The Indian Ocean is losing salinity at an "astonishing" rate, attracting the attention of scientists.
The salinity of ocean water determines how it accumulates in layers, how currents transport heat around the planet, and how easily nutrients reach the sunlit surface, where most marine life begins. For this reason, scientists are paying close attention to a remarkable transformation currently taking place off the coast of Western Australia, where the waters of the southern Indian Ocean are experiencing a rapid decrease in salinity.
Climate regulation
According to a study published by SciTechDaily, citing the journal Nature Climate Change, researchers from the University of Colorado Boulder reported that rising global temperatures over the past 60 years have altered major wind patterns and ocean currents. These shifts are leading to an increasing flow of fresh water into the southern Indian Ocean. Researchers warn that this trend could reshape how the ocean and atmosphere interact, impacting major circulation systems that regulate climate worldwide and intensifying pressure on marine ecosystems. Professor Wei Qing Han of the Department of Atmospheric and Oceanic Sciences said: "We are witnessing a large-scale shift in how fresh water moves through the ocean. This shift is taking place in a region that plays a pivotal role in global ocean circulation."
"Freshwater Complex"
The source of much of the fresh water flowing into a vast tropical region can be traced back to where surface water is naturally diluted by heavy rainfall. This region, stretching from the eastern Indian Ocean to the western Pacific Ocean in the tropical zones of the Northern Hemisphere, maintains its relative purity due to high rainfall and relatively low evaporation rates. Scientists often refer to it as the Indo-Pacific freshwater reservoir.
"Conveyor belt"
The "freshwater complex" is connected to the thermal and saline circulation, a global current system sometimes described as a "conveyor belt" because it transports heat, salt, and fresh water between ocean basins.
Meanwhile, warm surface waters from the Indian and Pacific Oceans feed pathways that ultimately affect conditions in the Atlantic Ocean. In the North Atlantic, the transported water cools, thickens, sinks, and then flows southward into the depths before eventually flowing back into the Indian and Pacific Oceans. Even slight changes in salinity can make a big difference, as salt helps determine the density of seawater, and density, in turn, influences the sinking and spreading processes that keep the system moving.
Drinking water for over 380 years
The waters off the coast of southwestern Australia are typically dry at the surface, with evaporation exceeding rainfall. Historically, this pattern has contributed to increased salinity. However, long-term observations indicate that this balance is changing. Han's team estimates that the area covered by saltwater in this region of the southern Indian Ocean has shrunk by about 30% over the past 60 years. They describe this as the fastest decline in salinity seen anywhere in the Southern Hemisphere. The study's lead researcher, Jingxin Chen, a visiting researcher in the Department of Atmospheric and Oceanic Sciences and chief scientist at the South China Sea Oceanographic Institute of the Chinese Academy of Sciences, said, "This increase in salinity is equivalent to adding about 60% of the freshwater in Lake Tahoe to the region annually." He added, "To put this in perspective, the amount of freshwater flowing into this ocean region is enough to supply the entire population of the United States with drinking water for more than 380 years."
less dense seawater
Researchers discovered that the increased flow of fresh water cannot be explained by local rainfall. By analyzing observational records along with computer simulations, they concluded that global warming is reshaping surface wind patterns across the tropical Indian and Pacific Oceans. These changing winds direct ocean currents in ways that transport more fresh water from the freshwater region of the Indian and Pacific Oceans to the southern Indian Ocean. As salinity levels decrease, seawater becomes less dense. Freshwater tends to remain above the salty, heavier water, increasing the separation between surface and deep layers. This increased stratification limits vertical mixing, the process that normally allows surface water to sink and deep water to rise, distributing heat and nutrients throughout the ocean.
Damage to marine life
Furthermore, reduced vertical mixing can harm marine life. When nutrient-rich water from the depths doesn't reach the sunlit surface, organisms in the upper layers have fewer resources to survive. At the same time, reduced mixing traps excess heat near the surface, further increasing temperatures for species already suffering from rising ocean temperatures. Chen concluded by saying that "salinity changes can affect plankton and seagrass, which form the basis of the marine ecosystem," explaining that "changes in the food web could have a far-reaching impact on ocean biodiversity."
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