Preliminary results from directly measuring middepth circulation in the tropical and South Pacific

As part of the World Ocean Circulation Experiment, 300 Autonomous Lagrangian Circulation Explorer floats were deployed in the tropical and South Pacific to measure the general circulation at 900 m depth. Most floats measured average currents over 25 submerged days between ascents to the surface for locating and data relay. By the end of 1997, over 12,500 observations spanning 840 float years were obtained. Accuracy of the average velocity is limited by variability, some of which is found to arise from steady flows with spatial variations smaller than averaging scales. Observed 25-day velocities are serially uncorrelated except near the equator where energetic zonal variability has seasonal timescales and appears to be described by a data-assimilating ocean circulation model run by the National Centers for Environmental Prediction. Space-time averages of float velocities disclose a general pattern of middepth flow that resembles the surface circulation with the strongest flows of O(3 cm/s) in the deep Antarctic Circumpolar Current and East Australian Current. The weaker elements of the flow, including an interior subtropical circulation with O(millimeters per second) flows, are described by an objective analysis whose accuracy is limited by sampling noise rather than uncertainty of the analysis parameters. The subtropical circulation has a gyres-within-a-gyre structure including boundary currents along Australia and east of New Zealand and a concentrated equatorial limb with surprising strength in the western basin. This current is also involved in a tropical gyre with generally eastward flow along the equator in the western basin at least. Because of strong seasonal-scale variability, the equatorial zonal currents are not well measured yet but data are still being accumulated. Comparison with Reid's [1997] absolute circulation shows more areas of agreement than of disagreement.