| 14 Nov 2025
The variability of Antarctic fast ice extent related to tropical sea surface temperature anomalies
Abstract. While numerous studies have examined the influence of meteorological variables on fast ice, the mechanistic linkages between fast-ice variability and large-scale climatic oscillations have remained inadequately explored. Empirical Orthogonal Function (EOF) analysis is applied to circumpolar Antarctic fast-ice extent data (March 2000–February 2018) to investigate seasonal-scale teleconnections between fast ice anomalies and tropical sea surface temperature (SST) variability. Results demonstrate a fluctuating but increasing trend in fast-ice extent during austral winter and spring, with spatially predominant anomalies concentrated in the West Antarctic. A physical pathway linking tropical and subtropical SST anomalies to fast-ice variability is elucidated through multiscale interactions: SST anomalies modulate outgoing longwave radiation (OLR), subsequently perturbing the 200 hPa geopotential height field and triggering atmospheric Rossby wave trains. These planetary waves propagate from the tropics towards the Antarctic coastal zone, generating anomalies in the Southern Annular Mode (SAM) and other patterns of mean-sea-level pressure (MSLP) and surface wind field. These atmospheric adjustments directly regulate fast-ice mechanical formation/disintegration processes while indirectly influencing thermodynamic ice evolution through air temperature modifications. Tropical SST anomalies predominantly exciting planetary waves during austral autumn, whereas the subtropical South Pacific SST dipole mode emerges as the primary forcing mechanism during austral winter and spring. Seasonal variations in atmospheric forcing on fast ice are identified. By tracing how remote SST forcing propagates through atmospheric wave dynamics to influence regional fast-ice conditions, this study advances process-level understanding of tropical and subtropical impacts on the Antarctic.
This research paper explores the global mechanisms that lead to variations in landfast ice around the Antarctic continent. This is an interesting submission and one which could make a strong contribution, but one which also needs revision. I outline below how such a revision should be undertaken.
Â
Â
As a broad comment, the focus of the investigation is on the variability breakdown via EOF analysis. As some background at an early stage in the paper, it would be very valuable to preset time series of the TOTAL fast ice area for the four seasons and their trends.
Â
Lines 43-46: In these opening lines in connection with the importance of the study it would be beneficial to reference the very recent studies of …
Luke G. Bennetts and Nathan J. Teder, 2025: Fast ice: The last line of defence for weakened Antarctic ice shelves. Nature Geoscience, 18, 574-575, and
Nathan J. Teder, Luke G. Bennetts, Phillip A. Reid, Robert A. Massom, Jordan P. A. Pitt, Theodore A. Scambos and Alexander D. Fraser, 2025: Large-scale ice-shelf calving events follow prolonged amplifications in flexure. Nat. Geosci., 18, 599-606.
Â
Line 50: The year of publication is 2023, not 2024.
Â
Line 51: ‘shrink’ (sp.)
Â
Line 53: The text here cites ‘Crocker & Wadhams, 1989a’ (and also below). However, no related ‘1989b’ is cited or in References. From the context here I suspect the authors may have wanted to also cite the paper …
Crocker GB, Wadhams P (1989) Modelling Antarctic fast-ice growth. J. Glaciol. 35: 3-8 doi: 10.3189/002214389793701590.
If this is correct please to make appropriate changes.
Â
Line 56: Paper is missing from References. Maybe you are referring to Shuki Ushio’s 2006 paper ‘Factors affecting fast-ice break-up frequency in Lützow-Holm Bay, Antarctica.’ Annals of Glaciology, 44, 177-182, doi: 10.3189/172756406781811835.
Â
Lines 72-74: On this important point, include references to ….
Pezza, A.B. et al., 2012: Climate links and recent extremes in Antarctic sea ice, high-latitude cyclones, Southern Annular Mode and ENSO. Climate Dyn., 38, 57-73, doi: 10.1007/s00382-011-1044-y,
Irving, D. & co-authors 2016. 'A new method for identifying the Pacific-South American pattern and its influence on regional climate variability', J. Climate 29, 6109–6125.
Â
Line 74: Another paper missing from the References! I imagine that ‘Clem et al., 2017’ is referring to
Kyle R. Clem, James A. Renwick and James McGregor, 2017: Large-scale forcing of the Amundsen Sea low and its influence on sea ice and West Antarctic temperature. Journal of Climate, 30, 8405-8424, doi: 10.1175/JCLI-D-16-0891.1.
Â
Lines 121-124: Strongly suggest discussing the EOFs first, and then the PCs. The EOFs are the more fundamental aspect. Similarly, flip the two columns in Figs. 1 & 2.
It is important to note that the sign of EOFs is arbitrary. If the sign is changed so does the sign of the PC and, e.g., ‘positive trends’ become ‘negative trends’. Broadly speaking the four EOF1s have similar structures, but also large differences in the key regions around 180 deg and from -90 deg to -10 deg. I presume the sign of the patterns was taken because they ‘look similar’, but it would be of interest to calculate the spatial cross-correlations of the four patterns. This would maybe be a path to make a more physically meaning statement about the relative changes.
Â
Lines 130-…: Related to the point above, the EOFs essentially identify the regions of (high) variability, rather than ‘positive anomalies’ etc. Please word these passages more appropriately.
Also, at line 130 (and 139, …) change ‘significant’ to something like ‘sizeable’. Only use the former word when referring to results of a statistical test of the null hypothesis.
Â
Line 144-145: Here the text speaks of the lack of statistical significance of the trends of the PC2 series. Similar test should be conducted for the PC1 series. One might guess that the winter and spring trends could be significant, placing any discussion on this on firmer ground.
Â
Lines 152-155: I am not sure that Fig. 3 is particularly useful in the paper, especially at the trends in PC2s are not significant (and a similar comment probably applies to most/all of the PC1s). Suggest deleting it.
Â
Line 167-169: The SST structure in MAM is suggestive of ENSO. But note that in the far eastern Pacific the significant regressions move off the Equator, and into the NH. The other three seasons in Fig. 4 show only scattered ‘significant’ areas, and probably less that the 5% of the globe that you would expect by chance. The regions of significance around Antarctica in JJA and SON are regions covered by sea ice. What does ‘SST’ mean here?!
Interesting to note that the eastern Pacific SSTA Shigeru Aoki in the 2017 paper (lines 77-79 above) showed strong ENSO structure with greater anomalies just to the SOUTH of the Equator in April and December (Figure 4 in the paper). (Also see the comment made in the paper at line 292-294.)
Â
Lines 176-179: The role of the ‘Amundsen Sea Low’ here is very interesting. Perhaps emphasise the points being made here by referencing study of Fogt, R. L., A. J. Wovrosh, et al., 2012 - (The characteristic variability and connection to the underlying synoptic activity of the Amundsen-Bellingshausen Seas Low. J. of Geophysical Research, 117, D07111, doi: 10.1029/2011JD017337) and commenting that the region is strongly tied to the location of the maximum cyclone system density and minimum cyclone central pressures, and is tied up with the SAM.
Â
Lines 183-185: Figure 7a is not mentioned or referred to in the paper. However, its very interesting structure shows a strong wavenumber three. This mode has a large influence on subantarctic conditions (e.g., Irving & co-authors 2015: A novel approach to diagnosing Southern Hemisphere planetary wave activity and its influence on regional climate variability. J. Clim., 28, 9041-9057). Some additional text on this aspect would be very valuable.
Â
Line 309: Unbalanced parentheses.