Effects on Ocean Biology Induced by El Niño‐Accompanied Positive Freshwater Flux Anomalies in the Tropical Pacific

Abstract

The El Niño and Southern Oscillation (ENSO) can induce large perturbations in freshwater flux (FWF, defined as precipitation minus evaporation) and ocean ecosystem in the tropical Pacific. However, how El Niño-induced FWF can affect the tropical Pacific ecosystem (e.g., chlorophyll) is still unknown. Here, a series of ocean-only experiments are performed using a coupled ocean-physical biogeochemistry model forced by prescribed climatological wind stress. Interannual FWF anomalies observed during the 1997-1998 El Niño event are imposed onto the ocean model in the tropical Pacific, and the related positive FWF anomalies are specified to be varying in its intensity, and the corresponding sensitivity experiments are performed to examine the ocean ecosystem response. In general, chlorophyll increases with the intensities of the El Niño-induced positive FWF anomalies at a certain strength but decreases when the positive FWF forcing is underrepresented (e.g., specified to be its half intensity). Additionally, chlorophyll in the eastern tropical Pacific keeps almost steady during the increasing stage of the El Niño-induced interannual FWF forcing and then rapidly increases after the peak stage of the FWF forcing. The phytoplankton budget and diagnostic analyses are conducted to understand the behavior of chlorophyll in response to varying intensities of FWF forcing. Chlorophyll response to varying intensities of FWF forcing depends on the iron concentration in the mixed layer, suggesting that the ocean ecosystem response may be shifted from one regime to another when FWF forcing reaches a certain intensity. These results offer insights into biophysical interactions and chlorophyll-induced feedback effects on ENSO.Plain Language Summary Chlorophyll is used to represent changes in phytoplankton biomass, and its variability is significantly modulated by multiscale physical processes. In the tropical Pacific, ENSO can induce large perturbations in freshwater flux (FWF) and ocean ecosystem. Given that FWF forcing affects sea surface salinity and buoyancy flux, it is reasonable to ask the following question: Can El Niño-induced FWF forcing affect the tropical Pacific ecosystem (e.g., phytoplankton and chlorophyll)? Here we examine the effect of FWF forcing on ocean biology using a coupled physics-biogeochemistry model, in which FWF forcing is explicitly represented with varying intensity. Results suggest that chlorophyll concentration in the eastern equatorial Pacific tends to increase in response to El Niño-accompanied positive FWF anomalies. However, a regime shift can occur when the FWF forcing intensity is underrepresented (e.g., specified to be its half intensity), with chlorophyll concentration tending to decrease instead. The behavior of chlorophyll in response to varying FWF forcing intensities is regulated by the iron concentration in the mixed layer, suggesting that the ocean ecosystem response may be shifted from one regime to another when FWF forcing reaches a certain intensity. These results offer insights into bi...

Attribution and Predictability of Climate‐Driven Variability in Global Ocean Color

“…These unanswered sources in TCV may constitute a quarter of midlatitude variability and nearly less than half of the tropical variabilities (Figure S7 in Supporting Information ). Potential sources of explanation go far beyond the simple physical climate metrics discussed in the present work and include previously identified mechanisms such as: internal dynamics of biogeochemical and ecological cycling on zooplankton grazing pressures on phytoplankton (Gorgues et al., 2010; Tian et al., 2021); anomalous phytoplankton blooms driven by volcanic ash increases in iron supply (Achterberg et al., 2013; Hamme et al., 2010); high contribution of the Amazon and Orinoco river plumes into the tropical Atlantic (Grodsky et al., 2008); widespread phytoplankton blooms triggered by Australian wild fires in the Southern Pacific (Tang et al., 2021); diversity and asymmetry of ENSO (Lee et al., 2014; Park et al., 2014); ENSO‐driven freshwater flux (Tian et al., 2020); nonlinearity of horizontal nitrate advection (Ham et al., 2021); nonlinear statistical approach (Martinez et al., 2020); and tropical instability wave driven nutrient supply (Evans et al., 2009) in the tropical Pacific phytoplankton variation.…”

Impacts of ENSO on Tropical Pacific Chlorophyll Biomass Under Historical and RCP8.5 Scenarios

“…Therefore, it is invaluable to fully understand ENSO‐related mechanisms that control phytoplankton biomass and community composition under various climate scenarios. Meanwhile, ENSO can also be modulated by changes in marine ecosystems in the tropical Pacific through a direct warming effect and an indirect cooling effect, since chlorophyll in phytoplankton affects the absorption of shortwave radiation and subsequently the vertical and meridional heat gradient (Heinemann et al., 2011; Lengaigne et al., 2007; Nakamoto et al., 2001; Park et al., 2014; Shi et al., 2023; Tian et al., 2020; Zhang et al., 2009, 2018). Though the overall effect of phytoplankton on ENSO remains uncertain, the facts of biologically induced changes in the physical environment are undeniable and have been proven to be a crucial factor in improving the accuracy of numerical models (Gnanadesikan et al., 2004; Manizza et al., 2005; Murtugudde et al., 2002).…”