978; Hamner & Hauri, 1981; Fu & Holt, 1982; Imberger & Johannes, personal communication). Indeed, the eddies shed by islands or headlands may be important as biological containers (e.g. Hamner & Hauri, 1977; Aldredge & Hamner, 1980) and in engineering applications (e.g. siting of outfalls).


Figure 13. Enhanced LANDSAT view of the study area on 1 November 1980. The field of view is that marked A in Figure 1. The results are interpreted in Figure 14. The white patches are coral reefs.

The existence of a wake in the lee of a coral reef in clear water (where serial observations show no visible structure) is also suggested from the following experiment. Large (2 x 2 m) sail drogues measuring currents at 6 m depth and tracked by radar, were released during slight wind conditions around Table Top Reef. This reef is nearly circular (Figure 17) and quite flat, submerged in 0.2-2 m of water, surrounded by well-mixed waters roughly 40 m deep. The reef slopes are steep so that the 40 m isobath is only 100 m or so from the reef edge. The trajectories of the drogues (Figure 17) suggest the existence of a fairly steady ambient eastward current ( ~30 cm s-i), with, in the lee of the reef, a clockwise-rotating eddy of diameter slightly smaller than that of the reef.

A field study of the circulation around Rattray Island was recently completed by one of us (EW). This island, marked A in Figure 20, is surrounded by turbid vertically well- mixed coastal waters 20-30 m deep. Aerial observations (see also Figure 20) reveal the existence of a wake located south-east of the island and made apparent by a colour front, the approximate location of which is shown in Figure 18. A number of drogues and moored


Shallow coostol woter (<IO m)

Figure 14. Interpretation sketch of Figure 13. The symbols are KI Island), A-E for features referred to in the text, RI (Rennel Island), SR (Shoal Reef), RO (Roberts Island).

(mooring site off Keats , DI (Dalrymple Island),

current meters were deployed at sites shown in Figure 18. We show, as an example, a typical synoptic view of the circulation during south-eastward prevailing currents. A clockwise-rotating eddy, of a length equal to l-2 times that of the island, is apparent. A strong velocity shear near the lateral edges (the colour fronts) is also apparent.

The length of the wakes in Figure 15 is l-3 times the width of the coral reefs or islands. No signs of instabilities such as Karman vortex streets is apparent. Contrary to the case of the Hawaiian islands, there is no likeliness of eddy generation by local wind conditions since the reefs are small and generally submerged, so that the wind is not modified measur- ably by the reefs. Comparison of the wake shape with that from laboratory experiments suggests a Reynolds number, R:

R=~elO (1) V

where U is the current speed, D the diameter of the island and v the eddy viscosity. Take U=O ‘4 m s-1, D = 1 x 103 m, one obtains v = 40 m* s-1. By the Reynolds analogy:

&=V (2)

where & is the horizontal exchange coefficient (eddy diffusion) around the islands or reefs. Implicit behind these calculations is the assumption that Coriolis effects are small, because the Rossby number &= U/Df, where f is the Coriolis parameter, is large (of the order of unity or larger).

River plumes and mixing

Figure 15. Close-up of Figure 13 for (a) Dalrymple Island, (b) Rennel Island, (c) Arden Island (left) and Shoal Reef, and (d) Roberts Island. See Figure 16 for an interpretation sketch and a scale (the same for all figures). The pixel size is ~80 tn. The scattered white patches south of Arden Island and Shoal Reefs are clouds.

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km Figure 16. Interpretation sketch of Figure 15(b). Depths (in m) have been added. Stipple indicates wake and eddy patches.


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Figure 17. Drogue trajectories around Table Top Reef on 8 December 1982. The local time (hours) is marked.

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0 km L

0 50 cm s-1 t

Figure 18. Stick plot of 10 mm averaged currents measured at 5 m above the sea floor in 20-30 m depth, at the sites marked (O), around Rattray Island at 1 I,10 hours local time on 4 December 1982. Also, drogue trajectories (0), with local time (hours). The position of the colour fronts is drawn from aerial observations and its precise extent south-east of the island could not be seen clearly. Calm weather pretiailed.

