Correlations between surface chlorophyll and sea surface height in the tropical Pacific during the 1997-1999 El Nino-Southern Oscillation event

Correlations between Sea-viewing Wide Field-of-view Sensor (SeaWiFS) surface chlorophyll and TOPER sea surface height (SSH) are examined in the tropical Pacific (30degreesS-30degreesN) using empirical orthogonal function (EOF) analysis both separately and jointly on the two fields. This analysis is done on data from September 1997, the start of data from the SeaWiFS satellite, through December 1999, a time period dominated by the El Nino-Southern Oscillation. Four distinct biological responses are observed. The dominant response is a symmetric off-equatorial chlorophyll increase during La Nina that extends between 2degrees and 18degrees latitude from the eastern Pacific to the date line. The chlorophyll mode is tightly correlated to 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 Nino is seen in a separate EOF mode. Using acoustic Doppler current profiler data from the Tropical Atmosphere-Ocean/Triton array, it is shown that the cessation of the El Nino 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 Nino. There is an off-equatorial bloom during the peak of the El Nino between 120degreesW-180degreesW and 8degreesN-15degreesN. This bloom occurs within the area covered by the previously mentioned La Nina 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 Nino results in a chlorophyll bloom that extends from the Philippines to 155degreesE between 0degreesN and 15degreesN. This bloom terminates in unison with the end of the El Nino when elevated SSH is reestablished in the western basin.