Sea Surface Chlorophyll Signature in the Tropical Pacific: Eastern and Central Pacific El Niño Events

We investigate the surface chlorophyll signature of different phases of El Niño-Southern Oscillation (ENSO) by applying an Agglomerative Hierarchical Clustering method to monthly maps of satellite-derived chlorophyll anomalies between September 1997 and December 2010. We identify five typical ENSO structures (see figure below) representative of the lonely 1997-1998 eastern Pacific El Niño (EPEN) of the period, four Central Pacific El Niño (CPEN), La Niña, transition situations between strong El Niño and La Niña, and intermediate conditions. Other satellite measurements (sea level anomaly, wind) and satellite-derived surface currents allow suggesting processes at work during ENSO phases. During the strong 1997-1998 EPEN, a large eastward shift of the oligotrophic warm pool, a deep nutrient pool (elevated sea level anomaly) and a reduction of equatorial upwelling (due to reduced trade winds) result in negative chlorophyll anomalies east of 170°E between 10°S and 10°N. During the following four CPEN events (2002-2003, 2004-2005, 2006-2007, and 2009-2010), westward surface currents in the central basin prevent the eastward spreading of low-chlorophyll waters that remain confined west of 160°W in the equatorial band. This core of negative anomalies corresponds to a region of elevated sea level anomaly indicating deep nutrient (nitrate and iron) sources. Negative chlorophyll anomalies that extend eastward from the main equatorial anomaly core probably result from reduced upward iron fluxes linked to the deepening of the Equatorial Undercurrent. During La Niña, positive chlorophyll anomalies are observed west of the date line because of the westward expansion of the equatorial upwelling. When strong La Niña follows strong El Niño, chlorophyll strongly increases in a narrow equatorial band (1998, 2010).