A phenomenon in the Indian Ocean that affects events in southeast Australia is helping to predict extreme weather up to six months in advance.
Image credit: Csiro
The phenomenon, the Indian Ocean Dipole, is the difference in sea-surface temperatures between the western and eastern part of the Indian Ocean, and until recently has been one of the most influential but the least understood natural forces affecting Australia’s climate.
An international team of scientists, led by CSIRO Wealth from Ocean Flagship’s Dr Wenju Cai, confirmed the link and have published their findings in the journal Nature Geoscience.
A better understanding of the relationship between the Indian Ocean Dipole and extreme weather events will enable farmers, industry, communities and governments to better anticipate and prepare for droughts and increased bushfire risk, up to six months in advance of the event.
Just as the El Niño Southern Oscillation (ENSO) affects weather patterns across the Pacific Ocean, the Indian Ocean Dipole influences weather and extreme events across the Indian Ocean. While ENSO fluctuates between 'El Nino', 'neutral' and 'La Nina' phases, the Dipole fluctuates between 'positive', 'neutral' and 'negative' phases approximately every three to eight years.
The positive phase is characterised by greater-than-average sea-surface temperatures, more rain in the western Indian Ocean region and cooler waters in the eastern Indian Ocean. It tends to cause droughts in East Asia and Australia, and flooding in parts of the Indian subcontinent and East Africa.
Positive Dipole activity has, to date, preconditioned major wildfires in southeast Australia, caused coral reef death across western Sumatra, and exacerbated malaria outbreaks in East Africa.
Dr Cai said the findings provide greater confidence in predicting extreme weather up to two seasons in advance, and furthermore, projecting positive IOD events into the future.
“Over the past 50 years, the Dipole has been trending upwards, increasing the number of positive events, occurring an unprecedented 11 times over the past 30 years,” Dr Cai said.
“For example, there were three consecutive positive Dipole events between 2006 and 2008, which preconditioned the catastrophic Black Saturday bushfires in Victoria.”
He said the increased frequency is due to the tropical Indian Ocean warming faster in the west than the east, due in part to the increasing temperature of Earth’s surface.
“This warming pattern will continue in the decades to come, according to the state-of-the-art global climate models used in the study,” Dr Cai said.
He said that as the warming pattern continues, future changes will include drier winter and spring seasons over southern Australia, particularly during positive Indian Ocean Dipole years.
Research into the Indian Ocean by CSIRO’s Wealth from Oceans Flagship enables better understanding of climate processes affecting Australia, detecting our changing climate, and reducing uncertainty in Australian climate projections.
Access the full paper at Nature Geoscience [external link].
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The Indian Ocean Dipole
The Indian Ocean Dipole refers to a pattern of sea surface temperature variations in the tropical Indian Ocean that varies from one year to another.
At its positive phase, the eastern Indian Ocean (off Sumatra-Java) is cooler than normal whereas the west is warmer than normal. These changes alter the atmospheric circulation, enhancing convection (hot and moist air rising) and rainfall over the west, suppressing convection and rainfall in the east Indian Ocean and surrounding countries including Australia.
The change in convection induces easterlies along the equator and alongshore Sumatar-Java, which causes upwelling (subsurface cold water to surface), further enhancing the sea surface temperature contrast between the west and the east.
Paper in Nature Geoscience
Nature Geoscience is a prestigious scientific journal that publishes high impact scientific information.
Dr Wenju Cai’s paper Projected response of the Indian Ocean Dipole to greenhouse warming was published in Nature Geoscience on 28 November 2013. It was featured on the front cover of the journal.
The paper outlined the findings of a complex computer simulation of the Indian Ocean Dipole using ocean and climate data, going back as far as 1854.
Over the past 31 years, the frequency of positive Indian Ocean Dipole events is unprecedentedly high, and the occurrences include 1982, 1987, 1991, 1994, 1997, 1999, 2002, 2004, 2006, 2007, 2008. These events contributed to major bushfires over southeast Australia including the Ash Wednesday in 1983, in addition to Black Saturday in 2009.
The paper was the result of collaboration between researchers from CSIRO’s Wealth from Oceans Flagship and Water for a Healthy Country Flagship, the Ocean University of China, the University of Exeter (UK), the Laboratoire d’Océanographie et du Climat (France), the First Institute of Oceanology (China), the and Japan Agency for Marine-Earth Science and Technology.
Dr Wenju Cai and CSIRO’s Wealth from Oceans Flagship
Dr Wenju Cai leads research on how the ocean is changing, the role of the ocean in climate variability that affects Australia climate and extreme events, and how climate variability will respond to greenhouse warming.
Research into the Indian Ocean by CSIRO’s Wealth from Oceans Flagship enables better understanding of climate processes affecting Australia, detecting our changing climate, and reducing uncertainty in Australian climate projections. Dr Cai’s work is partly funded by the Goyder Research Institute.
Oceans and climate
Monitoring, modelling and predicting change in the ocean is essential to climate and weather forecasting, climate adaptation, marine safety and conservation, national security and sustainable resource use on land and at sea.
Oceans play a fundamental role in the climate system, transporting heat, driving the hydrological cycle (hot moist air rises in the tropics, flows poleward delivering heat and rain en route) and moderating the rate at which carbon dioxide is rising in the atmosphere.
CSIRO scientists, engineers and technicians use innovative techniques to observe the ocean, often with Australian and international programs and colleagues. We study ocean currents, chemistry and productivity from satellite readings of sea height, temperature and colour, and use sensors on moored platforms and autonomous vehicles, and deployed from research and commercial vessels.
Models that link the ocean, atmosphere and terrestrial systems predict global phenomena such as carbon dioxide absorption and sea level rise associated with the enhanced greenhouse effect, and events that dictate Australia's long-term climate variability.
The models provide the basis of climate and weather forecasts that guide infrastructure and industry development, groundwater resources, wind prospecting, public health and safety and adaptation to climate change.