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  • View in gallery
    Fig. 1.

    (a) Locations of the Rottnest continental shelf, Western Australia measurement sites. The major current systems Leeuwin and Capes Currents are shown schematically as southward and northward flowing, respectively. The black squares denote the mooring sites off Two Rocks and in the Perth Canyon (Fig. 1b). The black triangle denotes the Rottnest Island meteorological station. HF radar shore stations were located at Guilderton and Fremantle. Bathymetry contours are in meters. (b) HF radar (WERA) operational coverage between 10 Mar 2010 and 30 Apr 2012. The color scale indicates the percentage of data recovery. The small black dots denote the HFR grid, and the white squares indicate the mooring sites. The larger black dots in the grid denote the two cross-shore transects where the effective Coriolis frequency (period) was estimated.

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    Fig. 2.

    Normalized wavelet spectra for wind from Rottnest Island meteorological station and currents at WATR200 (~30 m) for 22 Dec 2010 to 22 Dec 2012: (a) u (cross shore) wind component, (b) u (cross shore) current component, (c) υ (alongshore) wind component, (d) υ (alongshore) current component. The thick black line indicates the 95% confidence level, with red noise as the background spectrum. The blue line denotes the diurnal period. Four individual ADCP deployments are presented in (b) and (d). Lighter shadings on either end denote regions where edge effects become important.

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    Fig. 3.

    Rotary energy spectra calculated for the period 22 Dec 2010 to 22 Dec 2012 (wind) and the period 22 Dec 2010 to 15 Apr 2011 (currents): (a),(b) wind data from Rottnest Island meteorological station, (c),(d) currents at 30-m depth at WATR200, and (e),(f) currents at 132-m depth at WATR200. Clockwise components are denoted in (a),(c), and (e) and counterclockwise components are denoted in (b),(d), and (f).

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    Fig. 4.

    The 3-hourly snapshots of the 48-h, high-pass filtered, high-frequency HFR surface currents representing the mean daily cycle in February 2011. A vector corresponding to wind at Rottnest Island for each snapshot is shown in the bottom-left corner of each panel. Time flags represent local time (LT, +8 h UTC).

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    Fig. 5.

    Diurnal wind ellipse (black) and surface current ellipses (gray) calculated from the February 2011 data. Wind and current vectors at local time midnight are shown within each ellipse. For clarity, current ellipses denote every third HFR point that had at least two-thirds of temporal coverage in February 2011. All ellipses rotated counterclockwise.

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    Fig. 6.

    High-frequency currents (m s−1) between 1 Feb and 28 Feb 2011: WATR200 (a) cross-shore u component and (b) alongshore υ component; WATR500 (c) cross-shore u component and (d) alongshore υ component. Note that the panels have different depth scales.

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    Fig. 7.

    (a) Counterclockwise (gray) and clockwise (black) amplitudes of diurnal winds from the Rottnest Island meteorological station for 1 Feb to 19 Mar 2011; (b) counterclockwise amplitudes of HFR-measured diurnal surface currents at WATR200 (thick black line) and ADCP-measured diurnal currents at WATR200 (dashed line: 30-m depth; thin black line: 132-m depth) for 1 Feb to 19 Mar 2011; and (c) counterclockwise diurnal phases for the Rottnest wind data (gray line), HFR-measured surface currents at WATR200 (thick black line), and ADCP-measured currents at WATR200 (dashed line: 30-m depth; thin black line: 132-m depth). As the HFR and ADCP currents were mostly counterclockwise, only counterclockwise amplitudes and phases [(b) and (c), respectively] are shown; (d) the difference in phases between counterclockwise diurnal winds at Rottnest and counterclockwise diurnal HFR-measured currents (thick black line), and ADCP-measured currents at WATR200 (dashed line: 30-m depth; thin black line: 132-m depth).

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    Fig. 8.

    Sea surface temperature (color), altimetry (m; gray contours), daily averaged HFR surface currents (black arrows), and daily averaged winds from the Rottnest Island meteorological station (red arrows) for 4 days during the February 2011 Leeuwin Current marine heat wave. For clarity, only every second surface current vector is shown. The temperature scale is the same for all panels.

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    Fig. 9.

    February 2011. (a) Counterclockwise (red) and clockwise (blue) amplitudes of the diurnal winds for Rottnest Island; (b) counterclockwise (red) and clockwise (blue) amplitudes of the diurnal HFR surface currents at WACA200, WATR500 and WATR200; and (c) effective Coriolis period at WATR200 (blue line) and the corresponding planetary Coriolis period (red line). The black line denotes the diurnal period representing the wind forcing. (d) Counterclockwise amplitudes (m s−1) of the diurnal ADCP currents at WATR200. The white line denotes the mixed layer depth. (e) Vertical shear squared (s−2) for low-passed ADCP currents at WATR200. The white line denotes the mixed layer depth. HFR measurements were used to calculate the shear between the surface and 30-m depth when available. (f) Brunt–Väisälä frequency squared (s−2) at WATR200. The black line denotes the mixed layer depth. Temperature profile (°C) at (g) WATR100, (h) WATR150, and (i) WATR200. In (g),(h), and (i), the black line denotes the 23.3°C isotherm.

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    Fig. 10.

    The effective Coriolis period (h) estimated along the Two Rocks transect (upper panel) and the Perth Canyon transect (lower panel) between January and April 2011. The black contour lines denotes the 24-h period. See Fig. 1b for the transect positions. The mooring locations are also indicated.

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    Fig. 11.

    February 2011. (a) the effective Coriolis period at WATR200 (blue) and the corresponding planetary Coriolis period (red). The black line indicates the diurnal period representing the wind forcing. (b) The rate of work done by the local wind (surface energy flux, W m–2) at WATR200 calculated using the Rottnest Island wind data and the HFR surface currents data; (c) as in (b) but low pass filtered (“mean”); (d) as in (b) but high pass filtered. (e) Kinetic energy per unit volume (J m–3) at WATR200 estimated from the HFR surface currents data, and (f) vertical profile of kinetic energy per unit volume (J m–3) at WATR200 calculated from the ADCP data. The black line in (f) denotes the mixed layer depth. The periods when the effective Coriolis period was close to the diurnal period are shaded (see Figure 13c).

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    Fig. 12.

    Sea surface temperature (color), altimetry (m; gray contours), daily averaged HFR surface currents (black arrows), and daily averaged winds from the Rottnest Island meteorological station (red arrows) for 4 days in February and March 2012. For clarity, only every second surface vector is shown. The temperature scale is the same for all panels.

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    Fig. 13.

    February–March 2012. (a) Counterclockwise (red) and clockwise (blue) amplitudes of the diurnal winds for Rottnest Island; (b) counterclockwise (red) and clockwise (blue) amplitudes of the diurnal HFR surface currents at WACA200, WATR500, and WATR200; and (c) effective Coriolis period at WATR200 (blue) and the corresponding planetary Coriolis period (red). The black line denotes the diurnal period representing the wind forcing. (d) High-frequency cross-shore u component at WATR200; (e) high-frequency alongshore υ component at WATR200. The black line in (d) and (e) denotes the mixed layer depth. Temperature profile (°C) at (f) WATR100, (g) WATR150, (h) WATR200, and (i) WACA200. In (f),(g),(h), and (i), the black line denotes the 22°C isotherm.

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    Fig. 14.

    The effective Coriolis period (h) estimated along the Two Rocks transect (upper panel) and the Perth Canyon transect (lower panel) between January and April 2012. The black contour lines denotes the 24-h period. See Fig. 1b for the transect positions. The mooring locations are also indicated.

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    Fig. 15.

    As in Fig. 11 but for February–March 2012.

