The near‐surface circulation of the North Pacific using satellite tracked drifting buoys

Abstract

The trajectories of 16 satellite‐tracked drifting buoys from February 1976 through May 1980 and their derived velocities are used to construct a Lagrangian realization of the near‐surface circulation of the North Pacific subtropical gyre. A composite transit time around the periphery of this gyre is approximately 4.5 years at an average speed of 15 cm s−1. The details of the surface flow indicated regional differences in mesoscale energetics. Flow variability is low in the eastern and high in the western portions of the northern limb of the gyre. The highest mean speeds and minimum variances are found in the tropical currents of the southern limb. The drifter data is compared with the climatological mean field of dynamic topography (0/500 dbar). This comparison shows significant differences between drifter movement and inferred geostrophic flow. In the eastern mid‐latitude region the direction of the drifter motion and dynamic topography is in good agreement; however, the drifter speeds are 5 times greater. In the eastern and southern branches of the gyre, the current magnitudes of the drifters and geostrophic flow are comparable but their directions diverge. Comparison of the drifter trajectories and climatological seasonal sea level pressure maps reveal that the trajectory departures from the dynamic topography can often be related to atmospheric forcing. In the eastern mid‐latitudes, the large annual signal in the atmospheric westerlies correlates with a fivefold increase in drifter speeds from summer to winter. In the tropics, a small but persistent Ekman flow component causes the drifters to move northward from the equatorial regions where the mean geostrophic flow converges toward the equator. In the eastern limb of the gyre, response to the atmospheric forcing of the Pacific high pressure center causes the trajectories to cross isolines of dynamic topography. The drifters provide a unique insight into the spatial and temporal relationship of geostrophic and wind‐driven flow components that influence the near‐surface circulation of the subtropical gyre.