Variations of zonal currents in the central tropical Pacific
during 1970 to 1987: Sea level and dynamic height
measurements
Taft, B.A., and W.S. Kessler
Journal of Geophysical Research, 96(C7), 12,599-12,618
(1991)
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 Niños of
1972 and 1982, which were followed by near disappearance of the
currrent the following year; these fluctuations are clearly present
in both the surface current indices and the volume transport. In
1976 and 1986-1987 (moderate Niños) the same pattern is not
marked. The El Niño signal in NEC transport is weak. There
was a strong rise of equatorial sea level during the early phases
of the 1972 and 1982 Niños 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.
Dr. William S. Kessler
NOAA / PMEL / OCRD
7600 Sand Point Way NE
Seattle WA 98115 USA
