Propagation of Big Island eddies

Using satellite altimetry data, we have observed a series of anticyclonic eddies as they form at the Big Island of Hawaii and have tracked them as they move away from the island. While similar eddies have been observed near the Hawaiian Islands in previous studies, the fate of the anticyclonic eddies has previously been unclear. The eddies that we observed initially propagated to the southwest but consistently changed propagation direction to the northwest later in their lifetimes. This was intriguing to us, as theoretically, the decay of isolated anticyclonic eddies on a β plane should cause them to continually move toward the southwest. Such isolated eddy dynamics are unable to account for the observed change to northwestward eddy propagation, and the presence of the westward flowing North Equatorial Current turns out to be important to the Big Island eddy dynamics. The eddies are not passively advected by the North Equatorial Current; rather, the mean flow changes the propagation characteristics of the eddies. An existing theory that includes meridionally varying, purely zonal mean flow is shown to account for the observed propagation of the Big Island eddies if the zonal variation of the mean flow is considered.