Tag Archives: Climate

The 1991 Heard Island Feasibility Test

MV Cory Chouest the ship used for the experiments detailed below.  Image credit: US Navy (public domain, via Wikipedia).
MV Cory Chouest, the ship used for the experiments detailed below. Image credit: US Navy (public domain, via Wikipedia).

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,[1] 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.[1] 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.

Speed of sound measured at various depths in the Pacific Ocean north of Hawaii.  Image credit:  Nicoguaro (CC-BY-SA); data from the 2005 World Ocean Atlas.
Speed of sound measured at various depths in the Pacific Ocean north of Hawaii. Image credit: Nicoguaro (CC-BY-SA); data from the 2005 World Ocean Atlas.

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.[2] 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.[3] 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.[4] 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.

***
[1] 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

[2] 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.

[3] This sounds almost analogous to the upcoming VK0EK ham radio expedition to Heard Island, where radio operators (including myself) will have stations around the world listening for their signal.

[4] 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

Advertisements

Meteorology and its Application to Heard Island

Cloud vortices off Heard Island. Image credit: NASA GSFC (Terra/MODIS, 9/19/2011 for you satellite imagery junkies).

Meteorology is an important consideration for any outing, be it a trip to the grocery store or an extended expedition. You will prepare differently if the temperature is hot enough to melt butter in the shade than you would if it’s cold enough to flash-freeze a pot of boiling water.

There are two different things we will need to keep in mind for these preparations: climate and weather. Although they are related, they are fundamentally different. Climate is long-term, whereas weather is short-term. For instance, the average high temperature* for Minneapolis for January 4th is 24 F. That’s the climatological average; if you’re planning a trip to Minneapolis for January 4, 2016, that’s a ballpark of what to expect. Weather is more variable, of course. In 2015, the high temperature on January 4th was 12 F; in 2014, it was 35 F. That’s weather. Here’s an analogy: take two six-sided dice. On average, when you roll them, their sum will be seven (that’s the climate). However, you shouldn’t be surprised with a roll of three!

Last month at the Five Thirty Eight blog, Nate Silver did a great job analyzing the amount of weather at various cities across the US, and in general exploring this distinction and the limits of its usefulness. Unfortunately, his title is a bit misleading:

“Which City Has The Most Unpredictable Weather?”

His use of unpredictable has nothing to do with the skills or abilities of the local meteorologists.** Instead, it is a reflection of the variability away from the climate average. That is, if you use the climate average as a forecast, how likely is it to be correct? In some areas, such as Phoenix, AZ, the climate average high temperature is a very good predictor. In others, like Minneapolis, the departures from average can be larger and more frequent, as exemplified above.

So, now that we have that discussion out of the way, and deferring things like uncertainty to another post, what do we know about Heard Island?

Expect cool temperatures, wet weather, and plenty of wind. The vast icy waters surrounding the island keep conditions from changing much. Temperatures remain near freezing year-round, with monthly average temperatures in summer still only 5.2 C (~41 F). The ocean provides plenty of moisture too; a research expedition from 1948-1954 recorded precipitation on 75% of days at Atlas Cove, one of the more accessible parts of the island.

The volcano can make its own weather by disrupting the west-to-east winds. Windward locations tend to see more fog and rain (like the coastal parts of San Francisco), while leeward areas see more sun and warmer temperatures (like Berkeley or Palo Alto). Lenticular clouds, formed when the humid air is pushed up around the mountain, are also found at Heard Island. Other conditions can cause cloud vortices when the smooth flow of wind is disrupted while passing the island, as seen in the picture at the top of this post.

In the southern hemisphere, there is much less land to disrupt the smooth flow of air than there is in the northern hemisphere. Consequently, the winds tend to be stronger more consistent. At Atlas Cove, the average wind speeds are around 26-33.5 km/h (16-21 mph), with gusts recorded up to ~180 km/h (110 mph). That means we can expect damage similar to that found from a borderline EF1/EF2 tornado.***

In short, the climate of Heard Island is near-freezing, wet, and strongly influenced and moderated by the water around it. Be prepared for cold, snow, rain, and wind. There will be some sunshine in there too, but it won’t be the norm.

* The National Weather Service computes this over the 30-year period of 1981-2010.
** Variable would have been a more appropriate word, because devious has deviated from the sense which would be useful here.
*** Tornado categories use the Enhanced Fujita scale (0-5, 5 is strongest), and are estimates of wind speed based on damage. Obviously we’d rather use a direct measurement and have our camp sturdy enough to not have damage to use for estimating wind speed. More on the inner workings of the (enhanced) Fujita scale can be found at the NOAA Storm Prediction Center.

Global Warming, and Stephenson Glacier Retreat

Annual global surface temperature difference from the 20th century average.  2014 is the 38th straight year above average.  Image credit: NOAA.
Annual global surface temperature difference from the 20th century average. 2014 is the 38th straight year above average. Image credit: @NOAA.

Two things came to my attention today which are of particular interest.

First, NOAA has announced that globally, 2014 was the warmest year on record, and the 38th straight year of above-average temperatures. Continued action will be needed in 2015 to reverse this trend. Every delay makes fixing the situation more difficult.

Second, Mauri Pelto has written today about the retreat of Stephenson Glacier and the formation of a lagoon on Heard Island. In 1947-1948, when members of the Australian National Antarctic Research Expedition (ANARE) spent 15 months at Heard Island, they found Spit Point, on the southeast side of the island, was only accessible after crossing Stephenson Glacier. Imagery from LANDSAT shows substantial retreat, as do photographs from a 2004 expedition to Heard Island.

Landsat 2010 image, annotated by Mauri Pelto.  Arrows mark the toe of the glacier in 2001 (purple), 2010 (red), and 2013 (yellow).  Additional images are available on Mauri Pelto's blog.
Landsat 2010 image, annotated by Mauri Pelto. Arrows mark the toe of the glacier in 2001 (purple), 2010 (red), and 2013 (yellow). Additional images are available on Mauri Pelto’s blog.

Today, where once Stephenson Glacier met the ocean, there is now Stephenson Lagoon. The toe of the glacier has retreated inland, and to my eye appears to have moved about 4 km. With a warming at Atlas Cove of 1 °C over 1947-2001, the retreat is not surprising.