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Dynamical Response of the Oceanic Eddy Field to the North Atlantic Oscillation: A Model–Data Comparison

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  • 1 Center for Ocean–Atmospheric Prediction Studies, The Florida State University, Tallahassee, Florida
  • | 2 Laboratoire des Ecoulements Géophysiques et Industriels, Grenoble, France
  • | 3 Department of Oceanography, The Florida State University, Tallahassee, Florida
  • | 4 Center for Ocean–Atmospheric Prediction Studies, The Florida State University, Tallahassee, Florida
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Abstract

Observational studies have shown that in many regions of the World Ocean the eddy kinetic energy (EKE) significantly varies on interannual time scales. Comparing altimeter-derived EKE maps for 1993 and 1996, Stammer and Wunsch have mentioned a significant meridional redistribution of EKE in the North Atlantic Ocean and suggested the possible influence of the North Atlantic Oscillation (NAO) cycle. This hypothesis is examined using 7 yr of Ocean Topography Experiment (TOPEX)/Poseidon altimeter data and three ⅙°-resolution Atlantic Ocean model simulations performed over the period 1979–2000 during the French “CLIPPER” experiment. The subpolar–subtropical meridional contrast of EKE in the real ocean appears to vary on interannual time scales, and the model reproduces it realistically. The NAO cycle forces the meridional contrast of energy input by the wind. The analysis in this paper suggests that after 1993 the large amplitude of the NAO cycle induces changes in the transport of the baroclinically unstable large-scale circulation (Gulf Stream/North Atlantic Current) and, thus, changes in the EKE distribution. Model results suggest that NAO-like fluctuations were not followed by EKE redistributions before 1994, probably because NAO oscillations were weaker. Strong NAO events after 1994 were followed by gyre-scale EKE fluctuations with a 4–12-month lag, suggesting that complex, nonlinear adjustment processes are involved in this oceanic adjustment.

Current affiliation: Laboratoire des Ecoulements Géophysiques et Industriels, Grenoble, France

Corresponding author address: Thierry Penduff, LEGI-MEOM, BP53, 38041 Grenoble Cedex 9, France. Email: thierry.penduff@hmg.inpg.fr

Abstract

Observational studies have shown that in many regions of the World Ocean the eddy kinetic energy (EKE) significantly varies on interannual time scales. Comparing altimeter-derived EKE maps for 1993 and 1996, Stammer and Wunsch have mentioned a significant meridional redistribution of EKE in the North Atlantic Ocean and suggested the possible influence of the North Atlantic Oscillation (NAO) cycle. This hypothesis is examined using 7 yr of Ocean Topography Experiment (TOPEX)/Poseidon altimeter data and three ⅙°-resolution Atlantic Ocean model simulations performed over the period 1979–2000 during the French “CLIPPER” experiment. The subpolar–subtropical meridional contrast of EKE in the real ocean appears to vary on interannual time scales, and the model reproduces it realistically. The NAO cycle forces the meridional contrast of energy input by the wind. The analysis in this paper suggests that after 1993 the large amplitude of the NAO cycle induces changes in the transport of the baroclinically unstable large-scale circulation (Gulf Stream/North Atlantic Current) and, thus, changes in the EKE distribution. Model results suggest that NAO-like fluctuations were not followed by EKE redistributions before 1994, probably because NAO oscillations were weaker. Strong NAO events after 1994 were followed by gyre-scale EKE fluctuations with a 4–12-month lag, suggesting that complex, nonlinear adjustment processes are involved in this oceanic adjustment.

Current affiliation: Laboratoire des Ecoulements Géophysiques et Industriels, Grenoble, France

Corresponding author address: Thierry Penduff, LEGI-MEOM, BP53, 38041 Grenoble Cedex 9, France. Email: thierry.penduff@hmg.inpg.fr

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