We’ve just shoved off from Macquarie Wharf in Hobart The Captain is spending some time calibrating the ships compass. Then we’ll head down the Derwent Estuary and into the Tasman Sea.
Matthew Alford and his Leg I Team brought the ship onto Hobart Tuesday morning. They had their gear well organized for offloading, and after a few hours of work by the Revelle’s crew, our container was aboard and we started to assemble the Fast CTD system.
On Wednesday, Nicole Jones and her team from the University of Western Australia arrived and loaded the T-SHELF mooring arrays and bottom landers that we’ll be deploying tomorrow. Preparing instruments and assembling electronics has kept us busy till now.
Figure 1 a) The RV Revelle. B) Output of Harper Simmons global model of semi-diurnal baroclinic tidal generation and propagation. The beam heading from the Maquarie Ridge toward Tasmania motivates our study.
As we head out to sea, it’s hard to believe that the initial TTIDE proposal was written back in 2009. It had just been noted that both satellite evidence and numerical simulations suggested that an energetic (5 kW/m) beam of semi-diurnal baroclinic (internal) tidal energy is heading WNW from the Macquarie Ridge toward the east coast of Tasmania. When these mode-1 waves impact the Tasmanian continental slope, significant turbulent mixing, scattering to higher mode internal waves, and reflection are anticipated. On Leg II, we’ll focus on documenting the phenomena associated with mixing processes and quantifying the scattered high-vertical-mode waves. Preliminary modeling of the eastern continental slope’s response to the incoming tidal beam has been undertaken by Jody Klymak at U. Victoria using the MIT GCM in 2-D and 3- D (Fig 2a,). The results suggest that dissipation is concentrated in a number of regions along the coast, where bottom topography is favorable to the generation of lee waves or upslopebores.
Regions of active mixing should extend 200-300 m above the steeply sloping sea floor, with forward-scattered waves potentially extending even further into the water column (Fig. 2b). Our challenge is to resolve rapidly evolving phenomena like bores as they pass under the ship, held on station 1-2 km above. We have had past experience observing shoaling tides in the South China Sea (Fig.3). Here, a very energetic tide is arriving from Luzon Strait. The near sea floor isopycnals become distinctly non-sinusoidal, assuming a bore-like form. Breaking occurs in regions of high strain (isopycnal separation), where density gradients are low. The challenges in collecting time series such as this one include maintaining the ship on station for extended periods of time, developing a rapid vertical profiling CTD and turbulence sensing system, and operating it continuously so as to obtain a continuous time record of the deep phenomena.
Figure 2. a) The eastern slope of Tasmania with expected levels of depth-integrated turbulent dissipation indicated by color. TTide mooring lines will be concentrated around km 110 and km 305. b) A cross-section of the slope showing cross-slope currents (top) and anticipated dissipation rates (bottom). Model output courtesy of Jody Klymak, using the MIT GCM.
Figure 3. Observations of a shoaling internal tide on the western slope of South China Sea.The sea-floor is at ~700m (black line),appearing to vary as the ship shifts in position. Theblack lines in the top figure indicate the depths of surfaces of constant water density, as they get lifted up & down by the waves. Colors indicate cross-slope flow, as seen from the ship’s ADCP. Strong turbulence initiates (lower figure) as the density surfaces diverge from the sea floor, producing a region of low density-gradient. Roughly 200 CTD profiles were obtained during this 30-hour observation to document the shoaling tide and resulting turbulence.
The instruments we’ll be using on Leg II include the Hydrographic Doppler Sonar System (HDSS) on the Revelle, a nested set of 140 and 50 kHz Doppler sonars built into the Revelle’s hull, and the Fast-CTD, a Seabird 49 CTD that is vertically profiled at ~5 m/sec, to document the evolution of the density field as the waves pass by. To profile rapidly, the Fast CTD (Fig. 4) is housed in a streamlined lead-nosed package. The instrument is suspended from a thin spectra cable to minimize cable drag. Falling at 5 m/s (10kts), we will profile through the lower 200 m of the sea (the turbulent region) in ~40 sec. We’ll try to get within 20 m (4 sec) of the sea floor on each drop. This requires extreme vigilance on the part of the operators. Along with the CTD, the profiler contains a micro-conductivity cell to detect turbulence (0.1 m vertical resolution) and an inertial package to determine the spin-rate/ trajectory of the fish. New for this trip is an acoustic altimeter that should detect the sea floor from ranges up to 50 m. If it works, we’ll get ~10 seconds of advanced warning as the bottom approaches.
The Fast CTD is operated off a custom boom that will be mounted on the Revelle’s stern (Fig 4b). The boom is needed to keep the CTD cable from fouling the ship’s hull or propellers during operation. To manage the cable payout at high speeds, a motorized sheave is used at the end of the boom. The sheave can operate at extreme wire angles, which are common when the CTD is used in strong currents. The system has been extensively modified for the rough conditions expected off Tasmania. We’ll see how it all works in the next few weeks.
Figure 4. a) the Fast-CTD fish. The micro-conductivity probe is protected by the circular guard. The CTD and altimeter are housed within the fish. b) The motorized sheave manages cable tension on descent. The boom and the motorized sheave work in tandem to keep the fish clear of the Revelle’s hull & screws.
Figure 5. The control screen for the Fast-CTD. Winch status, control, and CTD raw data are displayed in real time.
The Fast CTD is operated from a console in the Revelle’s aft lab. A lab-view display controls the winch (Fig 5). Several video monitors and a CTD data display also are located at the operator’s station.
On station, the plan is to operate 24 hours a day. Typically we divide into two watches, each of which stands alternating 12-hour shifts (3 am-3 pm). Within each watch, crew rotates between hourly stints controlling the CTD, standing watch on the fantail to look for cable fouling, and serving as back-up / reserve.
Tomorrow we’ll start to deploy Nicole Jones’ “T-Shelf” moorings and Drew Lucas’ Wirewalker profilers on the Tasman shelf. We’ll then spend a day profiling near these moorings. Following this initial effort, we’ll move offshore and launch the Fast CTD. We’ll spend 24-36 hours at each site, exploring the various wave and turbulent phenomena associated with different topographic features. We’ll decide on sites to explore based on our initial findings and those of Matthew Alford’s team on Leg I.
– Rob Pinkel, The Revelle
The Tasman Tidal Dissipation Experiment//Supported by the National Science Foundation