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On the formation and spreading of the Bass Strait cascade
Pergamon
Continental Shelf Research, Vol. 14, No. 4, pp. 385-399. 1994 Copyright © 1994 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0278-4343/94 $6.00 + 0.00
On the formation and spreading of the Bass Strait cascade JOHN L . LUICK,* ROLF K A S E t a n d MATTHIAS TOMCZAK:~
(Received on 25 July 1991; in revised form 20 August 1992; accepted 1 October 1992) Abstract--The Bass Strait cascade is a wintertime downwelling caused by cooling of the shallow
waters of Bass Strait. During winter, a front separates the cold shelf water from the waters of the Tasman Sea. Continuous horizontal bands of downwelled water leading oceanward beneath the front imply that it can be transgressed near the bottom anywhere along its length. However, by far the greatest volume crosses at a breach at the northern end. Measured currents in eastern Bass Strait fit a predictable pattern: eastward toward the front, then as the front is approached, swinging north towards the breach. Flow northwards along the slope after downwelling is quantified using a simple analytic model. Cascade water found in the "far-field" was found only in small patches. One such patch was found to possess motion independent from the mean flow in which it was embedded.
INTRODUCTION
THE Bass Strait cascade is a downwelling current which originates in the shallow waters of Bass Strait and flows down the continental slope to depths of several hundred metres or more in the Tasman Sea (Fig. 1). As it downwells to meet its equilibrium density level, it turns left under the influence of geostrophy and continues northwards as a slope undercurrent. Recent studies (GODFREY et al., 1980, GIBBS et al., 1986) have demonstrated that the cascade is the result of wintertime cooling in Bass Strait relative to external waters, where mixing is not limited to the upper 80 m (the maximum depth of Bass Strait). Within the strait, the water rapidly mixes over the entire depth due to strong winds and tides. TOMCZAK (1985) postulated that the cascade is confined to a breach at the northeastern corner of the strait; a persistent thermohaline surface front exists across the entire eastern entrance except at that point. The front is positioned along the shelf break, at a depth of 100-200 m. Earlier studies (BOLAND, 1971; NEWELL, 1961) suggested that anomalies of temperature and salinity found hundreds of miles away in the Tasman Sea were traces of the cascade water. Within the "near field" at least, the cascade water is identifiable as a distinctive kink towards higher temperature and salinity in a T-S diagram (ToMcZAK, 1985), reminiscent of
*Ocean Sciences Institute, The University of Sydney, NSW 2006, Australia. Present address: National Tidal Facility, Flinders University, GPO Box 2001, Adelaide, SA, Australia. t Institut Meerskunde, Theoretische Ozeanog., Dusternbrooker Weg 20, D-2300, Kiel, Germany. $ Ocean Sciences Institute, The University of Sydney, NSW 2006, Australia. Present address: Department of Oceanography, Flinders University, GPO Box 2100, Adelaide, SA, Australia. 385
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the Mediterranean Water signal in the Atlantic (ZENK, 1975). The cause of this kink lies in the background profile of the Tasman Sea, in which salinity and temperature both d e c r e a s e with depth below the mixed layer. The salinity within Bass Strait may be higher or slightly lower than that of the contiguous ocean. The cold cascade water sinks down into ever colder and fresher surroundings, so when it reaches its density level, it is warmer and saltier than its surroundings. This study addressed several aspects of the cascade and slope current. The first was the thermohaline front across the eastern e n t r a n c e - - h o w realistic is the assumption that the existence of a surface front implies that the cascade outflow is confined to the northern end? Turning then to the point at which the cascade water has left the formation region, a mesoscale eddy was located just offshore of the slope. What effect would this have on transport to the cascade "downstream"? Also, once the cascade reaches this area, is it
Formation and spreading of the Bass Strait cascade
387
swept passively into the Tasman Sea by the E A C or does it continue to show an independent velocity signature? INSTRUMENTS AND METHODS A hydrographic survey was conducted using R.V. Franklin over the 10 day period from 4 to 13 July 1989. Cross-sections were performed either by lowering a Neil Brown Mark III CTD in the usual fashion, or by using "Bunyip", a Mark III CTD mounted in a hull, which was developed by the CSIRO Division of Oceanography, Hobart, Tasmania. Bunyip is a towed or "seasoar" type instrument, which rises or sinks according to the remotely controlled pitch of a pair of wings. Towing speed was 8 knots. Data from the seasoar were de-spiked and reduced to 2 m averages, or 20 s time intervals when depth remained constant, during post-cruise processing by CSIRO. When using the lowered CTD, a 12bottle General Oceanics rosette of Niskin bottles was used to collect water samples at 50 m (or less) intervals. A cracked conductivity cell contaminated the data from the lowered CTD. At the start of a cast, the reading was clearly in error when compared to the bottle salinities; as the CTD was lowered, the offset gradually disappeared. Below a depth which varied from 50 to 150 m depending on the station, the offset was negligible and the instrument appeared to follow the minor fluctuations faithfully (D. Vaudrey, personal communication). Problems were also experienced with the towed instrument. Biological fouling occasionally caused an abrupt apparent decrease in salinity. An R D I Acoustic Doppler Current Profiler (ADCP) was used to measure currents; ship velocities were subtracted using one of three methods: bottom track, GPS, or SatNav fixes. Processing and quality control of the A D C P data was provided by CSIRO Marine Labs in Hobart. A thermosalinograph provided a continuous record of surface conditions. The temperature and salinity was calibrated using the surface CTD temperatures and Niskin bottle salinities by least squares fit. Because of the wide range of water types in Bass Strait, which overlapped those of the open Tasman Sea, we felt that simply defining a range of temperature and salinity values would not make a conclusive test for cascade water. Instead, we wrote a simple program which flagged occurrences of the T - S "kink" mentioned in the introduction. It required at least three successive values in the correct sense, i.e. a layer at least 4 m thick with temperature and salinity increasing, embedded in a water mass whose temperature and salinity decrease with depth. (The total thickness of the layer, used below and in the figures, was approximated by doubling the thickness of the portion above the tip of the kink). The results were screened by eye to prevent possible "forgery" by foreign water masses. Another possible problem is that cascade water could escape the procedure due to a lack of contrast with the surroundings. This would lead to an unrealistically low quantities of cascade water. Again, the results were checked against the T - S curves, and certain layers were manually flagged. THERMOHALINE FRONT ACROSS EASTERN ENTRANCE As expected from TOMCZAK(1985), a surface thermohaline front was observed to extend across most of the eastern entrance of Bass Strait [Fig. 2(a),(b) and (c)]. He published
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Formation and spreading of the Bass Strait cascade
389
