Temporal Changes in Ocean Eddy Transports

Detlef Stammer Institut für Meereskunde, Universität Hamburg, Hamburg, Germany

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Carl Wunsch Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts

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Kyozo Ueyoshi Physical Oceanography Research Division, Center for Observations, Modeling and Prediction, Scripps Institution of Oceanography, La Jolla, California

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Abstract

New estimates from 11 yr of altimetric data are made of the global time-average variability kinetic energy and its decadal-scale variability. Making the approximation that the variability reflects primarily eddy motions, a time-mean, but spatially varying, eddy mixing coefficient is then estimated along with its changes over the last decade. With a record length more than 2 times that previously available, the time-mean variability kinetic energy KE is statistically more reliable and smoother in its spatial pattern. Minimum values of KE are present in the subpolar North Pacific Ocean and in the eastern South Pacific (both less than 100 cm2 s−2). In contrast to the North Pacific, the subpolar North Atlantic Ocean shows relatively enhanced KE. Eddy kinetic energy and eddy mixing appear to have declined during the last decade over large parts of the western Pacific Ocean, in some regions by as much as 50% of the time-mean value. Increased eddy variability can be found in the Kuroshio and Gulf Stream regions, as well as in the Agulhas region, east of Australia, and at several locations along the Antarctic Circumpolar Current. Somewhat enhanced eddy variability and eddy mixing are also apparent in the eastern tropical Pacific. A numerical simulation of the ocean circulation at 1° spatial resolution over a 10-yr period suggests that variations in eddy mixing of this order of magnitude measurably affect the deep temperature field in the vicinity of permanent frontal structures on a time scale of less than 4 yr. The meridional overturning circulation also reacts on these time scales. If persistent over longer periods in the ocean, these effects would be important for climate simulations.

Corresponding author address: Dr. Detlef Stammer, Institut für Meereskunde, Universität Hamburg, Bundenstrasse 53, 20146 Hamburg, Germany. Email: stammer@ifm.uni-hamburg.de

Abstract

New estimates from 11 yr of altimetric data are made of the global time-average variability kinetic energy and its decadal-scale variability. Making the approximation that the variability reflects primarily eddy motions, a time-mean, but spatially varying, eddy mixing coefficient is then estimated along with its changes over the last decade. With a record length more than 2 times that previously available, the time-mean variability kinetic energy KE is statistically more reliable and smoother in its spatial pattern. Minimum values of KE are present in the subpolar North Pacific Ocean and in the eastern South Pacific (both less than 100 cm2 s−2). In contrast to the North Pacific, the subpolar North Atlantic Ocean shows relatively enhanced KE. Eddy kinetic energy and eddy mixing appear to have declined during the last decade over large parts of the western Pacific Ocean, in some regions by as much as 50% of the time-mean value. Increased eddy variability can be found in the Kuroshio and Gulf Stream regions, as well as in the Agulhas region, east of Australia, and at several locations along the Antarctic Circumpolar Current. Somewhat enhanced eddy variability and eddy mixing are also apparent in the eastern tropical Pacific. A numerical simulation of the ocean circulation at 1° spatial resolution over a 10-yr period suggests that variations in eddy mixing of this order of magnitude measurably affect the deep temperature field in the vicinity of permanent frontal structures on a time scale of less than 4 yr. The meridional overturning circulation also reacts on these time scales. If persistent over longer periods in the ocean, these effects would be important for climate simulations.

Corresponding author address: Dr. Detlef Stammer, Institut für Meereskunde, Universität Hamburg, Bundenstrasse 53, 20146 Hamburg, Germany. Email: stammer@ifm.uni-hamburg.de

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