Deep Sea Research Part I: Oceanographic Research Papers
Influence of ENSO variability on sinking-particle fluxes in the northeastern equatorial Pacific
Highlights
► Background particle fluxes at the 10°N thermocline ridge show a marked seasonal variability. ► The enhanced particle fluxes in the cold season are attributed to the wind-driven mixing. ► Background flux variability commonly exceeds the variability of ENSO and post-ENSO signals.
Introduction
Previous studies based on time-series sediment-traps have sought to monitor the seasonal to interannual variations in fluxes of sinking particles from the surface ocean to the deep sea floor, and have made important contributions to our understanding of the biological processes induced by various types of climate forcing (Buesseler et al., 2007, Conte et al., 2001, Hebel and Karl, 2001, Honjo, 1997, Karl et al., 1996, Li et al., 2004, Unger et al., 2003). Especially in the equatorial Pacific, the sediment trap has been a useful tool in elucidating the response of oceanic and atmospheric system to El Niño–Southern Oscillation (ENSO) events (Dymond and Collier, 1988, Gupta and Kawahata, 2002, Gupta and Kawahata, 2007, Hernes et al., 2001, Honjo et al., 1995, Kawahata et al., 2000).
Sinking particle fluxes were monitored during the 1982/1983 strong El Niño, 1992 moderate El Niño, and 1984/1985 La Niña events, and subsequent post-ENSO periods along the ∼140°W meridian in the eastern equatorial Pacific (Dymond and Collier, 1988, Honjo et al., 1995). These studies reported that the moderate (1992) to strong (1982/1983) El Niño events led to a decrease or little change in particle fluxes, depending on the scale of the event. The 1984/1985 La Niña resulted in the opposite response of the environmental system; i.e., a 2- to 3-fold increase in particle fluxes compared with the post-ENSO period (Dymond and Collier, 1988). These observed trends indicate the occurrence of changes in surface-water productivity in response to ENSO-modulated oceanographic conditions; i.e., a suppressed upwelling rate of deep nutrient-rich water under El Niño conditions and enhanced upwelling under La Niña conditions (Chavez et al., 1999, Kang et al., 2008, McPhaden, 2004).
Exceptions to the above general trend have been reported from sites at 9°N and 11°N, where fluxes were between two and four times higher than the above values during the same El Niño events (Dymond and Collier, 1988, Honjo et al., 1995). These results were unexpected and are not fully understood, as both sites were located a short distance from the core region of ENSO effects, and the responses were the opposite to the trend expected for El Niño-induced oceanographic environments in the eastern equatorial Pacific. Both sites are located at the margin of the 10°N thermocline ridge (9°–13°N, 105°–140°W; Pennington et al., 2006), representing the region of divergence between the North Equatorial Counter Current (NECC) and the North Equatorial Current (NEC). Unlike the equatorial region, the 10°N thermocline ridge area shows distinct seasonal environmental changes arising from seasonal shifts in the zonal wind regime and resulting latitudinal shifts of the tropical gyres that influence the pattern of local upwelling (Donguy and Meyers, 1996, Fiedler and Talley, 2006, McGee et al., 2007, Pennington et al., 2006, Romero-Centeno et al., 2007). Thus, seasonal variations in particle fluxes in this region should be evaluated quantitatively to improve our understanding of the effect of ENSO on sinking particle fluxes. However, there exist no long-term monitoring data with which to quantify the seasonal background fluxes.
In the present study, we monitored particle fluxes using a time-series sediment trap in the western part of the 10°N thermocline ridge area for 5 years from July 2003 to July 2008. Kim et al. (2010) measured particle fluxes at the KOMO station (see below for station details) from July 2003 to June 2005, and found large seasonal variations of particle fluxes in the 10°N thermocline ridge area. The main objective of the present study is to document the natural seasonal variability of particle fluxes in the 10°N thermocline ridge area, thereby enabling a quantitative evaluation of the effect of ENSO on sinking particle fluxes.
Section snippets
Oceanographic and atmospheric setting at the 10°N thermocline ridge
The Korea Ocean Research & Development Institute has been operating a long-term monitoring station at 10.5°N, 131.3°W (KOMO station) to understand the physicochemical and biological characteristics of the 10°N thermocline ridge area (Fig. 1). The present study area is located in the high-nutrient and low-chlorophyll-a (HNLC) region that extends to 15°N (Karl, 1999, Murray et al., 1995). The surface environmental properties at KOMO are governed mainly by seasonal changes in the trade wind
Methods
A time-series sediment trap was operated at a depth of 4950 m, about 55 m above the seafloor, from July 2003 to July 2008 at KOMO station (Table 1, Fig. 1). Sinking particles were collected at monthly intervals in a bottle filled with a 5% formalin solution buffered with NaBO3 in filtered sea water, although sampling was bimonthly during the following periods due to temporary malfunctions of the trap rotating system: February–March 2004, September–October 2004, December 2005–January 2006, and
El Niño/La Niña conditions during the monitoring periods
El Niño/La Niña conditions are defined based on the Ocean Niño Index (ONI), which describes SST departure from long-term average values in the Niño 3.4 region (Fig. 2a; McPhaden, 2004). Based on ONI, El Niño/La Niña events are classified as weak (values of 0.5–1.0 or −0.5 to −1.0), moderate (1.0–1.5 or −1.0 to −1.5), or strong (>1.5 or < −1.5) (Yumul et al., 2010). During the monitoring periods, three El Niño/La Niña conditions were observed: weak El Niño (June 2004–February 2005), moderate El
Conclusions
The environmental properties of the surface ocean in the 10°N thermocline ridge showed a seasonal change related to movement of the ITCZ. Particle fluxes at the KOMO station, in the 10°N thermocline ridge, show clear seasonal fluctuations, ranging from high values in the cold season (25.3 mg m−2 d−1) to low values in the warm season (14.0 mg m −2 d−1). These seasonal fluctuations in particle fluxes are characterized by distinct differences in CaCO3 fluxes and chlorophyll-a concentrations between the
Acknowledgments
We are grateful for the very constructive comments by anonymous reviewers and the Editor. This study was supported by the Ministry of Land, Transportation, and Maritime Affairs of the Korean Government (PM55653), and by the Korea Ocean Research and Development Institute (PE98522, PE98445).
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