maps similar to Fig. 2, showing slightly weaker density and temperature fronts in the same position, along with a much stronger salinity front, during June-July 1981. At the time of our study, the offshore temperatures were over 17°C, or about 2 ° higher than during the prior study. These high temperatures seem to be associated with an abnormally high southward excursion of the East Australia Current (EAC). The presence of the saline E A C water also partly explains the reduced salinity gradient. The contrast across the front can also be affected by discharge of rivers into Bass Strait. Particularly noticeable in Fig. 2 is a source of fresh water along the northeast coast of Tasmania. The probable source is the T a m a r River, which empties into Bass Strait just to the west of the portion of Tasmanian coast visible on the bottom edge of the figure. The streamflow in the Tamar River is largely controlled by a spillway in its major feeder stream, the South Esk. Records from a gauge at the spillway revealed that the monthly average discharge for June 1989 (the cruise began 4 July 1989) was the highest June discharge since 1981. Also, the cumulative discharge for the January-June period of 1989 was the highest for that period of any of the years 1980-1990 for which records were obtained. Similar results were obtained from a check of the records of a nearby river (the North Esk) which was not controlled by a spillway. Hence, one would expect the waters of southern Bass Strait to be unusually fresh at the time of this survey. GODFREY et al. (1980) interpreted the surface temperature front as an indication of the location where Bass Strait water sinks beneath the surface Tasman Sea water. TOMCZAK (1985) modified this picture, implying that except in northern Bass Strait, the front extended to the bottom, and concluded that the cascade was confined to the northern portion. We tested the possibility that Bass Strait water might sink under the front elsewhere by looking for a cascade "signature" in cross-sections through the front. Water of Bass Strait origin was found just outside the thermohaline front, at CTD Stas 6 and 7. Several seasoar sections pass through the front, and in the two cases (Legs 5 and 6) in which the seasoar was allowed to dip through the 100~00 m layer, Bass Strait water was found in a nearly continuous band passing beneath the surface front to the outer portion of the section beyond the front. The strength of the inversions, as measured by the temperature anomaly, was less than one-sixth of that later found in the primary downwelling zone (I°C vs 6°C). Also the thickness of the layers was about one-tenth of those within the primary downwelling zone (5 m vs 50 m). Hence, water escapes through or under the front, though it appears to be less intense than at the primary downwelling zone. Further offshore, at depths below 200 m, inversions were found at a higher ot than were observed over the shelf in Leg 5 (Fig. 3). These deeper inversions may have originated to the north, in the primary downwelling zone, and then swept to the south by the larger scale flow. Despite the occasional leakage beneath the front, it does appear to largely "insulate ~' the offshore water from that of Bass Strait. The one seasoar section (Leg 6) that extended well into the region of lateral homogeneity in Bass Strait showed a remarkably gradual transition between the shelf and slope densities (Fig. 4), with vertical isohalines extending at least to the depth the seasoar reached (within 20 m from the bottom). Inside Bass Strait, near CTD Stas 12, 13 and 14 (Fig. 5), the A D C P - m e a s u r e d flow is eastward (towards the front). As it reaches the gradual transition zone of Leg 6 (Figs 4 and 5), it swings abruptly to the north. Along Leg 6, closer to the front, the flow is northward, i.e., aligned with the front. The deeper currents (not shown), though weaker, were of an identical pattern. Of course tidal, shelf wave, and to a lesser extent wind-driven, currents
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affect the measured currents in Bass Strait. However, JONES (1980) deduced a maximum M 2 tidal velocity of around 0.15 m s- 1 at the Kingfish B platform, which is considerably less than the currents along Leg 6. Downwelling was observed at the northernmost transect within Bass Strait (Leg 15, Fig. 6). Cascade water dominates over the entire inshore 20 km below the sharp pycnocline. Offshore flow was indicated not only by the hydrographic data, but also by the A D C P current vectors (not shown). The recent formation of the downwelled layer is evident in the numerous instances of at (but not in situ density) inversions (-< 0.05 at unit) seen within the cascade layer near the slope at approximately 250 m. Salinity spiking was discounted as an explanation of the inversions since salinity was nearly homogeneous, while temperature increased with depth. Such an inversion can be expected to disappear rapidly in the presence of internal motions, and indeed, no inversions -> 0.008 crt units were seen in the northern sections. MOVEMENT
ALONG THE SHELF-SLOPE
Previous studies (ToMCZAK, 1985) have inferred as subsurface pathway for the downwelled water based on distributions of temperature, salinity, oxygen and nutrients, and on dynamical considerations. The inferred pathway starts at the downwelling zone near Cape Howe and follows the isobaths north. At the time of the cruise, a mesoscale eddy of the East Australian Current was situated adjacent to the pathway, just north of the downwel|ing zone. The E A C eddy was of the warm-saline-core, anticyclonic variety (HuYER e t al., 1988; TOMCZAK, 1983). The map of the 250 m isotherms (Fig. 7) shows the location of the eddy on 3 July 1989 (the day prior to the cruise), and the surface currents, derived using an empirical method which assumes that the velocity is proportional to the separation of the 250 m isotherms. The location and intensity of the eddy are inferred from maps of satellitederived sea surface temperatures (SSTs). Maps of the SSTs over the previous 6 weeks show little change in the location of the eddy, whose radius is over 100 km. The location of the E A C eddy, with it's southward-flowing currents on the inshore side,
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did not block the cascade from penetrating north. The same program used to detect cascade water outside the thermohaline front (by seeking inversions) was used to detect it in the C T D and seasoar data in the northern sections. The program identified numerous small patches of thickness less than 15 m in sections both within the E A C eddy and to the north. A typical C T D cast in the latter zone would have two such patches between 200 and 350 m, with some being as thick as 50 m. The seasoar data gave a similar result. The frequency of occurrence of the patches north of Cape H o w e did not decrease with distance from the source, but there was no identifiable coherence between patches in successive C T D profiles, either towed or conventional. Two C T D transects cut obliquely through the northwest corner of the E A C eddy (Figs 8 and 9). Evidence of downwelling across the shelf b r e a k is seen in the southerly section (Fig. 8), in which the dashed isotherm sinks down from the shelf to 330 m depth. This isotherm, together with the presence of cascade water as depicted, indicates downwelling beneath the location of the front, as suggested by GODFREY 1980). The two available A D C P profiles on this transect show southward flow at the surface, in agreement with Fig. 7, beneath which the flow is offshore (eastward). The offshore flow supports the interpretation of downwelling in progress.
