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Leeuwin current observations on the Australian North West Shelf, May–June 1993
Deep-Sea
Research I, Vol.42, No. 3, pp. 285-305. 1995 Copyright 0 1995 ElsevierScienceLtd
Printed inGreatBritain. Allrights reserved 0%7-0637/95 $9.50+0.00
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Leeuwin Current observations on the Australian North West Shelf, May-June 1993 PETER
E .HOLLOWAY
*
(Received 11 November 1993; in revised form 13 July 1994;accepted 9 September 1994)
Abstract-Observations of the Leeuwin Current, the poleward flow of warm, low salinity water off the west coast of Australia, are described from acoustic Doppler current profiler (ADCP), temperature and salinity (CID) and moored current meter measurements from the Australian North West Shelf made in May and June 1993. Two cross-shelf and slope transects of ADCP and CTD measurements were completed at latitudes of approximately 17 and 19%. After the removal of the tidal signal, the ADCP measurements reveal the poleward flowing Leeuwin Current. The current is broad (250 km at 17’S), deep (at least 440 m) and relatively weak (typically 0.2 m s-‘), but the poleward transport of approximately 4 x 106 m3 s -’ is similar to values reported in the literature for the more intense part of the Leeuwin Current, between approximately 22 and 35’S, where a maximum of approximately 7 X 106 m3 s -’ has been observed. There is a significant flow reversal or undercurrent, of magnitude similar to the poleward transport, at the northern transect, but only a weak undercurrent is observed at the 19% transect. In the CID data the current is also seen as a low salinity core with salinity less than 35.2 but also with patches less than 35.0, possibly a result of a series of eddies. A current meter record from mid-way along the 19”s transect shows the poleward flow to be relatively persistent over all of May, but slightly stronger towards the end of May. There is only one significant reversal of flow towards the equator lasting a few days. Current meter data obtained between August 1983 and March 1985 from 120 m depth in the Timor Sea (approximately 12”S)are also reported and reveal a weak poleward flow for much of the year and strongest from January to April. For some of this time (March-April) the poleward flow is supported by the South East Trade winds. However, the poleward current weakens and turns equatorward in May/June to flow into the persistent winds from the east. Simultaneous current meter measurements from the shelf at 20’S show similar strength poleward flows from December to March.
1. INTRODUCTION
Leeuwin Current (LC) is the poleward flow of warm low salinity surface water found over the continental slope off the west coast of Australia and in the Great Australian Bight. It is now well recognised as an anomalous eastern boundary current as it flows poleward, instead of equator-ward like other eastern boundary currents, into the prevailing winds (CHURCH et al., 1989). The current is typically 50 km wide and 250 m deep and has its core around the shelf break (THOMPSON, 1984; SMITH et al., 1991). Although the first attempt of modelling the forcing of the LC was through specification of the large scale wind field over the Indian Ocean (THOMPSON and VERONIS, 1983), it is now generally considered that the current is driven by a steric height gradient that runs south from the North West THE
*Department of Geography and Oceanography, University College, University of New South Wales. Australian Defence Force Academy, Canberra ACT 2600, Australia. 285
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P. HOLLOWAY
Shelf (NWS), a result of temperature contrasts from north to south and possibly influenced by flow from the Pacific to Indian Oceans through the Indonesian Archipelago. The steric height gradient causes geostrophic flow of water eastwards from the Indian Ocean which is blocked by the Western Australian coast and so causes a poleward LC. Relatively deep surface-mixed layers (25 m in summer to 50 m deep in winter) mean that the momentum from the wind is distributed over a deep layer, and hence the predominantly equatorward blowing wind stress is too weak to overcome the steric height forcing (THOMPSON,1984, 1987; GODFREYand RIDGEWAY,1985; WEAVERand MIDDLETON,1989 and the discussion by BALEEN and RUTHERFORD,1990). There had been relatively few observational studies of the current until recent years. CRESSWELLand GOLDING(1980) presented some of the first direct observations of the current from hydrographic surveys and satellite-tracked drifting buoys, although they discuss earlier observations, and suggested the Leeuwin Current as the name for this poleward flow. Their observations showed the seasonal variability of the current with strongest flow from May to July and the existence of many cyclonic eddies on the seaward side of the current. Their study concentrated on the region south of about 27”s. Further observational studies included the analysis of satellite AVHRR images by LEGECKISand CRESSWELL (1981)) current meter and hydrographic observations by THOMPSON (1984) and current meter observations from the southern part of the NWS by HOLLOWAYand NYE (1985). By far the most comprehensive observational study was run during 1986 and 1987 and reported by SMITHet al. (1991). Their paper describes results from extensive current meter deployments and a number of repeated conductivity, temperature, depth (CTD) lines between 22 and 35%. At their Dongara (29”3O’S) section, Smith et al. describe a southward flow at the shelf edge penetrating to a depth of about 200 m, strongest in April and May, and characterized by warm temperatures and low salinity with a minimum of 35.4. Further south at 34% the flow is less intense but penetrates to a greater depth (230 m) than at 29”2O’S. Moving from north to south the salinity minimum weakens but the temperature contrast to the surrounding waters increases. SMITHet al. (1991) calculated the poleward transport of water from the geostrophic velocity sections and found maximum values of about 7 x lo6 m3 s-’. Strongest flow near the shelf break was seen during March to May. THOMPSON(1984) calculated a geostrophic poleward transport of 4 X lo6 m3 S-I off the West Australian coast. SMITHet al. (1991) produced maps of geopotential anomaly that show the LC is fed by water from the South East Indian Ocean and, particularly during March and to a lesser extent in August, from the NWS. While it is generally considered that the LC originates from the NWS, there have been very few observations reported from this region. HOLLOWAY and NYE (1985) used current meter observations from around 20% to reveal a seasonal cycle in the LC at the shelf break in 125 m water depth. The flow was strongest between March and June (based on observations over 19 months). CHURCHet al. (1989) discuss evidence for the LC on the NWS. They describe a broad, shallow (200 km wide by 50 m deep), poleward flow at around 20”s inferred from maps of steric height. GENTILLI (1972) used historical seasurface temperature observations to describe a broad “raft” of warm water spreading from the West Australia coast to 100”E and from Indonesia to 20%. He suggested this water had its origins from throughflow from the Pacific to Indian Ocean. During autumn and winter the warm water was seen to move southwards along the West Australian coast as far south as Cape Leeuwin (34%). RIDGWAYand LOCH (1987) computed average temperature/ salinity relationships for Australian waters, and these show low salinity water on the NWS,
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with higher salinity South East Indian Water to the west, suggesting a southward flow. Although several models of the LC are fed by geostrophic inflow from the west rather from the north (THOMPSON,1987), the existence of the LC as a warm and low salinity current indicates that at least some of the source water must come from the NWS region. This paper aims to provide further details on the dynamics of the northern extension of the LC and presents some new observations from the NWS of ADCP (acoustic Doppler current profiler) and CTD sections made across the continental shelf and slope at approximately 17 and 19”s during May and June 1993. In addition, data is presented from one current meter moored on the 19”s section during the period of ADCP and CTD measurements along with data from two moorings from the shelf run during 1983-84 at approximately 12 and 20%. 2. THE MEASUREMENT PROGRAM A measurement program was run during May and June 1993 on the NWS from the Royal Australian Navy’s ship H.M.A.S Moresby. Two cross-shelf sections of CTD and ADCP measurements were completed with station locations shown in Fig. 1. The northern transect is centred at 17’S and the southern transect at 19%. In addition a current meter mooring was maintained mid-way along the southern transect, in 100 m water depth, over the period 28 April to 2 June 1993. Although the mooring held two InterOcean “S4” current meters, only the meter at 30 m depth returned good data. The CTD/ADCP sections ran from close to the coast with an initial 30 nm (55 km) spacing which reduced to 5 nm (9.3 km) in deeper water. Two additional stations at around 30 nm separation were completed on the northern transect with deepest stations in 1600 m water depth. Measurements along the southern transect were made between 26 and 29 May 1993 and along the northern transect between 3 and 5 June 1993. The CTD used was a Yeo-Kal submersible data logger that operates only to 300 m. The instrument recorded a sample every second and was lowered at a nominal rate of 1 m s-l. The manufacturer’s specifications of accuracy of the temperature and salinity sensors are O.l”C and 0.05, respectively. The ADCP was an RDT manufactured instrument fitted with 150 kHz transducers that allow measurement over a depth range of nominally 350 m, although measurements up to 440 m were obtained. The instrument was operated only on station and was lowered over the side of the ship to a depth of about 2 m with the transducers looking downwards. The ADCP was run in a direct reading mode with data logged directly onto a computer. At each station data was collected for approximately 25 min. Bottom tracking, the strong acoustic reflection from the sea bed, is used to determine any movement or drift of the ship and hence compensate the data for such movement. However, this is achievable only in water depths up to approximately 550 m. In deeper water GPS navigation on the ship is used to compensate the ADCP data for ship’s movement. In addition the ADCP data is processed to remove tidal currents as discussed in section 5. 3. HYDROGRAPHIC DATA For each of the two transects, cross-shelf sections of salinity (Fig. 2) and in situ temperature (Fig. 3) are presented. Both sections show the existence of cores of low salinity water (less then 35.0) at a depth of about 100 m, although they are broken up into a
288
P. HOLLOWAY
16’
116OE
1 2o”
124’
Fig. 1. Map of the Australian North West Shelf showing the locations of CTD/ADCP stations (0) and current meter moorings (W).
series of patches. At the northern transect the low salinity water is about 100 km offshore of the shelf edge, whereas further south the core is close to the shelf edge. A broader band of water between 35.0 and 35.2 is seen to extend offshore from the shelf edge at both sections at depths of between about 80 and 200 m. A high salinity layer (35.4) is seen at the northern transect from the surface down to about 80 m depth. This is less evident at the southern transect, although the water increases in salinity from the shelf edge towards the coast with a maximum of 36.4. The high salinity water on the inner shelf is presumably a result of evaporation on the shelf in the absence of river input at the coast. If the low salinity water is interpreted as an indicator of the LC, as it is south of North West Cape (SMITH etal., 1991), then the salinity transects suggest the current exists at these sections and is further offshore at the more northern section, and possibly there is an equator-ward flow at the surface of the northern section corresponding to the high salinity surface water. The nature of the cores, being broken into a number of patches, possibly indicates eddying in the current flow. The temperature sections shown in Fig. 3 do not provide any indication of the LC in
Leeuwin
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Fig. 2.
