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

Abstract

The trajegtories 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 -•. 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 westefiies 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.

Fractal drifter trajectories in the Kuroshio extension

“…These data were included in the synthesis of the North Pacific subtropical circulation compiled by McNally et al (1983). Part of the data used here were also analyzed by Vastano et al (1985) and Kirwan and Cresswell (1982).…”

“…The first approach is essentially based on considering the "ensemble" behavior of many trajectories, with the aim of reconstructing the overall characteristics of the circulation. Some examples of these kinds of analyses are provided by Kirwan et al (1978), McNally et al (1983) and McNally and White (1985), who studied the circulation in the North Pacific; by Richardson (1983), who studied the mean kinetic energy distribution in the North Atlantic; by Molinari et al (1981), who analyzed the surface currents in the Caribbean Sea, by Schmitz et al (1981), Owens (1984), and Shaw and Rossby (1984), whose work was devoted to the study of the Gulf Stream and of the subtropical gyre with SOFAR floats; by Royer and Emery (1984), who studied the circulation in the Bering Sea, by Hofmann (1985), who analyzed the circulation in the Southern Ocean; and by Peterson (1985), who compared drifter data and current-meter measurements at Drake passage in the Southern Ocean. An extensive review of the drifter approach and of the technical problems related to the use of satellite-tracked drifters may be found in Kirwan and Cresswell (1982).…”

Measuring surface currents with Surface Velocity Program drifters: the instrument, its data, and some recent results

“…One of the earliest such deployments was in 1975 as part of the North Pacific Experiment (NORPAX). NORPAX drifters were 3 m long, 38 cm-diameter fiberglass cylinders, drogued at 30 m with a 9 m parachute (McNally et al, 1983). An array of 35 drifters, drogued with 200 m of polypropylene line and either a 25 kg weight or a windowshade sail, was deployed in the Gulf Stream region in 1976-1978(Richardson, 1980.…”

The Dispersal Barrier in the Tropical Pacific: Implications for Molluscan Speciation and Extinction

“…That some westward dispersal takes place on a smaller scale is indicated by the fact that the fish fauna of Johnston Island contains many species that are otherwise known only from the more northeasterly Hawaiian Islands (Randall et al, 1985). Circulation in this part ofthe North Pacific is such that most surface-water transport is from the east (McNally et al, 1983). Despite this and other local effects, it is reasonable to assume that populations in the eastern part of the range of many Western Pacific species are in genetic contact with, and from time to time are replenished by, larvae from westerly populations (see also Thresher and Brothers [1985] and Perron and Kohn [1985]).…”