Correlations between Surface Chlorophyll and Sea-Surface Height in the Tropical Pacific during the 1997/1998 ENSO Event

Cara Wilson and David Adamec

Journal of Geophysical Research, 106,31,175-31,188, 2001


Correlations between SeaWiFS surface chlorophyll and TOPEX/Poseidon sea surface height (SSH) are examined in the tropical Pacific(30°S-30°N) using empirical orthogonal function (EOF) analysis both separately and jointly on the two fields. This analysis is done on data from Sept. 1997, the start of data from the SeaWiFS satellite, through Dec. 1999, a time period dominated by the 1997/1998 ENSO oscillations. Four distinct biological responses are observed. The dominant response is a symmetric off-equatorial chlorophyll increase during La Niña that extends from the eastern Pacific to the dateline between 2-18° both north and south of the equator. This chlorophyll response is tightly correlated to the SSH, suggesting that the chlorophyll increase is a result of the shoaling thermocline, which increases the surface nutrient supply. The better known equatorial decrease in chlorophyll during El Niño is seen in a separate EOF mode. Using ADCP data from the TAO/TRITON array it is shown that the cessation of the El Niño equatorial chlorophyll minimum is tied to the recommencement of the iron-rich Equatorial UnderCurrent which occurs several months prior to the termination of the El Niño. There is also an asymmetric off-equatorial bloom during the peak of the El Niño between 120-180°W and 8-15°N. This bloom occurs within the area covered by the previously mentioned La Niña bloom, but it is more localized and its fluctuations appear correlated with changes in the North Equatorial Counter Current. The shoaling thermocline in the western warm pool during El Niño results in a chlorophyll bloom that extends from the Philippines to 155°E between 0-15°N. This bloom terminates in unison with the end of the El Niño when elevated SSH is reestablished in the western basin.

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