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Diurnal Sea Breezes Force Near-Inertial Waves along Rottnest Continental Shelf, Southwestern Australia

Hrvoje MihanovićSchool of Civil, Environmental and Mining Engineering, and The University of Western Australia Oceans Institute, The University of Western Australia, Crawley, Western Australia, Australia, and Institute of Oceanography and Fisheries, Split, Croatia

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Charitha PattiaratchiSchool of Civil, Environmental and Mining Engineering, and The University of Western Australia Oceans Institute, The University of Western Australia, Crawley, Western Australia, Australia

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Florence VerspechtSchool of Civil, Environmental and Mining Engineering, and The University of Western Australia Oceans Institute, The University of Western Australia, Crawley, Western Australia, Australia

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Abstract

Observations of upper-ocean dynamics close to the critical latitude (ratio of the local inertial to diurnal frequency is 0.94) from a range of platforms (surface currents using HF radar, moored instruments, and satellite remote sensing data) off southwest Australia indicated the presence of energetic, near-inertial waves generated through the diurnal–inertial resonance. During the austral summer, when southerly winds and land–sea breeze (LSB) system dominated the wind regime, strong counterclockwise diurnal motions (amplitudes surpassing 0.3 m s−1) penetrated to 300-m depth with diurnal vertical isotherm fluctuations up to 60 m. The upward phase propagation speed of ~140 m day−1, deep penetration of diurnal currents below the mixed layer, and the ~180° phase difference between the upper and lower water column suggested that the local LSB system caused the resonant diurnal motions. Relative vorticity fluctuations along two cross-shore transects indicated changes to the local effective Coriolis frequency by more than 50% (±0.5f). In the presence of strong and relatively consistent cross-shore diurnal wind forcing in the study area, the main factors that controlled the observed energetic but sporadic near-inertial oscillations were the Leeuwin Current strength and spatial–temporal variations. These variations controlled the effective Coriolis frequency and enabled the effective pumping of diurnal wind energy into the ocean particularly when the effective Coriolis frequency was ~24 h.

Denotes Open Access content.

Corresponding author address: C. Pattiaratchi, The University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia. E-mail: chari.pattiaratchi@uwa.edu.au

Abstract

Observations of upper-ocean dynamics close to the critical latitude (ratio of the local inertial to diurnal frequency is 0.94) from a range of platforms (surface currents using HF radar, moored instruments, and satellite remote sensing data) off southwest Australia indicated the presence of energetic, near-inertial waves generated through the diurnal–inertial resonance. During the austral summer, when southerly winds and land–sea breeze (LSB) system dominated the wind regime, strong counterclockwise diurnal motions (amplitudes surpassing 0.3 m s−1) penetrated to 300-m depth with diurnal vertical isotherm fluctuations up to 60 m. The upward phase propagation speed of ~140 m day−1, deep penetration of diurnal currents below the mixed layer, and the ~180° phase difference between the upper and lower water column suggested that the local LSB system caused the resonant diurnal motions. Relative vorticity fluctuations along two cross-shore transects indicated changes to the local effective Coriolis frequency by more than 50% (±0.5f). In the presence of strong and relatively consistent cross-shore diurnal wind forcing in the study area, the main factors that controlled the observed energetic but sporadic near-inertial oscillations were the Leeuwin Current strength and spatial–temporal variations. These variations controlled the effective Coriolis frequency and enabled the effective pumping of diurnal wind energy into the ocean particularly when the effective Coriolis frequency was ~24 h.

Denotes Open Access content.

Corresponding author address: C. Pattiaratchi, The University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia. E-mail: chari.pattiaratchi@uwa.edu.au

1. Introduction

Interaction between the atmosphere and ocean is of fundamental importance for understanding the dynamics and mixing in the upper ocean. In combination with the Coriolis force, surface currents generated by wind stress have a maximum response at the local, inertial frequency (Ekman 1905; Simpson et al. 2002; Kim and Crawford 2014). A resonance condition occurs when the period of wind forcing is close to the local, inertial frequency and occurs in coastal regions close to the critical latitude (30°N or 30°S ± 10°). These resonant wind current responses close to the critical latitude have been addressed in a variety of field and theoretical studies and have been shown to enhance the upper-ocean velocity field and vertical mixing (e.g., Simpson et al. 2002; Hyder et al. 2002, 2011; Zhang et al. 2009; Kim and Crawford 2014). In this paper, we present data obtained from a range of platforms: surface currents using HF radar (HFR), from moored instruments (bottom-mounted ADCP and thermistor strings), and remotely sensed sea surface temperature data from the Rottnest continental shelf, southwest Australia, to examine the influence of the strong land–sea breeze (LSB) cycle on the upper-ocean dynamics through the diurnal–inertial resonance in a region close to the critical latitude, where the local inertial period is 22.6 h.

In microtidal coastal regions, where tidal forcing is negligible, vertical mixing is strongly influenced by wind events such as extratropical storms (e.g., Ferré et al. 2005; Verspecht and Pattiaratchi 2010; Hopkins et al. 2014), tropical cyclones/hurricanes (e.g., Jacob et al. 2000; Zhang et al. 2014), and strong sea breezes (e.g., Pattiaratchi et al. 1997; Gallop et al. 2012). In deep waters off the continental shelf, vertical mixing contributes to the transport of nutrients across the pycnocline to the primary producers at the surface layer (Sharples and Tett 1994; Sharples et al. 2001; Burchard and Rippeth 2009). In strongly stratified systems, the pycnocline prevents diffusion of nutrients, oxygen, and phytoplankton, and in the absence of vertical mixing, this can cause the detrimental depletion of nutrients in the surface boundary layer (e.g., Sharples et al. 2001). In oligotrophic environments, processes that are able to transport nutrients from below the pycnocline to the upper ocean are important for the local primary production.

Wind-induced resonance occurs when either the broadbanded, open-ocean winds excite the near-inertial frequency more effectively than other frequencies or when the main wind forcing is at a single frequency, which causes the ocean to respond at the same frequency. The latter situation is proposed to occur off southwest Australia, where the diurnal LSB forcing is close to the local inertial frequency. Resonance of this kind has also been observed at the critical latitude off Namibia at 28.6°S (Simpson et al. 2002); the Thermaikos Gulf, northwest Aegean Sea (Hyder et al. 2002); the Texas–Louisiana shelf (Chen et al. 1996; DiMarco et al. 2000; Zhang et al. 2009, 2010; Gough et al. 2016); and at other critical latitudes around the world (Rosenfeld 1988; Lerczak et al. 2001; Hunter et al. 2007; Edwards 2008). Kim and Crawford (2014) used HFR-derived surface current data obtained along the west coast of the United States to assess the relation between LSB forced currents and latitudes between 32° and 47°N and concluded that the broadbanded, wind-driven, near-inertial currents at locations closer to critical latitude were 8 to 12 times stronger than at locations farther away from the critical latitude.

Alongshore coastal wind forcing at a frequency slightly lower than the inertial frequency (the “subinertial” case) causes strong pycnocline oscillations dominated by coastal-trapped waves (Clancy et al. 1979; Wolanski 1986; Rosenfeld 1988; Cushman-Roisin 1994). In contrast, for the superinertial case (forcing frequency is higher than the inertial frequency) waves mostly radiate away from the coast during the periodic coastal upwelling, and the cross-shore wind produces more pycnocline oscillations than the alongshore wind (Orlić and Pasarić 2011). Inertial currents oscillating in an unbounded ocean display no vertical variability, but when a coastal boundary is introduced, the zero-flux condition at the boundary changes the behavior of the inertial currents, resulting in a phase shift between the surface and bottom layers (Chen et al. 1996). The zero-flux condition induces an opposing cross-shelf sea surface slope at the coast, which acts as a pressure gradient through the water column. The wind forcing acts only at the surface, which causes a cross-shelf flow with a 180° phase shift at the interface between the upper mixed layer and the lower stratified layer to drive a reverse flow in the lower stratified layer (Craig 1989; Simpson et al. 2002). The different phases in the surface and bottom layer motion produce vertical shear, which enhances vertical water column mixing and turbulent dissipation (Simpson et al. 2002).

