Observations Of Wave Mean Flow Interaction In The Pacific Equatorial Undercurrent

Author : Rui Vasques de Melo Ponte
Publisher :
ISBN 13 :
Total Pages : 181 pages
Book Rating : 4.:/5 (184 download)

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Download or read book Observations and Modelling of Deep Equatorial Currents in the Central Pacific written by Rui Vasques de Melo Ponte and published by . This book was released on 1988 with total page 181 pages. Available in PDF, EPUB and Kindle. Book excerpt: 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. At all wavenumber bands in general, power levels decayed away from the equator over scales broader than the Kelvin wave scale, suggesting the presence of Rossby wave energy. Cross-spectral analysis showed Rossby and Kelvin wave motions to be dominant at the equator over the 933 sm and the 140-400 sm vertical wavelength bands, respectively. The latter agrees with the findings of Eriksen (1981) in the western Pacific, and thus seems to be a climatological feature of the deep equatorial Pacific fields. In an attempt to model the observed zonal velocity signals, alternative forcing mechanisms for the deep ocean (other than direct surface winds) were tried. The probable presence of deep energy sources at the ocean side walls (e.g., Kawase, 1987) was explored by considering the linear response of an equatorial ocean to a time varying zonal jet placed at the lateral boundaries. In another simple model, we examined the character of stationary Kelvin wave solutions obtained in the presence of vertically sheared mean westward flows. In this case, the waves are forced below the thermocline by a vertical velocity representing large scale convergence or divergence patterns associated with the upper ocean circulation. Results suggest that both ideas remain potentially important to the existence of deep baroclinic currents in the equatorial ocean.