Climate-scale Chlorophyll Patterns in the Tropical Pacific from a Multi-decadal Statistical Reconstruction

In the tropical Pacific, marine biological productivity is primarily controlled by variations in physical oceanography. Using the climate-quality SeaWiFS ocean color chlorophyll record in the tropical Pacific, we reconstructed the surface chlorophyll record back over five decades utilizing its strong correlation to the longer records of sea surface temperature and height. Results are not sensitive to parameters of the reconstruction but are limited by the stability of covariance estimates associated with the short possible training interval (11+ years). The reconstruction demonstrates greatest skill away from the coast and within 10^o of the equator where chlorophyll variance is greatest and primarily associated with El Niño Southern Oscillation (ENSO) dynamics. East and central Pacific-centered ENSO events are distinguished by their associated chlorophyll anomalies. While the overall patterns are consistent with what is known about the impact of ENSO on biology, a narrow equatorial band of productivity responds to lower frequency forcing from the rest of the area. Between 1958-1976, chlorophyll in the narrow band was that the rest of the equatorial cold tongue: between 1977-1995, equatorial band productivity was higher than the surrounding cold tongue. Comparisons with output from a fully-coupled biogeochemical model support the hypothesis that this anomalous band is controlled by low frequency changes in the depth of the Equatorial Undercurrent (EUC) that transports iron to the eastern equatorial Pacific. Changes in the depth and speed of the EUC have been linked to anthropogenic change.