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and southern
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terms of water mass characteristics. However, the steeply sloping isotherms at the shelf break on the southern transect do indicate a geostrophic poleward flow with maximum shear down to about 200 m depth. The shelf water, at both transects, is vertically homogeneous, and off the shelf there is a surface mixed-layer approximately 80 m deep, with the exception of a warm water (28°C) surface-intrusion 300 km offshore on the northern section. The temperature of the low salinity core can be seen from the temperature and salinity sections to be between 20 and 24”C, although the T/S relationship (not shown) is very noisy. The horizontal variations on short spatial scales seen in some of the isotherms are most likely a result of variability caused by internal wave activity, known to be large on the NWS (HOLLOWAY,1988). RIDCWAYand LOCH(1987) present maps of salinity on different temperature surfaces for the waters around Australia using historical data sets. Off the NWS between 15 and 20”s (the region of measurement in this study) the highest salinities seen are about 35.2 at 20”s on the 20°C temperature surface. Moving northwards, the salinity on this surface then rapidly drops to a value of approximately 34.8 at 15”s. Away from the continent the isohalines run approximately east-west, but closer to the shelf region they are tilted to the south, indicating a southward advection. On other surfaces the salinities are generally lower, and on all surfaces the salinity continues to decrease moving northwards with a minimum of approximately 34.4 on 22S”C at about 12”s. This picture then suggests that the water observed during the current study, with a salinity minimum of 35.0, consists of a mixture of water from both the west and north.
4. MOORED CURRENT METER DATA A 36 day record of currents, overlapping in time with the CTD/ADCP measurements, was obtained at 30 m depth from a mooring in 106 m water depth located along the southern transect at 19”12’S, 117”42’E (Fig. 1). The 15 min data is presented in Fig. 4 as low-pass filtered vector sticks where the filter was a half amplitude response at 48 h, t \
Fig. 4. Time series of low-pass filtered current vector-sticks from the current meter at 30 m depth in 106 m water depth on the southern transect. The filter had a half amplitude response at 48 h. Currents are rotated clockwise by 60” so that sticks perpendicular to the time axis represent flow parallel to the local bathymetry.
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removing tidal and inertial (35 h) period motion. The orientation of the plot (axis rotated 60” east of north) means sticks down the page are poleward and parallel to the local bathymetry. It is seen that for most of the time the flow was poleward with maximum speeds of about 0.15 m s-‘. The flow reversed only once during the 36 day measurement period, and the equatorward flow lasted for only about three days. The poleward flow appears to strengthen in the latter half of May, and it is during this time when the CID/ ADCP measurements were made at the southern transect (25-29 May). This indicates that the ADCP velocity measurements will be representative in strength of the LC at this location for most of May 1993. The average current over the 36 days is 0.048 m s-’ in the direction 252” east of north (poleward and parallel to the bathymetry). 5. PROCESSING
THE ADCP
DATA
Interest in the ADCP data is centred at defining the non-tidal velocity field. This involves removal of the tidal signal from the data and correction of data for ship’s movement when the water was too deep for bottom tracking (depths greater than approximately 550 m). All ADCP data files were averaged in time over the duration of data collection at each station. This was 25 (+2) min for each station. All data were collected and averaged over 8 m deep vertical bins. The uppermost bin was centred at 14 m depth, and the deepest bin at approximately 92% of the water depth (for transducers that are set at 20” off the centre line) with a maximum of around 440 m. The ship’s velocity was logged every 10 s from a GPS system and used to compute the average velocity of the ship over the same time period for which the ADCP data were averaged. Comparisons between bottom tracking and GPS navigation from 35 stations gave an average difference in speed of 0.021 m s-l and in direction of 3.8” where a typical value of the speed of the ship was 0.6 m s-l. Wherever possible the bottom tracking was used to correct for the ship’s movement, and only four stations required correction from the GPS navigation. Correction of the ADCP velocities for tidal currents is achieved by removing predicted tidal currents from the measurements. Results from a two-dimensional, non-linear, barotropic tidal model of the NWS region were supplied by the National Tidal Facility (B. Mitchell, personal communication). The model was run over the domain 13-21”S, 115125”E, on a resolution of 0.25” by 0.25” latitude and longitude and run simultaneously for M2, S2, K1 and 0, constituents. The model was calibrated against eight locations with tidal elevation information but not against tidal current information. The ratio of the rms difference in amplitude between the observed and modelled elevations to the mean observed amplitude and the rms phase differences between the observations and model are (0.0460.239 m, 4”), (0.040/0.808m, 5‘7, (0.030/0.230 m, 4”) and (0.013/0.149 m, 3”) for M2, S,, Kr and 0, constituents, respectively. The tidal ellipse properties from the current meter moored along the southern transect are listed in Table 1 for Mz, Sz, Kr and 0, constituents where inference has been used in the analysis to resolve Pi from Ki and Kz from S2 constituents giving a total of 38 constituents resolved in the analysis. These results can be compared to the model constituents from the nearest model grid-location given in Table 2. The agreement is very good for the largest constituent Mz with semi-major axis lengths differing by only 0.011 m s-r in 0.33 m s-r and phase by only 1”. The agreement between modelled and observed S2 currents is not as good with a difference in semi-major axis lengths of 0.046 m s-’ and in phase of 11”. For K, there is again good agreement in the
293
Leeuwin Current observations Table 1. Tidal current constituents derived from the current meter moored on the southern transect in 106 m water depth at location 19”12’S, 117’42’E. Semi-major and semi-minor axis lengths are a and b, respectively, g is phase lag relative to GMT, and 8 is the ellipse orientation measured in degrees anti-clockwise from east
Constituent M2 s2
Kl 0,
a (m s-l)
b (m s-l)
g
e
0.323 0.176 0.031 0.006
0.092 0.050 0.014 0.001
172 241 299 286
131 131 142 163
Table 2. Tidal current constituents from the tidal model for the location nearest to the current meter location on the southern transect. Semi-major and semi-minor axis lengths are a and b respectively, g is phase lag relative to GMT, and @is the ellipse orientation measured in degrees anti-clockwise from east
Constituent M2 s2
Kl 0,
a (m SK’)
b (m SK’)
g
e
0.338 0.222 0.032 0.016
0.096 0.059 0.021 0.007
171 230 250 269
130 130 81 146
semi-major axis lengths but phase difference by 78”. However, the K, (and 0,) currents are very weak (0.032 m s-i) so that the phase discrepancy is not important. The constituent values were also compared with results of an analytical model of cross-shelf variations in tides (BA~WXI and CLARKE,1982). There was close agrement between the numerical and analytical model results. Constituents from the nearest model grid-point to each measurement location were used to predict the tidal current at the mid-point time of the 25 min ADCP current average. Predictions of currents were performed with the tidal package of FOREMAN (1978) using the four constituents listed above plus K2 and N2 inferred from S2 and MZ, respectively from coastal ratios from Dampier (EASTON,1970). On the shelf the predicted tidal currents are strong and generally account for most of the measured ADCP current signal. However, in deeper water, where it is found that the alongshelf flows are strongest, the corrections from tidal currents are small. Table 3 presents a listing of the predicted tidal current for each station location at the time of ADCP measurements along with the resulting non-tidal near-surface current (bin 2 centred at 22.6 m). It must be recognised that the ADCP data could be affected, to some extent, by oscillatory currents associated with processes such as internal tides and shelf waves. However, it is expected that such currents would be weak, particularly in the deeper water offshore of the shelf break.
P. HOLLOWAY
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Table 3. Predicted tidal current speeds and directions for each ADCP station at the time of the ADCP current measurements along with the resulting de-tided ADCP current at a depth of 22.6 m (bin number 2). Dates are for the year 1993 and all times (GMT) are the time of tidal current prediction which are at the mid-point of the 25 min ADCP current average Tidal prediction Station number Transect A 1 2 3 4 5 6 7 8 9 10 11 12 13 14 16 18 Transect B 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
Date
26 May
27 May 28 May
29 May
3 June
4 June
5 June
6. ADCP
Non-tidal current
Time (GMT)
Speed (m s-‘)
Dir.