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Formation and spreading of the Bass Strait cascade 18
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Fig. 8. Temperature section, CTD casts 17-21. Also, two ADCP velocity profiles (note key--arrows refer to map plane, not plane of figure). A dashed line shows the deepest shelf isotherm. The two transects of Figs 8 and 9 cross the shelf b r e a k only 50 km apart, but by the m o r e northern transect (Fig. 9), the downwelling is greatly diminished and the flow at the shelf b r e a k is to the north. The A D C P profile available from well offshore of the b r e a k also shows northward flow. Thus, the northern limit of the "primary downwelling zone" (shaded in Fig. 1) has been defined. A substantial percentage of C T D s 19 and 23 is comprised of cascade water. These layers are indicated by vertical bars in the profiles in Figs 8 and 9. These two stations are just beyond the shelf b r e a k in their respective transects, and in both cases, they are the stations which contain the largest amount of cascade water, The fact that downwelling is nearly absent from Fig. 9 indicates that the water has downwelled to the south of the transect and has therefore penetrated north along the slope against the general circulation of the E A C eddy. The concentration of cascade water in a layer against the slope, as seen in these two transects, was less pronounced in the far-field zone to the north. The density driven flow down the continental slope under the influence of rotation has characteristics that are c o m m o n for a variety of geographical regions. The outflow of the Mediterranean W a t e r at Gibraltar is one of the better known examples. There, the strongly non-linear overflow creates mechanical mixing, forming a new characteristic water mass, as a mixture of Mediterranean and Atlantic waters. Consider an initially linear
394
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Fig. 9. As Fig. 8, except CTD casts 21-25. vertical density stratification and turbulent mixing confined ideally to a vertical depth range Zo + 6z. The density anomaly with respect to ambient water after complete homogenization is positive above z0 and opposite below. As this water mass moves away from the mixing region, it attains quasi-geostrophic equilibrium, resulting in either an anticyclonic vortex or (in conjunction with coriolis force and wall friction) a slope current. A transport estimate would be M = (g'6z2/f). For the Mediterranean outflow, the numbers would be g' = 0.002 m s -2, dz = 400 m , f = 10 -4, i.e. M = 3.2 x 106 m -3 s -1 . The actual outflow past Gibraltar was only about 1 Sv--the increase comes about from the entrainment of Atlantic water. The density contrast between the slope current at Sta. 19 and the ambient water is nearly the same as in the Mediterranean case, but the thickness of the layer is only about 100 m - - h e n c e , the transport is about a fifth of a Sverdrup. Figures 8 and 9 show that the point where the cascade reaches its equilibrium density level and becomes a slope current may be reached very rapidly. The deepest "shelf" isotherm (14.754 °) is at 333 m at CTD cast 19 (Fig. 8); it rises to 266 m at CTD cast 23 (Fig. 9), and by the time the isotherm reaches the northernmost station, CTD 45, it is at 182 m. VELOCITIES IN THE FAR-FIELD A D C P velocities at the offshore end of the northernmost transect showed consistent offshore flow near the surface, with a maximum of about 80 cm s -1 near 30 m, decreasing
Formation and spreading of the Bass Strait cascade
395
N A
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to 25% of that value at 350 m. Within this current (undoubtedly an extension of the E A C ) were e m b e d d e d numerous patches of cascade water. In the single instance where it was possible to compare C T D data with A D C P data at the same location, it was found that the speed of the patch was significantly different from that of the m e a n flow directly above and below. It also m o v e d independently to the area average at the same level. Because of the strong m e a n flow, the sequence of the events shown in Fig. 10 is significant. They begin with C T D 44, followed by C T D 45 about 1 h later. As the latter station was completed, GPS navigation became available, and a series of 20 A D C P profiles were obtained while the ship slowly steamed first west, then east, as the seasoar was readied for deployment. The vectors shown in Fig. 10 are the A D C P velocity "anomalies" at 188 m - - i . e , the area-averaged vector at that depth has been subtracted from each of the 20. O f particular interest are the large velocity anomalies observed in A D C P profiles immediately following C T D 45 (nos. 1, 2 and 3), which will be shown to be associated with a patch of cascade water. The T - S diagrams (Fig. 11) reveal inversions at 148 m and 212 m in C T D 44, and at 142, 170 and 188 m at C T D 45. N o n e of the inversions that fall within the t e m p e r a t u r e range observed in Bass Strait a p p e a r in both stations (separated by 10 k m ) - - i n other words, the horizontal scale of the patches is less than 10 km. The shallower inversions (at 148 and 142 m) are not of interest to this study. The T - S inversions of 170 and 188 m in C T D 45 were chosen for further examination because of the availability of simultaneous A D C P data. The speed [(u 2 + v2)1/2], salinity, and t e m p e r a t u r e profiles are shown in Fig. 12. These two figures show that the small ( < 1 0
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Formation and spreading of the Bass Strait cascade
397
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at 20 and 40 min following number 1). This implies that the two inversion layers are probably moving together and should be thought of as a single patch. To see how the speed of the patch compares with the neighboring water mass, the first three profiles are plotted along with a profile which is an areal average of the twenty profiles at each depth (Fig. 13). All three individual profiles showed a higher than average speed at the depth of the patch and an equal or lower than average speed above and below. The overall thickness of the anomalously moving layer is about 50 m. DISCUSSION
