Abstract:
Analysis of vertical profiles of absolute horizontal velocity collected in January 1981,
February 1982 and April 1982 in the central equatorial Pacific as part of the Pacific
Equatorial Ocean Dynamics (PEQUOD) program, revealed two significant narrow band
spectral peaks in the zonal velocity records, centered at vertical wavelengths of 560 and
350 stretched meters (sm). Both signals were present in all three cruises, but the 350 sm
peak showed a more steady character in amplitude and a higher signal-to-noise ratio.
In addition, its vertical scales corresponded to the scales of the conspicuous alternating
flows generically called the equatorial deep jets in the past (the same terminology will
be used here). Meridional velocity and vertical displacement spectra did not show any
such energetic features.
Energy in the 560 sm band roughly doubled between January 1981 and April 1982.
Time lagged coherence results suggested upward phase propagation at time scales of
about 4 years. East-west phase lines computed from zonally lagged coherences, tilted
downward towards the west, implying westward phase propagation. Estimates of zonal
wavelength (on the order of 10000 km) and period based on these coherence calculations,
and the observed energy meridional structure at this vertical wavenumber band,
seem consistent, within experimental errors, with the presence of a first meridional
mode long Rossby wave packet, weakly modulated in the zonal direction.
The equatorial deep jets, identified with the peak centered at 350 sm, are best defined
as a finite narrow band process in vertical wavenumber (311-400 sm), accounting
for only 20% of the total variance present in the broad band energetic background. At
the jets wavenumber band, latitudinal energy scaling compared well with Kelvin wave
theoretical values and a general tilt of phase lines downward towards the east yielded
estimates of 10000-16000 km for the zonal wavelengths. Time-lagged coherence calculations
revealed evidence for vertical shifting of the jets on interannual time scales.
Interpretation of results in terms of single frequency linear wave processes led to inconsistencies,
but finite bandwidth (in frequency and wavenumber) Kelvin wave processes
of periods on the order of three to five years could account for the observations. Thus,
the records do not preclude equatorial waves as a reasonable kinematic description of
the jets.
