New Delhi: A kilometre beneath the ocean’s surface, scientists have noticed a change that could impact weather, monsoons, and how the sea stores carbon and heat. Atlantic Intermediate Water, a large body of water spreading from the Southern Ocean and flowing into the Atlantic, Indian and Pacific oceans, has become saltier after decades of growing less salty, a new study found.
This sudden reversal began around 2015, coinciding with a sharp decline in Antarctic sea ice, a new study published in Nature Climate Change found. Researchers warn that as this layer gets saltier, it could change ocean circulation and the movement of heat, eventually even impacting global climates.
Kaushik Mishra, a PhD student at the Indian Institute of Science (IISc) said that the last time such an event happened was nearly 4,00,000 or 3,00,000 years ago, when the climate was transitioning from a glacial to an interglacial period. At the time, the change was gradual, spanning a few thousand years. But this time, in just about 10 years, the change in sea salinity is hard to ignore.
“Sitting in India, why do we care about it at all? The answer is that this intermediate water acts like a barrier between the upper and deeper oceans,” lead author Mishra told ThePrint.
Mishra, Bishakhdatta Gayen, a professor at IISC and the University of Melbourne, along with Alberto C Naveira Garabato, a professor at the University of Southampton, published their findings on 28 October.
When they first began looking into Antarctic Intermediate Water (AIW), its salinity was hardly their focus.
An unexpected discovery
Mishra and his team initially set out to study how the Antarctic Intermediate Water flows and mixes into the Indian Ocean, and how its properties change over time. As they examined recent measurements, they noticed a sudden reversal around 2015-2016. For decades, this mass of water had been growing fresher, but that changed dramatically after Antarctic sea ice declined.
“The Antarctic region is the freshwater engine of oceans around the globe. Melting Antarctic sea ice influences circulation thousands of kilometres away. Until 2016, the seasonal summer melt was triple the Amazon River’s entire annual flow of roughly 6,600 cubic km. From 2016, Antarctic sea ice shrank to record lows. This abruptly choked off that critical seasonal surge and the meltwater delivery dropped to roughly double the Amazon’s annual flow,” co-author professor Gayen said in a statement.
Sea ice forms during the Antarctic winter and melts as temperatures rise. When it melts, it releases freshwater into the ocean, which feeds into the AIW. This water then sinks beneath the surface and gradually moves northwards. Now that there is less freshwater, saltier water is moving upwards from the ocean’s depths, changing the balance between the different layers.
“If this barrier sort of reorganises itself, there will be a reorganisation in the ocean heat content. And 90% or more of the anthropogenic heat is stored in the global oceans,” said Mishra.
He explained that the team mainly used satellite observations of sea ice and measurements from Argo floats, small robotic instruments that record the ocean temperature and salinity at different depths. They found that AIW had become saltier by 0.0048 grams per kilogram per decade around the Southern Ocean.
“This might be a small change in absolute terms, but it affects the density of the seawater and how ocean layers mix,” said Mishra.
Also read: AI found a painting stolen in 1972. How Italy is using tech to recover missing art
Impact of the sudden change
The Antarctic Intermediate Water is also a part of the Atlantic Meridional Overturning Circulation (AMOC), a system of currents that redistribute heat across the Atlantic Ocean. As it changes, it could even influence this larger system, although this remains to be proven.
Researchers also highlight that changes in the ocean’s layers impact how much carbon the sea can store, but Mishra clarifies that their study had not quantified these effects.
For India, a saltier Antarctic Intermediate Water could eventually influence summer monsoons too, but Mishra said that just a decade of observations is not enough to make accurate predictions about the climate impacts of such a change. These findings bring current climate models into focus too.
“Our climate models have not yet learnt to factor in these changes. I think the oceanography community has to start tinkering and fine-tuning the ocean and climate models and definitely not wait for 20, 30 years more,” said Mishra.
He added that many of the impacts of such changes in the ocean would have already manifested themselves by the time humanity learns to foresee them.
For Mishra, the collaboration with the University of Melbourne and the University of Southampton also allowed him to explore a side of climate science that receives very little attention in India.
“In India, whenever you see climate news, it is mostly just focused on the monsoon. Very rarely do you see people working on research about Antarctica or the Southern Ocean sitting in India. But these changes will affect us, and soon,” he said.
(Edited by Ratan Priya)