Date of Award

Winter 2004

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Ocean & Earth Sciences

Committee Director

Larry P. Atkinson

Committee Member

Chester Grosch

Committee Member

Chester Grosch

Committee Member

Thomas C. Royer

Committee Member

Franklin B. Schwing

Abstract

The coastal ocean of northern Chile has persistent wind-driven upwelling that produces high nutrient and chlorophyll concentrations in a narrow band along the coast. The objective of this thesis is to study the low frequency temporal variability of the upwelling system, to understand the spatial and temporal changes in the wind field, and how these changes may affect the upwelling and anchovy variability. Data used in this thesis includes time series of wind, sea level, sea surface temperature, and atmospheric pressure at coastal stations, from 1960 to 2003, and oceanographic and acoustic cruises for the period 1993–2003. The time series are analyzed using the STL method (Seasonal Trend Decomposition procedures based on locally weighted regression and smoothed scatter plots (LOESS)), maxima entropy spectral analysis, wavelets, cross spectra, and EOFs (Empirical Orthogonal functions). The wind field from ERS and QuikSCAT satellite radar scatterometers was analyzed.

Variability occurred at four frequency bands: quasi-biennial (2–3 years), ENSO (3–8 years), decadal (9–12 years), and interdecadal (13–25 years). The origin of quasi-biennial variability could be explained by means of the Delayed Oscillator Mode. The STL trend shows three regime changes: before 1976 (cold), between 1976 and 1998 (warm), and after 1998 (cold). These regime changes correspond with the changes in anchovy and sardine populations in the Chile-Peru system. During the last five years, there has been a diminution of the upwelling index, SST, sea level, and atmospheric pressure, and more Lasker events (calms).

The spatial variability of wind stress in the study area is characterized by a minimum between two maximums located at 15°S and 30°S. The effect of the curvature of the coast, as well as the height of mountains near the coast, produces a unique system, where the wind is forced thermally, reaching a maximum in summer and a minimum in winter, in opposition to the oceanic pattern. In spring and summer the alongshore wind is accelerated by cross-shore pressure gradient changes, producing an intense divergence along the coast. Very near the coast, the wind stress is low, though it increases offshore. The combined effect of both processes produces a convergence outside of the upwelling front. On the scale of days, this convergence could be the origin of the mesoscale eddies that are observed in the zone. This pattern changes during ENSO events.

El Niño produced changes in the density, distribution, and the depth of the anchovy schools. The deepening of some schools in the coastal area placed them below normal fishing nets, explaining their fast recovery when the El Niño ends.

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DOI

10.25777/3788-gf67

ISBN

9780496939008

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Oceanography Commons

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