Based on observations, we have found that while downwelling mainly occurs in a well-defined zone in northeastern Bass Strait, it also can occur along the entire eastern entrance beneath a thermohaline front. ADCP currents, however, confirm a basic premise of TOMCZAK(1985): that flow along the front is to the north, until downwelling occurs at a primary downwelling zone. Recently downwelled water is characterized not only by a signature profile in the T - S plane, but by frequent (~t inversions. The strength of the front and hence the cascade is subject to large variations at all time scales. A mesoscale eddy of the East Australian Current, a frequent regional feature, did not prohibit the northward penetration of flow along the slope of downwelled water, but it appeared to temporarily interrupt the downwelling current. This might occur, for example, if penetration could only be achieved by a sufficient mass of cascade water. Smaller quantities would be lost by
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Fig. 13. ADCP speed profiles 1, 2 and 3 and the average profile using all twenty profilesin Fig. 10. dilution and entrainment into the eddy. GODFREY et al. (1986) detected large volumes of high salinity water within a mesoscale eddy located off Spencer Gulf (South Australia) and concluded that the salinity was due to entrainment of Spencer Gulf outflow. Slope current transport at a point just downstream of the primary downwelling zone was estimated to be about one-fifth Sverdrup. This assumed a density contrast of 0,2 kg m 3 and a thickness of 100 m. GODFREY et al. (1980) reported numerous mixed layers of this thickness along the slope nearby. Although some of the volume would be comprised of entrained water, we have also shown that not all the downwelled water enters the slope current to the north. Previous estimates refer to the volume leaving Bass Strait, much of which may pass out without feeding the slope current. This would depend on the prevailing air-sea t e m p e r a t u r e difference and the point at which the flow crossed the eastern entrance. Godfrey estimated 0.5 Sv for the volume leaving Bass Strait based on the mean drift of a salinity tongue in Bass Strait, and BAINES et al. (1991), using a series of current meters across the strait, measured a net flux through Bass Strait of 0.49 Sv, but it varied between - 0 . 8 Sv and 3.05 Sv. Our transport estimate was based on a simple analytical model of the dynamics of slope currents. The center of the current, which is assumed to be well mixed, is assumed to be at its equilibrium density level and it is assumed to be penetrating a linearly stratified medium. Therefore, the top half of the current represents a positive density anomaly and the b o t t o m half is a negative anomaly. Integration of the thermal wind equation from either the top or b o t t o m will result in either an eddy, or, where sidewall friction is present, a slope current. In the "far-field", a patch of cascade water had a motion independent from that of the mean flow in which it was e m b e d d e d . We offer two possible explanations for this. The first is that we happen to have observed the patch while it was undergoing a displacement due to
Formation and spreading of the Bass Strait cascade
399
an internal wave. PADMANet al. (1990) showed that under certain conditions, submerged rotating lenses can selectively absorb momentum from vertically propagating internal waves. The second explanation is that the patch may have acquired rotation, either while sinking, in the manner of "meddies" (BORMANSand TURNER,1990), or by the mechanical mixing process described above. Acknowledgements--Thanks are due to Mark Johnston of the Tasmanian Department of Resources and Energy, Rivers and Water Supply Division for the discharge data; Dave Vaudrey, Jeff Dunn, and Lindsay Pender of CSIRO Oceanography, Hobart, for providing the best possible CTD, ADCP and seasoar data sets (respectively) despite instrumental problems; Neil Lawson of Lawson and Treloar Pty. Ltd for the meteorological data from Kingfish B; the Australian Oceanographic Data Centre, for satellite SSTs; Myriam Bormans, for helpful insights; and to the captain and crew of R.V. Franklin. One author (Luick) was supported during this study by a National Research Fellowship grant from the Commonwealth of Australia.
REFERENCES BArnES P. G., G. HUBBERTand S. POWER(1991) Fluid transport through Bass Strait. ContinentalShelfResearch, 11,269-293. BOLAND F. M. (1971) Temperature-salinity anomalies at depths between 200 m and 800 m in the Tasman Sea. Australian Journal o f Marine and Freshwater Research, 22, 55-62. BORMANS M. and J. S. TURNER (1990) The formation of the doubly stable stratification in the Mediterranean Outflow. Deep-Sea Research, 37, 1697-1712. GIBBS C. F., R. A. COWDELLand A. R. COWDELL(1986) Seasonal variations of density patterns in relation to the Bass Strait Cascade. Australian Journal o f Marine and Freshwater Research, 37, 21-25. GODFREYJ. S., I. S. F. JONES, J. G. H. MAXWELLand B. D. SCOTT(1980) On the winter cascade from Bass Strait into the Tasman Sea. Australian Journal of Marine and Freshwater Research, 31,275-286. GODFREY J. S., D. J. VAUDREYand D. HAHN (1986) Observations of the shelf-edge current south of Australia, winter 1982. Journal of Physical Oceanography, 16, 668-679. HUYERA., R. L. SMITH,P. J. STABENO,J. A. CHURCHand N. WHITE (1988) Currents off south-eastern Australia: results from the Australian Coastal Experiment. Australian Journal o f Marine and Freshwater Research, 39, 245-288. JONES I. S. F. (1980) Tidal and wind-driven motions in Bass Strait. Australian Journal o f Marine and Freshwater Research, 31,109-117. NEWELL B. S. (1961) Hydrology of south-east Australian waters: Bass Strait and New South Wales tuna fishing area. CSIRO Australia Div. Fisheries and Oceanography Technical Paper 10. PADMANL., M. LEVINE,T. DILLON, J. MORISON and R. PINKEL (1990) Hydrography and microstructure of an arctic cyclonic eddy. Journal of Geophysical Research, 95,9411-9420. TOMCZAKM. JR. (1983) Water masses in East Australian Current Eddy H. CSIRO Marine Laboratory Report 161. TOMEZAKM. JR. (1985) The Bass Strait Water Cascade during winter 1981. Continental Shelf Research, 4,255278. ZENK W. (1975) On the Mediterranean outflow west of Gibraltar. "Meteor" Forsch.-Ergebnisse A, 16, 23-34.
