Rectified Effects of Interannual Chlorophyll Variability on the Tropical Pacific Climate Revealed by a Hybrid Coupled Physics‐Biology Model

In the boreal summer of 2021, the equatorial Atlantic experienced the strongest warm event, that is, Atlantic Niño, since the beginning of satellite observations in the 1970s. Such events have far‐reaching impacts on large‐scale wind patterns and rainfall over the surrounding continents. Yet, developing a paradigm of how Atlantic Niño interacts with the upper‐ocean currents and intraseasonal waves remains elusive. Here we show that the equatorial Kelvin wave associated with the onset of the 2021 Atlantic Niño modulated both the background flow and the eddy flux of the equatorial upper‐ocean circulation, causing an extremely weak and delayed tropical instability wave (TIW) season. TIW‐induced variations of sea surface temperature (SST), sea surface salinity, sea surface height, and eddy temperature advection were exceptionally weak during May to July, the climatological peak of TIW activity, but rebounded in August when higher than normal variability was observed. Moored velocity data at 23°W show that during the peak of the 2021 Atlantic Niño from June to August, the Equatorial Undercurrent was deeper and stronger than usual. An anomalously weak eddy momentum flux strongly suppressed barotropic energy conversion north of the equator from May to July, likely contributing to low TIW activity. Reduced baroclinic energy conversion also might have played a role, as the meridional gradient of SST was sharply reduced during the Atlantic Niño. Despite extremely weak TIW velocities, modest intraseasonal variability of chlorophyll‐a (Chl‐a) was observed during the Atlantic Niño, due to pronounced meridional Chl‐a gradients that partly compensated for the weak TIWs.

Section: Discussionmentioning

confidence: 95%

Modulation of Equatorial Currents and Tropical Instability Waves During the 2021 Atlantic Niño

Tuchen,

Perez,

Foltz

et al. 2023

“…Therefore, narrowing the model spread of chlorophyll projections can contribute to reducing the uncertainty in future projections. Additionally, a series of modeling studies have suggested that chlorophyll‐induced change in Q pen can substantially affect the tropical climate from mesoscale (i.e., tropical instability waves) to large‐scale (Park & Kug, 2014; Tian, Zhang, Wang, & Zhi, 2021), but the chlorophyll‐induced change in Q pen has not been adequately represented in the current ESMs as summarized in a recent study (Séférian et al., 2020) and Table S1 in Supporting Information .…”

Section: Resultsmentioning

confidence: 99%

“…Apart from hflx, downwelling solar radiation can penetrate through the bottom of the oceanic ML and subsequently enter the subsurface layers; this part of the shortwave is called Q pen , which instantaneously affects the subsurface temperature, vertical mixing/entrainment, and stratification. The extent to which solar radiation penetrates through the bottom of the ML ( Q pen ) is dependent on the downwelling shortwave radiation itself reaching the sea surface (SW), the ML depth ( H m ), and chlorophyll concentration in the open ocean (Chen et al., 1994; Morel, 1978; Ohlmann, 2003; Ohlmann et al., 1996; Kang et al., 2017; Shi et al., 2023; Zhang et al., 2009). Thus, Q pen is an essential heating quantity representing the interactions between the marine physical and biogeochemical processes.…”

Section: Introductionmentioning

confidence: 99%

Increasing Shortwave Penetration Through the Bottom of the Oceanic Mixed Layer in a Warmer Climate

Tian

2023

Self Cite

Shortwave penetration (Qpen) through the bottom of the oceanic mixed layer (ML) profoundly affects the thermal structure in the upper ocean and consequently contributes to sea surface temperature (SST) change, which has not been adequately understood under global warming. Here, using ensemble earth system model simulations under a high‐emissions scenario (SSP5‐8.5), we investigate the Qpen change and related effects on some oceanic parameters, which are regionally dependent. It is found that globally averaged Qpen typically increased by 2.49 ± 0.83 W m−2 in the second half of the 21st century, which is comparable to an increase in the net surface heat flux (3.01 ± 0.31 W m−2), corresponding to an increase in SST by 3.07 ± 0.83°C. Although there exist substantial intermodel uncertainties in the projected chlorophyll change, the shoaled ML contributes the most of the increase in Qpen in the global ocean, whereas in the tropical ocean, the reduction in the chlorophyll concentration plays an equivalent role with the mixed layer depth in determining Qpen change. The ML heat budget indicates that the enhanced Qpen leads to surface cooling through a decrease in the surface net surface heat flux. However, the cooling is compensated for by a warming effect from ocean dynamical change due to more shortwave penetration into the subsurface layer, leading to a small net effect on the ML heat budget. It is suggested that the impact of Qpen on the global oceanic ML heat balance needs to be adequately recognized.

“…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).…”

Section: Introductionmentioning

confidence: 99%

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

Chen,

Wu,

et al. 2024

The present study aims to investigate the influences of El Niño and Southern Oscillation (ENSO) events on chlorophyll biomass in the tropical Pacific under historical and RCP8.5 scenarios with simulations from the Community Earth System Model Large Ensemble (CESM‐LE) project. Large variance in surface chlorophyll concentrations is identified across the Peruvian Upwelling (PU), Equatorial Upwelling (EU), and Western Pacific (WP) regions within the tropical Pacific. Results suggest that responses of surface chlorophyll to ENSO in these regions are governed by different mechanisms. The increasing chlorophyll during La Niña is a consequence of increasing nutrients, influenced by local upwelling systems in the PU and EU regions, while the supplementary nutrients in the WP region arise from the eastern Pacific through stronger surface westward currents. Under RCP8.5 scenario, stronger water warming in the eastern equatorial Pacific leads to a remarkable reduction in the amplitudes of seasonal cycles and interannual variations of chlorophyll in both PU and EU regions. This results in less responsiveness of the chlorophyll biomass to El Niño and moderate La Niña compared to the historical period. However, though warming induces a decrease in chlorophyll concentrations in the WP region, the interannual variations of chlorophyll have shown an improvement in correlation with ENSO events. Meanwhile, despite the small phytoplankton‐dominated community being observed under future scenario, species dominance is likely to shift back to diatoms once extreme La Niña occurs, which is unseen in all El Niño and moderate La Niña cases.