Cross&ndash;Seasonal Impact of SST Anomalies over the Tropical Central Pacific Ocean on the Antarctic Stratosphere

Zi, Yucheng; Long, Zhenxia; Sheng, Jinyu; Lu, Gaopeng; Perrie, Will; Xiao, Ziniu

doi:https://doi.org/10.5194/egusphere-2025-2990

Preprints

https://doi.org/10.5194/egusphere-2025-2990

Preprints

01 Aug 2025

| 01 Aug 2025

Cross–Seasonal Impact of SST Anomalies over the Tropical Central Pacific Ocean on the Antarctic Stratosphere

Yucheng Zi, Zhenxia Long, Jinyu Sheng, Gaopeng Lu, Will Perrie, and Ziniu Xiao

Abstract. In this study we examine the cross–seasonal effects of boreal winter sea surface temperature (SST) anomalies over the central tropical Pacific (Niño4 region) on Antarctic stratospheric circulation and ozone transport during the subsequent austral winter using ERA5 reanalysis of 45 years (1980–2024). Our analyses show that warm (cold) SST anomalies in Niño4 region during December–February are associated with polar stratospheric warming (cooling), a weakened (strengthened) stratospheric polar vortex (SPV), and enhanced (suppressed) polar ozone concentrations during July–September of the subsequent year. This delayed response is mediated by a Pacific–South America (PSA) teleconnection, which excites planetary waves that propagate upward into stratosphere and modify the Brewer–Dobson circulation. In addition, as the influence of Niño4 SSTs on the PSA teleconnection pattern diminishes during July–September, surface heat feedback at mid and high latitudes becomes critically important for planetary waves. Specifically, persistent South Pacific SST warming and sea-ice loss over the Amundsen and Ross Seas reinforce planetary waves by releasing heat from ocean into atmosphere. A multivariate regression statistical model using predictors of boreal winter Niño4 SST, June PSA, June South Pacific SST, and May–June sea-ice concentration (SIC) indices explain approximately 35 % of the variance in austral winter stratospheric temperatures. These findings highlight a previously underexplored pathway through which tropical Pacific SST anomalies modulate Antarctic stratospheric dynamics and chemistry on seasonal timescales. This implies a new insight into tropical–polar coupling and provides a potential signal for extended–range forecasts of ozone depletion risk.

Received: 23 Jun 2025 – Discussion started: 01 Aug 2025

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Yucheng Zi, Zhenxia Long, Jinyu Sheng, Gaopeng Lu, Will Perrie, and Ziniu Xiao

Status: final response (author comments only)

RC1:
'Comment on egusphere-2025-2990', Anonymous Referee #1, 21 Aug 2025
General Comments:

This manuscript investigates the cross-seasonal influence of tropical central Pacific SST anomalies (Niño 4 region) on the Antarctic stratospheric circulation and ozone transport. While the topic is of interest and the use of ERA5 reanalysis over a 45-year period is a strength, the study suffers from several major conceptual, methodological, and interpretive flaws that significantly undermine its conclusions. The core argument regarding the delayed influence of boreal winter Niño 4 SSTs on austral winter stratospheric conditions is not convincingly supported by the presented evidence. The analysis relies heavily on correlation and composite methods without sufficient dynamical diagnostics or causal mechanisms. Furthermore, the inclusion of ozone-related content in the introduction and abstract is misleading, as the results pertaining to ozone are minimal and not integral to the main narrative. The manuscript requires substantial revision in both structure and scientific rigor to be considered for publication. Due to the significant issues outlined below, I cannot recommend this manuscript for publication in its current form.
Specific Comments:
Mismatch Between Introduction/Abstract and Actual Content. The introduction and abstract heavily emphasize ozone related processes, yet the results section contains very little substantive analysis on ozone. The sudden mention of “chemical influence” in the abstract (line 44) is unsupported by the rest of the abstract. The authors should refocus the manuscript to align with the actual findings—namely, the dynamical response of the stratosphere to tropical forcing—and remove extraneous discussions on ozone unless they are robustly analyzed and central to the story.

Misinterpretation of Polar Vortex Weakening. The claim that warm Niño4 events lead to a “weakened stratospheric polar vortex” (e.g., abstract, line 32) is not fully supported by Figure 2. The zonal wind anomalies show a dipole structure: weakened winds at mid-latitudes and strengthened winds at high latitudes. Figure 1c also shows the weakened zonal winds locate at mid-latitudes. This suggests a contraction or shift of the vortex rather than a uniform weakening. The authors should supplement their analysis with horizontal maps of geopotential height, temperature, and wind over the polar region to better characterize the vortex response.

Inadequate Explanation of Stratospheric Warming Mechanism. The proposed mechanism involving planetary wave propagation and E–P flux convergence (Section 4) does not adequately explain the polar stratospheric warming. Figure 6 shows wave activity and residual circulation anomalies primarily confined to mid-latitudes, with limited direct impact on the polar region. The authors need to provide clearer evidence to link mid-latitude wave activity to polar warming.

Overreliance on Correlation and Composite Analysis. The study relies heavily on statistical correlations and composite differences without sufficient dynamical or causal diagnostics. The discussion of sea-air and ice-air interactions is largely descriptive and lacks quantitative support. For instance, the claim that sea-ice loss enhances shortwave radiation absorption (lines 428–430) is not plausible during austral winter (JAS) when solar insolation is minimal.

Flawed Multivariate Regression Model. The multivariate regression model uses predictors that are highly correlated with each other, violating the assumption of independence in linear regression. The low explained variance (35%) further undermines the model’s utility. The authors should either use orthogonal predictors or apply methods more suitable for correlated predictors. A physical justification for including each predictor is also needed.

