the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 30 Jan 2026
Tropical Instability Vortices reduce Pacific Ocean ENSO-Driven CO2 outgassing
Abstract. The relationship between the intensity of Pacific Ocean Tropical Instability Vortices (TIVs), ENSO variability, and dissolved inorganic carbon (DIC) remains poorly constrained. Here, we use a 30-year-long eddy-resolving ocean biogeochemistry simulation to quantify the effects of TIVs on DIC budget components at both synoptic and interannual timescales. At synoptic scales, TIVs primarily influence DIC through advection, especially along the leading edge of the wave fronts, while vertical diffusion and biological processes play secondary roles. To investigate interannual variability, we develop a TIV index to classify strong and weak TIV phases within each ENSO state. In the upper 50 meters, TIV-driven advection shapes large-scale DIC transport pathways while enhancing, yet spatially confining, primary production. Consequently, during El Niño, TIVs tend to amplify oceanic CO2 uptake, associated with a 57 % decrease in CO2 partial pressure (pCO2). During La Niña, they suppress CO2 outgassing, even reversing the ocean's role from a source to a sink. TIVs also affect the upper thermocline carbon inventory by modulating both biological activity and lateral transport. Strong TIVs during El Niño reduce DIC inventories in the upper thermocline by 8.5 GtC due to increased vertical mixing and enhanced transport, while during La Niña, strong TIVs lead to a 77 % higher DIC accumulation compared to weak TIVs. These findings underscore the critical role of TIVs in regulating the equatorial Pacific carbon budget and highlight the need to accurately represent them in Earth system models.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2026-504', Anonymous Referee #1, 14 Apr 2026
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RC2: 'Comment on egusphere-2026-504', Anonymous Referee #2, 12 May 2026
This manuscript investigates the influence of Tropical Instability Vortices (TIVs) on dissolved inorganic carbon (DIC), air–sea CO2 fluxes, and upper thermocline carbon storage in the equatorial Pacific using an eddy-resolving ICON-O/HAMOCC simulation. The manuscript is generally well organized, and the use of a long high-resolution simulation is a clear strength of the study. However, my major concern is that, although the manuscript focuses on TIVs, I did not find convincing evidence demonstrating that the analyzed signals indeed represent robust TIV variability. In several places, the connection between the diagnosed carbon signals and TIW/TIV dynamics remains unclear. Since the entire study is built upon the identification and interpretation of TIV activity, the methodology and physical interpretation of the TIW/TIV signal need to be substantially clarified and strengthened before the subsequent carbon-cycle analyses can be fully convincing. I therefore recommend major revision before the manuscript can be considered for publication.
Major comments:
1. Lack of clear and convincing TIW/TIV signals. Although the manuscript is centered on TIWs/TIVs, I did not clearly see convincing TIW signals in the analyses and figures. In particular, the manuscript does not sufficiently provide the data processing details and demonstrate that the diagnosed variability is truly associated with TIW activity rather than broader interannual or large-scale circulation variability.
It appears that the analyses are primarily based on monthly data. I am concerned whether monthly data can adequately resolve TIW signals, whose typical timescales are approximately 20–40 days. Some papers use daily data and band-pass filter to get the TIW signal (Xue et al., 2020). I encourage the authors to provide more evidence to prove that the diagnosed variability is TIV signal instead of other signal.
2. Definition and validation of the TIV index. The calculation of the TIV index is not sufficiently described. In many previous studies, TIW activity is often diagnosed using filtered velocity anomalies (e.g., or SST’) or band-pass filtered variability on TIW timescales. In contrast, the manuscript defines the index using own method (Figs. 1-2). However, the rationale and implementation details of this method are not adequately explained. The authors should provide a detailed description of the methodology; a discussion of how this method compares with more conventional TIW diagnostics; evidence demonstrating that the resulting TIV index indeed captures realistic TIW variability.
At present, Figure 2 shows interannual variability of the TIV index, but it is unclear whether this index truly represents interannual TIW variability or contains contributions from broader large-scale circulation changes. Because this index appears to be central to the study, the methodology requires much stronger validation and justification.
3. Section 3 presents DIC budget analyses, which indicate that DIC tendency is mainly controlled by horizontal and vertical advection. However, I did not fully understand how these diagnostics are directly linked to TIW/TIV processes. Currently, the budget analysis mainly demonstrates general controls on DIC variability, but it does not clearly isolate TIW-related processes. I wonder whether the authors should instead: isolate TIW timescale variability using filtering, demonstrate that TIW signals dominate DIC tendency on 15–60 day timescales, and then diagnose the associated mechanisms.
4. The manuscript discusses enhanced northward DIC transport associated with TIWs/TIVs. However, the methodology used to diagnose this transport is not clearly described. From my understanding, if one aims to isolate TIW-induced carbon transport, it is usually necessary to use daily data, remove climatology and/or low-frequency variability, apply a 15–60 day band-pass filter.
