The mean structure and variability of the Mindanao Current at 8°N
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On the basis of eight surveys along 8 degrees N in the western Pacific Ocean taken between September 1987 and June 1990, the mean structure and transport of the Mindanao Current is presented. Direct acoustic Doppler current profiler (ADCP) velocity measurements show the Mindanao Current to be an intense 200- km-wide coastally trapped jet with a typical inshore velocity maximum of Im s(-1). Though the individual geostrophic velocity estimates were strongly contaminated by dynamic height noise, the survey averaged geostrophic and directly measured shears were identical to within the estimated errors. The ADCP mean velocity field was taken as a reference for the geostrophic flow estimate based on the mean density distribution. Relative to a bottom reference level, a barotropic correction was required near the coast to make the mean geostrophic and directly measured velocities agree. Integrated from the Mindanao Coast to 130 degrees E, the average transport above sigma(theta) = 26.7 kg m(-3) is 25 +/- 4 x 10(9) kg s(-1), while that between 0 and 2000 dbar is 39 +/- 10 x 10(6) kg s(-1) based on the mean ADCP-referenced geostrophic fields. The velocity structure of the Mindanao Current was remarkably stable from cruise to cruise. However, two cruises in the spring of 1989 sampled an: anomalous velocity structure offshore which is attributed to the presence or a 400-km-diameter warm-core feature containing South Pacific water masses. We speculate that this feature is either a pinched-off eddy or meander from the confluence of northern and southern hemisphere waters that occurs near 5 degrees N. The sense of circulation of the eddy is anticyclonic, opposite iri sign to the recirculation which has been historically termed the Mindanao Eddy. Only two of the eight cruises sampled northward thermocline flow offshore df the Mindanao Current; the average of all cruises contains no significant recirculation. Hence for the time period of these surveys the Mindanao Eddy and Undercurrent were essentially absent at 8 degrees N.