Ocean Surface Warming and Long-Term Variability in Rainfall in Equatorial Pacific Atolls

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This manuscript investigates the long-term variability of Sea Surface Temperature (SST) and rainfall across two Pacific atolls, offering valuable insights to enhance the climate resilience of small island nations. Despite its contributions, there are several critical concerns that must be addressed prior to publication:

1. The paper would benefit significantly from an in-depth exploration of the dynamic mechanisms underlying the correlations between SST and rainfall variabilities. Clarification is needed on why disparities exist between the two atolls and the reasons behind the temporal transitions in the variabilities of the phenomena under study. Additionally, it is imperative to elucidate how large-scale atmospheric and oceanic circulations, along with air-sea interactions, influence rainfall patterns in the equatorial Pacific atolls.

2. Employing the NINO index to quantitatively represent the intensity of El Niño-Southern Oscillation (ENSO) events would enhance the robustness of the analysis. The subjective nature of the estimations presented in Tables 4 and 5 warrants reconsideration for a more objective approach.

3. The caption for Table 8 contains a typographical error regarding the period under study, listed as “1951 to 20123.” This should be corrected to accurately reflect the intended time frame.

4. A reconsideration of the manuscript's title is recommended to more accurately capture the essence of the study. A suggested title is “Ocean Surface Warming and Long-term Variabilities of Rainfall in Equatorial Pacific Atolls.”

Author Response

Review No 1.

The authors wish to thank the reviewer for this review which has helped to strengthen the manuscript. Our responses are:

1. There are two points in three points in this comment.

• The dynamic mechanisms underlying the correlations between long-term SST variability and the variability of long-term rainfall is still an area of active of research with many unknowns as we have tried to exemplify with the references cited. Long term variability in SST and tropical atoll rainfall is driven by ENSO. The big unknown is the dynamics of ENSO which no current GCM predicts well because of the Pacific cold-tongue anomaly. This is made very clear by the IPCC in its latest report as we quote in the introduction:

“projected changes in hydrological drought are less certain due to uncertainties in future ENSO frequency and intensity [4].”

• Why do disparities exist between the two atolls? We have tried to make clear in Section 2.1 the fact that the islands are separated by 3,300 km of the equatorial Pacific and subject to differing major atmosphere-ocean interactions. We have slightly improved this section to clarify this:

“The atolls are separated by 3,300 km of the tropical Pacific and both have continuous monthly rainfall records from at least January 1951 [14]. Kiritimati, in the central Pacific, borders on the Pacific dry zone with annual rainfall influenced by the north-south migration of the ITCZ during ENSO events [2]. Tarawa, in the western Pacific, lies to the east of the Pacific warm pool and close to the confluence of the ITCZ and the SPCZ and can be impacted by the West Pacific Monsoon [2]. Annual rainfall there can vary with north-south movement of the SPCZ during ENSO events.

The atolls are subject to different major atmosphere-ocean systems, but their variability is driven by ENSO.

• Elucidation of large-scale atmosphere and oceanic circulations influence rainfall patterns.

The dominant large-scale atmosphere and oceanic circulation which influences rainfall patterns not just in the equatorial Pacific but across many parts of the world is ENSO which we discuss at length in this paper. As we point out above current GCMs do not handle it well. We also discuss the influence of the Pacific trade winds, the western Pacific monsoon, as well as the dominant ITCZ and SPCZ (the world’s largest rainfall band as well as IPO and the Pacific warm pool and cite the relevant extensive literature on them. However, the dominant large-scale atmosphere and oceanic circulation is ENSO which influences all of the above processes.  

