VARIATIONS OF ZONAL CURRENTS IN THE CENTRAL TROPICAL PACIFIC DURING 1970 TO 1987 - SEA-LEVEL AND DYNAMIC HEIGHT MEASUREMENTS

The annual and interannual variations of the major zonal currents of the central tropical Pacific for the period 1970-1987 are described by analyzing two data sets: sea level data from a few selected island stations and ship-of-opportunity expendable bathythermograph (XBT) observations. The annual cycles of geostrophic surface current indices (dynamic height or sea level difference between the ridge and the trough defining the current) in the North Equatorial Current (NEC) and North Equatorial Countercurrent (NECC) are found to be in phase (maxima in November), to be strong, and to explain about 50% of the total variance. The annual cycle of volume transport relative to 400 dbar in the NECC is also strong, but NEC volume transport has a weak annual cycle. The discrepancy between the NEC surface current and the transport is due to the variation of the surface current being larger than that of the deep currents. The annual cycle of the transport of the South Equatorial Current (SEC) is 6 months out of phase with that of the NEC and NECC. Phases of the annual cycles of the sea level and dynamic height indices of flow agree well in general, but there are significant differences between the annual amplitudes of the two types of measurements. These differences arise from a variety of sources: dynamic height reference level assumptions, longitudinal spacing of sea level gauges, meridional resolution of sea level array, and different signal to noise ratios for the two sets of indices. The NECC shows marked flow maxima during the peaks of the strong Ninos of 1972 and 1982, which were followed by near disappearance of the current the following year; these fluctuations are clearly present in both the surface current indices and the volume transport. In 1976 and 1986-1987 (moderate Ninos) the same pattern is not marked. The El Nino signal in NEC transport is weak. There was a strong rise of equatorial sea level during the early phases of the 1972 and 1982 Ninos followed by a fall the next year. This resulted in a decrease of the transport of the SEC in the first year followed by an increase to a maximum during the second year; the maximum coincides with a minimum in NECC transport.