the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 04 Sep 2025
The Northwest Pacific corals unravel the NPGO/Victoria Mode-related temperature back to the 19th century
Abstract. Atmospheric and oceanic phenomena in the Pacific Ocean, such as the North Pacific Gyre Oscillation (NPGO) and the Victoria Mode (VM), have significant implications for global climate variability through their linkage with temperature variability in the Central Pacific (CP). The NPGO and VM manifest as sea surface height and basin-wide sea surface temperature anomaly patterns, respectively, in the North Pacific. Although recent studies have drawn attention to their connections with surface temperature anomalies in the CP, their historical interactions remain poorly understood due to limited instrumental observations. Here, we present a long-term reconstruction of NPGO/VM-related temperature variability based on a coral Sr/Ca temperature proxy from the Northwest Pacific, covering the period from 1798 to 2014 Common Era. This reconstruction contributes to a better spatiotemporal understanding of historical interactions among NPGO, VM, and surface temperature anomalies in the CP over the past two centuries. The coral-inferred temperature record closely aligns with surface temperature variability in the CP, indicating the extra-tropical propagation of NPGO/VM-related temperature variations from the CP via an ocean-atmosphere bridge. Our reconstruction identifies pronounced negative-NPGO/cool-temperature phases in the late 1840s and during the 1860s–1900s, and a positive-NPGO/warm-temperature phase during the 2000s–2010s. We also find a significant correlation between surface temperature anomalies in the CP and interannual to decadal NPGO/VM variability throughout the past 200 years. This relationship lasted from the early 20th century until 2014, with the exception of the period from the 1970s to the 1990s, and strengthened from the 2000s onward. Given the strong linkage between the NPGO and the surface temperature anomalies in the CP, as well as the projected increase in CP-centred climate variability under future warming scenarios, the prominence of the NPGO in the North Pacific and beyond is expected to intensify.
Status: final response (author comments only)
- RC1: 'Comment on egusphere-2025-4054', Anonymous Referee #1, 28 Oct 2025
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RC2: 'Comment on egusphere-2025-4054', Anonymous Referee #2, 14 Nov 2025
Ito and colleagues present a valuable 217-year (1798-2014 CE) temperature reconstruction from Shiomichi Bay, Kikai Island, Japan, derived from three Porites coral cores. The coral record demonstrates connections between low-frequency temperature variability in the Northwest Pacific (NWP) and major Pacific climate modes, specifically the North Pacific Gyre Oscillation (NPGO) and Victoria Mode (VM), which may be connected to Central Pacific surface temperatures via an atmospheric teleconnection. The analytical approach is rigorous, employing high-precision ICP-OES methods and incorporating multiple calibration slopes through Monte Carlo simulation to address uncertainties. This reconstruction makes an important contribution by expanding the limited network of coral and marine-based paleoclimate records in the Northern Pacific region, particularly those examining NPGO and VM variability.
While I find the manuscript appropriate and relevant for publication in this journal, I believe that the manuscript may require substantial revisions to fully realize its potential. Specifically, the manuscript would benefit from additional analyses, content reorganization, and strengthened argumentation to better support its key claims. Below, I provide my main comments and suggestions.
General Comments
1. The introduction effectively establishes the importance of increasing the number of records in this region and the value of investigating past NPGO and VM variability beyond instrumental records. However, the manuscript would benefit from additional details from the literature. Specifically, clarification of the seasons and timescales on which the NPGO and VM typically operate would be valuable. Definitions could be made clearer throughout. The paper would also benefit from reorganization in the presentation of results and discussions.
2. While the rationale for comparing the record to surface temperatures (ST) due to the coral's location (inside a bay, surrounded by landmass) is understandable, the use of ST rather than sea surface temperature (SST) may somewhat obscure the primary focus of this paper, which is identifying connections between NWP climate and the NPGO and VM, since both are oceanic expressions of North Pacific variability.
3. Throughout the manuscript, NPGO and VM are used interchangeably, sometimes combined as NPGO/VM and other times described separately, which may confuse readers. While the close relationship between NPGO and VM is acknowledged, they are considered distinct modes, one described as SSH variability, while the other as SST, respectively. Therefore, discussing them individually and separately in a clearer manner would be beneficial.