Continental Shelf Research, Vol. 14, No. 4, pp. 385-399. 1994 Copyright © 1994 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0278-4343/94 $6.00 + 0.00
On the formation and spreading of the Bass Strait cascade JOHN L . LUICK,* ROLF K A S E t a n d MATTHIAS TOMCZAK:~
(Received on 25 July 1991; in revised form 20 August 1992; accepted 1 October 1992) Abstract--The Bass Strait cascade is a wintertime downwelling caused by cooling of the shallow
waters of Bass Strait. During winter, a front separates the cold shelf water from the waters of the Tasman Sea. Continuous horizontal bands of downwelled water leading oceanward beneath the front imply that it can be transgressed near the bottom anywhere along its length. However, by far the greatest volume crosses at a breach at the northern end. Measured currents in eastern Bass Strait fit a predictable pattern: eastward toward the front, then as the front is approached, swinging north towards the breach. Flow northwards along the slope after downwelling is quantified using a simple analytic model. Cascade water found in the "far-field" was found only in small patches. One such patch was found to possess motion independent from the mean flow in which it was embedded.
INTRODUCTION
THE Bass Strait cascade is a downwelling current which originates in the shallow waters of Bass Strait and flows down the continental slope to depths of several hundred metres or more in the Tasman Sea (Fig. 1). As it downwells to meet its equilibrium density level, it turns left under the influence of geostrophy and continues northwards as a slope undercurrent. Recent studies (GODFREY et al., 1980, GIBBS et al., 1986) have demonstrated that the cascade is the result of wintertime cooling in Bass Strait relative to external waters, where mixing is not limited to the upper 80 m (the maximum depth of Bass Strait). Within the strait, the water rapidly mixes over the entire depth due to strong winds and tides. TOMCZAK (1985) postulated that the cascade is confined to a breach at the northeastern corner of the strait; a persistent thermohaline surface front exists across the entire eastern entrance except at that point. The front is positioned along the shelf break, at a depth of 100-200 m. Earlier studies (BOLAND, 1971; NEWELL, 1961) suggested that anomalies of temperature and salinity found hundreds of miles away in the Tasman Sea were traces of the cascade water. Within the "near field" at least, the cascade water is identifiable as a distinctive kink towards higher temperature and salinity in a T-S diagram (ToMcZAK, 1985), reminiscent of
*Ocean Sciences Institute, The University of Sydney, NSW 2006, Australia. Present address: National Tidal Facility, Flinders University, GPO Box 2001, Adelaide, SA, Australia. t Institut Meerskunde, Theoretische Ozeanog., Dusternbrooker Weg 20, D-2300, Kiel, Germany. $ Ocean Sciences Institute, The University of Sydney, NSW 2006, Australia. Present address: Department of Oceanography, Flinders University, GPO Box 2100, Adelaide, SA, Australia. 385
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the Mediterranean Water signal in the Atlantic (ZENK, 1975). The cause of this kink lies in the background profile of the Tasman Sea, in which salinity and temperature both d e c r e a s e with depth below the mixed layer. The salinity within Bass Strait may be higher or slightly lower than that of the contiguous ocean. The cold cascade water sinks down into ever colder and fresher surroundings, so when it reaches its density level, it is warmer and saltier than its surroundings. This study addressed several aspects of the cascade and slope current. The first was the thermohaline front across the eastern e n t r a n c e - - h o w realistic is the assumption that the existence of a surface front implies that the cascade outflow is confined to the northern end? Turning then to the point at which the cascade water has left the formation region, a mesoscale eddy was located just offshore of the slope. What effect would this have on transport to the cascade "downstream"? Also, once the cascade reaches this area, is it
Formation and spreading of the Bass Strait cascade
387
swept passively into the Tasman Sea by the E A C or does it continue to show an independent velocity signature? INSTRUMENTS AND METHODS A hydrographic survey was conducted using R.V. Franklin over the 10 day period from 4 to 13 July 1989. Cross-sections were performed either by lowering a Neil Brown Mark III CTD in the usual fashion, or by using "Bunyip", a Mark III CTD mounted in a hull, which was developed by the CSIRO Division of Oceanography, Hobart, Tasmania. Bunyip is a towed or "seasoar" type instrument, which rises or sinks according to the remotely controlled pitch of a pair of wings. Towing speed was 8 knots. Data from the seasoar were de-spiked and reduced to 2 m averages, or 20 s time intervals when depth remained constant, during post-cruise processing by CSIRO. When using the lowered CTD, a 12bottle General Oceanics rosette of Niskin bottles was used to collect water samples at 50 m (or less) intervals. A cracked conductivity cell contaminated the data from the lowered CTD. At the start of a cast, the reading was clearly in error when compared to the bottle salinities; as the CTD was lowered, the offset gradually disappeared. Below a depth which varied from 50 to 150 m depending on the station, the offset was negligible and the instrument appeared to follow the minor fluctuations faithfully (D. Vaudrey, personal communication). Problems were also experienced with the towed instrument. Biological fouling occasionally caused an abrupt apparent decrease in salinity. An R D I Acoustic Doppler Current Profiler (ADCP) was used to measure currents; ship velocities were subtracted using one of three methods: bottom track, GPS, or SatNav fixes. Processing and quality control of the A D C P data was provided by CSIRO Marine Labs in Hobart. A thermosalinograph provided a continuous record of surface conditions. The temperature and salinity was calibrated using the surface CTD temperatures and Niskin bottle salinities by least squares fit. Because of the wide range of water types in Bass Strait, which overlapped those of the open Tasman Sea, we felt that simply defining a range of temperature and salinity values would not make a conclusive test for cascade water. Instead, we wrote a simple program which flagged occurrences of the T - S "kink" mentioned in the introduction. It required at least three successive values in the correct sense, i.e. a layer at least 4 m thick with temperature and salinity increasing, embedded in a water mass whose temperature and salinity decrease with depth. (The total thickness of the layer, used below and in the figures, was approximated by doubling the thickness of the portion above the tip of the kink). The results were screened by eye to prevent possible "forgery" by foreign water masses. Another possible problem is that cascade water could escape the procedure due to a lack of contrast with the surroundings. This would lead to an unrealistically low quantities of cascade water. Again, the results were checked against the T - S curves, and certain layers were manually flagged. THERMOHALINE FRONT ACROSS EASTERN ENTRANCE As expected from TOMCZAK(1985), a surface thermohaline front was observed to extend across most of the eastern entrance of Bass Strait [Fig. 2(a),(b) and (c)]. He published
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Formation and spreading of the Bass Strait cascade
389