Low-frequency background currents can also change the local relative vorticity and local effective Coriolis frequency (Kunze 1985; Lerczak et al. 2001). Kunze (1985) derived a dispersion relation for near-inertial internal waves propagating in geostrophic current shear and concluded that local relative vorticity ζ shifted the internal wave band’s lower bound from the planetary Coriolis frequency to the effective Coriolis frequency: feff = f + ζ/2. For the Southern Hemisphere, the effective frequency is shifted below (above) the Coriolis frequency in regions where low-frequency motions have positive (negative) vorticity. Counterclockwise (anticyclonic) eddies in the Southern Hemisphere have positive vorticity and therefore decrease the local Coriolis frequency, whereas clockwise (cyclonic) eddies increase the local Coriolis frequency.

This paper uses data from different platforms (HF radar, moored instruments, and satellite remote sensing) to examine the generation and influence of near-inertial oscillations along the Rottnest continental shelf, southwest Australia. In particular the following are examined: 1) the forcing, response, and the extent of the diurnal variability of the winds and the surface current field; 2) the vertical structure and horizontal extent of the rotatory motion; 3) the influence of the wind forcing and horizontal current shear on near-inertial motions and isotherm fluctuations in the vertical; and 4) the relationship between the wind forcing and the local effective Coriolis frequency in the presence of low-frequency motion.

The paper is arranged as follows: Section 2 describes the study area’s oceanographic and meteorological characteristics. Data collection and analysis techniques are outlined in section 3, with the results and discussion presented in section 4. The conclusions and implications of the results are presented in section 5.

2. Study site

The study site is the Rottnest continental shelf, southwest Australia (Fig. 1a). This microtidal region has a mean spring tidal range of ~0.6 m and diurnal tides (Pattiaratchi and Eliot 2009). Continental shelf processes, which are mainly wind driven, include mixing, circulation, and particulate resuspension dominated by LSB activity, particularly during the summer months (Pattiaratchi et al. 1997; Zaker et al. 2007; Verspecht and Pattiaratchi 2010; Gallop et al. 2012).

Fig. 1.
Fig. 1.

(a) Locations of the Rottnest continental shelf, Western Australia measurement sites. The major current systems Leeuwin and Capes Currents are shown schematically as southward and northward flowing, respectively. The black squares denote the mooring sites off Two Rocks and in the Perth Canyon (Fig. 1b). The black triangle denotes the Rottnest Island meteorological station. HF radar shore stations were located at Guilderton and Fremantle. Bathymetry contours are in meters. (b) HF radar (WERA) operational coverage between 10 Mar 2010 and 30 Apr 2012. The color scale indicates the percentage of data recovery. The small black dots denote the HFR grid, and the white squares indicate the mooring sites. The larger black dots in the grid denote the two cross-shore transects where the effective Coriolis frequency (period) was estimated.

Citation: Journal of Physical Oceanography 46, 11; 10.1175/JPO-D-16-0022.1

Local sea breezes, superimposed upon synoptic southerly winds, with speeds often >15 m s−1, occur almost daily during the austral spring, summer, and early autumn (October–April). The unusually strong alongshore sea breeze results from the interaction of the local sea breeze generated due to the land–sea temperature gradient and the synoptic pressure system (Pattiaratchi et al. 1997).

The Leeuwin Current system, which includes the Leeuwin Current (LC), the Leeuwin Undercurrent, and the Capes Current on the shelf (Pattiaratchi and Woo 2009), controls the circulation along the Australian west coast (Fig. 1a). The LC is a warm, low-salinity eastern boundary current, which flows poleward along the continental shelf edge. This is an anomalous direction for an eastern boundary current; although the equatorward winds provide conditions conducive for coastal upwelling, downwelling conditions are a feature of the region (Smith et al. 1991). The LC transports tropical waters southward from northern Australia and flows strongest (5–6 Sv) during the austral autumn/winter (April–September) when the opposing southerlies and sea breezes are weakest (Smith et al. 1991; Pattiaratchi and Woo 2009). The Leeuwin Current system also responds to the El Niño and La Niña cycles with the current being stronger (weaker) during La Niña (El Niño) events (Pattiaratchi and Buchan 1991). Of particular interest to this study, the region experienced a marine heat wave in February and March 2011, which was associated with the warming related to the La Niña event defined as Ningaloo Niña (Feng et al. 2013). This event increased the Leeuwin Current’s volume transport in February—an unusual event at this time of the year that resulted in unprecedented warm sea surface temperature anomalies (~5°C higher than normal) off Australia’s west coast, with widespread coral bleaching and fish kills (Feng et al. 2013; Doi et al. 2013; Kataoka et al. 2014).

The Capes Current (Fig. 1a) flows northward along the continental shelf, inshore of the Leeuwin Current (Pearce and Pattiaratchi 1999). The current originates with wind-driven upwelling of cooler, saltier waters around 34°S, which flows northward particularly during the summer (Gersbach et al. 1999). This water flows along the inner continental shelf and broadens as it moves north from 34°S. The Capes Current is strongest in spring and summer when sea breezes and southerly winds prevail. From November to March, the northward wind stresses push the Leeuwin Current offshore and reinforce the Capes Current (Pearce and Pattiaratchi 1999).

The Perth Canyon drops from the 200-m isobath, west of Rottnest Island, to ~4000 m within ~50 km (Fig. 1a). Rennie et al. (2006), using thermistor strings, observed the impact of sea breeze on the thermal structure in the Perth Canyon and observed a periodic variation at the diurnal frequency, which manifested in the upper 200 m of the water column. They also identified a phase relationship between the wind and ocean temperature during sea-breeze conditions. The sea breeze is highlighted in their study as a source of the strongest diurnal periodicities and isotherm displacements of up to 80 m over a 24-h period that could not be accounted for by the tide.

The 1 yr of current measurements from an ADCP mooring within our study area was used by Cresswell (2009) to highlight the current reversal and variability due to wind forcing and transport off the shelf. The ADCP data revealed periods of several days when cross-shelf flow dominated. Because of the coastal boundary effect, the surface and bottom layers were often out of phase. Cresswell (2009) used progressive vector diagrams to show the generation of current oscillations as loops and cusps and found that the LSB cycle produced baroclinic inertial waves.

3. Data collection and analysis

a. Data

Data presented here were collected as part of Australia’s Integrated Marine Observing System (IMOS) and include surface currents from HFR, vertical current profiles, and thermal structure from moored instruments and sea surface temperature from satellite remote sensing augmented by meteorological data. The HFR surface current data from February 2011 and February 2012 were used to examine the horizontal extent of the surface high-frequency current patterns. ADCP currents measured at 200 and 500 m (WATR200 and WATR500, respectively) during the 2010/11 austral summer were analyzed in detail, with a focus on the high-frequency variability observed in February and March 2011 (Table 1); this period was compared with a period during the 2011/12 austral summer (February–March 2012; Table 2), which also showed strong diurnal oscillations.

Table 1.

Mooring deployment details for austral summer 2010/11.

Table 1.
Table 2.

Mooring deployment details for austral summer 2011/12.

Table 2.

1) Meteorological data

Meteorological data were collected at the Rottnest Island meteorological station (32.0069°S, 115.5022°E), located 10–30 km from the Perth Canyon sites and 50–60 km from the mooring locations along Two Rocks (Fig. 1a). Wind speed and direction were recorded 10 m above the ground every 30 min, with short (almost insignificant) gaps. These data were interpolated and resampled into 1-h intervals to correspond with the radar data and other analyses. The coordinate system was rotated 25° counterclockwise to align with the coast to provide cross-shore and alongshore winds. Wind vectors are presented in subsequent figures in the direction the wind is blowing to but referred to in the text using meteorological convention.

