Climate-scale chlorophyll patterns in the tropical Pacific from a 51 year statistical reconstruction
Uz, Stephanie Schollaert
Busalacchi, Antonio J
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Understanding large, slow biological changes in the oceans has been hindered by a lack of spatial coverage by direct measurements and a lack of temporal coverage by satellite remote-sensing observations. Global ocean surface chlorophyll, a proxy for phytoplankton standing stock, has been derived from satellites for over a decade. With these measurements, the strong connection between ocean physics and biology has become clear and provided new insights about what drives seasonal and interannual biological processes. At longer time scales, however, there are many unanswered questions about the variability of phytoplankton in the ocean that plays a critical role in the carbon cycle as well as the marine food web. Statistical reconstructions have been used by others to extend physical climate variables in space and time. Taking advantage of the fact that physical forcing has been found to be the primary driver of biological primary production in the tropical Pacific, especially during El Niño, the most closely correlated physical variables are used as predictors in a statistical reconstruction to extend monthly chlorophyll anomalies from just over a decade to just over five decades between 1958-2008. The reconstructed chlorophyll is evaluated through leave-one-out-cross-validation, compared to several independent data sets: in situ samples, another ocean color satellite data set, model output from a dynamic, fully-coupled ocean circulation-biogeochemistry model. Highest skill in the tropical Pacific reconstruction is away from the coast and within 10o of the equator, including areas known as Niño 3/3.4/4. Over the half-century of chlorophyll anomalies, the most dominant climate pattern apparent in the reconstruction is associated with the interannual El Niño followed by the Pacific Decadal Oscillation. Biological distinctions emerged between the east Pacific El Niño events and those that only extend to the central Pacific. Chlorophyll anomalies were compared between regimes to ascribe physical forcing mechanisms. While the overall patterns were consistent with what is known about the impact of ENSO on biology, with the PDO primarily serving to amplify or damp ENSO, a narrow equatorial band consistently displayed an inverse response to the rest of the equatorial cold tongue: lower values during the PDO cool phase between 1958-1976, higher values during the PDO warm phase between 1977-1995. A likely explanation for this anomaly is linked to variability in the depth of the Equatorial Undercurrent that transports iron to the high-nutrient, low-chlorophyll east Pacific. These and other ideas are explored to demonstrate the feasibility and utility of reconstructing ocean color chlorophyll to address open questions about large-scale, low frequency primary production that forms the base of the marine food web and plays an important role in Earth’s climate system.