maps similar to Fig. 2, showing slightly weaker density and temperature fronts in the same position, along with a much stronger salinity front, during June-July 1981. At the time of our study, the offshore temperatures were over 17°C, or about 2 ° higher than during the prior study. These high temperatures seem to be associated with an abnormally high southward excursion of the East Australia Current (EAC). The presence of the saline E A C water also partly explains the reduced salinity gradient. The contrast across the front can also be affected by discharge of rivers into Bass Strait. Particularly noticeable in Fig. 2 is a source of fresh water along the northeast coast of Tasmania. The probable source is the T a m a r River, which empties into Bass Strait just to the west of the portion of Tasmanian coast visible on the bottom edge of the figure. The streamflow in the Tamar River is largely controlled by a spillway in its major feeder stream, the South Esk. Records from a gauge at the spillway revealed that the monthly average discharge for June 1989 (the cruise began 4 July 1989) was the highest June discharge since 1981. Also, the cumulative discharge for the January-June period of 1989 was the highest for that period of any of the years 1980-1990 for which records were obtained. Similar results were obtained from a check of the records of a nearby river (the North Esk) which was not controlled by a spillway. Hence, one would expect the waters of southern Bass Strait to be unusually fresh at the time of this survey. GODFREY et al. (1980) interpreted the surface temperature front as an indication of the location where Bass Strait water sinks beneath the surface Tasman Sea water. TOMCZAK (1985) modified this picture, implying that except in northern Bass Strait, the front extended to the bottom, and concluded that the cascade was confined to the northern portion. We tested the possibility that Bass Strait water might sink under the front elsewhere by looking for a cascade "signature" in cross-sections through the front. Water of Bass Strait origin was found just outside the thermohaline front, at CTD Stas 6 and 7. Several seasoar sections pass through the front, and in the two cases (Legs 5 and 6) in which the seasoar was allowed to dip through the 100~00 m layer, Bass Strait water was found in a nearly continuous band passing beneath the surface front to the outer portion of the section beyond the front. The strength of the inversions, as measured by the temperature anomaly, was less than one-sixth of that later found in the primary downwelling zone (I°C vs 6°C). Also the thickness of the layers was about one-tenth of those within the primary downwelling zone (5 m vs 50 m). Hence, water escapes through or under the front, though it appears to be less intense than at the primary downwelling zone. Further offshore, at depths below 200 m, inversions were found at a higher ot than were observed over the shelf in Leg 5 (Fig. 3). These deeper inversions may have originated to the north, in the primary downwelling zone, and then swept to the south by the larger scale flow. Despite the occasional leakage beneath the front, it does appear to largely "insulate ~' the offshore water from that of Bass Strait. The one seasoar section (Leg 6) that extended well into the region of lateral homogeneity in Bass Strait showed a remarkably gradual transition between the shelf and slope densities (Fig. 4), with vertical isohalines extending at least to the depth the seasoar reached (within 20 m from the bottom). Inside Bass Strait, near CTD Stas 12, 13 and 14 (Fig. 5), the A D C P - m e a s u r e d flow is eastward (towards the front). As it reaches the gradual transition zone of Leg 6 (Figs 4 and 5), it swings abruptly to the north. Along Leg 6, closer to the front, the flow is northward, i.e., aligned with the front. The deeper currents (not shown), though weaker, were of an identical pattern. Of course tidal, shelf wave, and to a lesser extent wind-driven, currents
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affect the measured currents in Bass Strait. However, JONES (1980) deduced a maximum M 2 tidal velocity of around 0.15 m s- 1 at the Kingfish B platform, which is considerably less than the currents along Leg 6. Downwelling was observed at the northernmost transect within Bass Strait (Leg 15, Fig. 6). Cascade water dominates over the entire inshore 20 km below the sharp pycnocline. Offshore flow was indicated not only by the hydrographic data, but also by the A D C P current vectors (not shown). The recent formation of the downwelled layer is evident in the numerous instances of at (but not in situ density) inversions (-< 0.05 at unit) seen within the cascade layer near the slope at approximately 250 m. Salinity spiking was discounted as an explanation of the inversions since salinity was nearly homogeneous, while temperature increased with depth. Such an inversion can be expected to disappear rapidly in the presence of internal motions, and indeed, no inversions -> 0.008 crt units were seen in the northern sections. MOVEMENT
ALONG THE SHELF-SLOPE
Previous studies (ToMCZAK, 1985) have inferred as subsurface pathway for the downwelled water based on distributions of temperature, salinity, oxygen and nutrients, and on dynamical considerations. The inferred pathway starts at the downwelling zone near Cape Howe and follows the isobaths north. At the time of the cruise, a mesoscale eddy of the East Australian Current was situated adjacent to the pathway, just north of the downwel|ing zone. The E A C eddy was of the warm-saline-core, anticyclonic variety (HuYER e t al., 1988; TOMCZAK, 1983). The map of the 250 m isotherms (Fig. 7) shows the location of the eddy on 3 July 1989 (the day prior to the cruise), and the surface currents, derived using an empirical method which assumes that the velocity is proportional to the separation of the 250 m isotherms. The location and intensity of the eddy are inferred from maps of satellitederived sea surface temperatures (SSTs). Maps of the SSTs over the previous 6 weeks show little change in the location of the eddy, whose radius is over 100 km. The location of the E A C eddy, with it's southward-flowing currents on the inshore side,
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did not block the cascade from penetrating north. The same program used to detect cascade water outside the thermohaline front (by seeking inversions) was used to detect it in the C T D and seasoar data in the northern sections. The program identified numerous small patches of thickness less than 15 m in sections both within the E A C eddy and to the north. A typical C T D cast in the latter zone would have two such patches between 200 and 350 m, with some being as thick as 50 m. The seasoar data gave a similar result. The frequency of occurrence of the patches north of Cape H o w e did not decrease with distance from the source, but there was no identifiable coherence between patches in successive C T D profiles, either towed or conventional. Two C T D transects cut obliquely through the northwest corner of the E A C eddy (Figs 8 and 9). Evidence of downwelling across the shelf b r e a k is seen in the southerly section (Fig. 8), in which the dashed isotherm sinks down from the shelf to 330 m depth. This isotherm, together with the presence of cascade water as depicted, indicates downwelling beneath the location of the front, as suggested by GODFREY 1980). The two available A D C P profiles on this transect show southward flow at the surface, in agreement with Fig. 7, beneath which the flow is offshore (eastward). The offshore flow supports the interpretation of downwelling in progress.