Speed (m s-‘)
Dir.
0047 0341 0710 0822 0928 2336 0047 0156 0315 0441 0828 0940 2342 0056 0202 0319
0.39 0.44 0.35 0.48 0.33 0.16 0.06 0.16 0.25 0.14 0.11 0.15 0.10 0.03 0.02 0.04
150 119 340 325 316 301 230 157 141 121 344 326 307 281 142 120
0.35 0.16 0.09 0.15 0.13 0.16 0.28 0.26 0.32 0.24 0.19 0.38 0.16 0.17 0.06 0.12
320 216 232 204 258 257 280 275 290 240 229 317 250 234 267 153
0021 0250 0436 0530
0.20 0.35 0.59 0.53 0.27 0.13 0.32 0.43 0.17 0.13 0.28 0.42 0.36 0.28 0.13 0.12 0.23 0.16 0.08 0.14 0.09
109 325 308 300 287 213 151 121 100 14 336 322 315 305 275 183 132 116 75 327 311
0.01 0.09 0.07 0.05 0.17 0.15 0.13 0.24 0.08 0.03 0.01 0.14 0.21 0.27 0.12 0.19 0.15 0.19 0.06 0.42 0.30
6 12 101 197 217 186 194 293 290 295 37 98 26 48 39 335 42 312 229 140 141
0638 0744 0922 0010 0116 0220 0324 0429 0538 0639 0744 0851 0004 0112 0220 0442 0604
NON-TIDAL
CURRENTS
The non-tidal component of the ADCP measured currents, calculated as described in section 5, are displayed in several forms. Figure 5 shows vector sticks of currents along the
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Leeuwin Current observations
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Fig. 5. Vector sticks of current along the northern transect for different depth levels. Currents are averaged over 20 m depth (10 m each side of the indicated depth). Distance is that from the coast. North is perpendicular to the time axis.
&
0
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Southern Transect -10
- so
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;;; g E 5
- 200
_.I,+
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\i
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\
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^\ -..__
- 400 40
O-
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cm/s
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,
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Distance (Mometres) Fig. 6. Vector sticks of current along the southern transect for different depth levels. Currents are averaged over 20 m depth (10 m each side of the indicated depth). Distance is that from the coast. North is perpendicular to the time axis.
8
Leeuwin
Station
Current
observations
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locations
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200 z Jz z -300
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Station
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SOUTHERN SECTION VELOCITY (cm/s) (positive is equatorward)
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Fig. 7. Cross-shelf section of de-tided longshelf velocity component along the northern and southern transects. Alongshelf is defined as a clockwise axis rotation of 70” east of north for the southern transect and of 30”for the northern transect. Positive flow is equatorward.
5 n
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northern transect at different depths, and Fig. 6 shows the vector sticks from the southern transect. Values are displayed for every 50 m depth and are calculated as averages over a 20 m depth range, where the indicated depth is the mid-point value. Along the southern transect the strongest flow is seen over the slope region. The flow is fairly uniform over the top 100 m, consistent with the observed deep surface mixed-layer. Although there is some variability in the flow direction along the transect, there is a predominance of poleward flow in the upper layers. Between 150 and 250 m there is again variability in direction along the transect, but the flow is now predominantly equatorward. The deeper flow is significantly weaker and flowing poleward. At the northern transect the flow is more variable in direction and strength across the transect with surface flow poleward around the shelf break and equatorward further offshore. Towards the end of the transect, 360 km from the coast, the flow is strongest (0.4 m s-l) and directed onshore. The currents are relatively uniform with depth over the upper 100 m. The patterns suggest eddying motion rather than a spatially uniform flow. Below 250 m depth the flow is weaker in strength but more uniform in direction and flowing poleward. Contoured cross-shelf sections of the longshelf velocity components are plotted in Fig. 7 for both transects. The orientation defining alongshelf is chosen for each section to reflect the local bathymetry, and a clockwise rotation of 30”for the northern transect and 70” for the southern transect are used. The southern transect shows predominantly poleward flow strongest over the shelf, at 0.34 m s-i, where the water is approximately 150 m deep. In deeper water the flow is still poleward but weaker, with the exception of a ring of equatorward flow between 100 and 300 m deep. At the northern transect the flow is spatially more variable. As seen in the southern transect there is a poleward flow over the shelf break in water 125 m deep. Over the slope from the surface down to approximately 250 m depth the flow is equatorward with a maximum of 0.33 m s-’ and with maxima occurring in several distinct cores. Below 250 m the flow is weaker but poleward across the entire section. In addition, at the outer part of the section the flow is also poleward in the upper 200 m. As a means of quantifying the strength of the poleward flowing LC, the velocity profiles at each station along each of the two transects are depth integrated to define the net poleward and net equatorward transports (m2 s-l) at each station. Velocity profiles are integrated from the surface to the maximum depth (92% of the water depth or 440 m) and across all stations, as shown in Fig. 7. Values are linearly interpolated between stations. The resulting cross-shelf sections of transport are plotted in Fig. 8 for each transect where poleward transport (the LC) is defined as positive and equatorward transport as negative. The integral of these curves then gives the volume transport (m3 s-i) across each section. For the southern transect a well defined poleward current is seen with a peak transport of 50 m2 s-l occurring at a distance of 200 km from the coast, and the width of the current can
Fig. 8. Cross-shelf section of poleward (positive) and equatorward (negative) transports for the northern and southern transects. Values are computed by depth-integrating the de-tided ADCP current profiles at each station and calculating net positive and net negative transports. Also poleward and equatorward transports are estimated for Dongara (29”3O’S) computed from current meter data presented by SMITH et al. (1991). The Dongara data are an average over the period 1 March-12 May 1987 and are from seven different meters in four moorings.
299
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100
NORTHERN
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TRANSECT
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200
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Fig. 8.
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P. HOLLOWAY
be defined from the transport distribution as approximately 120 km. The poleward volume transport across the section is 4.2 x lo6 m3 s-i , and the equatorward transport is 0.4 x lo6 m3 s-1 , given a net poleward transport of 3.8 x lo6 m3 s-l. For the northern transect the transport shows three distinct cores in the poleward direction, with the largest transport in the furthest offshore core approximately 350 km from the coast. The width of the poleward current is 225 km but could well continue beyond the end of the measurements. The opposing equatorward flow is in a well defined current core with a width of 100 km. There is little transport either equatorwards or polewards on the shelf at either section. There is a poleward volume transport through the northern section of 3.7 x lo6 m3 s-’ and an equatorward transport of 3.3 x 106m3s-’ , giving a net poleward transport of 0.4 x lo6 m3 s-l. The poleward volume transport across each transect is then approximately equal, but slightly larger at the southern transect. The transport at the outer edge of measurements from the northern transect is well above zero, indicating that measurements were not made across the full width of the current, and hence the southward transport is probably larger than the above figures indicate, 7. SATELLITE
DERIVED
SEA
SURFACE
TEMPERATURES
Figure 9 shows an AVHRR image of the NWS region obtained from the NOAA11 satellite at 08222 on 26 May 1993, coincident in time with the beginning of the southern transect measurements and 10 days prior to the completion of the northern transect measurements. Typical of low latitude regions, the surface waters only show weak variations (about 2°C) in temperature. However, the image does provide some useful information. The warm water off the NWS can be seen to exhibit substantial spatial variability by about + 1°C. The warmest water (28°C) is seen north of 18”s and offshore of 120”E. This water is intruding eastward producing a surface temperature field with patchiness and evidence of eddying motions. This is consistent with the near surface ADCP velocities shown in Figs 5 and 6, particularly for the northern transect, where flow reversals are seen along the length of the transect. The more uniform poleward surface flow seen at the southern transect is consistent with there being less variation in temperature than at the northern transect. A relatively cold strip of water is seen along the coast and inner shelf, approximately 2°C colder than the offshore water. This is presumably a result of winter cooling of the shallow shelf waters. 8. EARLIER
CURRENT
MEASUREMENTS
Moored current meter data obtained during the period August 1983 to March 1985 is considered from two locations on the NWS. Current meters from a single mooring at Jabiru location (11”48’S, 125”9’E) in a depth of 120 m at the northern end of the NWS provide a nearly continuous record from October 1983 to March 1985. Neil Brown Acoustic Current meters were used at depths of 15,48,82 and 116 m and sampled 5 min vector averages. Not all current meters successfully operated at all times, and a depth averaged current time series is calculated as the simple mean of all current records available at each time. In addition, an offshore buoy at the same location recorded wind velocity data at a height of 4.3 m above sea level and at a sampling interval of either 30 or 60 min. Some current meter data from North Rankin (19”37’S, 116”6’E) at the southern end of the NWS was also recorded during this period. In 123 m water depth, currents were
Leeuwin Current observations
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Leeuwin Current observations
t \
Jabiru Winds
10
c E
0 -10 30_
Jabiru Cum’ItS
1 t E,
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Fig. 10. Time series of low-pass filtered, depth-averaged currents and winds from Jabiru location (11“48’S, 125YE) in a water 120 m deep and from North Rankin location (19’37’S, 116”6’E) in a water 123 m deep. The filter has a half amplitude response at 96 h. Currents are rotated clockwise by 50”at Jabiru and by 25”at North Rankin so that sticks perpendicular to the time axis represent flow parallel to the local bathymetry.
