| 22 Aug 2025
Thermodynamic and dynamic drivers underlying extreme central Arctic sea ice loss
Abstract. Variability of sea ice in the central Arctic is pivotal to the entire Arctic region, yet in-depth understanding of its characteristics and underlying mechanisms remains elusive. We investigate the characteristics and mechanisms of extremely low sea ice concentration (SIC) events (ELSEs) in the central Arctic by analysing the reanalysis data. First, we define the area where climatological SIC exceeds 90 % as the central Arctic. Moreover, this paper primarily investigates the relative variations in sea ice, i.e., the standardized sea ice anomalies, which highlight common characteristics of sea ice variations. Based on the Empirical Orthogonal Function method, we identify the first two dominant modes of relative variations of ELSEs within the central Arctic: the East–West Seesaw Mode (EWSM) and the Pacific–Atlantic Seesaw Mode (PASM). By using the sea ice budget diagnostic method for comparison, it is found that the thermodynamic (dynamic) contributions of EWSM and PASM account for 68 % (32 %) and 72 % (28 %), respectively. Thermodynamically, both the EWSM and the PASM are dominated by local diabatic heating, with their maintenance and intensification facilitated by water vapor and cloud feedbacks. Dynamically, sea ice advection plays an important role in the formation of the two modes. In particular, the upper tropospheric divergence anomalies over the North Pacific induce Rossby wave train that modulate the EWSM. Overall, this study elucidates the key drivers of ELSEs in the central Arctic, which enriches our knowledge of the complex cryosphere processes.
Introduction
This manuscript explores the changes to central Arctic sea ice concentration (SIC) by identifying episodes of intense SIC loss ELSEs or Extreme Loss Sea ice Events. This is done through a rare event threshold and is inferred from the NSIDC SIC record. Secondly, the manuscript performs Empirical Orthogonal Function analysis upon the SIC fields to identify two dominant modes of SIC anomaly. Finally, the authors budget the two modes of SIC using PIOMAS and explore the connections to atmospheric feedbacks. The paper is well written, thorough and the visual representations are well produced. However, there are some considerations to make before progressing with this manuscript, specifically with the clarity of methodology.
Novelty
The novelty in this paper is rooted in the identification of modes of SIC ELSEs. Namely, the East-West seesaw (EWSM) and Pacific-Atlantic seesaw (PASM) modes. These are interesting concepts for explaining the differing behaviours of the Arctic sea ice system during loss events. As well as the interpretation of atmospheric feedbacks that induce these modes.
Positioning
The paper is well placed within the literature to move forward with identifying system changes to the central Arctic sea ice regime. However, there are several key papers within the field that are not cited. See references.
General Comments
The authors should confirm the prevalence and trend of SIC ELSEs using an alternative SIC product, e.g. OSISAF-401. I imagine this to have minimal change in event distribution; however, the impact upon EOF modes may be stronger. The work is robust in using multiple reanalysis datasets for atmospheric variables; the same approach should be taken concerning the SIC product.
It is not obvious to me where the SIC budget is drawn from. Are the drift products used within the budget all from PIOMAS, or are they observational? If the budget is solely constructed from PIOMAS, how well does PIOMAS align with the observations, not only on the existence of ELSEs but also the distributions? If PIOMAS does not produce any ELSEs, how can the sea ice budget be well constrained or relevant for these modes? The whole methodological section on budgeting needs to make clear what is used and why.
There is a clear change in the behaviour of ELSEs of the PASM in Figure 2c. Naturally, there should be a discussion about the shifting seasonality of this phenomenon that the work is missing.
The brief section on Arctic amplification in the introduction is well read but needs to be restructured for clarity.
Technical Comments:
L27-28: Remove “a greater” in the sentence on Arctic Amplification
L35: “Significant declining trend” not a “significant decline trend”
L38-39: “acts as a pivotal modulator to drive the evolution” should be “acts as a pivotal modulator of the evolution”
L43: “What’s more” is too informal
L119: “Cases when the principal components” should be “Cases in which …”
L290: remove “directly”
L440: “Whatever” should be However or Nevertheless
Figures/Tables:
Figure 11 – either the stippling needs to change to a more appropriate and eye-catching colour, or the colourmaps. The cyan is lost in 11c/11f in the low advection regions
Figure S11 - is a very strong plot demonstrating the modal deformation anomalies. What is the equivalent plot for EWSM?
References
Holland, P. R., & Kimura, N. (2016). Observed concentration budgets of Arctic and Antarctic sea ice. Journal of Climate, 29(14), 5241-5249.
Massonnet, F., Vancoppenolle, M., Goosse, H., Docquier, D., Fichefet, T., & Blanchard-Wrigglesworth, E. (2018). Arctic sea-ice change tied to its mean state through thermodynamic processes. Nature Climate Change, 8(7), 599-603.