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Internal Tide Observations from the Australian North West Shelf in Summer 1995

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Internal Tide Observations from the Australian North West Shelf in Summer 1995 1182 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 31 Internal Tide Observations from the Australian North West Shelf in Summer 1995 PETER E. HOLLOWAY AND PAUL G. CHATWIN* School of Geography and Oceanography, University College, University of New South Wales, Australian Defence Force Academy, Canberra, Australia PETER CRAIG CSIRO Marine Research, Hobart, Tasmania, Australia (Manuscript received 7 January 2000, in ®nal form 6 July 2000) ABSTRACT Observations are presented of the internal tide over the continental shelf and slope from a cross section on the Australian North West Shelf. Data collected from moored instruments and repeated pro®le measurements during the summer months of 1995 show an energetic, large amplitude, shoreward propagating semidiurnal internal tide. Multiple generation sites are suggested, coinciding with near-critical bottom slopes. In water less than approximately 200 m deep, the vertical structure of the internal tide is predominantly a ®rst vertical mode, whereas in deeper water over the slope the vertical structure is more complicated with currents and vertical displacements intensi®ed in the lower part of the water column. The internal tide is largely con®ned to a region approximately 100 km wide, from water depths between 70 and 1000 m. Strong generation and dissipation of the internal tide energy is observed over this region and there is evidence that the dissipated energy impacts on the vertical mixing of the density ®eld, particularly near the shelf break and upper continental slope. Even though the diurnal barotropic tidal currents are weak, a diurnal internal tide is observed and appears to be generated over a section of the continental slope that is at the critical slope for the K1 tidal frequency. The M4 harmonic is also observed and this results from nonlinear interactions of the M2 baroclinic tide. 1. Introduction from the Bay of Biscay and suggest wave propagation Internal tides over continental shelf and slope regions along characteristics. In this situation, the signal propa- are characterized by high spatial and temporal variability, gates along a narrow beam with signi®cant vertical phase have phases that are not locked to that of the barotropic propagation and re¯ects off the sea surface and seabed, tide and propagate both shoreward and seaward. The consistent with simple theoretical models of internal wave waves are often energetic and can signi®cantly contribute generation over critical slopes (e.g., Baines 1982). How- to ocean mixing (e.g., Pingree et al. 1986). Most obser- ever, in many instances, and perhaps in regions of less vations have shown internal tides at semidiurnal frequen- steep topography, internal tides are well described in terms cies rather than diurnal. An exception are the observations of vertical modes, usually dominated by the ®rst mode. presented by Leaman (1980) of a diurnal internal tide, at For example, Sherwin (1988) analyzes observations from near-critical bottom slope, off the west Florida continental the Marlin Shelf north of Ireland and Rosenfeld (1990) shelf. The propagation of internal tides, seaward from steep analyzes observations from the shelf off northern Cali- continental slope topography, can be described in terms fornia. In both cases the vertical structure of the internal of energy propagation along internal wave characteristics, tide is dominated by the ®rst vertical mode. While internal that is, along the path of the group velocity vector. For wave ``beams'' can be described as the sum of high-order example, Pingree and New (1989) present observations vertical modes, there seems to be little observational ev- idence of transition from regions dominated by high modes to those dominated by low modes. Although numerous surveys of internal tides near con- * Current af®liation: Fisheries Research Services Marine Labora- tory, Aberdeen, United Kingdom. tinental margins have been reported in the literature [see Huthnance (1989) for a review], most studies have been limited in their spatial and/or temporal coverage. In an Corresponding author address: Dr. Peter Holloway, School of Ge- attempt to help rectify this gap in observational detail and ography and Oceanography, University College, University of New South Wales, Australian Defence Force Academy, Canberra ACT to address some of the questions raised above, a ®eld 2600, Australia. program was run on the Australian North West Shelf E-mail: [email protected] (NWS), obtaining detailed observations of the internal tide q 2001 American Meteorological Society Unauthenticated | Downloaded 09/26/21 12:12 AM UTC MAY 2001 HOLLOWAY ET AL. 1183 TABLE 1. Details of moored instrumentation. Water Sampling Moor- depth Current meter depths Temperature sensor depths interval ing (m) Instrument type (m) (m) (min) C12 750 Aanderaa RCM5/5S current meters 150, 300, 450, 600 150, 300, 450, 600 5 C10 300 Interocean S4 current meters 40, 90, 140, 190, 240, 290 40, 90, 140, 190, 240, 290 5 C6A 125 RDI 150 KHz ADCP every 4 from 22±114 5 C6B 125 Steedman Acoustic current meters 12, 39, 66, 120 12, 39, 66, 120 2 C6C 125 Aanderaa thermistor chain every 10 from 20±120 10 C2 65 Steedman Acoustic current meters 25, 55 25, 55 5 C2 65 Applied Micro Systems Tide gauge 5 over a cross section where energetic internal waves have In this paper, observations are analyzed in order to previously been reported (Holloway 1984, 1985, 1988, de®ne the vertical structure of the internal tide in terms 1994). The aim was to measure the internal tide from of both vertical displacement and horizontal currents at offshore of the generation region and to track the complete M2 (12.42 h), S 2 (12.00 h), K1 (23.93 h), and M 4 (6.21 cycle of evolution of the internal tide as it propagated h) tidal frequencies. Also investigated are the changes shoreward and ®nally dissipated over the outer continental in horizontal structure across the continental slope, and shelf. Measurements were obtained from moored current the energetics of the internal tide. meters, an acoustic Doppler current pro®ler (ADCP), and a thermistor chain as well as from detailed shipborne CTD 2. Measurement program and ADCP pro®le measurements, all made during the sum- mer months (Jan±Mar) of 1995 when the internal tide is Moorings containing current meters, ADCPs, therm- strongest (see Holloway 1988). istor chains, and water level recorders were deployed FIG. 1. Map showing bathymetry, mooring, and CTD/ADCP stations on the Australian North West Shelf. Unauthenticated | Downloaded 09/26/21 12:12 AM UTC 1184 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 31 FIG. 2. A cross section showing moorings and instrumentation. on the NWS in January 1995. A number of instrument tions. Locations are referred to as C1 to C13, where all failures reduced the array to four mooring locations and are CTD stations and some are mooring locations as instruments at depths as indicated in Table 1. Bathym- well. Despite instrument failures, the mooring array pro- etry, CTD, and mooring locations are shown in Fig. 1, vides results ranging from 750-m depth on the slope and Fig. 2 is a cross section showing instrument posi- into 65-m depth on the outer shelf. Moorings provide good vertical coverage, particularly at location C6 where the ADCP gives current measurements every 4 TABLE 2. Details of CTD pro®le measurements. Local times are m from 26 to 114 m in 125-m water depth. The moored used. The asterisks indicate a mooring location in addition to the CTD measurements. instruments collected data for approximately 2 months at sampling rates of 2, 5, or 10 min. (Table 1). Water Distance Repeat A detailed CTD survey was conducted in January Loca- depth from C1 cycle 1995 during the mooring deployment. At each of 13 tion Start time Finish time (m) (km) (min) locations in a section across the shelf and slope (Fig. C1 1300 15 Jan 0100 16 Jan 65 0 30 1), repeated CTD pro®les were measured over a 13-h C2* 0200 16 Jan 1500 16 Jan 65 10.5 30 C3 1600 16 Jan 0500 17 Jan 70 18.8 30 period in order to measure one complete cycle of the C4 2200 17 Jan 1100 18 Jan 86 27.0 30 semidiurnal tide. At most locations pro®les were mea- C5 1230 18 Jan 0130 19 Jan 116 35.0 30 sured every 30 min, but less frequently at deep stations. C6* 0730 17 Jan 1830 17 Jan 124 43.3 30 Simultaneously, the ship's hull-mounted ADCP provid- C7 1830 14 Jan 0730 15 Jan 132 53.3 30 ed current pro®les in 8-m bins from 16 to approximately C8 0330 19 Jan 1230 19 Jan 162 63.5 30 C9 1800 19 Jan 0700 20 Jan 242 76.3 30 230 m depth, or to 14% of depth above the seabed in C10* 0830 20 Jan 2130 20 Jan 302 89.3 60 shallower water, for example, to 86 m in 100 m. Table C11 1930 13 Jan 0830 14 Jan 416 103.8 60 2 lists the times and dates the pro®les were measured, C12* 0000 21 Jan 1300 21 Jan 764 122.8 60 and the repeat frequency of the pro®les. Note that it C13 1500 21 Jan 0430 22 Jan 1382 140.8 90 took 7 days to complete the section from C1 to C13. Unauthenticated | Downloaded 09/26/21 12:12 AM UTC MAY 2001 HOLLOWAY ET AL. 1185 TABLE 3. Barotropic tidal current properties. Ellipse parameters are semimajor (a) and semiminor (b) axis lengths in cm s21, where 1ye (b) indicates anticlockwise and 2ye(b) clockwise rotation of the velocity vector, phase (g) in local time, 8 h ahead of UTC, and orientation (u) in degrees anticlockwise from true north.
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