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Formation and spreading of the Bass Strait cascade 18
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Fig. 8. Temperature section, CTD casts 17-21. Also, two ADCP velocity profiles (note key--arrows refer to map plane, not plane of figure). A dashed line shows the deepest shelf isotherm. The two transects of Figs 8 and 9 cross the shelf b r e a k only 50 km apart, but by the m o r e northern transect (Fig. 9), the downwelling is greatly diminished and the flow at the shelf b r e a k is to the north. The A D C P profile available from well offshore of the b r e a k also shows northward flow. Thus, the northern limit of the "primary downwelling zone" (shaded in Fig. 1) has been defined. A substantial percentage of C T D s 19 and 23 is comprised of cascade water. These layers are indicated by vertical bars in the profiles in Figs 8 and 9. These two stations are just beyond the shelf b r e a k in their respective transects, and in both cases, they are the stations which contain the largest amount of cascade water, The fact that downwelling is nearly absent from Fig. 9 indicates that the water has downwelled to the south of the transect and has therefore penetrated north along the slope against the general circulation of the E A C eddy. The concentration of cascade water in a layer against the slope, as seen in these two transects, was less pronounced in the far-field zone to the north. The density driven flow down the continental slope under the influence of rotation has characteristics that are c o m m o n for a variety of geographical regions. The outflow of the Mediterranean W a t e r at Gibraltar is one of the better known examples. There, the strongly non-linear overflow creates mechanical mixing, forming a new characteristic water mass, as a mixture of Mediterranean and Atlantic waters. Consider an initially linear
394
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Fig. 9. As Fig. 8, except CTD casts 21-25. vertical density stratification and turbulent mixing confined ideally to a vertical depth range Zo + 6z. The density anomaly with respect to ambient water after complete homogenization is positive above z0 and opposite below. As this water mass moves away from the mixing region, it attains quasi-geostrophic equilibrium, resulting in either an anticyclonic vortex or (in conjunction with coriolis force and wall friction) a slope current. A transport estimate would be M = (g'6z2/f). For the Mediterranean outflow, the numbers would be g' = 0.002 m s -2, dz = 400 m , f = 10 -4, i.e. M = 3.2 x 106 m -3 s -1 . The actual outflow past Gibraltar was only about 1 Sv--the increase comes about from the entrainment of Atlantic water. The density contrast between the slope current at Sta. 19 and the ambient water is nearly the same as in the Mediterranean case, but the thickness of the layer is only about 100 m - - h e n c e , the transport is about a fifth of a Sverdrup. Figures 8 and 9 show that the point where the cascade reaches its equilibrium density level and becomes a slope current may be reached very rapidly. The deepest "shelf" isotherm (14.754 °) is at 333 m at CTD cast 19 (Fig. 8); it rises to 266 m at CTD cast 23 (Fig. 9), and by the time the isotherm reaches the northernmost station, CTD 45, it is at 182 m. VELOCITIES IN THE FAR-FIELD A D C P velocities at the offshore end of the northernmost transect showed consistent offshore flow near the surface, with a maximum of about 80 cm s -1 near 30 m, decreasing
Formation and spreading of the Bass Strait cascade
395
N A
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to 25% of that value at 350 m. Within this current (undoubtedly an extension of the E A C ) were e m b e d d e d numerous patches of cascade water. In the single instance where it was possible to compare C T D data with A D C P data at the same location, it was found that the speed of the patch was significantly different from that of the m e a n flow directly above and below. It also m o v e d independently to the area average at the same level. Because of the strong m e a n flow, the sequence of the events shown in Fig. 10 is significant. They begin with C T D 44, followed by C T D 45 about 1 h later. As the latter station was completed, GPS navigation became available, and a series of 20 A D C P profiles were obtained while the ship slowly steamed first west, then east, as the seasoar was readied for deployment. The vectors shown in Fig. 10 are the A D C P velocity "anomalies" at 188 m - - i . e , the area-averaged vector at that depth has been subtracted from each of the 20. O f particular interest are the large velocity anomalies observed in A D C P profiles immediately following C T D 45 (nos. 1, 2 and 3), which will be shown to be associated with a patch of cascade water. The T - S diagrams (Fig. 11) reveal inversions at 148 m and 212 m in C T D 44, and at 142, 170 and 188 m at C T D 45. N o n e of the inversions that fall within the t e m p e r a t u r e range observed in Bass Strait a p p e a r in both stations (separated by 10 k m ) - - i n other words, the horizontal scale of the patches is less than 10 km. The shallower inversions (at 148 and 142 m) are not of interest to this study. The T - S inversions of 170 and 188 m in C T D 45 were chosen for further examination because of the availability of simultaneous A D C P data. The speed [(u 2 + v2)1/2], salinity, and t e m p e r a t u r e profiles are shown in Fig. 12. These two figures show that the small ( < 1 0
396
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Formation and spreading of the Bass Strait cascade
397
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at 20 and 40 min following number 1). This implies that the two inversion layers are probably moving together and should be thought of as a single patch. To see how the speed of the patch compares with the neighboring water mass, the first three profiles are plotted along with a profile which is an areal average of the twenty profiles at each depth (Fig. 13). All three individual profiles showed a higher than average speed at the depth of the patch and an equal or lower than average speed above and below. The overall thickness of the anomalously moving layer is about 50 m. DISCUSSION
