鈥淚t was blowing a perfect hurricane, the squalls coming down in regular gusts鈥� the sea at this time running mountains high, and very broken.鈥�
That was one of the written in the logbook of a ship called the Catherine Hill, before a giant storm drove it aground north of Sydney, Australia, on 21 June 1867. That storm and others like it are now being studied to help weather forecasters predict future recurrences.
Australia鈥檚 eastern seaboard is lashed by monster storms called 鈥渆xtreme east coast lows鈥� about once every 10 to 20 years. Their infrequency makes them notoriously difficult to predict. For example, meteorologists failed to foresee the severity of , which caused the Pasha Bulker coal ship to run aground 100 kilometres north of Sydney.
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鈥淢ost forecasters would only see one such event during their careers, and nobody working on the forecast desk at the time had ever seen anything like it,鈥� says at Macquarie University in Sydney.
Most of what we know about these storms is based on satellite data, which only goes back to 1979.
Historical storms
To build a more comprehensive picture, Browning and his colleague pieced together data going back almost two centuries.
First, they collected temperature and pressure measurements from a handful of weather stations that have records back to the 1850s. They also studied coastal erosion patterns to see when major storm damage occurred.
Lastly, they cross-checked this information with tales of big storms, ship logbook extracts, and daily weather reports published in the Sydney Morning Herald newspaper from 1831 onwards. This helped them reconstruct some of Australia鈥檚 worst maritime storms.
The most famous was the storm of 20 August 1857. This sank the Dunbar ship just outside Sydney, killing more than 120 people. One described how 鈥淭he decks burst up from the pressure of the water, the ship was rent into a thousand pieces, and all on board鈥� were hurried into the foaming and terrific sea.鈥�
Foaming seas
Browning and Goodwin found that severe storms are most likely to hit the east coast when shifts occur in two related weather patterns, both of which affect sea surface temperatures: the El Ni帽o Southern Oscillation (ENSO) and the Interdecadal Pacific Oscillation (IPO).
Conditions are most ripe when the ENSO switches to a La Ni帽a phase and the IPO switches from positive to negative. La Ni帽as warm the sea surface around Australia, and are stronger and more frequent during negative IPO phases.
When the warm sea air collides with cold air on land, the resulting storms can be so intense they are known as 鈥渨eather bombs鈥�, says Browning. Because cold air is denser and heavier, it forces the warm air upwards. Rising air releases moisture in the form of rain, as well as lots of energy, which causes strong winds and ocean waves up to .
Browning presented the findings at the (AMOS) conference in Sydney last week.
Winter nights
The researchers also found that extreme east coast lows are more common in southern hemisphere autumn and winter, particularly during June, and tend to peak at night. At these times, air over the sea cools more slowly than air over land, creating temperature differences.
Browning and Goodwin now plan to refine their predictions using weather observations from old ship logbooks. of the National Institute of Water and Atmospheric Research in New Zealand has digitised more than 130,000 logbook images from whaling, trade and migrant ships that visited the region from the 18th century onwards. She also presented her work at the AMOS conference.
Browning hopes the new insights will help meteorologists better predict monster storms, so that ships can avoid them and flood warnings can be put in place along the coast.
Fortunately, Browning says we are not likely to see any such storms for at least a decade, because the IPO is about to switch into its less-risky positive phase.
