Two independent data sets are used to diagnose the wind effect on the mesoscale activity in the North Atlantic Ocean. The oceanic surface variability is described with TOPEX/Poseidon sea surface height measurements from October 1992 to September 1997. Spatial extensions and temporal variations of high mesoscale activity are compared with the zonal and meridional components of the ERS-1 wind stress, the Ekman pumping, and the zonal Sverdrupian velocity. Regions with high mesoscale surface variability seem to be correlated to regions with eastward Sverdrupian velocities. When the Sverdrupian velocity is eastward and increases with time, the mesoscale activity is intensified, indicating a growth of instability rates. Theoretical and numerical results based on the interplay of barotropic eastward Sverdrup flows and westward propagation of long Rossby waves suggest that the wind controls western regions where baroclinic instability is favored. Interactions between the eddy field and the mean circulation are therefore analyzed to try to extract the instability mechanism responsible for the growth of the mesoscale activity along the Gulf Stream and the North Atlantic Current. Our results seem to suggest that the seasonal changes of mesoscale activity, associated with eastward currents, are linked to seasonal changes of the eastward Sverdrupian velocity. ¿ 1999 American Geophysical Union