The variability of ocean currents

Throughout the past 4 years the investigation of midocean low‐frequency mesoscale currents by American physical oceanographers has accelerated and intensified. These oceanic motions (referred to as ‘eddies’), which are apparently to some extent dynamically analogous to the atmospheric synoptic scale motions, are characterized by wavelengths of the order of a few hundred kilometers and periods of a few months, with longer time and space scales predominant above the main thermocline. Substantial energy is contained in the barotropic and first baroclinic vertical modes with rms speeds an order of magnitude greater than those characteristic of the mean circulation. The midoceanic eddies are of interest not only because they represent the dominant energy containing scales but because of possible energy transformations between the eddies and motions on longer and shorter time and space scales both locally and at a distance. The determination of the interaction of the eddies and the general circulation through mechanisms of transport and transfer of, e.g., heat, momentum, and vorticity, is of particular importance. This review will be primarily concerned with progress in the study of the variability of the midoceanic current field but will also include some related studies of the low‐frequency variability of the western and northwestern boundary currents of the main subtropical gyres. Some references to non‐U.S. research are given as appropriate because of importance or direct relationship.