measured at 23,53,82 and 120 m depths from August 1983 to March 1984 and at 35,65 and 95 m depths from December 1984 to March 1985. All measurements were 5 min vector averages. Again a depth average current is calculated. Mooring locations are shown in Fig. 1. Figure 10 shows vector-stick time series of the depth averaged, low-pass filtered currents and winds. The filter was designed to remove super-inertial motion and has a halfamplitude response at 96 h where the inertial period is 57 h at the Jabiru location and 35 h at the North Rankin location. Prior to filtering, short gaps were replaced by zero values to provide a simple interpolation. The maximum gaps are shorter than the period at which the filter has its half amplitude response so that any bias produced by the adding of zeros will be minimal. The orientation of the plots is so that sticks perpendicular to the time axis are parallel to the local bathymetry. Alongshelf flow is defined by a rotation of 50” and 25” east of north at Jabiru and North Rankin, respectively. At Jabiru the flow is seen to be predominantly poleward for most of the measurement period. Maximum poleward currents are about 0.2 m s-l and occur in March and April 1984. The mean depth-averaged current over the 18 months of measurement is 0.033 m s-l in the direction 208” east of north, which is a poleward flow. The LC appears to be present at this northern location, particularly from January to April inclusive. During the winter months the flow is weaker and more variable although still often in a poleward direction. The winds at Jabiru are characterized by a distinct seasonal cycle with the North West Monsoons blowing from the west from December to March followed by the South East Trades blowing from the east from April through to October but strongest from April to July. During March and April
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the winds support the poleward currents at Jabiru but from May to July the poleward flow weakens, despite the strong winds, and at times flows equatorward into the prevailing South East Trades blowing from the east. At the North Rankin location the LC is seen, most predominantly, from December to March. The poleward flow is of similar strength to that observed at Jabiru. More extensive observations of the LC from the North Rankin region are discussed by HOLLOWAY and NYE (1985). 9. DISCUSSION In order to compare the strength and width of the LC off the NWS to that observed further south, data from SMITHet al. (1991) are used to construct a cross-shelf section of transport similar to those for the two transects from the NWS. Data from their Dongara array at latitude 29”3O’Sis used. Average alongshelf currents from 1 March to 12 May 1987 from seven current meters in four moorings are contoured and presented in Smith’s Fig. 5. Here data from their Fig. 5 was used to approximate the poleward and equatorward transports with the results presented in Fig. 8. It can be seen that the peak poleward transport is approximately 100 m2 s-l, about double that found at transect A, and that the width of the current is only 50 km, much narrower than the broad flow found on the NWS. The net poleward volume transport is found by integrating under the curve as 2.1 x lo6 m3 S-‘, although it is clear that their observations do not completely cross the current. The return equatorward transport is much weaker than this. SMITHet al. (1991) computed volume transports for a number of different times and sections between 25 and 35”s from geostrophic calculations. The values vary in time and location ranging from approximately 2 to 7 X lo6 m3 s-l. The alongshelf transports are, however, relatively uniform between 25 and 34%. Smith et al. suggest there is some seasonal variation in the transport with significant onshore transport and feeding of the LC from the subtropical offshore water during the late winter (August and September). Then in early autumn (March) the current is also fed by water from the NWS. The calculated transports on the NWS, reported in this paper, are then of similar strength to those observed further south although not as strong as the peak values reported by SMITHet al. (1991). This suggests that the LC is continuous well up onto the NWS and that a significant fraction of the volume of the LC originates from the NWS, at least as far as latitude 17”s during the reported time period of May/June. The observations presented in this paper have provided a snapshot of the structure of the LC on the NWS. Although the ADCPKTD observations were taken over a period of 10 days, each section has been interpreted as providing a synoptic picture of the current flow. It has previously been suggested that at least some of the LC water originates on the NWS and limited observations from the southern end of the NWS have supported this. The observations reported in this paper have revealed a broad, deep and slow moving LC over the continental slope region of the NWS at latitudes of approximately 17 and 19”s. Additionally, the measured poleward volume transport of about 4 X lo6 m3 s-l is similar to values previously reported from further south where the LC is narrow and swiftly flowing. At 17”s the LC is approximately 250 km wide (and possibly wider), is seen from the surface to a depth of at least 440 m, and appears to exist as an eddying or meandering flow rather than a single current core as seen at 19”s and further south of North West Cape. The moored current meter data also shows that the LC exists as far north as 12”S, although it was not possible to estimate the volume transport at this location.
Leeuwin Current observations
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It is clear that additional observations are warranted from the NWS to further trace the origins of the LC. Acknowledgements-A
number of individuals and organizations have made invaluable contributions to this project. All field work from H.M.A.S. Moresby was made possible through generous cooperation and collaboration with Cmdr David Knight and Leut. Mike Evans from the Royal Australian Navy Hydrographic Office and from the assistance of the Officers and Seaman on H.M.A.S. Moresby. Mike Evans also provided the CTD data. Expert assistance was provided by Tony Veness who prepared and maintained all equipment and by Ray Lawton who prepared the current meter moorings. Bill Mitchell from the National Tidal Facility at Flinders University provided the tidal model results for the North West Shelf. Mark Orzechowski wrote many new algorithms and expertly carried out most of the data processing. Ian Shepherd and Tony Webb most generously carried out all processing of the satellite image. Current meter data from Jabiru location were provided by BHP Petroleum and from North Rankin by Woodside Offshore Petroleum, and both data sets were collected by Steedman Science and Engineering.
REFERENCES BAITISTID.
A. and A. J. CLARKE(1982) A simple method for estimating barotropic tidal currents on continental margins with specific application to the Mz tide off the Atlantic and Pacific Coasts of the United States.
Journal of Physical Oceanography,
l&8-16.
BATTEENM.
L. and M. J. RUTHERFORD (1990) Modeling studies of eddies in the Leeuwin Current: the role of thermal forcing. Journal of Physical Oceanography, 20, 1484-1520. CHURCHJ. A., G. R. CRESSWELLand J. S. GODFREY(1989) The Leeuwin Current. In: Polewardflows along eastern ocean boundaries, S. J. NESHYBA,CH. N. K. MOOERS,R. L. SMITHand R. T. BARBER,editors, Coastal and Estuarine Studies, 34, Springer-Verlag, New York, pp. 230-254. CRESSWELL G. R. and T. J. GOLDING(1980) Observations of a south-flowing current in the southeastern Indian Ocean. Deep-Sea Research, 27, 449-466. EASTONA. K. (1970) Tides around Australia. Horace Lamb Centre for Oceanographical Research, Flinders University of South Australia, Research Report 37.326 pp. FOREMAN M. G. G. (1978) Manual for tidal current analysis and prediction. Institute of Ocean Sciences, Patricia Bay. Sydney, B.C. Canada, Pacific Marine Science Report 78-6,70 pp. GENTILLIJ. (1972) Thermal anomalies in the Eastern Indian Ocean. Nature, Physical Science, 238,93-95. GODFREYJ. S. and K. R. RIDGWAY(1985) The large-scale environment of the poleward-flowing Leeuwin Current, Western Australia: longshore steric height gradients, wind stresses and geostrophic flow. Journal of Physical Oceanography,
15,481-495.
HOLLOWAY P. E. and H. C. NYE (1985) Leeuwin Current and wind distributions on the southern part of the Australian North West Shelf between January 1982 and July 1983. Australian Journal of Marine and Freshwater Research, 36, 123-137.
HOLLOWAY P. E. (1988) Climatology of internal tides at a shelf-break location on the Australian North West Shelf. Australian Journal of Marine and Freshwater Research, 39, l-18. LEGECKISR. and G. CRESSWELL (1981) Satellite observations of sea surface temperature fronts off the coast of western and southern Australia. Deep-Sea Research, Z&297-306. RIDGWAYK. R. and R. G. LOCH(1987) Mean temperature-salinity relationships in Australian waters and their use in water mass analysis. Australian Journal of Marine and Freshwater Research, 38,553-567. SMITH R. L., A. H. HUYER,J. S. GODFREY and J. A. CHURCH(1991) The Leeuwin Current off Western Australia, 1986-1987. Journal of Physical Oceanography, 21,322-345. THOMPSON R. 0. R. Y. and G. VERONIS(1983) Poleward boundary current off Western Australia. Australian Journal of Marine and Freshwater Research, 34, 173-185.
THOMPSON R. 0. R. Y. (1984) Observations of the Leeuwin Current off Western Australia. Journal of Physical Oceanography,
14,623-628.
THOMPSONR. 0. R. Y. (1987) Continental-shelf-scale
model of the Leeuwin Current. Journal of Marine
Research, 45,813-827.
WEAVERA. J. and J. H. MIDDLETON(1989) On the dynamics of the Leeuwin Current. Journal of Physical Oceanography,
19.626648.