4. Reorganization of Figures – Mentioning figures in chronological order would improve readability. Some examples (but not all) where figures appear out of sequence include:
- Figs. 1b, 1c, and 1e might be better placed in the latter part of the paper as results, as they are discussed toward the end
- Fig. 7 is mentioned before Fig. 6c
- Fig. A9 is mentioned before Figs. A6 and A7
5.Reorganization of Text – Several sections of the text would benefit from relocation or enhancement. A thorough revision is recommended. Some examples include:
- Lines 225-240 appear more suitable for the Discussion section
- Lines 259-263 could be placed earlier when describing the study site in the Methods section
- Some portions of the Results section would benefit from additional interpretation. For example, lines 269-271 present a correlation between the Kikai-ST_coral and Palmyra records, but do not discuss the implications
- Section 4.4 (Lines 350-365) may be better suited to the Methods section or presented as part of the Results, as some sentences introduce results for the first time in the Discussion.
6. The manuscript’s central conclusions become diluted across the Discussion section. I recommend restructuring the Discussion section to highlight and prioritize the most significant findings of the study.
Specific Comments
1. Study Site Description – A close-up map of the study site showing the precise coral collection location would help readers understand that samples came from within the bay rather than more open environments (as in Kawakubo et al., 2017). Including locations of other Kikai Island corals used for comparison would also be valuable. A more detailed description of reef conditions, submersion history, and potential local factors affecting Sr/Ca variability would enhance the manuscript.
2. Selection of Sampling Tracks – Before examining the relationship between the coral reconstruction and NPGO/VM, it would be valuable to provide additional justification for the sampling approach used for Core 2. I appreciate the authors showing the X-ray images with all sampling tracks. However, based on these images, it remains unclear how sampling along extending corallites or the maximum growth axis was ensured to yield reliable Sr/Ca ratios (see e.g., DeLong et al., 2013). Some portions of the core appear to show off-axis growth. Given that the coral paleoclimate community typically exercises considerable care in sampling track selection to avoid spurious measurements unrelated to climate/environmental change, additional documentation of the track selection criteria would strengthen confidence in the results.
3. Several overlapping tracks are present throughout the cores. While the Sr/Ca ratios for overlapping tracks between Cores 1 and 2, and between Cores 2 and 3, are shown, it would be helpful to see raw Sr/Ca data plotted for all overlapping tracks, particularly for Core 2 where annual bands and growth axes appear less distinct. Additional explanation of how tracks were identified for Core 2 under these conditions would be valuable. In Figure 2b, higher variance appears in the data during time periods represented by Core 2, and clarification of whether this reflects sampling considerations or natural climate signals would be helpful.
4. Section 2.3 Chronology:
- Clarification would be helpful regarding the use of OISST v2.1 for delineating coral Sr/Ca seasonal cycles while HadCRUT5 is used for analysis. Are SST values from 1981-2015 similar between these datasets? If so, what motivated the choice to switch datasets? Consideration of ERSSTv5 reanalysis data (or other SST data products) might also be worth discussing.
- Additional detail on the conversion of Sr/Ca from the depth domain to the time domain would be beneficial. Specifically, were values interpolated to 12 months per year? Explicit mention in the Methods section would be helpful. The citation to Ito et al., 2020 also appears to be missing from the references.
- Further explanation of the time assignment for Sr/Ca values beyond the instrumental period would be valuable. Was this based on climatology? If software such as QAnalySeries was used for assigning tie points, this should be mentioned with appropriate detail.
5. The assignment of single peaks as summer (August) or winter (February) may warrant reconsideration, as the timing of highest/lowest temperatures can vary between years. This could potentially contribute to the observed lag between coral-derived reconstructions and NPGO or VM indices. Defining seasons more broadly (e.g., JJAS for summer or DJFM for winter) based on SST climatologies and performing analyses accordingly might provide more robust results.
6. Additional justification would be helpful for comparing summer and winter seasons to annual instrumental data rather than season-to-season comparisons. Furthermore, the observation that summer temperature matches annual instrumental temperature better than annual coral temperature does (Fig. A6) warrants explanation.
7. Clarification of whether NPGO or VM expression is more pronounced during boreal summer in the Northwest Pacific would strengthen the manuscript. While NPGO and VM are typically more strongly linked to winter climate patterns with the largest seasonal variance in winter, this paper compares coral-based summer temperature reconstruction to annual temperatures. If the expression or impact is indeed more evident in boreal summer for this region, this should be explicitly stated to justify the analytical approach.
8. Choice of 8–30 year bandpass filter and 30-year running correlations (Fig. 7) – Additional rationale for these frequency ranges would strengthen the analysis. Does the coral record show significant power at these timescales? An explanation of why this frequency range (8-30 years) is particularly important for understanding NPGO and VM variability would be beneficial.
9. The manuscript would be strengthened by comparing the temperature record to other major Pacific climate modes (ENSO/PDO/IPO) and explicitly discussing why NPGO and VM appear to be more dominant at this site. Currently, the relative influence of NPGO/VM compared to other major modes of Pacific climate variability is not directly addressed. Clarification of why this coral record preferentially captures NPGO and VM variability would enhance the interpretations.
