About ten months ago, the Heard Island Expedition team launched the first of our eleven Argo buoys into the Indian Ocean. The buoys are equipped with conductivity-temperature-depth (CTD) instruments, and spend most of their time drifting about 1 km beneath the ocean surface. Every ten days, they dive to 2 km, then record CTD data as they ascend to the surface. At the surface, they relay the data via satellite over the course of a day before returning to 1 km depth. With a large network of these buoys, scientists can gather important data on currents under the ocean, as well as changes in temperature and salinity profiles.
Over time, ocean currents move the buoys. None of our eleven buoys are where they started, and some have moved far away from where they entered the ocean. We deployed two batches of buoys: six before reaching Heard Island from Cape Town, and five more on our voyage on to Fremantle/Perth.
I have obtained the latest position data (as of Jan 14, 2017) for the eleven buoys. Their tracks are shown in the figure at the top of this post. Tracks are colored by buoy, reusing the colors for the first and second batch. Some of the buoys have moved more than 1500 km as the albatross flies, with path lengths approaching 3000 km!
The CTD data are also interesting. For instance, here are the temperature/depth and salinity/depth profiles measured by buoy 5902454 (dark blue path on second leg of map above).
Around December 1, buoy 5902454 encountered a different water mass with colder, saltier water throughout much of the 2 km water column.
Generally for these buoys, the surface water temperatures reflect the seasonal variations (warmer in Austral summer, colder in winter), while the deep water shows less variation—but sometimes there are shifts between different water masses.
On January 10, 1992, on a voyage from Hong Kong to Tacoma, Washington, the cargo vessel Ever Laurel encountered rough seas and a container was washed off the ship. The container broke open and released its contents: 28,800 yellow rubber duckies and other floating bath toys. Since then, the duckies have been floating around, moved by wind and wave, and washed up on coasts around the world. By tracking the date and location of washed-up duckies, oceanographers can get a sense for the speed and direction of surface circulation at an oceanic scale. It’s like having 28,800 messages in bottles dumped from the same known location at the same known time.
Oceanographers sometimes want to be more precise in their measurements. The duckies probably floated very high in the water (at least at first), so that the wind could easily affect their direction and speed. Additionally, the rubber duckies are hard to track while they are at sea because they are small, few, and far between.
When more precise measurements are required, oceanographers turn to specially-designed drift buoys. These maintain a lower profile above water, and have a large “holey sock” sea anchor tethered to them in order to more accurately measure the ocean surface currents and not the wind. The buoys also have a thermometer—and sometimes additional sensors for salinity or barometric pressure—and a radio transmitter to establish the buoy’s position (by Doppler shift from 401.65 MHz, not GPS) and relay data via satellite back to the operations center.
Different floats can be used to measure temperature and salinity profiles, rather than surface currents. Argo floats are autonomous diving instruments, which can maintain neutral buoyancy and perform controlled ascent/descent to 2000 m. These floats make their temperature, pressure, and salinity measurements during a 6–12 hour ascent. Upon reaching the surface, they transmit their GPS location and the recorded data back to the operations center via satellite. Argo floats are not cheap, with each carrying a price tag of around $15k.
On the Heard Island Expedition, our team will be deploying both of these types of instruments. These measurements will improve understanding of ocean circulation, heat content, and salinity, as well as providing ground-truth sea surface temperature measurements for use in weather forecasting models. No rubber duckies will be deployed, but we’ll document any we find washed up on the beaches.
Batten the hatches and hang on to the hand rails, because this installment of science at/on/near Heard Island is going to be a wild ride! We’ll explore a paper entitled The Heard Island Feasibility Test, and along the way we’ll make ports of call in climate science, oceanography, and physics. I encourage you to check out a copy of the paper, either at your local (research) library or online. It’s really well-written! There’s also a pre-experiment lecture given by the study’s lead author which is freely available online, and details the rationale behind the study and the expected results.
In 1991, scientists were concerned about global warming. They were very interested in measuring the ocean temperature—oceans can store much more heat than the atmosphere, so while the atmosphere may not warm quickly in a changing climate, the oceans are likely to capture most of the heat. Additionally, water has a high heat capacity (the amount of energy it takes to raise its temperature by a degree), which is why it takes so long to bring a pot of water water to a boil on the stove.
Measuring the ocean temperature seems fairly straightforward: put a thermometer in the ocean, and log the temperature. Scatter a bunch of stations around the world and it’s done, right? Wrong.
The problem with using a thermometer (or many thermometers) to measure the ocean temperature is that there are many small-scale features which can influence the measured temperature. The variability of these measurements is likely to be quite high, and they each measure only a small place— extrapolating to the whole ocean isn’t necessarily justified.
How, then, can a measurement be made which yields an average temperature over a huge volume of ocean?
Sound. Ocean temperatures can be measured with sound. This is an amazing world in which we live!
In water, the speed of sound will vary depending on temperature, pressure, and (to a limited extent) salinity, and be in the ballpark of 1.48 km/s. With variations in speed of 4–5 m/s/°C, a +5 m°C (0.005 °C) change in temperature results in a -0.1 s change in travel time over a 10 Mm (10,000 km) path. Have an acoustic source emit a signal, measure the signal at a distant receiver, and the time delay will yield an apparent average speed of sound. Shifts in these speeds due to warming of about 5 m°C/yr would theoretically produce measurably earlier arrival times.