2) Moored time series data

The moorings were located ~40–80 km offshore, 200 km south of the critical latitude for diurnal–inertial resonance at 30°S (Fig. 1). The positions of the four northern moorings on the continental shelf off Two Rocks (Fig. 1; Tables 1 and 2) were in water depths of 100, 150, 200, and 500 m, respectively (WATR100–WATR500) and a southern mooring in the Perth Canyon was in water depth of 200 m (WACA200). Data from nearshore (~50-m water depth) moorings WATR050 and NRSROT were not used here; however, the mooring locations are shown in all relevant figures for reference.

Bottom-mounted, upward-looking acoustic Doppler current profilers (ADCP) were moored on the continental shelf off Two Rocks in 200- and 500-m water depths (WATR200 and WATR500, respectively, during the 2010/11 austral summer and at WATR200 in 200-m water depth during the 2011/12 austral summer; Fig. 1a). Mooring configurations, ADCP instrument type, and sampling configurations are presented in Tables 1 and 2. The coordinate system was rotated 25° counterclockwise so that u and υ represent the cross-shore and alongshore components, respectively.

The thermistors (Seabird Electronics SBE 39, SBE 39plus, and RBR XR420) were fixed onto the mooring lines at the Perth Canyon site and at the four sites on the continental shelf off Two Rocks (Fig. 1; see also Tables 1 and 2). Changes in depths with time due to mooring inclinations were taken into account using the pressure sensors associated with the thermistors.

3) HF radar data

Australian Coastal Ocean Radar Network (ACORN) operates surface current mapping HF radar systems as part of the Australian IMOS. This facility provides hourly, time-resolved coastal ocean surface current maps using a high-frequency, phased array wave radar—the Wellen Radar (WERA)—which is shore based and provides a reliable dataset of surface currents (Gurgel et al. 1999). The Rottnest shelf WERA HF radar system transmits at a frequency of 8.5125 MHz with a bandwidth of 33 kHz. High-frequency radar systems were first used to measure surface currents more than three decades ago (Barrick et al. 1977; Hammond et al. 1987; Shay et al. 2007) and are now widely used around the world with a high level of accuracy (Paduan and Washburn 2013).

Two WERA HFR shore stations were located at Guilderton and Leighton Beach in Fremantle (Fig. 1a) and the radar coverage area overlapped the mooring sites (Fig. 1b). The WERA instrument system had a maximum offshore range of ~180 km. The temporal resolution for the HFR measurements was 1 h, and the surface current vectors were derived over a regular grid with a horizontal resolution of 4 × 4 km. WERA HFR data were reprocessed using ACORN-developed software, which included an improved analysis algorithm to step from raw spectra to surface current components and provide quality control flags on the data points (Heron and Prytz 2011).

b. Analysis techniques

The Rottnest Island wind vector W time series for February 2011 and February and March 2012 were used to calculate the wind stress vector τ. The wind stress was calculated using the quadratic law τ = ρaCD|W|W, with air density ρa = 1.25 kg m−3 and the drag coefficient CD calculated according to Anderson (1993). The unfiltered wind vectors W and current vectors u were used to estimate the rate of work done by the local wind at WATR200, (surface energy flux, W m−2), and KE per unit volume KE = (1/2)ρ0|u|2 (J m−3), where the density ρ0 was set to 1023.25 kg m−3, which represented the average density at 30-m depth at WATR200 for the study period.

Each vector time series (winds and currents) was subjected to a 48-h, Butterworth filter (half-power point = 48 h) to separate the time series into high- and low-frequency components. This allowed for the examination of the high-frequency signal close to the diurnal band that is addressed in this paper.

Rotary energy spectra were calculated for the Rottnest Island wind data (22 December 2010–22 December 2012) following Gonella (1972) and using the fast Fourier transform (FFT) method and a 150-day, half-overlapping Hamming window. Similarly, rotary spectra were calculated for the ADCP currents (22 December 2010–15 April 2011) with a 19-day overlapping Hamming window.

Wavelet analysis was used for the wind data from the Rottnest Island meteorological station and all available current data (30-m depth) at the WATR200 ADCP station between 22 December 2010 and 22 December 2012 (four ADCP deployments). Wavelet analysis examined spectra moving in time, rather than one spectrum for the whole dataset, which revealed the seasonal changes in the dominant frequencies. The analyses were based on wavelet methods given in Torrence and Compo (1998) and Grinsted et al. (2004), with wavelet bias corrections from Liu et al. (2007) and Veleda et al. (2012).

The WERA HFR-derived surface current vectors with a large geometrical dilution of precision (GDOP) caused by poor intersecting beam geometry were excluded from the analysis (Chapman et al. 1997; Shay et al. 2007). The resulting HFR data were examined for spikes, with anomalous values removed from the dataset. The data were then spatially interpolated with the condition that the missing grid point within the area had to have a small GDOP and at least three contemporaneous hourly measurements within the 7-km radius. Short, nonoperational periods (maximum of 3 h) were also interpolated in time. The coordinates were then rotated 25° counterclockwise so that u and υ represent cross-shore and alongshore surface components, respectively. The percentage of HFR data operational coverage over the Perth Canyon and Two Rocks mooring sites was ~99% (Fig. 1b) from 10 March 2010 to 30 April 2012, based on 14 691 operational temporal points: that is, temporal points with at least one gridpoint measurement (out of an overall 18 784 hourly points available during the analyzed period; the radars were not operational through the entire period).

HFR surface currents were presented as a temporal evolution of surface currents at positions corresponding to the three mooring sites (WACA200, WATR200, and WATR500; Fig. 1b) in the 2010/11 and 2011/12 austral summers. The HFR surface current data were also used to estimate the surface KE per unit volume and the rate of work done by the local wind at WATR200 plus the local effective Coriolis frequency (period) along two transects corresponding to mooring sites (see Fig. 1b).

Complex demodulation (Emery and Thomson 1998) was used to obtain the clockwise and counterclockwise phases and amplitudes of the Rottnest Island wind data, ADCP current measurements, and the HFR currents data. We used 15–35-h, bandpassed filtered winds and currents (Nam and Send 2013) to extract the phases and amplitudes of the 24-h harmonics. Two diurnal periods of 48 h were overlapped and slid in time with 6-h time steps (Emery and Thomson 1998; Jarosz et al. 2007).

Inertia–gravity waves can be generated when the forcing frequency is larger than the lowest frequency allowed. If the vertical shear is negligible the effective Coriolis frequency is the lowest frequency allowed (Federiuk and Allen 1996). However, if vertical shear is large the lowest frequency is (Federiuk and Allen 1996)
e1
Therefore, vertical shear and stratification can decrease the lowest allowed frequency (increase the longest allowed period). The vertical shear and buoyancy frequency were estimated using HFR and ADCP data and the thermistors.

4. Results and discussion

a. Wind forcing

The diurnal LSB system off the Western Australian coast is among the globally strongest (frequently > 10 m s−1 with maxima reaching 20 m s−1) and most consistent, resulting from a combination of forcing through the synoptic system and the diurnal heating–cooling cycle (Pattiaratchi et al. 1997; Masselink and Pattiaratchi 2001).

Wavelet analysis of 2 yr of measured wind data (22 December 2010–22 December 2012) from the Rottnest Island meteorological station revealed energy, with a period of 1 day, was present in austral spring, summer, and early autumn but absent in winter (Figs. 2a,c). Energy in the weather band (periods 5–20 days) was present during winter. The energy at the diurnal frequency was higher in the u (cross shore) component than in the υ component due to cross-shore nature of the LSB in the region. Similarly the ADCP currents at 30-m water depth at WATR200 (Fig. 1b) indicated higher energy in the diurnal band during the summer months with energy at the diurnal frequency being higher in the u (cross shore) component than in the υ component (Figs. 2b,d).

Fig. 2.
Fig. 2.

