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Western boundary currents (WBCs) modulate the global climate and dominate regional ocean dynamics. Despite their importance, few direct comparisons of the kinematic structure of WBCs exist, due to a lack of equivalent sustained observational data sets. Here we compare multiyear, high-resolution observations (1km, hourly) of surface currents in two WBCs (Florida Current and East Australian Current) upstream of their separation point. Current variability is dominated by meandering, and the WBCs exhibit contrasting time-mean velocities in a Eulerian coordinate frame. By transforming to a jet-following coordinate frame, we show that the time-mean surface velocity structure of the WBC jets is remarkably similar, considering their distinct local wind, bathymetry, and meandering signals. Both WBCs show steep submesoscale kinetic energy wavenumber spectra with weak seasonal variability, in contrast to recent findings in other ocean regions. Our results suggest that it is the mesoscale flow field that controls mixing and ocean dynamics in these regions.

Plain Language Summary Western boundary currents (WBCs) are warm, strong currents that flow along the western sides of the world's ocean basins. They dominate the climate and ecology of some of the most densely populated coastlines on Earth. Considering their importance, there is surprisingly little knowledge of how different WBCs compare. In this paper we use high-resolution measurements to study the mean structure and variability of two WBCs, in the North Atlantic and South Pacific, and show that while they differ in their variability, they are remarkably similar in their time-mean. We discovered this by transforming the coordinate frame in which we viewed them, so that the variability of the meandering currents does not affect the mean calculation. We quantify the amount of variability at different spatial scales and show that for these WBC regions, variability is dominated by the large-scale flow field, with no seasonal cycle at the smaller scales, in contrast to studies of other ocean current regimes. These high-resolution results provide an updated view of WBC systems globally and can be used by numerical modelers to evaluate the performance of their models, helping to improve forecasts of a changing climate.