Research I, Vol.42, No. 3, pp. 285-305. 1995 Copyright 0 1995 ElsevierScienceLtd
Printed inGreatBritain. Allrights reserved 0%7-0637/95 $9.50+0.00
0967-0637(95)oooo4-6
Leeuwin Current observations on the Australian North West Shelf, May-June 1993 PETER
E .HOLLOWAY
*
(Received 11 November 1993; in revised form 13 July 1994;accepted 9 September 1994)
Abstract-Observations of the Leeuwin Current, the poleward flow of warm, low salinity water off the west coast of Australia, are described from acoustic Doppler current profiler (ADCP), temperature and salinity (CID) and moored current meter measurements from the Australian North West Shelf made in May and June 1993. Two cross-shelf and slope transects of ADCP and CTD measurements were completed at latitudes of approximately 17 and 19%. After the removal of the tidal signal, the ADCP measurements reveal the poleward flowing Leeuwin Current. The current is broad (250 km at 17’S), deep (at least 440 m) and relatively weak (typically 0.2 m s-‘), but the poleward transport of approximately 4 x 106 m3 s -’ is similar to values reported in the literature for the more intense part of the Leeuwin Current, between approximately 22 and 35’S, where a maximum of approximately 7 X 106 m3 s -’ has been observed. There is a significant flow reversal or undercurrent, of magnitude similar to the poleward transport, at the northern transect, but only a weak undercurrent is observed at the 19% transect. In the CID data the current is also seen as a low salinity core with salinity less than 35.2 but also with patches less than 35.0, possibly a result of a series of eddies. A current meter record from mid-way along the 19”s transect shows the poleward flow to be relatively persistent over all of May, but slightly stronger towards the end of May. There is only one significant reversal of flow towards the equator lasting a few days. Current meter data obtained between August 1983 and March 1985 from 120 m depth in the Timor Sea (approximately 12”S)are also reported and reveal a weak poleward flow for much of the year and strongest from January to April. For some of this time (March-April) the poleward flow is supported by the South East Trade winds. However, the poleward current weakens and turns equatorward in May/June to flow into the persistent winds from the east. Simultaneous current meter measurements from the shelf at 20’S show similar strength poleward flows from December to March.
1. INTRODUCTION
Leeuwin Current (LC) is the poleward flow of warm low salinity surface water found over the continental slope off the west coast of Australia and in the Great Australian Bight. It is now well recognised as an anomalous eastern boundary current as it flows poleward, instead of equator-ward like other eastern boundary currents, into the prevailing winds (CHURCH et al., 1989). The current is typically 50 km wide and 250 m deep and has its core around the shelf break (THOMPSON, 1984; SMITH et al., 1991). Although the first attempt of modelling the forcing of the LC was through specification of the large scale wind field over the Indian Ocean (THOMPSON and VERONIS, 1983), it is now generally considered that the current is driven by a steric height gradient that runs south from the North West THE
*Department of Geography and Oceanography, University College, University of New South Wales. Australian Defence Force Academy, Canberra ACT 2600, Australia. 285
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Shelf (NWS), a result of temperature contrasts from north to south and possibly influenced by flow from the Pacific to Indian Oceans through the Indonesian Archipelago. The steric height gradient causes geostrophic flow of water eastwards from the Indian Ocean which is blocked by the Western Australian coast and so causes a poleward LC. Relatively deep surface-mixed layers (25 m in summer to 50 m deep in winter) mean that the momentum from the wind is distributed over a deep layer, and hence the predominantly equatorward blowing wind stress is too weak to overcome the steric height forcing (THOMPSON,1984, 1987; GODFREYand RIDGEWAY,1985; WEAVERand MIDDLETON,1989 and the discussion by BALEEN and RUTHERFORD,1990). There had been relatively few observational studies of the current until recent years. CRESSWELLand GOLDING(1980) presented some of the first direct observations of the current from hydrographic surveys and satellite-tracked drifting buoys, although they discuss earlier observations, and suggested the Leeuwin Current as the name for this poleward flow. Their observations showed the seasonal variability of the current with strongest flow from May to July and the existence of many cyclonic eddies on the seaward side of the current. Their study concentrated on the region south of about 27”s. Further observational studies included the analysis of satellite AVHRR images by LEGECKISand CRESSWELL (1981)) current meter and hydrographic observations by THOMPSON (1984) and current meter observations from the southern part of the NWS by HOLLOWAYand NYE (1985). By far the most comprehensive observational study was run during 1986 and 1987 and reported by SMITHet al. (1991). Their paper describes results from extensive current meter deployments and a number of repeated conductivity, temperature, depth (CTD) lines between 22 and 35%. At their Dongara (29”3O’S) section, Smith et al. describe a southward flow at the shelf edge penetrating to a depth of about 200 m, strongest in April and May, and characterized by warm temperatures and low salinity with a minimum of 35.4. Further south at 34% the flow is less intense but penetrates to a greater depth (230 m) than at 29”2O’S. Moving from north to south the salinity minimum weakens but the temperature contrast to the surrounding waters increases. SMITHet al. (1991) calculated the poleward transport of water from the geostrophic velocity sections and found maximum values of about 7 x lo6 m3 s-’. Strongest flow near the shelf break was seen during March to May. THOMPSON(1984) calculated a geostrophic poleward transport of 4 X lo6 m3 S-I off the West Australian coast. SMITHet al. (1991) produced maps of geopotential anomaly that show the LC is fed by water from the South East Indian Ocean and, particularly during March and to a lesser extent in August, from the NWS. While it is generally considered that the LC originates from the NWS, there have been very few observations reported from this region. HOLLOWAY and NYE (1985) used current meter observations from around 20% to reveal a seasonal cycle in the LC at the shelf break in 125 m water depth. The flow was strongest between March and June (based on observations over 19 months). CHURCHet al. (1989) discuss evidence for the LC on the NWS. They describe a broad, shallow (200 km wide by 50 m deep), poleward flow at around 20”s inferred from maps of steric height. GENTILLI (1972) used historical seasurface temperature observations to describe a broad “raft” of warm water spreading from the West Australia coast to 100”E and from Indonesia to 20%. He suggested this water had its origins from throughflow from the Pacific to Indian Ocean. During autumn and winter the warm water was seen to move southwards along the West Australian coast as far south as Cape Leeuwin (34%). RIDGWAYand LOCH (1987) computed average temperature/ salinity relationships for Australian waters, and these show low salinity water on the NWS,
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with higher salinity South East Indian Water to the west, suggesting a southward flow. Although several models of the LC are fed by geostrophic inflow from the west rather from the north (THOMPSON,1987), the existence of the LC as a warm and low salinity current indicates that at least some of the source water must come from the NWS region. This paper aims to provide further details on the dynamics of the northern extension of the LC and presents some new observations from the NWS of ADCP (acoustic Doppler current profiler) and CTD sections made across the continental shelf and slope at approximately 17 and 19”s during May and June 1993. In addition, data is presented from one current meter moored on the 19”s section during the period of ADCP and CTD measurements along with data from two moorings from the shelf run during 1983-84 at approximately 12 and 20%. 2. THE MEASUREMENT PROGRAM A measurement program was run during May and June 1993 on the NWS from the Royal Australian Navy’s ship H.M.A.S Moresby. Two cross-shelf sections of CTD and ADCP measurements were completed with station locations shown in Fig. 1. The northern transect is centred at 17’S and the southern transect at 19%. In addition a current meter mooring was maintained mid-way along the southern transect, in 100 m water depth, over the period 28 April to 2 June 1993. Although the mooring held two InterOcean “S4” current meters, only the meter at 30 m depth returned good data. The CTD/ADCP sections ran from close to the coast with an initial 30 nm (55 km) spacing which reduced to 5 nm (9.3 km) in deeper water. Two additional stations at around 30 nm separation were completed on the northern transect with deepest stations in 1600 m water depth. Measurements along the southern transect were made between 26 and 29 May 1993 and along the northern transect between 3 and 5 June 1993. The CTD used was a Yeo-Kal submersible data logger that operates only to 300 m. The instrument recorded a sample every second and was lowered at a nominal rate of 1 m s-l. The manufacturer’s specifications of accuracy of the temperature and salinity sensors are O.l”C and 0.05, respectively. The ADCP was an RDT manufactured instrument fitted with 150 kHz transducers that allow measurement over a depth range of nominally 350 m, although measurements up to 440 m were obtained. The instrument was operated only on station and was lowered over the side of the ship to a depth of about 2 m with the transducers looking downwards. The ADCP was run in a direct reading mode with data logged directly onto a computer. At each station data was collected for approximately 25 min. Bottom tracking, the strong acoustic reflection from the sea bed, is used to determine any movement or drift of the ship and hence compensate the data for such movement. However, this is achievable only in water depths up to approximately 550 m. In deeper water GPS navigation on the ship is used to compensate the ADCP data for ship’s movement. In addition the ADCP data is processed to remove tidal currents as discussed in section 5. 3. HYDROGRAPHIC DATA For each of the two transects, cross-shelf sections of salinity (Fig. 2) and in situ temperature (Fig. 3) are presented. Both sections show the existence of cores of low salinity water (less then 35.0) at a depth of about 100 m, although they are broken up into a
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16’
116OE
1 2o”
124’
Fig. 1. Map of the Australian North West Shelf showing the locations of CTD/ADCP stations (0) and current meter moorings (W).
series of patches. At the northern transect the low salinity water is about 100 km offshore of the shelf edge, whereas further south the core is close to the shelf edge. A broader band of water between 35.0 and 35.2 is seen to extend offshore from the shelf edge at both sections at depths of between about 80 and 200 m. A high salinity layer (35.4) is seen at the northern transect from the surface down to about 80 m depth. This is less evident at the southern transect, although the water increases in salinity from the shelf edge towards the coast with a maximum of 36.4. The high salinity water on the inner shelf is presumably a result of evaporation on the shelf in the absence of river input at the coast. If the low salinity water is interpreted as an indicator of the LC, as it is south of North West Cape (SMITH etal., 1991), then the salinity transects suggest the current exists at these sections and is further offshore at the more northern section, and possibly there is an equator-ward flow at the surface of the northern section corresponding to the high salinity surface water. The nature of the cores, being broken into a number of patches, possibly indicates eddying in the current flow. The temperature sections shown in Fig. 3 do not provide any indication of the LC in
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Fig. 2.