Normalized wavelet spectra for wind from Rottnest Island meteorological station and currents at WATR200 (~30 m) for 22 Dec 2010 to 22 Dec 2012: (a) u (cross shore) wind component, (b) u (cross shore) current component, (c) υ (alongshore) wind component, (d) υ (alongshore) current component. The thick black line indicates the 95% confidence level, with red noise as the background spectrum. The blue line denotes the diurnal period. Four individual ADCP deployments are presented in (b) and (d). Lighter shadings on either end denote regions where edge effects become important.

Citation: Journal of Physical Oceanography 46, 11; 10.1175/JPO-D-16-0022.1

The LSB cycle indicated counterclockwise rotation (Figs. 3, 4) with the offshore extent exceeding 150 km as revealed by the surface current response that extended to the limit of the HFR coverage (Figs. 4, 5). This compares with the offshore extent of sea breezes to 250 km reported along the east and west coasts of the United States by Edwards (2008) and Lerczak et al. (2001), respectively.

Fig. 3.
Fig. 3.

Rotary energy spectra calculated for the period 22 Dec 2010 to 22 Dec 2012 (wind) and the period 22 Dec 2010 to 15 Apr 2011 (currents): (a),(b) wind data from Rottnest Island meteorological station, (c),(d) currents at 30-m depth at WATR200, and (e),(f) currents at 132-m depth at WATR200. Clockwise components are denoted in (a),(c), and (e) and counterclockwise components are denoted in (b),(d), and (f).

Citation: Journal of Physical Oceanography 46, 11; 10.1175/JPO-D-16-0022.1

Fig. 4.
Fig. 4.

The 3-hourly snapshots of the 48-h, high-pass filtered, high-frequency HFR surface currents representing the mean daily cycle in February 2011. A vector corresponding to wind at Rottnest Island for each snapshot is shown in the bottom-left corner of each panel. Time flags represent local time (LT, +8 h UTC).

Citation: Journal of Physical Oceanography 46, 11; 10.1175/JPO-D-16-0022.1

Fig. 5.
Fig. 5.

Diurnal wind ellipse (black) and surface current ellipses (gray) calculated from the February 2011 data. Wind and current vectors at local time midnight are shown within each ellipse. For clarity, current ellipses denote every third HFR point that had at least two-thirds of temporal coverage in February 2011. All ellipses rotated counterclockwise.

Citation: Journal of Physical Oceanography 46, 11; 10.1175/JPO-D-16-0022.1

b. HF radar surface currents

To examine the mean response of the coastal surface ocean to the LSB cycle, the 48-h, high-pass filtered HFR surface currents for the entire month of February 2011 were used to construct mean values over a diurnal cycle: that is, the mean values at a particular time stamp. For example, 0130 local time (LT) is the mean value recorded at that time stamp for the whole month of February 2011 (Fig. 4a). Individual mean hourly snapshots contained points that had at least 10 out of 28 measurements representing that particular hour (Fig. 4).

The mean daily wind cycle starts at 0130 LT with a weak land breeze, that is, northeasterly winds (offshore) that reach a maximum speed of 3.4 m s−1 at 0730 LT (Fig. 4c). The land breeze decreases in strength by 1030 LT (Fig. 4d) and then switches direction to the sea breeze by 1330 LT (Fig. 4e). The sea breeze (from the southwest) then increases in speed with a maximum speed 5.4 m s−1 at 1630 LT (Fig. 4f) and then decreases by the end of the cycle (Fig. 4h). Note that the wind speeds were means of high-passed winds over 1 month with the instantaneous wind speeds being much higher for a particular day. The mean daily cycle of high-frequency surface currents reveals an almost instantaneous response of the surface currents to changes in the wind direction and a strong counterclockwise rotation that corresponds to the diurnal rotation of the wind vector. There is a clear offshore intensification of the surface currents, with speeds > 0.3 m s−1 in the western part of the radar domain (Fig. 4).

The diurnal ellipse characteristics of the Rottnest Island winds in February 2011 indicated a rectilinear ellipse with counterclockwise rotation with the major axis tilted 35° counterclockwise from the east (Fig. 5), reflecting the direction of the LSB system. The diurnal surface current ellipses for the same month indicated circular motion in the deeper water above the Perth Canyon (Fig. 1a), while on the continental shelf region in water depths < 50 m, the circulation was less circular (Fig. 5). The surface currents rotated counterclockwise across the whole region. The current amplitudes were up to a factor of 5 larger in the deeper offshore regions compared to those on the continental shelf (Fig. 5). Numerical model simulations by Davies and Xing (2005) indicated that the near-surface response to wind forcing is independent of the magnitude of forcing but rather increases with distance offshore. Similarly, Kaplan et al. (2005) reported, through the analysis of HFR-derived surface currents, that in Bodega Bay (northern California) the near-inertial variance was at maximum offshore, with significant decrease in near-inertial energy over the inner shelf region. The more polarized, decreased surface current magnitudes in the inner shelf region are a feature of the interaction of inertial motions with the coastal boundary (Kaplan et al. 2005).

c. Two Rocks ADCP currents

The rotary energy spectra of the currents at WATR200 indicated a narrow diurnal peak in the kinetic energy (KE) density of the positive, counterclockwise, rotary component at 30 and 132 m below the surface (Figs. 3d,f) and a negligible peak in the negative, clockwise component (Figs. 3c,e). These results indicated that the diurnal currents were enhanced at the surface and could be due to direct wind forcing. Another mechanism that may enhance the surface currents is subharmonic parametric instability (SHPI) in the tides, where the instability grows if the energy level of the internal waves is sufficiently strong (Simmons 2008). Here, when the internal tide crosses the turning latitude (for frequencies lower than the M2, the turning latitudes are located at ±28.8°) of its subharmonic, instabilities will develop where energy is transmitted to a subharmonic with high-mode vertical structure and to super harmonics (Simmons 2008). Off the coast of Namibia (~30°S) both wind forcing (Simpson et al. 2002) and the SHPI mechanism (Simmons 2008) indicated a strong maximum. The results of Simmons (2008; Fig. 3) indicate an absence of SHPI energy along the east coast of Australia, most likely due to the dominance of diurnal tides in this region (Pattiaratchi and Eliot 2009). Thus, the diurnal currents in the study region results were enhanced at the surface due to direct wind forcing.

Vertical profiles of the high-frequency currents from the shelf mooring (WATR200; Figs. 6a,b) and deep mooring (WATR500; Figs. 6c,d) at Two Rocks in February 2011 revealed a diagonal banding in the upper 200–300 m. At each depth level, a daily change in direction (alternating between positive and negative) was observed in both cross-shore u and alongshore υ components (Fig. 6). The banding strength in the time series varied; however, the high-frequency current amplitudes were >0.3 m s−1 at both sites (at 200- and 500-m water depths).

Fig. 6.
Fig. 6.

High-frequency currents (m s−1) between 1 Feb and 28 Feb 2011: WATR200 (a) cross-shore u component and (b) alongshore υ component; WATR500 (c) cross-shore u component and (d) alongshore υ component. Note that the panels have different depth scales.

Citation: Journal of Physical Oceanography 46, 11; 10.1175/JPO-D-16-0022.1

A counterclockwise rotation in the upper 200–300 m, where the u component leads the υ component by π/2 (or 90°), can be seen in Fig. 6. A counterclockwise rotation in the Southern Hemisphere (with the u component leading υ; Simpson et al. 2002) indicates upward phase propagation (Lerczak et al. 2001; Nam and Send 2013). This banding feature, which is caused by obliquely propagating waves in a continuously stratified fluid, defined as inertia–gravity waves, is often observed around critical latitudes (Kundu 1990; Sutherland 2010) and was observed to 300-m water depth at WATR500 (Figs. 6c,d)—a much greater depth than the Ekman depth of ~70 m at this location.

