New ocean observations improve understanding of motion

Oceanographers commonly calculate large scale surface ocean circulation from satellite sea level information using a concept called “geostrophy,” which describes the relationship between oceanic surface flows and sea level gradient. Conversely, researchers rely on data from in-water current meters to measure smaller scale motion. New research led by University of HawaiÊ»i at Mānoa oceanographer Bo Qiu has determined from observational data the length scale at which using sea level height no longer offers a reliable calculation of circulation.

Upper-ocean processes dissipate heat, transport nutrients and impact the uptake of carbon dioxide—making circulation a critical driver of biological activity in the ocean. The movement of water in the ocean is determined by many factors including tides; winds; surface waves; internal waves, those that propagate within the layers of the ocean; and differences in temperature, salinity or sea level height. Additionally, like high and low pressure systems seen on TV weather maps, the ocean is full of eddies, slowly swirling masses of water.

“As length scales become smaller from several hundred miles to a few tens of miles, we discovered the point at which geostrophic balance becomes no longer valid—meaning that sea level is no longer useful for calculating ocean circulation,” said Qiu, a professor at the UH Mānoa School of Ocean and Earth Science and Technology and lead author of the study. “That is due to the presence of oceanic internal wave motions which essentially disrupts the motion that would be caused by geostrophy.”

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