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Investigaciones marinas

versión On-line ISSN 0717-7178

Investig. mar. v.30 n.1 supl.Symp Valparaíso ago. 2002

http://dx.doi.org/10.4067/S0717-71782002030100034 

Large-Scale Chlorophyll
Variability Along the Eastern Pacific
Coastal Margin

Andrew Thomas1, Peter Brickley1,
P. Ted Strub2

1School of Marine Sciences, University of Maine,
Orono, Maine, U.S.A., 04469-5741,
E-mail: thomas@maine.edu
2College of Oceanic and Atmospheric Sciences,
Oregon State University, Corvallis, Oregon, U.S.A.
97331-5503, E-mail: tstrub@coas.oregonstate.edu

The objectives of this work are to quantify the large scale variability of chlorophyll along the Eastern Pacific coastal margin from 55oN in the California Current, through central America and down to 55oS in the Humboldt Current using satellite data. Special attention will be paid to variability associated with the 1997-98 El Nino event and comparisons between hemispheres.

Previous work has shown dramatic changes in both the concentrations and time / space patterns of chlorophyll concentration in the Pacific Ocean eastern boundary current (EBC) regions as a function of season and latitude. Interannual variability, especially during El Nino events is often stronger than seasonal variability. These EBC regions are characterized by latitudinally and seasonally varying equatorward alongshore wind stress which drives offshore Ekman transport and coastal upwelling of cold nutrient-rich subsurface water. The biological response to this upwelling is high concentrations of chlorophyll along the both the North and South American Pacific coasts, which in turn support highly productive fisheries. The advent of satellite data potentially allowed interannual variability to be quantified during both the 1983 and the 1997-98 events, first using Coastal Zone Color Scanner (CZCS) data and most recently, Ocean Color Temperature Sensor (OCTS) and SeaWiFS data. Limited coverage of the southern hemisphere Pacific EBC region by the CZCS, however, prevented any quantitative examination of the 1982-83 El Nino in this region. The time series afforded by the OCTS and the SeaWiFS instrument, despite the time gap between their missions, provides our first opportunity to carry out direct and concurrent comparisons of chlorophyll interannual variability between the California Current System and the Humboldt Current System, especially that which is associated with the 1997-98 El Nino event.

OCTS data and the first 4.5 years of SeaWiFS data are used to quantify mean seasonal and then interannual variability of large-scale surface chlorophyll patterns along the full latitudinal extent of the eastern Pacific basin focusing on the upwelling regimes in the Humboldt and California Currents and along the central American coast. Data are used first to contrast latitudinal differences in seasonality within each region and then to document differences between the northern and southern hemisphere. Interannual variability evident in the data, quantified by removal of mean seasonal cycles, is dominated by the 1997-98 El Nino. SeaWiFS data first become available in September 1997 and the first 5-6 months are coincident with strong hydrographic anomalies in both EBC regions. Contrasts between the first (September 1997 - August 1998) and the latter three years document the extent to which chlorophyll patterns were affected by El Nino conditions. These illustrate differences between the northern and southern hemisphere in the timing and magnitude of El Nino impacts on chlorophyll concentration and the development of a "normal" seasonal biomass cycle. EOF decomposition of the image time series highlights the time/space patterns of anomalous conditions. SeaWiFS chlorophyll patterns are compared to coincident large-scale wind forcing, altimeter fields of transport and dynamic height, and satellite-measured surface temperature to show linkages between physical forcing and biological response. These highlight temporal and latitudinal variability in the El Nino signal as well as recurrent differences in biological-physical coupling. In general, large-scale chlorophyll patterns show a stronger relationship to transport and surface temperature anomalies than to changes in wind forcing, consistent with previous work which has shown that El Nino chlorophyll anomalies are not the result of local changes in upwelling, but are associated with distant forcing and changes these impose on subsurface hydrography.

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