The low-pass filtered temperature measurements were used to define the mixed layer depth (MLD) as the depth where the difference between the uppermost available layer and lower layers was >0.5°C (Monterey and Levitus 1997). The MLD at WATR200 in February 2011 fluctuated between 30 and 100 m and was deepest around 15 February 2011 due to the marine heat wave associated with Ningaloo Niña (Feng et al. 2013). This warming was strongest between 17 and 25 February 2011, and the water column was less stratified than it was at the beginning of the month. Deep diurnal banding (between 200 and 300 m) was observed at WATR500 during this time (Figs. 6c,d). The rotary energy spectra for the upper (30 m) and lower (132 m) layers at WATR200 both revealed a peak at the diurnal frequency for the counterclockwise component, but not for the clockwise component (Figs. 3c–f), indicating the presence of the diurnal signal through the water column. However, the cross-shore and alongshore components showed a pronounced phase shift of 180° below the mixed layer when compared to the surface. Downward energy flux and upward phase propagation were visible, and the vertical phase speed around 10 February 2011 was estimated to be 130–140 m day−1, which resembled the phase speed observed in De Soto Canyon, Gulf of Mexico (138 m day−1), in summer (Jarosz et al. 2007), but was about twice as fast as the speed observed close to Mission Beach, California (68 m day−1; Lerczak et al. 2001).

The LSB system is modulated by the west to east propagation of the high pressure system (Verspecht and Pattiaratchi 2010), and during the period 1 February to 19 March 2011, the LSB system reflected a periodicity of around 5–6 days (Fig. 7a). During strong diurnal wind forcing periods, corresponding to larger amplitudes in Fig. 7a, the counterclockwise wind phase was relatively constant around −90° and only fluctuated about 30° from this value when the diurnal wind energy was low (Fig. 7c). The current phases from the HFR and ADCP measurements varied slightly. They gradually decreased from 1 to 26 February 2011 (Fig. 7c) and then remained around a relatively constant value. The counterclockwise phase at two depths (30 and 132 m) from the ADCP data at WATR200 revealed a distinct phase difference between the water column’s upper and lower layers (Figs. 7c,d), which was close to 180° for most of the study period. These observations are consistent with the mechanism of resonant response to the LSB with a coastal boundary, proposed for regions close to the critical latitude where it was also found that the directions and phases of surface currents match those of wind stress at the diurnal frequency at the surface with an 180° phase difference between upper and lower layers (e.g., Simpson et al. 2002; Edwards 2008; Zhang et al. 2009; Nam and Send 2013).

Fig. 7.
Fig. 7.

(a) Counterclockwise (gray) and clockwise (black) amplitudes of diurnal winds from the Rottnest Island meteorological station for 1 Feb to 19 Mar 2011; (b) counterclockwise amplitudes of HFR-measured diurnal surface currents at WATR200 (thick black line) and ADCP-measured diurnal currents at WATR200 (dashed line: 30-m depth; thin black line: 132-m depth) for 1 Feb to 19 Mar 2011; and (c) counterclockwise diurnal phases for the Rottnest wind data (gray line), HFR-measured surface currents at WATR200 (thick black line), and ADCP-measured currents at WATR200 (dashed line: 30-m depth; thin black line: 132-m depth). As the HFR and ADCP currents were mostly counterclockwise, only counterclockwise amplitudes and phases [(b) and (c), respectively] are shown; (d) the difference in phases between counterclockwise diurnal winds at Rottnest and counterclockwise diurnal HFR-measured currents (thick black line), and ADCP-measured currents at WATR200 (dashed line: 30-m depth; thin black line: 132-m depth).

Citation: Journal of Physical Oceanography 46, 11; 10.1175/JPO-D-16-0022.1

d. Local effective Coriolis frequency

The wind-forcing frequency must be near the local effective Coriolis frequency to generate near-inertial oscillations (Kunze 1985; Nam and Send 2013). Data collected from austral summers (February 2011 and February–March 2012) were used to examine the relation between the wind forcing and the local effective Coriolis frequency.

In February and March 2011, the region experienced a marine heat wave associated with the Ningaloo Niña event resulting in ocean temperatures ~5°C higher than normal (Feng et al. 2013). Time series maps combining sea surface temperatures, altimetry data, and the daily averaged surface currents obtained from HFR (Fig. 8) were used to examine the synoptic-scale dynamics in February 2011. In early February 2011, a system of two counterrotating eddies dominated the synoptic-scale dynamics (Fig. 8a). A clockwise eddy, located to the west of the study region, impacted the deeper Two Rocks moorings (WATR150–WATR500) with southward-flowing currents under the influence of the Leeuwin Current (eastern rim of the clockwise eddy). The continental shelf region zone was under the influence of the northward-flowing Capes Current (WATR050 and WATR100). A counterclockwise eddy was present to the northwest, and the warmer Leeuwin Current water was advected along the western and southern perimeters of this eddy and then entrained into the southern clockwise eddy and advected south (Fig. 8a). The clockwise eddy was dominant in the Perth Canyon until about 12 February 2011. After this date, the strengthening Leeuwin Current (Figs. 8b–d) transported warm water southward across the study region (Figs. 9g,h,i), which reduced the buoyancy frequency through the water column at WATR200 (Fig. 9f) and dissipated the clockwise and counterclockwise eddies.

Fig. 8.
Fig. 8.

Sea surface temperature (color), altimetry (m; gray contours), daily averaged HFR surface currents (black arrows), and daily averaged winds from the Rottnest Island meteorological station (red arrows) for 4 days during the February 2011 Leeuwin Current marine heat wave. For clarity, only every second surface current vector is shown. The temperature scale is the same for all panels.

Citation: Journal of Physical Oceanography 46, 11; 10.1175/JPO-D-16-0022.1

Fig. 9.
Fig. 9.

February 2011. (a) Counterclockwise (red) and clockwise (blue) amplitudes of the diurnal winds for Rottnest Island; (b) counterclockwise (red) and clockwise (blue) amplitudes of the diurnal HFR surface currents at WACA200, WATR500 and WATR200; and (c) effective Coriolis period at WATR200 (blue line) and the corresponding planetary Coriolis period (red line). The black line denotes the diurnal period representing the wind forcing. (d) Counterclockwise amplitudes (m s−1) of the diurnal ADCP currents at WATR200. The white line denotes the mixed layer depth. (e) Vertical shear squared (s−2) for low-passed ADCP currents at WATR200. The white line denotes the mixed layer depth. HFR measurements were used to calculate the shear between the surface and 30-m depth when available. (f) Brunt–Väisälä frequency squared (s−2) at WATR200. The black line denotes the mixed layer depth. Temperature profile (°C) at (g) WATR100, (h) WATR150, and (i) WATR200. In (g),(h), and (i), the black line denotes the 23.3°C isotherm.

Citation: Journal of Physical Oceanography 46, 11; 10.1175/JPO-D-16-0022.1

The influence of the subinertial shear on the effective Coriolis period was substantial. At the start of February 2011, the clockwise (cyclonic) eddy increased (in a negative sense) the relative vorticity at WATR200 and decreased the local Coriolis period to less than 17 h on 8 February 2011 (Fig. 9c). As the clockwise eddy moved southward, the vorticity became positive, which increased the local Coriolis period to around 24 h on 10 February 2011. As the diurnal wind was also strong at the time (Fig. 9a), energetic counterclockwise diurnal oscillations below the MLD at WATR200 were also observed (Fig. 9d).

The change in the local vorticity shifted the internal wave band’s lower bound, which allowed the intermittent, resonant generation of near-inertial waves and their deep penetration below the MLD. The effective Coriolis frequency’s temporal variation likely affected the drift of phase variation of counterclockwise current at WATR200 (Fig. 7c); however, the moorings at WATR050 and WATR150 were mostly in superinertial regime during February 2011 (and also April 2011; see Fig. 10) because of the Leeuwin Current’s more onshore position and the weaker and narrower Capes Current (Fig. 8b). The resulting counterclockwise shear decreased the local effective Coriolis frequency in that part of the Two Rocks transect.

