Flooding and landslides in Assam and Bihar have killed dozens of people and forced hundreds of thousands from their homes. Meanwhile, thousands of miles away in Australia, a period of hot, dry weather has led to a spate of bushfires.Both weather events have been linked to higher-than-usual temperature differences between the two sides of the Indian Ocean – something meteorologists refer to as the Indian Ocean Dipole.
What exactly is the dipole and how does it work?
The Indian Ocean Dipole – often called the “Indian Niño” because of its similarity to its Pacific equivalent – refers to the difference in sea-surface temperatures in opposite parts of the Indian Ocean that is Western Arabian Sea and the Eastern Indian Ocean, south of Indonesia. Temperatures in the eastern part of the ocean oscillate between warm and cold compared with the western part, cycling through phases referred to as “positive”, “neutral” and “negative”.
The dipole’s positive phase this year – the strongest for six decades – means warmer sea temperatures in the western Indian Ocean region, with the opposite in the east.The result of this unusually strong positive dipole this year has been higher-than-average rainfall and floods in eastern part of the country, Assam and Bihar and droughts in the Indian Peninsula.
Dr Andrew Turner, a lecturer in monsoon systems at the UK’s University of Reading, “When an Indian Ocean dipole event occurs, the rainfall tends to move with the warm waters, so you get more rainfall than normal over the Eastern part of the country and in East Asian Countries. On the other hand, in the east of the Indian Ocean, sea surface temperatures will be colder than normal and that place will get a reduced amount of rainfall.”
Andrew Watkins, head of long-range forecasts at the bureau, said the dipole was crucial to understanding the heatwave.
“The key culprit of our current and expected conditions is one of the strongest positive Indian Ocean dipole events on record,” he says.
“A positive IOD means we have cooler than average water pooling off Indonesia, and this means we see less rain-bearing weather systems, and warmer than average temperatures for large parts of the country.”
However, a negative dipole phase would bring about the opposite conditions – warmer water and greater precipitation in the eastern Indian Ocean, and cooler and drier conditions in the west.A neutral phase would mean sea temperatures were close to average across the Indian Ocean.
Heavy downpours have devastated parts of Assam over the last two months, with the Assam seeing up to 300% above average rainfall between May and mid-July.
Assam and Bihar have been hit with flash floods and landslides hitting communities across the region.
Several states in India are facing flood-like situation due to incessant rains. From Kerala in the South to Assam in the Northeast, floods have wreaked havoc.
Many rivers are flowing above the danger mark, aggravating an already dangerous situation. The India Meteorological Department (IMD) has warned of fairly widespread rainfall in Punjab, Haryana, Chandigarh, Delhi, Uttar Pradesh and East Rajasthan on Tuesday and Wednesday.
In Gujarat and parts of Madhya Pradesh, there is forecast for heavy rains till Friday.
Extreme climate and weather events caused by the dipole are predicted to become more common in the future as greenhouse gas emissions increase.
In a 2014 study published in Nature, scientists in Australia, India, China and Japan modelled the effects of CO2 on extreme Indian Ocean dipoles, such as those in 1961, 1994 and 1997.
Assuming emissions continue to go up, they projected that the frequency of extreme positive dipole events would increase this century from one every 17.3 years to one every 6.3 years.
“The countries in the west of the Indian Ocean, so on the African coast, are going to see much, much more flooding and heavy rainfall relating to these events,” says Dr Turner. “You’re going to get more damaging impacts on crops and on infrastructure and flooding.
“On the other hand, in the east of the Indian Ocean, islands on the west side of Indonesia are going to see a greater chance of drought and reduced rainfall.”
