Implications of changing El Niño patterns for biological dynamics in the equatorial Pacific Ocean

Abstract

[1] El Niño events are known to strongly affect biological production and ecosystem structure in the tropical Pacific. Understanding and predicting biological processes in this area are hampered because the existing in situ observing system focuses primarily on physical measurements and does not observe key biological parameters; the only high spatial and temporal resolution biology-related observations are from the global array of ocean color satellites which provide an estimate of surface chlorophyll concentrations only. Since the 1990s, an apparent shift of the El Niño maximum sea-surface temperature (SST) warm anomaly from the eastern to the central equatorial Pacific has frequently been observed. Satellite observations show significant changes in chlorophyll-a (Chl-a), new production (NP) and total primary production (PP) in the equatorial Pacific associated with these new central Pacific (CP) El Niño events (also called El Niño Modoki) relative to eastern Pacific El Niños. During CP-El Niños, NP, Chl-a and PP in the central basin are depressed relative to EP-El Niños and lower values of Chl-a and PP coincide spatially with higher SST in the central Pacific. While surface Chl-a, and integrated NP and PP over the entire equatorial band, decrease during both CP and EP-El Niños, the magnitude of this decrease seems to depend more on the intensity than type of event. The changing spatial patterns have significant implications for equatorial biological dynamics if, as has been suggested, CP-El Niños increase in frequency in the future. Citation:

Impact of El Niño Variability on Oceanic Phytoplankton

“…The results presented in Figure 3 are consistent with the different physical regimes and global climatological relationships previously demonstrated between open ocean satellite surface observations of chlorophyll and subsurface parameters of MLD, thermocline and nutricline depths at global scale (Wilson and Coles, 2005;Messié and Chavez, 2012;Brewin et al, 2014) and in the Equatorial Pacific (Turk et al, 2011;Gierach et al, 2012;Radenac et al, 2012;Lee et al, 2014). In coastal regions, phytoplankton production can be modified further by local supply of nutrients through coastal upwelling, riverine input (e.g., Turner et al, 2003) or atmospheric dust deposition (Abram et al, 2003;Jickells et al, 2005).…”

“…Furthermore, the biological and physical response patterns to each type of El Niño observed in the present work over the satellite record of 1997-2012 are consistent with contemporary case studies of specific EP and CP El Niño events in the Equatorial Pacific (Turk et al, 2011;Gierach et al, 2012;Radenac et al, 2012), the Indian Ocean (Webster et al, 1999), the continental U.S. (Yu et al, 2012;Yu and Zou, 2013), and the global oceans (Behrenfeld et al, 2001;Chavez, 2012, 2013). In addition, the response patterns observed for the physical variables have also been shown to persist over decadal timescales during the period 1979-2014 (see Section Physical Forcing Mechanisms Associated with El Niño Variability).…”

Unravelling Eastern Pacific and Central Pacific ENSO Contributions in South Pacific Chlorophyll-a Variability through Remote Sensing

“…In between, there is an El Niño phase of CP. A recent study of CP influence on Chl-a distribution is in agreement with our negative amplitude evolution of Chl-aA in the western side of the tropical Pacific [16]. Further, following the pattern described by [14], we observe a propagating negative (positive) Chl-aA pattern, in the case of an El Niño (La Niña) event, linked to the central Pacific and focused on the western and central Tropical Pacific.…”

“…Finally, we discuss decadal-scale changes in Chl-a concentrations in the context of recent increases in CP El Niño frequency-a mode with a clear and strong decadal peak. We demonstrate that at least since the late 1990s, the Chl-a signal has been more connected with the CP El Niño structure than with the classical EP ENSO mode-consistent with the increased frequency and intensity of the CP mode over the past 30 years [4,13,16].…”

“…CP El Niño displays a strong decadal periodicity [11,12], and has increased in intensity in recent decades, unlike EP El Niño [13]. Recent works reveal that CP El Niño events induce low values of remotely sensed phytoplankton indicators in the eastern part of the WPWP [14][15][16] while the examination of the strongest CP and EP El Niños to date (i.e., 1997-1998 EP El Niño and 2009-2010 CP El Niño) suggests that horizontal processes (e.g., intrusion of nutrient-deficient warm pool waters) dominate during a CP event while vertical processes (e.g., nutricline suppression) and mixing play a larger role in an EP event [17].…”