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terms of water mass characteristics. However, the steeply sloping isotherms at the shelf break on the southern transect do indicate a geostrophic poleward flow with maximum shear down to about 200 m depth. The shelf water, at both transects, is vertically homogeneous, and off the shelf there is a surface mixed-layer approximately 80 m deep, with the exception of a warm water (28°C) surface-intrusion 300 km offshore on the northern section. The temperature of the low salinity core can be seen from the temperature and salinity sections to be between 20 and 24”C, although the T/S relationship (not shown) is very noisy. The horizontal variations on short spatial scales seen in some of the isotherms are most likely a result of variability caused by internal wave activity, known to be large on the NWS (HOLLOWAY,1988). RIDCWAYand LOCH(1987) present maps of salinity on different temperature surfaces for the waters around Australia using historical data sets. Off the NWS between 15 and 20”s (the region of measurement in this study) the highest salinities seen are about 35.2 at 20”s on the 20°C temperature surface. Moving northwards, the salinity on this surface then rapidly drops to a value of approximately 34.8 at 15”s. Away from the continent the isohalines run approximately east-west, but closer to the shelf region they are tilted to the south, indicating a southward advection. On other surfaces the salinities are generally lower, and on all surfaces the salinity continues to decrease moving northwards with a minimum of approximately 34.4 on 22S”C at about 12”s. This picture then suggests that the water observed during the current study, with a salinity minimum of 35.0, consists of a mixture of water from both the west and north.
4. MOORED CURRENT METER DATA A 36 day record of currents, overlapping in time with the CTD/ADCP measurements, was obtained at 30 m depth from a mooring in 106 m water depth located along the southern transect at 19”12’S, 117”42’E (Fig. 1). The 15 min data is presented in Fig. 4 as low-pass filtered vector sticks where the filter was a half amplitude response at 48 h, t \
Fig. 4. Time series of low-pass filtered current vector-sticks from the current meter at 30 m depth in 106 m water depth on the southern transect. The filter had a half amplitude response at 48 h. Currents are rotated clockwise by 60” so that sticks perpendicular to the time axis represent flow parallel to the local bathymetry.
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removing tidal and inertial (35 h) period motion. The orientation of the plot (axis rotated 60” east of north) means sticks down the page are poleward and parallel to the local bathymetry. It is seen that for most of the time the flow was poleward with maximum speeds of about 0.15 m s-‘. The flow reversed only once during the 36 day measurement period, and the equatorward flow lasted for only about three days. The poleward flow appears to strengthen in the latter half of May, and it is during this time when the CID/ ADCP measurements were made at the southern transect (25-29 May). This indicates that the ADCP velocity measurements will be representative in strength of the LC at this location for most of May 1993. The average current over the 36 days is 0.048 m s-’ in the direction 252” east of north (poleward and parallel to the bathymetry). 5. PROCESSING
THE ADCP
DATA
Interest in the ADCP data is centred at defining the non-tidal velocity field. This involves removal of the tidal signal from the data and correction of data for ship’s movement when the water was too deep for bottom tracking (depths greater than approximately 550 m). All ADCP data files were averaged in time over the duration of data collection at each station. This was 25 (+2) min for each station. All data were collected and averaged over 8 m deep vertical bins. The uppermost bin was centred at 14 m depth, and the deepest bin at approximately 92% of the water depth (for transducers that are set at 20” off the centre line) with a maximum of around 440 m. The ship’s velocity was logged every 10 s from a GPS system and used to compute the average velocity of the ship over the same time period for which the ADCP data were averaged. Comparisons between bottom tracking and GPS navigation from 35 stations gave an average difference in speed of 0.021 m s-l and in direction of 3.8” where a typical value of the speed of the ship was 0.6 m s-l. Wherever possible the bottom tracking was used to correct for the ship’s movement, and only four stations required correction from the GPS navigation. Correction of the ADCP velocities for tidal currents is achieved by removing predicted tidal currents from the measurements. Results from a two-dimensional, non-linear, barotropic tidal model of the NWS region were supplied by the National Tidal Facility (B. Mitchell, personal communication). The model was run over the domain 13-21”S, 115125”E, on a resolution of 0.25” by 0.25” latitude and longitude and run simultaneously for M2, S2, K1 and 0, constituents. The model was calibrated against eight locations with tidal elevation information but not against tidal current information. The ratio of the rms difference in amplitude between the observed and modelled elevations to the mean observed amplitude and the rms phase differences between the observations and model are (0.0460.239 m, 4”), (0.040/0.808m, 5‘7, (0.030/0.230 m, 4”) and (0.013/0.149 m, 3”) for M2, S,, Kr and 0, constituents, respectively. The tidal ellipse properties from the current meter moored along the southern transect are listed in Table 1 for Mz, Sz, Kr and 0, constituents where inference has been used in the analysis to resolve Pi from Ki and Kz from S2 constituents giving a total of 38 constituents resolved in the analysis. These results can be compared to the model constituents from the nearest model grid-location given in Table 2. The agreement is very good for the largest constituent Mz with semi-major axis lengths differing by only 0.011 m s-r in 0.33 m s-r and phase by only 1”. The agreement between modelled and observed S2 currents is not as good with a difference in semi-major axis lengths of 0.046 m s-’ and in phase of 11”. For K, there is again good agreement in the
293
Leeuwin Current observations Table 1. Tidal current constituents derived from the current meter moored on the southern transect in 106 m water depth at location 19”12’S, 117’42’E. Semi-major and semi-minor axis lengths are a and b, respectively, g is phase lag relative to GMT, and 8 is the ellipse orientation measured in degrees anti-clockwise from east
Constituent M2 s2
Kl 0,
a (m s-l)
b (m s-l)
g
e
0.323 0.176 0.031 0.006
0.092 0.050 0.014 0.001
172 241 299 286
131 131 142 163
Table 2. Tidal current constituents from the tidal model for the location nearest to the current meter location on the southern transect. Semi-major and semi-minor axis lengths are a and b respectively, g is phase lag relative to GMT, and @is the ellipse orientation measured in degrees anti-clockwise from east
Constituent M2 s2
Kl 0,
a (m SK’)
b (m SK’)
g
e
0.338 0.222 0.032 0.016
0.096 0.059 0.021 0.007
171 230 250 269
130 130 81 146
semi-major axis lengths but phase difference by 78”. However, the K, (and 0,) currents are very weak (0.032 m s-i) so that the phase discrepancy is not important. The constituent values were also compared with results of an analytical model of cross-shelf variations in tides (BA~WXI and CLARKE,1982). There was close agrement between the numerical and analytical model results. Constituents from the nearest model grid-point to each measurement location were used to predict the tidal current at the mid-point time of the 25 min ADCP current average. Predictions of currents were performed with the tidal package of FOREMAN (1978) using the four constituents listed above plus K2 and N2 inferred from S2 and MZ, respectively from coastal ratios from Dampier (EASTON,1970). On the shelf the predicted tidal currents are strong and generally account for most of the measured ADCP current signal. However, in deeper water, where it is found that the alongshelf flows are strongest, the corrections from tidal currents are small. Table 3 presents a listing of the predicted tidal current for each station location at the time of ADCP measurements along with the resulting non-tidal near-surface current (bin 2 centred at 22.6 m). It must be recognised that the ADCP data could be affected, to some extent, by oscillatory currents associated with processes such as internal tides and shelf waves. However, it is expected that such currents would be weak, particularly in the deeper water offshore of the shelf break.
P. HOLLOWAY
294
Table 3. Predicted tidal current speeds and directions for each ADCP station at the time of the ADCP current measurements along with the resulting de-tided ADCP current at a depth of 22.6 m (bin number 2). Dates are for the year 1993 and all times (GMT) are the time of tidal current prediction which are at the mid-point of the 25 min ADCP current average Tidal prediction Station number Transect A 1 2 3 4 5 6 7 8 9 10 11 12 13 14 16 18 Transect B 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
Date
26 May
27 May 28 May
29 May
3 June
4 June
5 June
6. ADCP
Non-tidal current
Time (GMT)
Speed (m s-‘)
Dir.
Speed (m s-‘)
Dir.
