"This means that changes in the heat content of the deep ocean, which is an important part of the three-dimensional global heat distribution, are far more sensitive to air–sea thermal interchanges [than previously believed]," Shuhei Masuda of the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) told environmentalresearchweb. "Our findings require a re-assessment of the role of the Southern Ocean in determining the impact of atmospheric warming on deep ocean waters."

Ice formation and heat exchange with the atmosphere around Antarctica create particularly dense water masses. These sink to the depths to form Antarctic Bottom Water. The northwards spread of this water over much of the world's ocean floor helps to drive the global thermohaline circulation.

Bottom waters in the Pacific Ocean warmed 0.003–0.01 °C from 1985 to 1999, according, in part, to measurements taken during the World Ocean Circulation Experiment.

Scientists have estimated that heat transport via mass movement through the ocean would take several centuries. For example, an abyssal current flowing at 0.001 m/s would take more than 350 years to travel 12,000 km. The new mechanism proposed by the team sees a reduction in the formation of Antarctic Bottom Water, due to the input of more heat from the atmosphere, alter isopycnal surfaces in the deep ocean. These isopycnal changes are transmitted as internal oceanic waves and affect deep waters in the whole North Pacific. The resultant weaker deep water current reduces abyssal cooling and enables geothermal heating to warm the bottom waters.

"Although I am not sure at this stage how this teleconnection puts an impact on future climate (e.g. whether it accelerates or decelerates global warming), these findings should be a steady forward step to resolve these issues," said Masuda.

Together with colleagues from JAMSTEC; Kyoto University, Japan; and Environmental Satellite Applications, UK, Masuda used an ocean data assimilation system based on a 4-dimensional variational (4D-VAR) adjoint approach, to examine the effect of changes in heat flux off the Adélie Coast of Antarctica on bottom water temperature at 47° N–170° E.

"I think the 4D-VAR data synthesis approach is nowadays the only way to resolve the difficulties evolving in the sporadic ocean observations/imperfect model simulations to represent the deep ocean properties," said Masuda. "An adjoint sensitivity analysis, which can be employed by using a part of the 4D-VAR system, has great advantages for pursuing the possible physical mechanism of any climate changes based on the primitive equations for ocean representation."

According to Masuda, the findings are the first step in revealing the changes in properties of the global deep ocean. Now the team plans to investigate the Atlantic and Indian oceans, where bottom-water warming has also been observed, as well as to work on discovering the impact of this rapid teleconnection on future climate.

The researchers reported their work in Sciencexpress.

Link:  http://environmentalresearchweb.org/cws/article/news/43057