10. Have the authors considered comparing their record directly to the Victoria Mode Dipole Index (VDMI) (Wen et al., 2024, Dyn.) to further verify the relationship of their coral record to the Victoria Mode?
11. The observation that the coral record leads the instrumental record warrants further discussion. Could this result from age model uncertainties or from comparing summer coral values to annual instrumental records? Additional analysis of this temporal relationship would be valuable.
12. Given the proximity of this coral core to other records from Kikai Island, the differences in captured variability (e.g., winter records compared to Kawakubo et al., 2017) merit additional discussion. Potential explanations related to location or site-specific characteristics would be valuable to include.
13. Section 4.1 – While growth-related biases in Sr/Ca depending on season are discussed (with faster coral growth during summer than winter), the manuscript would benefit from systematic documentation of this effect. Providing information on average growth rates would be helpful. Currently, the lack of quantitative growth rate data makes evaluation of potential growth biases challenging. Analysis of relationships between Sr/Ca and annual growth rates could help ascertain whether growth biases are present in the record.
14. Section 4.2, Lines 311-313 – The attribution of poor correlation to insufficient quality of instrumental ST datasets prior to 1950 would be strengthened by consideration of alternative SST products. Additionally, Figure 7 shows strong relationships between CP-ST_HadCRUT5 and the coral-based reconstruction both before and after 1950. If HadCRUT5 data quality was indeed poor prior to 1950, the strong pre-1950 correlation at these frequencies requires explanation.
15. Additional comparison with the Palmyra record might be preferable to the Clarion δ18O record, given the Palmyra record's stronger documented relationship with the NPGO (as shown in Figure 6).
16. Line 455 – The claim that this record is based on "an oceanic temperature proxy" may require qualification, given that comparisons were made to surface temperatures (HadCRUT5), which blend land and ocean measurements.
Minor/Technical Corrections (Typographical errors):
- Fig. A1 – Adding year labels next to annual growth bands (at 5- or 10-year intervals) would help readers connect Sr/Ca data with positions on the coral slab.
- Table 1 caption – "Fig. A2" should be corrected to "Fig. A4."
- Line 253 – Table A4 could not be located in the manuscript.
- Lines 318-321 – The sentence structure could be improved by emphasizing the significance of findings before citing figure numbers, rather than using primarily descriptive phrasing.
- Line 464 – The citation should read "Yeh et al., 2011."
- Fig. A4 – Clarification of what "recomputed datasets" represent would be helpful. The magenta and light blue lines are difficult to distinguish on the plot; using dashed lines might improve visibility.
- Fig. A5 – Including slope values directly on the plot or providing equations from the cited papers would facilitate reference.
References:
DeLong, K. L., Quinn, T. M., Taylor, F. W., Shen, C. C., & Lin, K. (2013). Improving coral-base paleoclimate reconstructions by replicating 350 years of coral Sr/Ca variations. Palaeogeography, Palaeoclimatology, Palaeoecology, 373, 6-24.
Wen, T., Ding, R., Shi, L., Ji, K. (2024). A new dipole index for the North Pacific Victoria mode. Clim Dyn 62, 8169–8179. https://doi.org/10.1007/s00382-024-07330-y
Citation: https://doi.org/10.5194/egusphere-2025-4054-RC2
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- 1
Summary
Ito et al. present a highly novel coral record of Sr/Ca that monitors North Pacific Gyre Oscillation/Victoria Mode temperature variability from 1798 to 2014. The authors identify an important scientific question: the NPGO/VM has a tight and intensifying connection with the central Pacific in future simulations with strong greenhouse gas forcing, but how has this relationship changed over the common era? This record provides insight during an important gap in instrumental and coral observations and will be of interest to the paleoceanographic and coral communities. This record documents a variable relationship with instrumental observations from the central equatorial Pacific at 8-30 year timescales (sometimes the correlation between these two regions reverses sign, sometimes is significant, sometimes not). The manuscript is well structured and does a great job of acknowledging other paleo-derived reconstructions of NPGO variability, but could benefit from streamlining the central message of this analysis.
