Impact of strong El Nino events (1997/98 and 2009/10) on sinking particle fluxes in the 10 degrees N thermocline ridge area of the northeastern equatorial Pacific

Abstract

Sinking particle fluxes during two strong El Nino events (1997/98 and 2009/10) were monitored at a station (10 degrees 30N, 131 degrees 20W) located in the 10 degrees N thermocline ridge area (9 degrees-13 degrees N, 105 degrees-140 degrees W) of the northeastern equatorial Pacific. The 1997/98 El Nino event resulted in two- to fourfold higher organic carbon and biogenic silica fluxes than the non-ENSO levels. In combination, these findings suggest an increase in the productivity of surface waters. The strengthening of the North Equatorial Countercurrent, due to intensified westerlies and the resulting shoaling of the thermocline, likely induced upwelling of nutrient-enriched subsurface water and enhanced biological productivity. In contrast, the 2009/10 El Nino event was only accompanied by a substantial increase in planktonic foraminifera, with minor increases in organic carbon and biogenic silica fluxes. The distinct differences in the particle composition and the biological response of the marine ecosystem during the two El Nino events are attributed to different oceanographic and atmospheric conditions generated by two different types of El Nino: the cold-tongue (CT) type and the warm-pool (WP) type. Our results, together with the findings of a previous study of the 1982/83 strong El Nino event, suggest that enhanced biological productivity occurs in the 10 degrees N thermocline ridge area under strong El Nino conditions. Our study also provides insight into equatorial biological dynamics under the extreme CT- and WP-type El Nino conditions. This is important because the WP-type El Nino conditions are projected to increase in frequency in the future. (c) 2012 Elsevier Ltd. All rights reserved.