Currently, it appears that the manuscript mainly analyzes monthly meridional transport during periods of strong TIV index values. It is therefore unclear whether the diagnosed transport anomalies are truly TIW-related or instead reflect larger-scale interannual variability. In particular, the manuscript needs to clarify: whether the analyzed meridional transport is based on anomalies or total fields, whether temporal filtering was applied, and whether the diagnosed transport specifically represents TIW-induced eddy transport. At present, I do not find the evidence sufficiently convincing that the diagnosed northward transport is directly caused by TIWs/TIVs.
5. The manuscript states that TIWs/TIVs enhance northward DIC transport, but the physical mechanism responsible for this transport is not sufficiently explained. This is a physical process that has been discussed in previous studies, and the authors should cite the relevant literature and describe the mechanism more clearly. In particular, the manuscript should clarify why TIWs/TIVs generate meridional transport, whether the diagnosed transport is mainly associated with , , or their covariance , and how TIW dynamics physically modulate the meridional redistribution of DIC.
More broadly, several parts of the manuscript mainly present statistical relationships rather than mechanistic explanations. I suggest that the authors revise the text to provide more physical details on how TIWs/TIVs drive horizontal and vertical transport, and how these transport processes subsequently influence DIC variability. Without such mechanistic clarification, it is difficult for readers to understand how the reported carbon-cycle responses are dynamically linked to TIW/TIV processes.
Specific comments:
- Terminology: TIV vs TIW. The manuscript consistently uses “Tropical Instability Vortices (TIVs)”. However, in most of the literature, the commonly used terminology is “Tropical Instability Waves (TIWs)”. The authors should clarify why they chose “TIV” instead of the more standard “TIW”.
- The introduction would benefit from adding a brief paragraph explaining the broader scientific motivation for studying the influence of Pacific TIWs/TIVs on DIC and the carbon cycle. At present, the manuscript moves rather directly into the analyses without sufficiently establishing why this topic is important. For example, the authors could briefly discuss: the importance of the tropical Pacific in the global carbon cycle and air–sea CO₂ exchange, the strong physical and biogeochemical variability associated with equatorial dynamics, and the potential role of TIWs/TIVs in modulating carbon transport, DIC redistribution, biological production, and air–sea CO₂ fluxes.
- Lines 32-34: The manuscript should more clearly explain the physical background and mechanism responsible for the northward transport of high-nutrient/high-DIC waters and the southward transport of relatively low-nutrient waters associated with TIWs/TIVs.
- Line 70: Is it a global model or regional model. I can not find this informaiton here. I suggest including a brief summary of the model configuration in the main text, while providing a more detailed description in the Appendix or Supplementary Materials. This would greatly improve the clarity of the study.
- Line 134–135: Why are only negative advection terms considered here? Please provide a physical explanation and supporting references or evidence.
- Lines 148-149: It is difficult to assess whether the model accurately simulates the relevant physical fields because no observational or reference “truth” is provided for comparison. I suggest that the authors add additional model validation, at least in the Appendix. For example, they could compare observed and simulated velocity profiles, as well as temperature and salinity fields, to demonstrate the model’s ability to reproduce the key physical structures relevant to TIW/TIV dynamics and carbon transport.
- Fig. 5: The authors should provide the residual term of the budget analysis, preferably in the Appendix or Supplementary Materials, in order to demonstrate that the budget is reasonably closed.
- Line 201: Why is the budget in Figure 5 calculated for the mixed layer, while the budget here uses 50 m depth? Please clarify the rationale and consistency.
- Line 217–218: Why do El Niño and La Niña conditions produce different TIW characteristics and consequently different DIC impacts? What specific TIW properties differ between El Niño and La Niña periods?
- Line 228: The manuscript should provide a clearer physical explanation for why the SSH pattern shown here would lead to the accumulation or retention of low-DIC and warm waters in this region. At present, the dynamical linkage between the SSH anomaly and the tracer distribution is not sufficiently explained. In addition, the SSH anomalies appear relatively small (approximately 0.01–0.02 m). It is therefore unclear whether anomalies of this magnitude are dynamically strong enough to generate the reported changes in circulation and tracer transport. The authors should provide stronger evidence, quantitative analysis, or supporting references to justify this interpretation.
- Line 230: It is not clear whether the diagnosed transport is directly caused by the TIWs/TIVs themselves, or whether it mainly reflects broader circulation changes that tend to co-occur during periods of strong TIW/TIV activity. In other words, the manuscript should carefully distinguish between transport driven directly by TIW/TIV eddy processes and transport associated with the large-scale circulation background under conditions favorable for TIW/TIV development.
- Line 273: The manuscript states that TIWs/TIVs influence or modify the thermocline structure, but the physical mechanism responsible for this effect is not sufficiently explained. At present, this statement is presented almost as a conclusion or assumed background knowledge, which may make it difficult for readers unfamiliar with TIW dynamics to fully understand the interpretation. I suggest that the authors provide a clearer physical explanation of how TIWs/TIVs can modify thermocline depth or thermocline structure, and support this discussion with relevant references from previous studies.