2. There is a misunderstanding here. The ENSO intensities in Tables 4 and 5 are quantitative measures from the Oceanic Nino Index (the SST anomaly in the Nino3.4 region) as given in reference [24]. We now clarify this in the captions by adding “…intensity of the year of the event determined by the Oceanic Nino Index (ONI) [24]…”

 We also now give footnotes to the ENSO intensities for El Niño:

Table 4: ¹ Very Strong ONI > 2, Strong 1.5< ONI < 2, Moderate, 1.0 < ONI < 1.5, Weak, 0.5 < ONI < 1.0

And for La Niña:

Table 5: ¹Strong ONI < -1.5, Moderate -1.5 < NI < -1.0, Weak -1.0 < NI < -0.5

3. Thank you, a typographic error. Caption in Table 8 now changed to: “for the period 1951 to 2023”
4. An excellent suggestion: Title now changed to: Ocean surface warming and long-term variability of rainfall in equatorial Pacific atolls

Reviewer 2 Report

Comments and Suggestions for Authors

 The article is very interesting in terms of the results and conclusions. All conclusions are based on the field data obtained in the period 1951 to 2023. The above conclusion largely contradicts the well-established opinion about the dynamics of precipitation and some other atmospheric parameters in connection with global warming. You can treat the results in different ways, but the conclusions seem quite reasonable to me. There are some comments. The main ones are related to errors in determining various values. The data is taken from 1951. How were the measurements performed (for example, temperature, precipitation) and what is the accuracy of the measurements? It is necessary to give measurement errors in the text in the form as shown in Tables 3, 6, 7. Incomprehensible large errors in the linear domain shown in Tables 3, 6 and 7 (for example, 1.1± 0.5, 0.8±0.4, etc.). This means that changes occur in the range 0.6-1.6 and 0.4-1.2, etc. This is a very large spread. Please explain why there is such a spread? To explain the nature of such a spread. And I ask you to answer the main question: are the contradictory conclusions given in the conclusion related to such uncertainties, such variations?

Author Response

Review No 2.

The authors wish to thank the reviewer for this review which has helped to strengthen the manuscript. We are particularly pleased that the reviewer found that “the conclusions seem quite reasonable”.

1. The reviewer is concerned with the magnitude of the errors in Tables 3, 6 and 7. These errors are not in any way related to measurement errors of long-term rainfall or long-term SST. They are the standard errors (as mentioned in the Table captions) from least squares regression fits of the data series. Despite the large errors the trends are significant at the 5% level. Even though the annual SST have relatively small IoV (compared to annual rainfall) the trends have large errors because of the ENSO-related fluctuations of  SST. The enormous IoV in annual rainfall means that over 70 years of data a significant trend cannot be statistically detected. What we have shown here is that a modest 1.0°C variation in annual SST causes about 1,200 mm variation in annual rainfall. To clarify this, we have added in the discussion section:

4.2.1. Errors in Trends

Although the identified regression trends in SST in Tables 3, 6 and 7 were significant, or highly significant they were associated with large statistical standard errors. These standard errors scale with the relatively modest IoVs of the Nino region SST time series (Table 1) so that the trend in SST in the Nino4 region has the smallest standard error and is highly significant. These variations are driven by episodic ENSO events. Despite an apparent trend in rainfall in Kiritimati (Figure 5 (a)), the extremely large IoVs of longer-term rainfall in both atolls (Table 1), which are also driven by ENSO events, precludes any statistically significant trends. We have shown here that a modest 1.0° in annual SST produces about 1,200 mm variation in annual rainfall in the atolls.

2. The role of uncertainties in variations.

GCMs predict that with increasing SST annual rainfall should be increasing in the tropical Pacific. We find here that SST has increased over the last 70 years by about+1.0°C/100 years but there is no statistically significant increase in annual rainfall in two atolls in two different zones in the equatorial Pacific. The reason is that the ENSO produced fluctuations in SST are relatively small, buffered by the vast thermal mass of the Pacific Ocean, so a trend can just be detected in SST over the 70 years. For annual rainfall, however, the ENSO-produced variation in rainfall is huge so it is impossible to detect a statistically significant trend in annual rainfall over 70 years. The work shows that it is not slow progressive increase in SST that is the key factor in long-term atoll rainfall, but it is the swings in SST produced by ENSO cycles which drive the large fluctuations in atoll rainfall.

We were able to show that with increasing SST, extreme large annual rainfall events (greater than 95^th percentile) only became apparent after 1987 in both atolls consistent with projections, but variability has not yet increased.

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

no more concerns

Reviewer 2 Report

Comments and Suggestions for Authors

The corrected version of the article can be published.