This value of 4, is an order of magnitude larger than that (-5 rn2 s-l), derived by Okubo (1974) from dye spreading rates for a patch about 1000 m, and this discrepancy is due to the enhanced mixing by the secondary circulation around coral reefs. This value of & is also, for the same reasons, much larger than the lateral eddy diffusivity, D,, that one would expect in a ‘ river’ (Bligh Entrance) in the absence of islands, bends and secondary circulation (Fischer, 1981):

D,=O.15 H U*EO.4mZs-l (3)


Figure 19. Raw LANDSAT band 4 view of the Tomes Strait area near Cape York on 13 November 1981. The location of the area is that marked B in Figure 1.

where H is the depth (30 m) and U* is the shear velocity. This value of 4, is much smaller than the longitudinal dispersion, K,, due to secondary circulation operating at large time scales in a channel (Bligh Entrance) (Fischer, 1981):

K _O.lu2lP 1 = 6 x 104m2 s-1

H U*

where Wis the width of the channel (lo4 m).

The result:

is expected, as mixing is related to the eddy size (of the order of the depth for D,, of the island size for 4, of the channel width for K,). Hence, island wake mixing plays a dominant role in mixing at scales of a few kilometres, providing a mechanism for enhancing mixing at intermediate scales in the cascade from large to small scales.

In Torres Strait, near Thursday Island, the currents are primarily tidal and very strong. The corresponding values of the bulk Reynolds number should be much higher and the wakes (by comparison with laboratory experiments such as plates l-3 in Batchelor, 1967) should become wavy and develop instabilities, as, indeed, is apparent from Figure 19. The shape of the wakes suggests Reynolds number values of the order of 50-100. For Hz20 m, DU 103 m and tidally-predicted Uzl m s-1, L& = v c- 10-20 rnz s-1.

The vortex shed by an island is deformed by the flow-field, the vorticity being


Figure 20. (a) Raw LANDSAT band 4 view on 26 May 1975 of the area around Hook Reef (20%). (b). Raw LANDSAT band 4 view on 26 May 1975 of the GBR coastal waters near 2O”S, an area of high turbidity with numerous islands. ‘A’ refers to Rattray Island.

strengthened as the vortex lines are stretched. At the same time, the vorticity diffuses sideways and the eddy is weakened. J. Imberger (personal communication) has shown that the ratio of these two opposing actions depends on the ratio:

p= E DID

(7)

and modelled such flow in a Hele-Shaw apparatus. He showed that there exists a critical value, PC =l such that if P> PC, the eddy exists; P < PC, diffusion prevents eddy formation One finds P-O.9 in Bligh Entrance, and Pzl. 3 in Torres Strait, implying that eddies are shed by the reefs but also weakened by diffusion. The use of (7) implies that islands of order of 10 km on a shallow continental shelf may not shed an eddy.

Another type of eddy structure occasionally visible from satellite is generated by the oscillating tidal currents through rear openings. As the water jet develops, the separation


of the jet from the boundaries (the reef) generates a vortex pair which advances with the leading edge of the developing plume (Ozsoy, 1977, Wilkinson, 1978). This vortex pair is visible in Figure 20(a). Figure 20(b) shows that such vortex pairs are also shed by the jet flow between islands (constrictions), while eddies are also generated by flow separation at headlands.

Eddies must be assumed to exist on the lee of reefs everywhere on the GBR, but are not visible in satellite imagery due to clearer offshore waters.

