Look closely and you may see changes under way at your local beach as the El Nino gets going. Photo: John Veage
El Niños are well known for unleashing droughts, hotter-than-usual weather and paradoxically worse frosts in Australia, but they also drive remarkable - if temporary - changes to global sea levels.
While the shifts under way are so far barely discernible along the Australian coast, pointers of the forces are more evident in countries to Australia's north.
"The western Pacific has already dropped about 20cm below its normal height, and the eastern Pacific - because it's warmed up, it's expanded – is 20-30cm higher than normal," Andrew Watkins, head of climate prediction services at the Bureau of Meteorology, said.
"You literally start to see less water in the western pacific flowing through the Indonesian Archipelago."
Watched warily by farmers and climate scientists alike, El Niños alter the Pacific Ocean and atmospheric circulation in ways that tend to dry western regions such as eastern Australia while leading to soaking rains over nations on the ocean's eastern fringe.
Easterly trade winds that normally act to bank up water on the western Pacific stall or even reverse. As in a giant bath tub, some of the water sloshs back, lifting sea-levels in the east.
Another effect of the wind shift is that cooler, deep water that would normally rise in the eastern Pacific, stays deep, leaving the surface free to warm up and store more of the sun's energy as heat, Dr Watkins said.
"As the water warms it expands, and it can't go down or sideways, only up," he said. "Hence, the sea-level rise."
The El Niño effect on our oceans, though, doesn't stop there.
Moninya Roughan, an oceanographer at the University of NSW, said El Niño events can last 12-18 months, but the effects for coastal south-eastern Australia will take a while longer to fully play out.
The university's studies show the volume of water being moved by the main East Australian Current "increases approximately six to nine months after an El Niño event decays", Associate Professor Roughan said.
"As the El Niño event ends, we will see a southward penetration of the East Australian Current down the coast of Australia," she said, adding "we see that the [current] remains closer to the coast".
Warmer seas further south, such as along the southern NSW coast and Tasmania, will be welcomed by swimmers if not by marine life that is already threatened by the spread of warmer-climate species.
Global sea levels
While El Niños, may lower western Pacific sea levels, the effect on average global sea levels is to nudge them noticeably higher.
Satellite data from France's Centre National d'Etudes Spatiales reveal the impact of El Niños in lifting sea levels. Conversely, La Niñas - the opposite weather pattern in the equatorial Pacific with the west warming and the east cooling - work to lower them.
John Church, an oceanographer at the CSIRO, said the impact of El Niño-La Niña events on sea levels has less to do with changes in the warmth of oceans than in how much water ends up falling and remaining on land rather than ending up in the seas.
"There's probably a smaller element of thermal expansion but the dominant component is this land water storage issue for mass changes in the ocean," Dr Church said.
The big La Niña events in 2010-11, for instance, resulted in heavy rains over inland Australia, large enough to stall the background global sea-level rise of about 3.2mm a year caused by climate change melting land-based ice.
A recent paper published in Geophysical Research Letters, though, found the pace of global sea-level rise can be masked by the El Niño-La Niña pivot. Once the land water storage effect is stripped out, sea-level rises are accelerating.
The researchers, from Beijing's University of the Chinese Academy of Sciences, and Hokkaido University in Japan, found that the increase in global mean sea levels has accelerated to 4.4mm (+/-0.5mm) a year since 2010.
The source of the rising sea levels is the increasing loss of ice mass in Antarctica, Greenland and other glaciers and ice caps, as shown in this chart of gigatonne-losses since 2003:
(Sydney Morning Herald)
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