The amount of coldest deep ocean water in the Southern Ocean, called Antarctic Bottom Water, has been decreasing for decades, researchers say.

Oceanographers from the National Oceanographic and Atmospheric Administration and the University of Washington said Antarctic Bottom Water has been disappearing at an average rate of almost 9 million tons per second over the past few decades, equivalent to about 50 times the average flow of the Mississippi River or about a quarter of the flow of the Gulf Stream in the Florida Straits, NOAA reported.

Antarctic Bottom Water is formed around Antarctica where seawater is cooled by the overlying air and made saltier by ice formation, then sinks to the seafloor and spreads northward, filling most of the deep oceans around the world as it slowly mixes with warmer waters above it.

"Because of its high density, Antarctic Bottom Water fills most of the deep ocean basins around the world, but we found that the amount of this water has been decreasing at a surprisingly fast rate over the last few decades," UW oceanographer Sarah Purkey said.

Previous studies have shown that the bottom water has been warming and freshening over the past few decades.

Researchers say their findings are a concern because the world's deep ocean currents transporting waters of varying temperatures play a critical role in transporting heat and carbon around the planet, regulating our climate.

"We are not sure if the rate of bottom water reduction we have found is part of a long-term trend or a cycle," co-author Gregory C. Johnson, a NOAA oceanographer, said.

"We need to continue to measure the full depth of the oceans, including these deep ocean waters, to assess the role and significance that these reported changes and others like them."