Discussion

The northern Gulf of Papua covers about 3.7 x 1010 m*. If the thickness of the low salinity upper layer is 10 m, using a mean salinity of 32.5%0 for this layer relative to a base salinity of 35.5%0 (northern Coral Sea upper water), the volume of freshwater in this upper layer is about 6.3 x 1010 m3, or about 1.8 months of river flow, the mean river runoff being around 13 000 m3 s-1. This very rough calculation suggests that the salinity distribution in the Gulf of Papua probably represents an integral of two months or so of river runoff. This would tend to smooth out in the Gulf of Papua the effects of the already small seasonal variations. This long time of residence indicates that net currents are very small in the Gulf of Papua, in general agreement with the findings from surface and bottom drifters (MacFarlane, 1980).

The bulk of the freshwater from Papua New Guinea rivers moves east along the coast of Papua New Guinea, and may be responsible for some of the low salinity surface patches further east in the Coral Sea (Donguy & Henin, 1975). Some brackish river water, principally from the Fly River, intrudes on the Great Barrier Reef shelf, primarily through Bligh Entrance. Water at all stations in the northern half of the study area was less saline than in the open ocean (northern Coral Sea), indicating that river runoff does influence all stations in the northern half of the area including Bligh Entrance.

The water at the south-western end of the area, i.e. near Thursday Island, was more saline and warmer (except after the passage of a cyclone in April 1982) and probably came from the northern GBR shelf.

We expect that the general character of the salinity distribution described from the four cruises (except for exceptional floods) is typical for Bligh Entrance, with lower actual values for salinity during the period January-September when the runoff may be greater. The occasional strong bursts of northward currents through Bligh Entrance will help flush out from the Great Barrier Reef shelf some of the intrusive brackish water. However, enhanced mixing and trapping by secondary circulation (wake effect) around coral reefs increase the horizontal diffusion coefficient, inhibit the flushing, and diminish the range of the salinity fluctuations. As a result, the temporal salinity fluctuations in Bligh Entrance are expected to be smooth and primarily low-frequency in nature. Indeed, only ‘small’ salinity fluctuations (up to 1.5%0), several days to several weeks in duration are observed, uncorrelated with the temperature variations which are also primarily low-frequency in nature (Figure 21).

The mechanism of enhanced mixing due to the secondary circulation behind reefs is one explanation for the large differences between the cross-shelf gradients in temperature, in, respectively, the northern region (12”s) of the Great Barrier Reef south of Cape York (where reef density throughout the width of the continental shelf is high and cross-shelf temperature gradients small = 0.5 “C 80 km-i) and the central region (18”s) of the Great Barrier Reef (where reef density is low and temperature gradients in the same time of


31’ I Nov. Dec. JOll. Feb. March Apr.

1981 1982

Trme (months)

Figure 21. Time series of half-hourly temperature (raw data) and salinity (detrended raw data until mid- January, 1982; detrended and law-pass filtered thereafter, because of fouling of the conductivity sensor aliasing the data) at the Pearce Cay current meter (5 m above the bottom in 23 m depth).

the year are high 3 ’ C 80 km-i; Wolanski et al., 1981). This mechanism also enhances the dispersal of fish larvae and may explain the contrast in the cross-shelf gradients of the structure of reef fish communities in, respectively, the central region (18”S, where these gradients are large; Williams, 1982) and the northern region of the Great Barrier Reef (12”S, where these gradients are small; D. Williams, unpublished data).

Acknowledgements

The river discharge data were kindly made available by the Department of Minerals and Energy of Papua New Guinea, Dr R. Higgins of Ok Tedi Mining Ltd. and Mr J. A. H. Brown of the Snowy Mountains Engineering Corporation. Dr R. E. Thomson provided a profiler for one cruise. It is a pleasure to thank a number of our colleagues at the Australian Institute of Marine Science, particularly Mr D. van Senden, Mr R. McAllister, Mr M. Jones, MS P. Brodie, MS M. Thyssen, MS L. Howlett and MS P. Caterer. This research was supported by the Australian Institute of Marine Science. One of us (EW) wishes to acknowledge discussions with Prof. J. Imberger. This is contribution no. 141 from the Australian Institute of Marine Science.

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river plumes, coral reefs and mixing in the gulf of papua and the northern great barrier reef

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