One potential problem with all this is the part about receiving a sound signal 10 Mm away from its source. However, the temperature and pressure profile of the ocean cause a minimum in sound velocity at a depth of 500–1,000 m (for mid/low-latitude oceans). This low-velocity region, termed a SOFAR channel acts as a waveguide or a duct, where sounds within it tend to stay within rather than dispersing. Low-frequency sounds (50–100 Hz)are not attenuated or absorbed much by the water, so long-distance reception of these sounds might be possible.
The feasibility test was designed as a proof-of-concept for ocean-wide acoustic reception. Using powerful low-frequency transducers on loan from the U.S. Navy, the scientists would be able to send the signals and have receivers around the world listening for them. Unfortunately for the scientists, the transducers could only operate to a depth of 300 m. That meant that a high-latitude site needed to be found, where the SOFAR channel—that special place which enables long-distance reception—is much closer to the surface.
Heard Island was chosen as a transmission site, because the direct sound paths (mostly, but not entirely, great circles) would reach across both the Pacific and Atlantic oceans.
No major field work is complete without a little drama, though. Late in the planning and preparation phase, the US National Marine Fisheries Service notified the researchers that permits were required to mitigate threats to marine mammals from the powerful sounds. The Australians (Heard Island is an Australian territory) required the permits too. A second vessel was chartered and biologists were assembled to monitor marine mammal activity and fulfill the responsibilities associated with the permits.
The two ships sailed as originally scheduled on January 9, 1991, but neither the American nor Australian permits had been issued. With a scheduled transmission start of January 26th, there wasn’t much room for delay. Fortunately, the permits arrived just in time: January 18th and January 25th. I bet the scientists were very tense during the voyage from Perth/Fremantle (Australia) to Heard Island.
An unscheduled 5-minute equipment test the day before the first scheduled transmission was received in Bermuda, and shortly thereafter at Whidbey Island (near Seattle, and almost 18 Mm away). Basic feasibility was already shown!
Signals were sent in a 1-hour-on, 2-hours-off pattern. Some of the transmissions were a continuous-wave (CW) 57 Hz tone (to avoid 50 Hz and 60 Hz power noise), while others were a mixture of several different frequencies near 57 Hz. For details on these transmission modes I refer you to the paper.
Transmissions for the experiment were aborted on the 6th day—ahead of schedule—when a gale and 10-m swells caused one acoustic source to be lost from the string and fall to the ocean floor. The other sources were badly damaged. Conditions in the Southern Ocean can make field work there very difficult.
One thing I found surprising, but makes plenty of sense upon consideration, was that rather than staying in one fixed location, the ship towed the sources along at 3 kt (5.5 km/h, 3.5 mph). This makes sense once you think about the wind and waves in the Southern Ocean, and how, to maintain control of the ship, the vessel must be underway. Being broadside to the swell in a high sea is extremely dangerous.
In this experiment, the receivers were sensitive enough to detect the Doppler shift from the ship’s movement. In fact, the Doppler shift combined with the known path of the ship (from GPS) allowed the azimuth of the signals to be determined. For many of the signals, it was on the expected heading (not quite a great circle due to the non-spherical Earth and the inconsistent depth of the SOFAR channel). At Whidbey Island receiver array, though, the signals arrived from a bearing of 215°, not the 230° predicted. In that case, the signal appears to have taken a longer path southeast of New Zealand, rather than through the Tasman Sea and between Australia and New Zealand.
Fortunately for all involved, there was little impact noticed on the marine mammals. Despite the low observed impacts, the authors make recommendations for the Acoustic Thermometry of Ocean Climate project to reduce adverse effects to marine life. Using shorter-range transmit/receive pairs, the total power needed can be reduced significantly. Additionally, with temperate waters having a deeper SOFAR channel, the transmitters can be bottom-mounted at depths of around 0.5–1 km, which will help physically separate them from the near-surface-dwelling marine mammals.
In short, the Heard Island Feasibility Test was a resounding (pardon the pun) success. Ocean acoustic temperature measurement is possible, and measurements were made in the North Pacific for a decade, from 1996–2006.
This paper was a really interesting one, and fairly accessible (scientifically) to someone not in the field of signal processing or oceanography. I enjoyed reading it, and suggest you take a look at it if you’re at all interested. My summary here has skipped over large parts which detail the nature of the propagation and the signal processing aspects.
 Munk, W. H., Spindel, R. C., Baggeroer, A., Birdsall, T. G. (1994) “The Heard Island Feasibility Test” J. Acoust. Soc. Am.96 (4), p. 2330–2342. DOI 10.1121/1.410105
 This phenomenon is analogous to atmospheric ducting of radio waves, which can cause TV and FM radio stations to be heard far beyond their normal range, and for weather radar to pick up ground clutter far from the station.
 Bowles, A. E., Smultea, M., Würsig, B., DeMaster, D. P., Palka, D. (1994) “Relative abundance and behavior of marine mammals exposed to transmissions from the Heard Island Feasibility Test” J. Acoust. Soc. Am.96 (4), p. 2469–2484. DOI 10.1121/1.410120