Currents observed north of Oahu during the first five years of HOT

Abstract

Shipboard ADCP sections from 30 Hawaii Ocean Time-series cruises over a 5-year period show a mean flow 15 degrees north of west, reaching a maximum of 17 cm s − 1 about 50 km north of Oahu. This North Hawaiian Ridge Current is highly variable from cruise to cruise and from year to year. It was generally strong in the sections 1988–1991 and weak or absent in 1992 and 1993. There is no prominent annual cycle. Unresolved short-period oscillations are so energetic as to obscure the mean current in individual sections. The 5-year mean is highly significant, however, at five times the estimated standard error. Currents also were measured with a shipboard ADCP during the nominally 3-day occupations of the HOT Station ALOHA. The mean over 32 cruises was near 6 cm s −1 in magnitude at all depths from 20 to 200 m, but the median was near zero. Hence, the northern boundary of the North Hawaiian Ridge Current was usually south of ALOHA. The inter-cruise standard deviation of velocity varied from 22 cm s −1 at 20 m to 11 cm s −1 at 200 m for the zonal component, and 13 cm s −1 to 8 cm s − 1 for the meridional component. Intra-cruise variability during the 17 cruises with the best sampling in depth and time was dominated by a linear trend, near-inertial oscillations, and diurnal and semidiurnal tides. Each of these four elements had an rms value near 6 cm s −1 at 20 m, decreasing to 2–4 cm s −1 at 250 m. The vertical shear field was dominated by the diurnal tide and near-inertial oscillations; the semidiurnal tide and the trend had larger vertical scales of variability and consequently made smaller contributions to the shear.