Fig. 10.
Fig. 10.

The effective Coriolis period (h) estimated along the Two Rocks transect (upper panel) and the Perth Canyon transect (lower panel) between January and April 2011. The black contour lines denotes the 24-h period. See Fig. 1b for the transect positions. The mooring locations are also indicated.

Citation: Journal of Physical Oceanography 46, 11; 10.1175/JPO-D-16-0022.1

The land–sea breeze system dominated the wind regime throughout February 2011 (Fig. 9a). The surface currents measured by HFR indicated large, counterclockwise oscillations in the surface currents at the three ADCP mooring sites (Fig. 9b). From 9 February 2011 onward, banding was seen extending through the whole water column in the 200-m ADCP current profiles (Figs. 6a,b) and down to 250–300 m in the 500-m ADCP current profiles (Figs. 6c,d). Counterclockwise motions were dominant at WATR200, and the upper mixed layer and lower layer were clearly separated (Fig. 9d). The 100- and 150-m moorings (Figs. 9g,h, respectively) measured diurnal isotherm oscillations, with brief vertical isotherm fluctuations reaching almost 50 m at the 100-m mooring (WATR100) on 11 February 2011. As the thermocline deepened (because of the warmer Leeuwin Current waters flowing south), the 200-m Two Rocks mooring measured isotherm oscillations of about 15–20 m around 22–23 February 2011 (Fig. 9i).

The above discussion considered the effect of horizontal shear on the effective Coriolis frequency on diurnal–inertial motion. Previous studies (Kunze 1985; Federiuk and Allen 1996; Nam and Send 2013) have demonstrated that vertical shear may also play a significant role in modulating the inertial–gravity wave energy. The low-frequency vertical shear and buoyancy frequency estimated from the HFR and ADCP data and thermistor measurements indicated that the vertical shear squared was much smaller than buoyancy frequency squared for majority of the time (Figs. 9e,f). Hence, the lowest frequency allowed was the effective Coriolis frequency [see Eq. (1)]. However, on 9–10 February 2011 and just below relatively shallow mixed layer on 23–24 February 2011, the ratio between vertical shear squared and buoyancy frequency squared was relatively high (Figs. 9e,f): in the first case (9–10 February 2011), it enabled resonance in the middle layer (decreasing the lowest frequency allowed toward diurnal frequency), while on 23–24 February 2011 (when the effective Coriolis frequency was close to the diurnal; Fig. 9c), it also decreased the lowest frequency allowed, thus shifting it away from the diurnal resonance, which therefore was mostly pronounced at the surface and in the mixed layer.

The surface energy flux during February 2011 was mostly negative due to opposing directions of southward-flowing Leeuwin Current and southerly winds (Fig. 11b). The influence of the Leeuwin Current may be seen as the negative component in the low-passed energy flux (Fig. 11c). Diurnal oscillations of surface KE per unit volume were highest between 12 and 15 February 2011 (oscillating between 40 and 250 J m−3) and around 23–25 February 2011, when they fluctuated between 0 and 60 J m−3 (Fig. 11e). The energetic oscillations occurred when the oscillatory component of the energy flux (Fig. 11d) and the effective Coriolis period before/during those two periods was close to 24 h (Fig. 11a).

Fig. 11.
Fig. 11.

February 2011. (a) the effective Coriolis period at WATR200 (blue) and the corresponding planetary Coriolis period (red). The black line indicates the diurnal period representing the wind forcing. (b) The rate of work done by the local wind (surface energy flux, W m–2) at WATR200 calculated using the Rottnest Island wind data and the HFR surface currents data; (c) as in (b) but low pass filtered (“mean”); (d) as in (b) but high pass filtered. (e) Kinetic energy per unit volume (J m–3) at WATR200 estimated from the HFR surface currents data, and (f) vertical profile of kinetic energy per unit volume (J m–3) at WATR200 calculated from the ADCP data. The black line in (f) denotes the mixed layer depth. The periods when the effective Coriolis period was close to the diurnal period are shaded (see Figure 13c).

Citation: Journal of Physical Oceanography 46, 11; 10.1175/JPO-D-16-0022.1

The conditions were markedly different in February–March 2012 (Fig. 12). In early February 2012, a single counterclockwise eddy, centered on the Perth Canyon, dominated the synoptic-scale dynamics (Fig. 12a). The continental shelf region zone was under the influence of the northward-flowing Capes Current. With time, the counterclockwise eddy dissipated, and the southward Leeuwin Current was established in the offshore region (Fig. 12d).

Fig. 12.
Fig. 12.

Sea surface temperature (color), altimetry (m; gray contours), daily averaged HFR surface currents (black arrows), and daily averaged winds from the Rottnest Island meteorological station (red arrows) for 4 days in February and March 2012. For clarity, only every second surface vector is shown. The temperature scale is the same for all panels.

Citation: Journal of Physical Oceanography 46, 11; 10.1175/JPO-D-16-0022.1

Several episodes of energetic diurnal motions extending through the whole water column occurred at WATR200 in February and March 2012 (Figs. 13d,e). The vorticity along the Two Rocks transect was positive at deeper stations more often (resulting in a lower effective Coriolis frequency; Fig. 14) than it was in February 2011 because of the Leeuwin Current’s more offshore position (cf. Fig. 8 and Fig. 12). Such conditions were more favorable for effective pumping of the diurnal wind energy into the ocean.

Fig. 13.
Fig. 13.

February–March 2012. (a) Counterclockwise (red) and clockwise (blue) amplitudes of the diurnal winds for Rottnest Island; (b) counterclockwise (red) and clockwise (blue) amplitudes of the diurnal HFR surface currents at WACA200, WATR500, and WATR200; and (c) effective Coriolis period at WATR200 (blue) and the corresponding planetary Coriolis period (red). The black line denotes the diurnal period representing the wind forcing. (d) High-frequency cross-shore u component at WATR200; (e) high-frequency alongshore υ component at WATR200. The black line in (d) and (e) denotes the mixed layer depth. Temperature profile (°C) at (f) WATR100, (g) WATR150, (h) WATR200, and (i) WACA200. In (f),(g),(h), and (i), the black line denotes the 22°C isotherm.

Citation: Journal of Physical Oceanography 46, 11; 10.1175/JPO-D-16-0022.1

Fig. 14.
Fig. 14.

The effective Coriolis period (h) estimated along the Two Rocks transect (upper panel) and the Perth Canyon transect (lower panel) between January and April 2012. The black contour lines denotes the 24-h period. See Fig. 1b for the transect positions. The mooring locations are also indicated.

Citation: Journal of Physical Oceanography 46, 11; 10.1175/JPO-D-16-0022.1

The counterclockwise diurnal amplitudes of the HFR surface currents at WATR200 indicated three pronounced peaks: around 8 February, 3–4 March, and 24 March 2012 (Fig. 13b). The effective Coriolis period at station WATR200 was >24 h at the beginning and the end of February 2012 and through most of March 2012 and was close to 24 h during the three pronounced peaks mentioned above (Fig. 13c). For these three episodes the ratio between vertical shear and buoyancy frequency was insignificant (not shown), and the effective Coriolis frequency was the lowest frequency allowed. Therefore, when the counterclockwise diurnal wind amplitude peaks corresponded to the 24-h effective Coriolis period, resonance occurred. Strong diagonal diurnal banding was observed for the cross-shore and alongshore current components at WATR200 (Figs. 13d,e), with near-bottom, cross-shore currents flowing in the opposite direction to the cross-shore wind stress. LSB circulation was well developed in February and March 2012 but was especially energetic when maximum diurnal currents occurred (Fig. 13a).

