Impact of the North Atlantic Oscillation on the oceanic eddy flow: dynamical insights from a model-data comparison

Observational studies based on altimeter data have shown that in many regions of the World Ocean the oceanic eddy kinetic energy (EKE) significantly varies on interannual timescales. Comparing altimeter-based EKE maps for 1993 and 1996, Stammer and Wunsch (1998) discuss a significant meridional redistribution of EKE in the North Atlantic and speculated about the possible influence of the NAO cycle. This hypothesis is examined using 7 years of T/P altimeter data and three 1/6 o -resolution Atlantic Ocean model simulations from the French CLIPPER numerical experiment performed over the period 1979-2000. The subpolar-subtropical meridional contrast of EKE actually appears to vary on interannual timescales in the real ocean, and the model reproduces it realistically. The NAO cycle forces the meridional contrast of energy input by the wind. Our analysis suggests that after 1993, the large amplitude of the NAO cycle induces an adjustment of the large-scale circulation (Gulf Stream/North Atlantic Current), of its baroclinically unstable character, and, in turn, of the distribution of EKE. Model results suggest that before 1994, most NAO-like fluctuations were not followed by EKE redistributions, probably because NAO oscillations were weaker. Strong NAO index modulations are followed by gyre-scale EKE fluctuations with a 9-10 months lag, suggesting that complex, nonlinear adjustment processes are involved in this oceanic adjustment