0047 0341 0710 0822 0928 2336 0047 0156 0315 0441 0828 0940 2342 0056 0202 0319
0.39 0.44 0.35 0.48 0.33 0.16 0.06 0.16 0.25 0.14 0.11 0.15 0.10 0.03 0.02 0.04
150 119 340 325 316 301 230 157 141 121 344 326 307 281 142 120
0.35 0.16 0.09 0.15 0.13 0.16 0.28 0.26 0.32 0.24 0.19 0.38 0.16 0.17 0.06 0.12
320 216 232 204 258 257 280 275 290 240 229 317 250 234 267 153
0021 0250 0436 0530
0.20 0.35 0.59 0.53 0.27 0.13 0.32 0.43 0.17 0.13 0.28 0.42 0.36 0.28 0.13 0.12 0.23 0.16 0.08 0.14 0.09
109 325 308 300 287 213 151 121 100 14 336 322 315 305 275 183 132 116 75 327 311
0.01 0.09 0.07 0.05 0.17 0.15 0.13 0.24 0.08 0.03 0.01 0.14 0.21 0.27 0.12 0.19 0.15 0.19 0.06 0.42 0.30
6 12 101 197 217 186 194 293 290 295 37 98 26 48 39 335 42 312 229 140 141
0638 0744 0922 0010 0116 0220 0324 0429 0538 0639 0744 0851 0004 0112 0220 0442 0604
NON-TIDAL
CURRENTS
The non-tidal component of the ADCP measured currents, calculated as described in section 5, are displayed in several forms. Figure 5 shows vector sticks of currents along the
295
Leeuwin Current observations
Northern Transect -10
- 50
-100
-150
2
2 i 5 -200
- 250
- 300
- 350
- 400
1
400
I
I
,
1
300
200
100
0
Distance (kilometres)
Fig. 5. Vector sticks of current along the northern transect for different depth levels. Currents are averaged over 20 m depth (10 m each side of the indicated depth). Distance is that from the coast. North is perpendicular to the time axis.
&
0
296
P. HOLLQWAY
Southern Transect -10
- so
-100
-150
;;; g E 5
- 200
_.I,+
- 250
- 300
\i
‘-
\
- 350
1
^\ -..__
- 400 40
O-
I
cm/s
,
400
I
300
,
I
I
200
100
0
Distance (Mometres) Fig. 6. Vector sticks of current along the southern transect for different depth levels. Currents are averaged over 20 m depth (10 m each side of the indicated depth). Distance is that from the coast. North is perpendicular to the time axis.
8
Leeuwin
Station
Current
observations
297
locations
100
200 z Jz z -300
320
240 Distance
Station
160
80
z
0
(kilometres) locations
-100
- 200 z _c -300
SOUTHERN SECTION VELOCITY (cm/s) (positive is equatorward)
320
240 Distance
160
500 80
0
(kilometres)
Fig. 7. Cross-shelf section of de-tided longshelf velocity component along the northern and southern transects. Alongshelf is defined as a clockwise axis rotation of 70” east of north for the southern transect and of 30”for the northern transect. Positive flow is equatorward.
5 n
298
P. HOLLOWAY
northern transect at different depths, and Fig. 6 shows the vector sticks from the southern transect. Values are displayed for every 50 m depth and are calculated as averages over a 20 m depth range, where the indicated depth is the mid-point value. Along the southern transect the strongest flow is seen over the slope region. The flow is fairly uniform over the top 100 m, consistent with the observed deep surface mixed-layer. Although there is some variability in the flow direction along the transect, there is a predominance of poleward flow in the upper layers. Between 150 and 250 m there is again variability in direction along the transect, but the flow is now predominantly equatorward. The deeper flow is significantly weaker and flowing poleward. At the northern transect the flow is more variable in direction and strength across the transect with surface flow poleward around the shelf break and equatorward further offshore. Towards the end of the transect, 360 km from the coast, the flow is strongest (0.4 m s-l) and directed onshore. The currents are relatively uniform with depth over the upper 100 m. The patterns suggest eddying motion rather than a spatially uniform flow. Below 250 m depth the flow is weaker in strength but more uniform in direction and flowing poleward. Contoured cross-shelf sections of the longshelf velocity components are plotted in Fig. 7 for both transects. The orientation defining alongshelf is chosen for each section to reflect the local bathymetry, and a clockwise rotation of 30”for the northern transect and 70” for the southern transect are used. The southern transect shows predominantly poleward flow strongest over the shelf, at 0.34 m s-i, where the water is approximately 150 m deep. In deeper water the flow is still poleward but weaker, with the exception of a ring of equatorward flow between 100 and 300 m deep. At the northern transect the flow is spatially more variable. As seen in the southern transect there is a poleward flow over the shelf break in water 125 m deep. Over the slope from the surface down to approximately 250 m depth the flow is equatorward with a maximum of 0.33 m s-’ and with maxima occurring in several distinct cores. Below 250 m the flow is weaker but poleward across the entire section. In addition, at the outer part of the section the flow is also poleward in the upper 200 m. As a means of quantifying the strength of the poleward flowing LC, the velocity profiles at each station along each of the two transects are depth integrated to define the net poleward and net equatorward transports (m2 s-l) at each station. Velocity profiles are integrated from the surface to the maximum depth (92% of the water depth or 440 m) and across all stations, as shown in Fig. 7. Values are linearly interpolated between stations. The resulting cross-shelf sections of transport are plotted in Fig. 8 for each transect where poleward transport (the LC) is defined as positive and equatorward transport as negative. The integral of these curves then gives the volume transport (m3 s-i) across each section. For the southern transect a well defined poleward current is seen with a peak transport of 50 m2 s-l occurring at a distance of 200 km from the coast, and the width of the current can
Fig. 8. Cross-shelf section of poleward (positive) and equatorward (negative) transports for the northern and southern transects. Values are computed by depth-integrating the de-tided ADCP current profiles at each station and calculating net positive and net negative transports. Also poleward and equatorward transports are estimated for Dongara (29”3O’S) computed from current meter data presented by SMITH et al. (1991). The Dongara data are an average over the period 1 March-12 May 1987 and are from seven different meters in four moorings.
299
Leeuwin Current observations
100
NORTHERN
400
TRANSECT
300
200
300
200
100
0
-50 400
Distance
(kilometres)
Fig. 8.
100
0
300
P. HOLLOWAY
be defined from the transport distribution as approximately 120 km. The poleward volume transport across the section is 4.2 x lo6 m3 s-i , and the equatorward transport is 0.4 x lo6 m3 s-1 , given a net poleward transport of 3.8 x lo6 m3 s-l. For the northern transect the transport shows three distinct cores in the poleward direction, with the largest transport in the furthest offshore core approximately 350 km from the coast. The width of the poleward current is 225 km but could well continue beyond the end of the measurements. The opposing equatorward flow is in a well defined current core with a width of 100 km. There is little transport either equatorwards or polewards on the shelf at either section. There is a poleward volume transport through the northern section of 3.7 x lo6 m3 s-’ and an equatorward transport of 3.3 x 106m3s-’ , giving a net poleward transport of 0.4 x lo6 m3 s-l. The poleward volume transport across each transect is then approximately equal, but slightly larger at the southern transect. The transport at the outer edge of measurements from the northern transect is well above zero, indicating that measurements were not made across the full width of the current, and hence the southward transport is probably larger than the above figures indicate, 7. SATELLITE
DERIVED
SEA
SURFACE
TEMPERATURES
Figure 9 shows an AVHRR image of the NWS region obtained from the NOAA11 satellite at 08222 on 26 May 1993, coincident in time with the beginning of the southern transect measurements and 10 days prior to the completion of the northern transect measurements. Typical of low latitude regions, the surface waters only show weak variations (about 2°C) in temperature. However, the image does provide some useful information. The warm water off the NWS can be seen to exhibit substantial spatial variability by about + 1°C. The warmest water (28°C) is seen north of 18”s and offshore of 120”E. This water is intruding eastward producing a surface temperature field with patchiness and evidence of eddying motions. This is consistent with the near surface ADCP velocities shown in Figs 5 and 6, particularly for the northern transect, where flow reversals are seen along the length of the transect. The more uniform poleward surface flow seen at the southern transect is consistent with there being less variation in temperature than at the northern transect. A relatively cold strip of water is seen along the coast and inner shelf, approximately 2°C colder than the offshore water. This is presumably a result of winter cooling of the shallow shelf waters. 8. EARLIER
CURRENT
MEASUREMENTS
Moored current meter data obtained during the period August 1983 to March 1985 is considered from two locations on the NWS. Current meters from a single mooring at Jabiru location (11”48’S, 125”9’E) in a depth of 120 m at the northern end of the NWS provide a nearly continuous record from October 1983 to March 1985. Neil Brown Acoustic Current meters were used at depths of 15,48,82 and 116 m and sampled 5 min vector averages. Not all current meters successfully operated at all times, and a depth averaged current time series is calculated as the simple mean of all current records available at each time. In addition, an offshore buoy at the same location recorded wind velocity data at a height of 4.3 m above sea level and at a sampling interval of either 30 or 60 min. Some current meter data from North Rankin (19”37’S, 116”6’E) at the southern end of the NWS was also recorded during this period. In 123 m water depth, currents were
Leeuwin Current observations
301
4
303
Leeuwin Current observations
t \
Jabiru Winds
10
c E
0 -10 30_
Jabiru Cum’ItS
1 t E,
O-
-30 E:
-30’
I
‘A’S’O’N 1983
D’J’F
I
M
I
A
I
M
I
J
I
1984
J
I
A
I
S
I
D
II
N
D’J
I
F
1
1985
Fig. 10. Time series of low-pass filtered, depth-averaged currents and winds from Jabiru location (11“48’S, 125YE) in a water 120 m deep and from North Rankin location (19’37’S, 116”6’E) in a water 123 m deep. The filter has a half amplitude response at 96 h. Currents are rotated clockwise by 50”at Jabiru and by 25”at North Rankin so that sticks perpendicular to the time axis represent flow parallel to the local bathymetry.