- Section 3 states that the budget analysis is “integrated within the mixed layer”, but the specific definition of mixed-layer depth (MLD) is currently unclear. The authors should clarify the MLD criterion used in the study (e.g., temperature- or density-based threshold).
- The term “SoSi (sources and sinks)” is repeatedly used to represent the biogeochemical contribution in the DIC budget. The authors are encouraged to explicitly clarify in the Methods section which bgc processes are included in SoSi.
- The manuscript discusses changes in the “upper thermocline”, but the exact depth range corresponding to the thermocline is not consistently defined. Providing a clearer definition or representative depth range would improve clarity.
Citation: https://doi.org/10.5194/egusphere-2026-504-RC2
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- 1
Review of “Tropical instability vortices reduce Pacific Ocean ENSO-driven CO2 outgassing” by Casaroli et al.
This study investigates the DIC variability in the eastern equatorial Pacific as a function of the activity of tropical instability vortices and ENSO phase, using simulation results from a state-of-the-art ocean physical-biogeochemical model. Their approach which extracts intrinsic TIV variability independent of ENSO and examines composites of strong and weak TIV regimes separately for El Niño and La Niña phases, is unique. Their findings, showing that the contributions from TIV regimes differ substantially between ENSO phases, are important and appropriate for publication in this journal. I have one major concern, as described below, which I hope can be addressed before publication.
To my understanding, the mean states differ among the composites: upper-layer stratification and thermocline depth as well as EUC depth vary between ENSO phases; differences in TIV activity imply that the SEC, NECC, and/or other currents as well as meridional density gradients, associated with the TIV generation, are remarkable. These differences in the mean states are not explicitly described in the present manuscript but could affect their analysis, particularly the estimates of the advection term. Thus, I suggest a more careful interpretation of the advection term. Note that the drivers of TIVs would be the mean states; hence, descriptions such as “TIV-driven” might be misleading. In addition, TIVs sometimes enhance northward transport, sometimes enhance southward transport, and sometimes trap tracers locally. The physical interpretation should account for both the mean state differences and the TIV activity differences. I suggest revising the descriptions regarding these points.
Specific comments
L27-28: Most of these references investigated tropical instability waves (TIWs). I suggest separating the references between TIWs and TIVs and adding a description of the relationship between these phenomena. For this purpose, the description in Zheng et al. (2016), already cited, and a recent analysis by Toyoda et al. (2023, doi:10.1038/s41598-023-41159-5) may be useful. In addition, many other studies on TIWs exist, and the references in the manuscript do not fully cover the literature. Thus, I suggest using “e.g.,” before the references, here and in L25. Please also check similar reference descriptions (e.g., L150-151 and L181).
L44: CO2 -> CO2
Section 2.2: To my understanding, the authors aim to analyze the variabilities of SST anomaly and TIV index on a similar (interannual) time scale, although the treatment of these variables differs. I suggest adding an explanation of the influence of this difference.
L97: I consider that “minimum” might more appropriately be “mean”.
L104: I understand that Figure 2 (upper panel) shows the “intrinsic TIV variability independent of ENSO”. If so, I think that the figure caption and title need to be improved.
L126: What is the baseline of the sea surface temperature anomaly? Is seasonality included?
L159: Although the distribution of mixed layer depth is partly shown in Fig. 6, a brief explanation of the meridional distribution of mixed layer depth and its influence on the analysis would help in understanding the subsequent interpretation of the results.
Figure 8c, d: Please add an explanation for why the mean El Niño lines are not located between the strong and weak TIV composite lines.
L231: “(( ))” -> “( )”
Figure 10: Using latitude on the y-axis, as in other figures, would improve readability.
L243: What is the intrinsic dynamical difference between strong and weak TIVs? Why does one enhance the mean advection term while another traps carbon locally?
L247 “strong TIVs disrupt near-equatorial northward flow”: Please add an explanation of how the flow field changes and why.
L258 “strong TIVs cause shoaling of the equatorial thermocline during El Niño”: How can this causal relationship be understood dynamically?
L308-309: Why does “the remineralization of POM is also shifted south of this point” suggest “a larger production of particulate organic matter in strong TIVs north of 3°N”? Please add an explanation.
L323 “silicate is advected zonally”: The difference in the mean current as well as TIV intensity can affect this.
L328 “south of the equator”, L332 “across the equator”, L341 “cross-equatorial exchange”: To my understanding, the authors do not explicitly conduct analyses supporting these statements.
Figure 16: I wonder how the integrated DIC concentration (or anomaly) over this vertical-meridional cross section differs between weak and strong TIV composites. The analyzed terms are conservative when integrated over the domain, except for the SoSi term. The discussion of dominant terms might be meaningful when the overall (integrated) DIC concentration does not differ significantly between the TIV regimes.