Arbitrary Definition of Warm/Cold Years. The use of ±0.5 standard deviations to define warm/cold years is arbitrary. The authors should provide a justification or conduct a sensitivity test (e.g., using ±1σ) to ensure the robustness of the composite results.

Inaccurate Figure Captions. Several figure captions (e.g., Figures 4, 7) use the phrase “same as” despite the panels showing different variables or time periods. This is confusing and unprofessional. The captions should be rewritten to accurately describe each panel.

Others:
Use consistent formatting for ENSO indices: e.g. “Niño4” → “Niño 4” (with space).

Subscript the “3” in “TCO3” (line 131).

Avoid using asterisks in line 171 when they are also used in line 185.

L149-149: It is recommended to specify the timescale by filtering interannual component.
Citation: https://doi.org/10.5194/egusphere-2025-2990-RC1
RC2:
'Comment on egusphere-2025-2990', Anonymous Referee #2, 22 Aug 2025
General comments:
This study investigates the cross–seasonal effects of boreal winter sea surface temperature anomalies over the central tropical Pacific on Antarctic stratospheric circulation and ozone transport during the subsequent austral winter. By exciting the PSA wave train, SST anomalies in the Nino4 region led to warmer SSTs in the southeastern Pacific, accelerating sea ice melt. The subsequent release of ocean heat maintained atmospheric high-pressure anomalies during July–September, which in turn strengthened planetary wave anomalies. Furthermore, multivariate regression statistical model was used to verify the combined importance of both tropical forcing and mid–latitude atmospheric responses in stratospheric temperature predictability. These results further help to improve modeling and forecasting of future changes in the polar vortex. Some additional analyses are needed to verify these results.
Major comments:
All conclusions are obtainedfrom the reanalysis dataset. Can these results also be identified in CMIP6 simulations? Furthermore, I suggest that the author perform sensitivity experiments using numerical models to further support their conclusions.

Although the climatological geopotential height at 100 hPa is characterized by a wave-1 pattern, geopotential height anomalies appear to show a wave-2 pattern. Does the wave-2 component of geopotential heightanomalies exhibit an in-phase or out-of-phase relationship with the climatological pattern? How do the vertical propagations of different planetary wave components change?

The author used ±5 standard deviation as a threshold to select warm and cold nino4 years. Whether the results are sensitive to the value of thresholds?

Why was the SST region of 160°W–130°W and 30°S–60°S selected as the SST index related to the loss of sea icein Amundsen and Ross Seas? It seems that SST in this area has a weak relationship with sea ice reduction in Amundsen and Ross Seas. It might be more suitable to use SST averaged over 50°S–65°S instead.

The authors conclude that surface heat flux is largely associated with sea-ice loss, where the ocean releases heat through sea-ice changes to heat the lower atmosphere, thereby maintaining high-pressure anomalies that strengthen planetary waves and ultimately weaken the However, the results from multi-regression analysis show that the contribution of sea ice is relatively weak. How can this phenomenon be explained? Why is the contribution of sea ice relatively weak if it is considered a key factor?

Why does the net surface heat flux remain persistently negative in certain regions (approximately 40°–60°S, 90°–140°W), indicating that the ocean continuously absorbs heat from the atmosphere, while the SST anomalies in these regions are weakening? Does this imply that the weakening of SST is controlled by oceanic heat transport?

Minor comments:
Suggest adding the composited difference of the wave reflective index to Figure 6.

It is necessary to providethe correlation coefficients between the monthly PSA index, SIC in the Amundsen Sea and Ross Sea, and the SST_SP andthe July-September mean T_10-30

Line 73: In addition to other factors, sea ice has been recognized as an important influence on theSPV (e.g., Rea et al., 2024; Song et al., 2025; Sun et al., 2015), with important implications for Southern Hemisphere climate variability.

Reference:
Rea, D., et al., Interannual Influence of Antarctic Sea Ice on Southern Hemisphere Stratosphere‐Troposphere Coupling, Geophysical Research Letters, 51, e2023GL107478, https://doi.org/10.1029/2023GL107478, 2024.
Song, J., et al., Impact of Early Winter Antarctic Sea Ice Reduction on Antarctic Stratospheric Polar Vortex, JGR Atmospheres, 130, e2024JD041831, https://doi.org/10.1029/2024JD041831, 2025.
Sun, L., et al. Mechanisms of Stratospheric and Tropospheric Circulation Response to Projected Arctic Sea Ice Loss, Journal of Climate, 28, 7824–7845, https://doi.org/10.1175/JCLI-D-15-0169.1, 2015.
Line 116: “muiti-regression”-> “multi-regression”

Line 202: “bule”-> “green”

Line 442: “clod Niño4 years”-> “cold Niño4 years”
Citation: https://doi.org/10.5194/egusphere-2025-2990-RC2

Yucheng Zi, Zhenxia Long, Jinyu Sheng, Gaopeng Lu, Will Perrie, and Ziniu Xiao

Data sets

ERA5 analysis H. Hersbach et al. https://doi.org/10.24381/cds.bd0915c6

Niño index National Oceanic and Atmospheric Administration https://psl.noaa.gov/data/timeseries/month/DS/Nino4

Yucheng Zi, Zhenxia Long, Jinyu Sheng, Gaopeng Lu, Will Perrie, and Ziniu Xiao

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Short summary

We investigated how boreal winter sea surface temperatures anomaly in the central tropical Pacific impact the Antarctic stratosphere months later. Using 45 years of reanalysis data, we found that warmer sea surface lead to a warmer Antarctic stratosphere and increased ozone during the subsequent austral winter. This link, driven by the planetary waves and reinforced by regional sea-ice loss, provides a new way to make long-range forecasts for stratospheric conditions and ozone.


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