Contrasting land carbon uptake responses to El Ni&ntilde;o&ndash;Southern Oscillation across North America

Sun, Wu; Foster, Kelsey; Shiga, Yoichi; Merder, Julian; Randazzo, Nina; Michalak, Anna

doi:10.5194/egusphere-2026-880

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https://doi.org/10.5194/egusphere-2026-880

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13 Apr 2026

| 13 Apr 2026

Status: this preprint is open for discussion and under review for Atmospheric Chemistry and Physics (ACP).

Contrasting land carbon uptake responses to El Niño–Southern Oscillation across North America

Wu Sun, Kelsey Foster, Yoichi Shiga, Julian Merder, Nina Randazzo, and Anna Michalak

Abstract. El Niño–Southern Oscillation (ENSO) drives year-to-year variability in the land carbon sink. While responses of land carbon uptake to ENSO have been documented at aggregated continental scales over North America, these responses arise from ENSO's climate impacts that vary across subcontinental regions. Disentangling diverse regional responses is crucial for attributing ENSO's impacts on carbon uptake to climatic drivers. Here, we characterize ENSO-driven carbon–climate interactions across North America, a data-rich continent with prominent responses to ENSO, by leveraging fine-resolution (1° × 1°) top-down estimates of land carbon fluxes derived from tall-tower atmospheric CO₂ observations and spaceborne chlorophyll fluorescence measurements. We identify regions with distinct ENSO responses in the timing, direction, and magnitude of carbon uptake anomalies. Notably, regions where El Niño boosts carbon uptake, including the Pacific–Mountainous West and subtropical lands, are consistent with ENSO-driven shifts in the position of the subtropical jet. We further uncover contrasting patterns in how energy and water limitations mediate ENSO's impacts on carbon uptake across regions. These findings reveal key regional mechanisms connecting ENSO-driven climate variability with continental-scale carbon uptake responses and highlight the need to reassess tropical carbon–climate feedbacks in light of compensatory extratropical responses.

Received: 13 Feb 2026 – Discussion started: 13 Apr 2026

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Wu Sun, Kelsey Foster, Yoichi Shiga, Julian Merder, Nina Randazzo, and Anna Michalak

Status: open (until 25 May 2026)

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RC1:
'Comment on egusphere-2026-880', Anonymous Referee #3, 28 Apr 2026 reply
This study provides a comprehensive and detailed analysis of the regional terrestrial carbon flux response to ENSO across North America. By leveraging the available surface CO₂ measurement network, the authors analyze the lagged response of carbon fluxes to ENSO at the regional scale and provide detailed interpretations of the underlying mechanisms. The manuscript is also generally well written. Despite including a wide range of methods and results, the paper guides the reader clearly and sequentially, making it easy to follow. In addition, the authors provide an excellent synthesis of previous studies, and at times the manuscript reads almost like a review paper on ENSO responses over North America. I believe this study makes a significant contribution to expanding our understanding of the relationship between ENSO and the North American carbon cycle, and it will likely attract strong interest from the carbon cycle community.
Before publication, I encourage the authors to consider the following comments, which are intended to further improve the quality of this work.
Analysis period. The choice of the 2007–2015 period for the analysis likely reflects data availability; however, I suggest that the authors explicitly describe the rationale for selecting this time period in the manuscript.

Spatial and temporal coverage of observations. Since the availability of observational data varies across sites, it would be helpful to discuss how this heterogeneity may influence the estimated NBP variations. At a minimum, I suggest including a supplementary figure demonstrating that, in regions sensitive to ENSO influences, the sensitivity of atmospheric CO₂ concentration observations to surface net CO₂ fluxes does not vary substantially across years. A brief discussion of the potential impacts of observational coverage on the results would further strengthen the analysis.

Definition of ENSO-sensitive regions. The approach of using four ENSO indices to identify regions where correlations are both significant and consistent (in sign) is not entirely clear to me. These four indices do not represent the same aspect of ENSO. Specifically, ONI and Niño 3.4 primarily reflect SST anomalies, whereas SOI and MEI v2 capture atmospheric circulation and the coupled ocean–atmosphere state.

Given this, selecting regions where correlations are consistent across all four indices appears to effectively isolate cases where SST anomalies are strong, the atmospheric response is also strong, and the two are tightly coupled. In that sense, this approach may not necessarily identify regions with robust and consistent NBP responses to ENSO in a general sense, but rather regions that are particularly sensitive to specific ENSO expressions (e.g., canonical or EP-type ENSO).

If this is the authors’ intention, I encourage them to clarify the physical interpretation of this selection criterion and provide justification for adopting this approach. If not, it may be more appropriate to define ENSO-sensitive regions based on a general ENSO index primarily tied to SST anomalies. This could potentially lead to a broader spatial extent of the selected regions and may help reduce uncertainties in the inversion results by increasing the effective spatial coverage.

Please clarify how the authors calculate NBP anomalies from FLUXCOM NEE product “NBP anomalies were calculated from the FLUXCOM-X-BASE upscaled net ecosystem exchange (NEE) product (Nelson et al., 2024) and aggregated over the same North American domain.”

Equations in the Appendix. Several equations in the Appendix appear to be missing “+” signs. I suggest that the authors check the equations and correct them where necessary.

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Citation: https://doi.org/10.5194/egusphere-2026-880-RC1

Wu Sun, Kelsey Foster, Yoichi Shiga, Julian Merder, Nina Randazzo, and Anna Michalak

Supplement

https://doi.org/10.5194/egusphere-2026-880-supplement

Model code and software

Code for the analysis of ENSO impact on North American land carbon uptake Wu Sun and Julian Merder https://gitlab.com/wusun/enso-impact-na-carbon-flux

Wu Sun, Kelsey Foster, Yoichi Shiga, Julian Merder, Nina Randazzo, and Anna Michalak

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Short summary

El Niño weakens the global land carbon sink but boosts uptake over North America. Using regional top-down carbon flux estimates, we find that El Niño enhances carbon uptake in western and subtropical North America but suppresses uptake in mid-continental regions, reflecting control by the Pacific jet and region-specific impacts of energy and water availability. This spatially refined understanding of El Niño impacts on North American carbon uptake helps better inform carbon–climate feedbacks.


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