Biological response to the 1997–98 and 2009–10 El Niño events in the equatorial Pacific Ocean

Abstract

Changes in the physical environment associated with eastern Pacific (EP)‐El Niño and central Pacific (CP)‐El Niño events affect the biological response in the equatorial Pacific Ocean differently. However, such responses have not been adequately investigated, especially in terms of the relevant physical processes. This paper addresses the mechanistic differences in the biological response of the equatorial Pacific Ocean during the strongest CP‐ and EP‐El Niño to date (i.e., 1997–98 EP‐El Niño and 2009–10 CP‐El Niño) using satellite data and water mass pathway analysis based on an ocean reanalysis product. The 1997–98 EP‐El Niño was associated with a larger reduction of chlorophyll‐a (chl‐a) in the eastern equatorial Pacific (EEP) and the 2009–10 CP‐El Niño was associated with a larger reduction of chl‐a in the central equatorial Pacific (CEP). These biological responses were dependent on the strength and extent of westerly wind anomalies and their impact on horizontal and vertical processes. Horizontal advection was the primary contributor to differences in chl‐a between the two El Niño events in the CEP, whereas vertical advection and mixing were the dominant processes in the EEP.

Multiyear predictability of tropical marine productivity

“…4 B and C). Such a trans-Pacific propagation of anomalies was proposed to drive the major differences in surface chlorophyll observed between 1997-1998 and 2009-2010 ENSO events (23).…”

“…In the eastern Pacific, the latter are injected from deeper layers to the upper ocean by the equatorial upwelling, the intensity of which is strongly modulated by changes in wind strength related to wind-SST Bjerknes feedbacks (21). In the central and western Pacific, the westward advection of nutrients in response to changes in zonal currents during the ENSO phases explains most of the variations in NPP with a time lag of several months (4,20) to a decade (22,23). Simultaneously to these first-order mechanisms, local limitation of specific nutrients plays an important role in setting the NPP variability.…”

A closer look at the central Pacific El Niño and warm pool migration events from 1982 to 2011

“…Using surface chlorophyll levels as a proxy for biological activity, both CP and EP El Niños result in a reduction in biological productivity, but with varying spatial patterns in the Pacific Ocean between 10 N and 10 S [Radenac et al, 2012]. The maximum chlorophyll reduction occurs in the CP region during CP El Niños and in the EP region during EP El Niños [Radenac et al, 2012;Gierach et al, 2012]. While a 30 year record is too brief to draw any conclusions about climate change, our findings are consistent with a recent study of ENSO that suggested that the relative frequency of EP and CP El Niños may vary with global warming [Yeh et al, 2009].…”

The impact of advection on stratification and chlorophyll variability in the equatorial Pacific

“…El Niño-related decreases in chl a have been attributed to influxes of nutrient-poor waters and attributed to influxes of nutrient-poor waters and/or barrier layer properties from the western warm pool. These attributions are based on spatial pattern matching of winds, surface currents, and chl a anomalies [Radenac et al, 2012], modeling of advection of the oligotrophic/mesotrophic front [Messié and Chavez, 2012], and water mass pathway analyses during El Niño events [Gierach et al, 2012]. We note that the region is not always characterized by strong El Niño/La Niña states.…”