measured at 23,53,82 and 120 m depths from August 1983 to March 1984 and at 35,65 and 95 m depths from December 1984 to March 1985. All measurements were 5 min vector averages. Again a depth average current is calculated. Mooring locations are shown in Fig. 1. Figure 10 shows vector-stick time series of the depth averaged, low-pass filtered currents and winds. The filter was designed to remove super-inertial motion and has a halfamplitude response at 96 h where the inertial period is 57 h at the Jabiru location and 35 h at the North Rankin location. Prior to filtering, short gaps were replaced by zero values to provide a simple interpolation. The maximum gaps are shorter than the period at which the filter has its half amplitude response so that any bias produced by the adding of zeros will be minimal. The orientation of the plots is so that sticks perpendicular to the time axis are parallel to the local bathymetry. Alongshelf flow is defined by a rotation of 50” and 25” east of north at Jabiru and North Rankin, respectively. At Jabiru the flow is seen to be predominantly poleward for most of the measurement period. Maximum poleward currents are about 0.2 m s-l and occur in March and April 1984. The mean depth-averaged current over the 18 months of measurement is 0.033 m s-l in the direction 208” east of north, which is a poleward flow. The LC appears to be present at this northern location, particularly from January to April inclusive. During the winter months the flow is weaker and more variable although still often in a poleward direction. The winds at Jabiru are characterized by a distinct seasonal cycle with the North West Monsoons blowing from the west from December to March followed by the South East Trades blowing from the east from April through to October but strongest from April to July. During March and April
304
P. HOLLOWAY
the winds support the poleward currents at Jabiru but from May to July the poleward flow weakens, despite the strong winds, and at times flows equatorward into the prevailing South East Trades blowing from the east. At the North Rankin location the LC is seen, most predominantly, from December to March. The poleward flow is of similar strength to that observed at Jabiru. More extensive observations of the LC from the North Rankin region are discussed by HOLLOWAY and NYE (1985). 9. DISCUSSION In order to compare the strength and width of the LC off the NWS to that observed further south, data from SMITHet al. (1991) are used to construct a cross-shelf section of transport similar to those for the two transects from the NWS. Data from their Dongara array at latitude 29”3O’Sis used. Average alongshelf currents from 1 March to 12 May 1987 from seven current meters in four moorings are contoured and presented in Smith’s Fig. 5. Here data from their Fig. 5 was used to approximate the poleward and equatorward transports with the results presented in Fig. 8. It can be seen that the peak poleward transport is approximately 100 m2 s-l, about double that found at transect A, and that the width of the current is only 50 km, much narrower than the broad flow found on the NWS. The net poleward volume transport is found by integrating under the curve as 2.1 x lo6 m3 S-‘, although it is clear that their observations do not completely cross the current. The return equatorward transport is much weaker than this. SMITHet al. (1991) computed volume transports for a number of different times and sections between 25 and 35”s from geostrophic calculations. The values vary in time and location ranging from approximately 2 to 7 X lo6 m3 s-l. The alongshelf transports are, however, relatively uniform between 25 and 34%. Smith et al. suggest there is some seasonal variation in the transport with significant onshore transport and feeding of the LC from the subtropical offshore water during the late winter (August and September). Then in early autumn (March) the current is also fed by water from the NWS. The calculated transports on the NWS, reported in this paper, are then of similar strength to those observed further south although not as strong as the peak values reported by SMITHet al. (1991). This suggests that the LC is continuous well up onto the NWS and that a significant fraction of the volume of the LC originates from the NWS, at least as far as latitude 17”s during the reported time period of May/June. The observations presented in this paper have provided a snapshot of the structure of the LC on the NWS. Although the ADCPKTD observations were taken over a period of 10 days, each section has been interpreted as providing a synoptic picture of the current flow. It has previously been suggested that at least some of the LC water originates on the NWS and limited observations from the southern end of the NWS have supported this. The observations reported in this paper have revealed a broad, deep and slow moving LC over the continental slope region of the NWS at latitudes of approximately 17 and 19”s. Additionally, the measured poleward volume transport of about 4 X lo6 m3 s-l is similar to values previously reported from further south where the LC is narrow and swiftly flowing. At 17”s the LC is approximately 250 km wide (and possibly wider), is seen from the surface to a depth of at least 440 m, and appears to exist as an eddying or meandering flow rather than a single current core as seen at 19”s and further south of North West Cape. The moored current meter data also shows that the LC exists as far north as 12”S, although it was not possible to estimate the volume transport at this location.
Leeuwin Current observations
305
It is clear that additional observations are warranted from the NWS to further trace the origins of the LC. Acknowledgements-A
number of individuals and organizations have made invaluable contributions to this project. All field work from H.M.A.S. Moresby was made possible through generous cooperation and collaboration with Cmdr David Knight and Leut. Mike Evans from the Royal Australian Navy Hydrographic Office and from the assistance of the Officers and Seaman on H.M.A.S. Moresby. Mike Evans also provided the CTD data. Expert assistance was provided by Tony Veness who prepared and maintained all equipment and by Ray Lawton who prepared the current meter moorings. Bill Mitchell from the National Tidal Facility at Flinders University provided the tidal model results for the North West Shelf. Mark Orzechowski wrote many new algorithms and expertly carried out most of the data processing. Ian Shepherd and Tony Webb most generously carried out all processing of the satellite image. Current meter data from Jabiru location were provided by BHP Petroleum and from North Rankin by Woodside Offshore Petroleum, and both data sets were collected by Steedman Science and Engineering.
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A. and A. J. CLARKE(1982) A simple method for estimating barotropic tidal currents on continental margins with specific application to the Mz tide off the Atlantic and Pacific Coasts of the United States.
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l&8-16.
BATTEENM.
L. and M. J. RUTHERFORD (1990) Modeling studies of eddies in the Leeuwin Current: the role of thermal forcing. Journal of Physical Oceanography, 20, 1484-1520. CHURCHJ. A., G. R. CRESSWELLand J. S. GODFREY(1989) The Leeuwin Current. In: Polewardflows along eastern ocean boundaries, S. J. NESHYBA,CH. N. K. MOOERS,R. L. SMITHand R. T. BARBER,editors, Coastal and Estuarine Studies, 34, Springer-Verlag, New York, pp. 230-254. CRESSWELL G. R. and T. J. GOLDING(1980) Observations of a south-flowing current in the southeastern Indian Ocean. Deep-Sea Research, 27, 449-466. EASTONA. K. (1970) Tides around Australia. Horace Lamb Centre for Oceanographical Research, Flinders University of South Australia, Research Report 37.326 pp. FOREMAN M. G. G. (1978) Manual for tidal current analysis and prediction. Institute of Ocean Sciences, Patricia Bay. Sydney, B.C. Canada, Pacific Marine Science Report 78-6,70 pp. GENTILLIJ. (1972) Thermal anomalies in the Eastern Indian Ocean. Nature, Physical Science, 238,93-95. GODFREYJ. S. and K. R. RIDGWAY(1985) The large-scale environment of the poleward-flowing Leeuwin Current, Western Australia: longshore steric height gradients, wind stresses and geostrophic flow. Journal of Physical Oceanography,
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HOLLOWAY P. E. and H. C. NYE (1985) Leeuwin Current and wind distributions on the southern part of the Australian North West Shelf between January 1982 and July 1983. Australian Journal of Marine and Freshwater Research, 36, 123-137.
HOLLOWAY P. E. (1988) Climatology of internal tides at a shelf-break location on the Australian North West Shelf. Australian Journal of Marine and Freshwater Research, 39, l-18. LEGECKISR. and G. CRESSWELL (1981) Satellite observations of sea surface temperature fronts off the coast of western and southern Australia. Deep-Sea Research, Z&297-306. RIDGWAYK. R. and R. G. LOCH(1987) Mean temperature-salinity relationships in Australian waters and their use in water mass analysis. Australian Journal of Marine and Freshwater Research, 38,553-567. SMITH R. L., A. H. HUYER,J. S. GODFREY and J. A. CHURCH(1991) The Leeuwin Current off Western Australia, 1986-1987. Journal of Physical Oceanography, 21,322-345. THOMPSON R. 0. R. Y. and G. VERONIS(1983) Poleward boundary current off Western Australia. Australian Journal of Marine and Freshwater Research, 34, 173-185.
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THOMPSONR. 0. R. Y. (1987) Continental-shelf-scale
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WEAVERA. J. and J. H. MIDDLETON(1989) On the dynamics of the Leeuwin Current. Journal of Physical Oceanography,
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