Synoptic time and length scales of motion in the north equatorial current system of the Pacific Ocean

Abstract

In the summer of 1967 two deep-moored buoys measured subsurface temperature data for nearly 2 months in the North Equatorial countercurrent near 119°W. The power spectral density functions calculated from these time series indicate that most of the variance in temperature of the thermocline occurred near three principal time periods corresponding to the semidiurnal and diurnal tidal motions and to the inertial oscillation. The inertial periodicity is thought to be a manifestation in the sensors of the mooring motion induced by horizontal inertial currents. In the spring of 1969 two more buoys were moored near the location of the 1967 stations; the power spectral density functions from the newer buoys show approximately the same concentration of variance in temperature as the earlier observations did. The cross-correlation function between the two buoys provided phase information from which the zonal wavelength L of the inertial motion was calculated to have been approximately 110 km. In all the moored power spectra the inertial spectral peak was consistently at a frequency significantly higher than the local inertial frequency f. However, drogue measurements obtained during the 1969 expedition suggest an inertial frequency substantially closer to the local inertial period, though still higher. The finiteness of L was found to induce in the dispersion relation for inertial waves a frequency shift that can account for the observed frequency shift between the theoretical inertial frequency and the observed frequency obtained by drogue (Lagrangian) measurements. However, the frequency shift of the inertial wave as measured in the moored power spectra (a Eulerian measurement) could not be accounted for by the finiteness of L, this result suggesting that the inertial wave was Doppler shifted by the mean flow past the fixed moorings.