Between 2 and 6 March 2012, the diurnal wind ellipse was less polarized than usual, with the counterclockwise wind following a more circular path (not shown). The LSB also induced near-inertial oscillations more effectively when the MLD was smaller (e.g., at the start of March 2012 when the MLD was around 50–55 m).

Diurnal temperature oscillations were observed in February and March 2012 and were vigorous at WATR150 around 4 March 2012 and from 18 to 20 March 2012 (the isotherm range reached 60 m; Fig. 13g). Sporadic diurnal temperature oscillations were present at all Two Rocks sites. A prolonged period of diurnal temperature variability was also observed at WACA200 in March 2012, with the vertical isotherm fluctuations reaching 20 m (Fig. 13i).

The surface energy flux during this period was mostly positive, with diurnal bursts surpassing 0.04 W m−2 during the following periods: 4–8 February 2012, 13–15 February 2012, 29 February–5 March 2012, and 22–24 March 2012 (Fig. 15). Maximum value of diurnal bursts reached 0.1 W m−2 (Fig. 15b), more than 5 times larger than the flux observed close to Huntington Beach, California (~33.63°N), by Nam and Send (2013), who also suggested that wind stress enhanced the currents. The diurnal oscillations of surface KE per unit volume were also higher on these dates, except between 13 and 15 February 2012 (Fig. 15e). The analysis of the effective Coriolis period (Fig. 15a) indicated large diurnal surface KE peaks occurred when the effective period was close to 24 h. The peaks were not observed between 13 and 15 February 2012 when the effective Coriolis period was much shorter (<20 h; Fig. 15a), representing subinertial conditions. These results are similar to those observed in February 2011 (Fig. 11) where the diurnal oscillations occurred when the oscillatory component of the energy flux (Fig. 15d) and the effective Coriolis period before/during those two periods was close to 24 h (Fig. 15a).

Fig. 15.
Fig. 15.

As in Fig. 11 but for February–March 2012.

Citation: Journal of Physical Oceanography 46, 11; 10.1175/JPO-D-16-0022.1

The surface KE per unit volume showed strong diurnal fluctuations between 1 and 4 March 2012. On 4 March 2012, the fluctuations surpassed 400 J m−3, which was more than twice as large as the near-surface KE that Nam and Send (2013) observed. Similar surface KE values were observed on 4 February 2012 and somewhat smaller between 18 and 20 March 2012; however, the diurnal variability was less and the rate of work done by the local wind was much larger (in absolute sense) than it was on 4 March 2012. The surface energy flux was high and positive (up to 0.3 W m−2) on 4 February 2012 due to the constructive influence of strong southerly winds and a large, counterclockwise eddy to the west of WATR200 (Fig. 12a); however, the rate of wind work at the surface was high but negative on 19 March 2012 because of the opposing effect of the strong southerly winds and the southward-flowing Leeuwin Current (Fig. 12c). The KE’s temporal evolution at WATR200 showed the KE increased in depth during periods with diurnal banding, especially between 2 and 4 March 2012 (Fig. 15c) when the effective Coriolis period was almost exactly 24 h (Fig. 15a).

The low-frequency surface current systems observed during the February 2011 were different from those observed during February–March 2012. In 2011, the Ningaloo Niña event strengthened the Leeuwin Current (Feng et al. 2013). The mean low-frequency southerly wind component was weaker in February 2011 (4.1 m s−1) than it was in February 2012 (5.4 m s−1), which caused a weaker and narrower Capes Current, fewer counterclockwise mesoscale eddies, and a more onshore position of the Leeuwin Current (Fig. 8). Such surface dynamics resulted in a negative relative vorticity and a higher effective Coriolis frequency at depths greater than 150 m along the Two Rocks transect and at 200 m along the Perth Canyon transect in February 2011 (Fig. 10); however, at depths between 50 and 150 m, the system was mainly in the superinertial regime (especially along the Two Rocks transect), where resonance or freely propagating waves may be expected to occur (Fig. 10).

The region with the superinertial regime was shifted farther offshore in February–March 2012 (Fig. 14), and when the effective Coriolis period was close to 24 h, strong, deep diurnal banding was observed at WATR200 (Figs. 13d,e), especially between 2 and 4 March 2012 when the effective period was almost exactly 24 h. The downward energy and upward phase propagation (Figs. 13d,e) confirmed the surface was the energy source for diurnal motions, which was consistent with the resonant response to LSB forcing at the time. The resonant excitation was strongest at the start of March 2012 when the ratio between the surface KE and the work done by the local wind on the surface water was the largest (Fig. 15); this finding supports the notion that low-frequency shears created the conditions that allowed a surface current resonant response to the LSB.

5. Conclusions and implications

This paper presented analysis of a set of multiplatform data with wide spatial and temporal resolution to examine the diurnal LSB influence on the surface and subsurface currents and temperature structure along the Rottnest continental shelf in southwestern Australia. The main conclusions were as follows:

  1. The study region is subjected to strong LSB forcing along an axis oriented ~35°–45° counterclockwise from the east, with counterclockwise rotation extending >150 km offshore.

  2. The surface diurnal current response in deeper water was circular and counterclockwise, with current amplitudes > 0.3 m s−1. The current amplitudes were stronger in the offshore region when compared to the inner shelf that had weaker currents and more eccentric amplitudes due to the interaction of inertial motions with the coastal boundary.

  3. Energy in the diurnal band was observed up to 300-m depth and penetrated deep below the mixed layer. Vertical isotherm fluctuations up to 60 m accompanied these motions with an upward phase propagation speed of ~140 m day−1.

  4. The upward phase propagation and deep penetration of diurnal currents below the mixed layer and the ~180° phase difference between the upper and lower water column suggested the local LSB system caused the resonant diurnal motions.

  5. The LSB forcing in the study region area is slightly subinertial, and the rate of wind energy transfer at the surface became more efficient when the effective Coriolis period was ~24 h and the mixed layer was shallower (e.g., at the start of March 2012). The effective Coriolis frequency was also highly dependent on the wider-scale ocean dynamics (such as the marine heat wave in 2011) and the low-frequency southerly winds.

In the absence of strong tidal mixing, the intermittent, resonant LSB forcing of near-inertial waves and isotherm oscillations can result in vertical mixing across the pycnocline. The Rottnest continental shelf is oligotrophic due to the dominance of the Leeuwin Current that inhibits large-scale upwelling, with low productivity despite episodic upwelling events during the summer months (Hanson et al. 2005). There is considerable conjecture with regard to the supply of nutrients to the upper ocean in this region (Feng et al. 2009). Vertical mixing has been proposed as an important mechanism for delivering nutrients, from below the Leeuwin Current, to the surface mixed layer (Thompson et al. 2011). Lucas et al. (2014) also studied the diurnal–inertial resonance in the Benguela upwelling system off Namibia and demonstrated that wind forcing enhanced phytoplankton productivity through a diurnal resonance mechanism embedded in the upwelling’s low-frequency vertical flux forced by Ekman dynamics. Thus, enhanced surface currents’ shear and mixing through the water column through diurnal–inertial resonance could be an important mechanism for nutrient supply to the upper ocean during the summer months in the study region, where low-frequency upwelling episodes driven by strong southerly winds occur often in summer concurrently with land–sea breezes.

Acknowledgments

Hrvoje Mihanović acknowledges support received through the Group of Eight (Go8) European Fellowship during his residence at The University of Western Australia (Go8 European Fellowship 2011). We are indebted to Arnstein Prytz for his work on the ACORN analysis algorithm and quality control for the WERA data. The authors thank John Simpson for his early comments on the work and review. The oceanographic data (moorings, HF radar, and satellite SST and altimetry) used in this paper were collected as part of the Integrated Marine Observing System (IMOS), funded by the Australian Government through the National Collaborative Research Infrastructure Strategy and the Super Science Initiative. The meteorological data were obtained from the Bureau of Meteorology.

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