From a conceptual point of view the modulated response of subtropical gyres is of considerable importance because it helps explain the abrupt change in the vorticity of the western boundary current off the coasts. This phenomenon, which will be confirmed for longer periods, allows to solve a mystery that has persisted since the work of the Norwegian oceanographer and meteorologist Harald Ulrik Sverdrup (1888-1957). Sverdrup has been appointed director of the prestigious Scripps Institution of Oceanography (SIO) in California from 1936 to 1948. During this period he developed his theory of the circulation of the oceans. As we will see, this theory based on the formation of steady surface currents driven by wind stress only partially explains the functioning of the subtropical gyres. In particular, it does not apply to the change in vorticity of western boundary currents when they merge with the subtropical gyres or, conversely, when they deviate to migrate poleward as do the drift currents in the northern hemisphere, or the circumpolar currents in the southern hemisphere. Indeed, any reasoning implying a purely inertial behavior (accelerated by the Coriolis force alone) of the gyre is incomplete because it must involve the modulated response of subtropical gyres and the associated geostrophic forces (combination of Coriolis force and gravity) around the gyres.
Sverdrup (1947) and Stommel (1948) theories do not explain more the total transport of western boundary currents, which can be understood only by considering the modulated response of subtropical gyres. Whether the change in vorticity or the magnitude of the flow of western boundary currents, attempts at explanation have been given since the pioneering work of Sverdrup, but unconvincingly.
Munk, W. H. On the wind-driven ocean circulation, J. Meteorol., Vol. 7,1950
Stommel, H., The westward intensification of wind-driven ocean currents, Trans. Amer. Geophys. Union, 1948, 29, 202-206.
Sverdrup, H.U., Wind-Driven Currents in a Baroclinic Ocean; with Application to the Equatorial Currents of the Eastern Pacific, Proc. Natl. Acad. Sci., 1947, 33, 318-326.