Major Comments
1. The introduction attempts to differentiate between reconstructions of NPGO and Victoria Mode variability (~line 65-70) and states that this coral record will be the first record of Victoria Mode variability. However, the rest of the analysis compares the Kikai Island record and instrumental SSTs to the NPGO index by DiLorenzo et al. 2008. If the authors want to make the case that their record is the first of Victoria Mode variability (not NPGO), they should compare their record to the Victoria Mode. At the least, they could describe how the timeseries of the NPGO and VM indices differ. Ding et al. 2014 show that the NPGO and VM are different at decadal timescales (which would certainly apply when using the 8-30 year bandpass filter), writing “The NPGO exhibits a significant (at the 90% level) spectral peak around 10 years that is absent from the VM; instead, the VM tends to have higher power at interannual time scales, with a peak at periods of 3–4 years (but not significant at the 90% level). “
2. The authors should clarify some of the assumptions that they make in their interpretation of the Kikai Sr/Ca index over the instrumental era.
Firstly, the authors focus on 8-30 year bandpass filtered summer Sr/Ca measurements to derive their Kikai-ST_coral record. This relationship has the highest agreement with annual instrumental data in the late 20th century, but I am concerned that this filtering may prevent capturing meaningful signals about the Victoria mode. For example, winter time SST variability is often noisier than summer SST variability at higher latitudes– the VM even shows high variance in late winter and early spring SSTs (Ding et al. 2014). Eliminating winter observations and bandpass filtering for 8-30 years may be smoothing out potentially important signals.
Secondly, the authors demonstrate that their 8-30 year bandpass filtered summer Sr/Ca Kikai-ST record has stronger coherence with 8-30 year bandpass filtered SST (all months) observations from the central equatorial Pacific than the Kikai region (Figure 3). Why might this record not represent local conditions as well? Additionally, in Figure A8, the authors compare the Kikai summer Sr/Ca record with Palmyra Sr/Ca record and find a much weaker relationship. This is surprising given that Nurhati et al. 2011 published a strong agreement between Palmyra Sr/Ca and the NPGO index. Why might the relationship between Kikai-ST_coral have a stronger correlation with uncertain instrumental SST observations than another coral Sr/Ca record?
Thirdly, the authors describe that the NPGO annual index lags the Kikai-ST_coral record by 1 year. Ceballos et al. 2009 describe that the NPGO can lead KOE variability by 2-3 years, but what processes might cause SSTs in the Northwestern Pacific to lead NPGO variability by a year? COuld this unusual lead-lag relationship be partially caused by the authors focus on summer Sr/Ca values?
3. I was also curious about the authors interpretation of the Kikai Sr/Ca index prior to the instrumental era (e.g. Figure 6). From 1798-1920, the reconstructed NPGO index is virtually always in a negative phase. Would the analysis be better served by detrending their coral record? Physically, it does not seem realistic for the NPGO/VM to be in a centuries long negative phase.
4. In the abstract and introduction, the authors mention set up the reader to think about how the relationship between the central equatorial Pacific and NPGO/VM changed over the 19th and 20th centuries as context for the projected changes over the 21st centuries. The authors don’t focus on this as much as implied earlier on– the authors show a plot of 8-30year bandpass filtered CP SST observations with their 8-30year Sr/Ca record over the instrumental era. Given the uncertainties in SSTs in the early 20th century, wouldn’t it make more sense to use the Nurhati et al. 2011 record? The authors could also loop in the Sanchez et al. 2016 Clarion record into this analysis. Is Figure 7 (the running correlation plot) substantially different if using all months from the Kikai Sr/Ca record? In addition, the authors are uniquely well suited to comment on how Kikai Sr/Ca variability changes during other periods of substantial volcanic or aerosol forcing for additional impact, if they want. The response of the climate system to the Mystery and Tambora eruptions is of interest to many in the paleoclimate and paleoceanographic communities.
Minor Comments
– Figure 1 B,C,E, the word “annual” is misspelled
— How does 1B differ from 1C? The heading is exactly the same?
–The authors should indicate that they limit the analysis to the year 2014 to heighten similarity with their Sr/Ca record or use the latest available data through 2025. The authors should consider using the most recent decades– it would double the available years for their SSH analysis.
Citations:
Ceballos, Lina I., Emanuele Di Lorenzo, Carlos D. Hoyos, Niklas Schneider, and Bunmei Taguchi. "North Pacific Gyre Oscillation synchronizes climate fluctuations in the eastern and western boundary systems." Journal of Climate 22, no. 19 (2009): 5163-5174.
Ding, Ruiqiang, Jianping Li, Yu‐heng Tseng, Cheng Sun, and Yipeng Guo. "The Victoria mode in the North Pacific linking extratropical sea level pressure variations to ENSO." Journal of Geophysical Research: Atmospheres 120, no. 1 (2015): 27-45.
Nurhati, Intan S., Kim M. Cobb, and Emanuele Di Lorenzo. "Decadal-scale SST and salinity variations in the central tropical Pacific: Signatures of natural and anthropogenic climate change." Journal of Climate 24, no. 13 (2011): 3294-3308.