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Ocean circulation is being altered by warmer seasons

In a process known as convection, warm and salty ocean water cools at the surface and gains density, causing it to sink deep into the sea. Researchers are now reporting that freshwater from melting glaciers and other sources has been significantly disrupting this deep ocean circulation process.

Experts at the GEOMAR Helmholtz Centre for Ocean Research Kiel focused their study on long-term observations in the Labrador and Irminger Seas.

“For various periods over the last 60 years, we have been able to combine important processes: atmospheric variability, such as the North Atlantic Oscillation, water and air temperatures, the occurrence of fresh surface water, and the duration of convection,” explained lead author Dr. Marilena Oltmanns.

The analysis revealed a distinct correspondence between sea surface temperatures in the Irminger Sea in the summer, the amount of freshwater on the surface layer, and the atmospheric conditions and onset of convection in the following winter.

“In case that warm summers with increased surface freshwater occur within extended warm periods, the ocean loses less heat in the following winter,” said Dr. Oltmanns. “As a result, the fresh surface layer that formed in summer remains stable for a longer time resulting in a delayed onset of convection.”

Freshwater is normally cooled and mixed by convection each winter. However, when convection is delayed, more freshwater remains near the surface and merges with additional freshwater in the spring.

“This effect could add up in future warm periods and thus weaken the convection – especially with regard to the rising temperatures and increased melting,” explained Dr. Oltmanns.

The research highlights the importance of long-term observations of global ocean circulation, particularly in regions that are frequently infiltrated with freshwater.

“Only through long-term measurement programs the connection between the complex oceanic and atmospheric processes can be identified,” said study co-author Dr. Johannes Karstensen.

“Thus, the continuous funding of personnel, ships and material is important – which in this case was provided through the support of individual projects of the German Science Foundation DFG, the German Federal Ministry of Education and Research.”

The study has been published in the journal Nature Climate Change.

By Chrissy Sexton, Staff Writer

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