Analysis and Interpretation of Deep Equatorial Currents in the Central Pacific

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Abstract
Analysis of vertical profiles of absolute horizontal velocity collected in January 1981, February and April 1982 in the equatorial central Pacific revealed two significant narrowband spectral peaks in zonal velocity, centered approximately at vertical wavelengths of 560 and 350 stretched meters (sm). Energy in the 560 sm band roughly doubled between the first and last cruises. Tim-lagged coherence results suggested upward phase propagation at periods of about 4 years. East–west phase fines computed from coherence over zonal separations tilted downward towards the west, implying westward phase propagation and zonal wavelengths on the order of 10 000 km. The peak that was centered at 350 sm occured at the vertical scales of the conspicuous alternating flows in the records, generically called the equatorial deep jets in the past (the same terminology is used here). It showed a more steady character in amplitude and a higher signal-to-noise ratio in comparison with the 560 sm peak. The deep jets were best defined as a finite narrowband process in vertical wavenumber (311–400 sm), accounting for 20% of the total variance present in the broad-band energetic background. At the jets' wavenumber band, latitudinal energy scaling compared reasonably well with Kelvin wave theoretical values and a general tilt of phase lines downward towards the east yielded estimates of 10 000–16 000 km for the zonal wavelengths. Time-lagged coherence calculations revealed evidence for vertical shifting of the jets on interannual time scales. Interpretation of both signals in terms of equatorial waves was ambiguous, because of their relatively long spatial and temporal scales compared to the records. The simplest hypothesis of linear waves in a resting basic state ocean could not be rejected, but more complicated physics cannot be ruled out. At most wavenumber bands, power levels decayed away from the equator over scales broader than the Kelvin wave scale. Within ½° of the equator, zonal current led (lagged) vertical displacement by π/2 with depth for the 933 sm (140–400 sm) band. The result at the 140–400 sm band agrees with the findings of Eriksen (1981) in the western Pacific, and thus seems to be a general feature of the deep equatorial Pacific fields.

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