Ash from Chile’s Puyehue-Cordón Caulle volcano getting drawn into a low pressure system in the Atlantic Ocean.Volcanic Ash and the Pacific Ring of Fire

The ‘Pacific Ring of Fire’ is home to 75% of the world’s active volcanoes. Adjacent to Australia, the ring of fire starts in New Zealand, moves northwards thorough the Pacific Islands into Papua New Guinea and Indonesia. The ring of fire then follows the outer edge of the Pacific finishing in Chile.
Most readers would be aware that a volcano in Chile caused air travel disruptions during June over southern Australia and New Zealand. Many people I have spoken to were surprised that the Bureau of Meteorology has a section whose function is to monitor the movement of volcanic ash.
This section is called the Volcanic Ash Advisory Centre (VAAC) and is one of nine such offices that monitor global volcanic ash movements. The VAAC is based in Darwin and the majority of volcanic ash advisories that it issues are for areas to the north of Australia where there are many active volcanoes.
The VAAC monitor ash clouds by viewing specific wavelengths from satellites. They also track the wavelengths from satellite sensors that show sulphur dioxide (SO2). A lot of SO2 is released when a volcano erupts and if there is SO2 in the atmosphere it is highly likely that there will be volcanic ash in the same area.
The eruption of Mount Pinatubo in the Philippines in 1991 was thought to have released 20 million metric tonnes of SO2 into the atmosphere. The data from satellites along with forecast upper winds and complex dispersion models are used to predict the movement of the ash and provide warning to the aviation industry.
In a collaboration between Indonesian vulcanologists (who make eruption predictions for some Indonesian volcanoes) and the VAAC, it is now possible for the aviation industry to be warned of likely volcanic ash problems before the eruption has even occurred.
Satellite picture showing pumice raft near Tonga. Courtesy of NASA Earth Observatory. / Sailing through a pumice raft near Tonga. At times, out to the horizon looked like a desert the photographer said. Floating pumice partially fills North Bay along the southern coast of Kadavu, Fiji.
Volcanic eruptions can send ash very high into the atmosphere. The Mount Pinatubo eruption reached as high as 45 kilometres into the air; almost to the top of the stratosphere. The particles from Mount Pinatubo’s eruption that went into the stratosphere had an effect on the world’s climate. The particles changed the albedo (reflectivity) of that zone so more sunlight was reflected back to space. This cooled the planet by about 0.5°C for the years immediately after the eruption.
When a volcano erupts, vast amounts of ash are put into the atmosphere. Particles larger than half a millimetre (500 microns) generally fall to ground within the first hour. During the first 24 hours most of the ash greater than 25 microns falls to ground. However, as Australia and New Zealand saw during June, the finer ash particles can remain airborne for many days. In fact the very small particles can remain airborne for years.
Volcanoes can affect the mariner. It is obvious that if you are in a vessel in the vicinity of an erupting volcano, there may be hazards that could be catastrophic, such as: violent explosions; rock missiles (up to 80km from the volcano); pyroclastic flows; and tsunami. Further from the eruption the ash which is very abrasive can block vents and filters and cause rapid wear and tear if ingested into engines.
Another problem that may be encountered a long way from a volcano is pumice. Pumice is solidified lava ‘foam’. It is light and also very abrasive; you can buy pumice in chemists to scratch off hard skin. However, being light, pumice, at least initially, floats and can form pumice rafts that drift with the ocean currents. Navigating through these rafts is a bit like sailing through sandpaper. The pumice is very abrasive and smaller particles will block water intakes.

Darlington Jetty, Tasmania chartHigh tides in July

The cold and windy conditions that southeastern Australia experienced in early July highlighted the effect of meteorological conditions upon the mainly astronomical tides. During June 28-30 the southeast of Australia’s weather pattern was controlled by a very strong high pressure system of 1043 hPa in the Tasman Sea. Within a few days the high was replaced with a low pressure system of around 980 hPa that brought the adverse weather to several states.
high tides chartsThe sea level changes with the changing air pressure and winds. A high pressure pushes down on the ocean surface much like several people sitting on one end of a waterbed. As the weight of the high pressure was taken off the ocean (the glass falling) the weight was taken off the ocean (people began to get off the waterbed); the sea level rises.
On the graph above, the blue lines are the predicted astronomical tide, the red line is the actual sea level recorded. In the early part of the graph with the high pressure system in the area the recorded sea level was below the predicted tide. Later in the week under the influence of the low the sea level was very much above the predicted astronomical tide; by up to 600mm.
The graph is from the Bureau of Meteorology’s Maria Island station on Tasmania’s east coast. Graphs that show similar readings cover Tasmanian, Victorian and the south of New South Wales coastal areas.
The wind would also have added and subtracted to the sea level with the northerly winds from the high, blowing water away from the southeastern coastlines further reducing the sea level and the very strong southerly and southwesterly winds from low pushing water onto the Victorian and Tasmanian coastlines further adding to the sea level.