Based on observations, we have found that while downwelling mainly occurs in a well-defined zone in northeastern Bass Strait, it also can occur along the entire eastern entrance beneath a thermohaline front. ADCP currents, however, confirm a basic premise of TOMCZAK(1985): that flow along the front is to the north, until downwelling occurs at a primary downwelling zone. Recently downwelled water is characterized not only by a signature profile in the T - S plane, but by frequent (~t inversions. The strength of the front and hence the cascade is subject to large variations at all time scales. A mesoscale eddy of the East Australian Current, a frequent regional feature, did not prohibit the northward penetration of flow along the slope of downwelled water, but it appeared to temporarily interrupt the downwelling current. This might occur, for example, if penetration could only be achieved by a sufficient mass of cascade water. Smaller quantities would be lost by
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Fig. 13. ADCP speed profiles 1, 2 and 3 and the average profile using all twenty profilesin Fig. 10. dilution and entrainment into the eddy. GODFREY et al. (1986) detected large volumes of high salinity water within a mesoscale eddy located off Spencer Gulf (South Australia) and concluded that the salinity was due to entrainment of Spencer Gulf outflow. Slope current transport at a point just downstream of the primary downwelling zone was estimated to be about one-fifth Sverdrup. This assumed a density contrast of 0,2 kg m 3 and a thickness of 100 m. GODFREY et al. (1980) reported numerous mixed layers of this thickness along the slope nearby. Although some of the volume would be comprised of entrained water, we have also shown that not all the downwelled water enters the slope current to the north. Previous estimates refer to the volume leaving Bass Strait, much of which may pass out without feeding the slope current. This would depend on the prevailing air-sea t e m p e r a t u r e difference and the point at which the flow crossed the eastern entrance. Godfrey estimated 0.5 Sv for the volume leaving Bass Strait based on the mean drift of a salinity tongue in Bass Strait, and BAINES et al. (1991), using a series of current meters across the strait, measured a net flux through Bass Strait of 0.49 Sv, but it varied between - 0 . 8 Sv and 3.05 Sv. Our transport estimate was based on a simple analytical model of the dynamics of slope currents. The center of the current, which is assumed to be well mixed, is assumed to be at its equilibrium density level and it is assumed to be penetrating a linearly stratified medium. Therefore, the top half of the current represents a positive density anomaly and the b o t t o m half is a negative anomaly. Integration of the thermal wind equation from either the top or b o t t o m will result in either an eddy, or, where sidewall friction is present, a slope current. In the "far-field", a patch of cascade water had a motion independent from that of the mean flow in which it was e m b e d d e d . We offer two possible explanations for this. The first is that we happen to have observed the patch while it was undergoing a displacement due to
Formation and spreading of the Bass Strait cascade
399
an internal wave. PADMANet al. (1990) showed that under certain conditions, submerged rotating lenses can selectively absorb momentum from vertically propagating internal waves. The second explanation is that the patch may have acquired rotation, either while sinking, in the manner of "meddies" (BORMANSand TURNER,1990), or by the mechanical mixing process described above. Acknowledgements--Thanks are due to Mark Johnston of the Tasmanian Department of Resources and Energy, Rivers and Water Supply Division for the discharge data; Dave Vaudrey, Jeff Dunn, and Lindsay Pender of CSIRO Oceanography, Hobart, for providing the best possible CTD, ADCP and seasoar data sets (respectively) despite instrumental problems; Neil Lawson of Lawson and Treloar Pty. Ltd for the meteorological data from Kingfish B; the Australian Oceanographic Data Centre, for satellite SSTs; Myriam Bormans, for helpful insights; and to the captain and crew of R.V. Franklin. One author (Luick) was supported during this study by a National Research Fellowship grant from the Commonwealth of Australia.
REFERENCES BArnES P. G., G. HUBBERTand S. POWER(1991) Fluid transport through Bass Strait. ContinentalShelfResearch, 11,269-293. BOLAND F. M. (1971) Temperature-salinity anomalies at depths between 200 m and 800 m in the Tasman Sea. Australian Journal o f Marine and Freshwater Research, 22, 55-62. BORMANS M. and J. S. TURNER (1990) The formation of the doubly stable stratification in the Mediterranean Outflow. Deep-Sea Research, 37, 1697-1712. GIBBS C. F., R. A. COWDELLand A. R. COWDELL(1986) Seasonal variations of density patterns in relation to the Bass Strait Cascade. Australian Journal o f Marine and Freshwater Research, 37, 21-25. GODFREYJ. S., I. S. F. JONES, J. G. H. MAXWELLand B. D. SCOTT(1980) On the winter cascade from Bass Strait into the Tasman Sea. Australian Journal of Marine and Freshwater Research, 31,275-286. GODFREY J. S., D. J. VAUDREYand D. HAHN (1986) Observations of the shelf-edge current south of Australia, winter 1982. Journal of Physical Oceanography, 16, 668-679. HUYERA., R. L. SMITH,P. J. STABENO,J. A. CHURCHand N. WHITE (1988) Currents off south-eastern Australia: results from the Australian Coastal Experiment. Australian Journal o f Marine and Freshwater Research, 39, 245-288. JONES I. S. F. (1980) Tidal and wind-driven motions in Bass Strait. Australian Journal o f Marine and Freshwater Research, 31,109-117. NEWELL B. S. (1961) Hydrology of south-east Australian waters: Bass Strait and New South Wales tuna fishing area. CSIRO Australia Div. Fisheries and Oceanography Technical Paper 10. PADMANL., M. LEVINE,T. DILLON, J. MORISON and R. PINKEL (1990) Hydrography and microstructure of an arctic cyclonic eddy. Journal of Geophysical Research, 95,9411-9420. TOMCZAKM. JR. (1983) Water masses in East Australian Current Eddy H. CSIRO Marine Laboratory Report 161. TOMEZAKM. JR. (1985) The Bass Strait Water Cascade during winter 1981. Continental Shelf Research, 4,255278. ZENK W. (1975) On the Mediterranean outflow west of Gibraltar. "Meteor" Forsch.-Ergebnisse A, 16, 23-34.