Graduate Thesis Or Dissertation
 

Propagation and forcing of high frequency sea level variability in the eastern equatorial Pacific

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  • Sea level and wind data from coastal and island stations from Buenaventura, Colombia (4°N) to Callao, Peru (12°S) have been analyzed for the 1979-1984 time period, to describe the seasonal and interannual variations in the characteristics of short time scale variability (1-2 weeks). Auto- and cross spectral analyses are used to make comparisons between Austral summers and winters as well as interannual comparisons between the 1982-1983 El Nino/Southern Oscillation (ENSO) period and non-ENSO years. The principal results show weak evidence of local forcing of the sea level by the alongshore wind during the whole year without significant differences between summer and winter seasons. The alongshore coherence and phase spectra between the sea level series show evidence of poleward propagating fluctuations at speeds of 2.6-3.0 ms^-1 during winter, but no propagation is evident during summer. There is also a large energy increase in coastal sea level, especially in the 8-11 day band, during the 1982-1983 ENSO episode. This increase is associated with a non-dispersive, poleward propagation of events at speeds of 3.11-3.6 ms^-1. The propagating fluctuations are superimposed on a weak, locally forced variability. The only plausible source for the observed coastal fluctuations appears to be trapped waves in the equatorial waveguide. Additional equatorial data from subsurface pressure gauges in the Galapagos Islands and inverted echo sounders at 3°N,95°W, 3°N,85°W and 2°S,85°W are used to explore the possibility that the coastal signal during the 1982-1983 ENSO episode is connected to the arrival of energy in the form of equatorially trapped Rossby-gravity (Yanai) and Kelvin waves, and to determine the respective contributions of the two wave types. Cross spectral analyses, frequency domain EOF analysis and the characteristics of equatorial waves demonstrate that equatorially trapped Yanai waves are the principal source of the propagating signals in the coastal sea levels during the 1982-1983 ENSO. During the 1982-1983 ENSO, between 64% and 91% of the coastal SLH variability in the 8-11 day period band is associated with antisymmetric variability across the equator. Phase in this band is zonally invariant along the equator but is poleward propagating along the coast, consistent with the conversion of stationary, equatorial lanai waves into coastal trapped waves.
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