| 27 Jul 2026
Evaluation of subseasonal sea ice changes in the Community Earth System Model version two
Abstract. Forecasting Arctic sea ice is a complex, open problem in polar science, exacerbated by climate change and Arctic amplification. Arctic sea ice dynamics are inherently interconnected to atmosphere and ocean dynamics. This means that coupled climate models are currently the best available tools to model and forecast sea ice. However, climate models consistently underrepresent sea ice processes on subseasonal timescales, contributing to forecasting challenges. In this study, we evaluate the Community Earth System Model Version Two's (CESM2) skill in representing Arctic sea ice by comparing model output to reanalysis and observational data. Doing so establishes CESM2's viability as a tool for studying Arctic sea ice processes and identifies places where model improvements are needed. While sea ice processes in CESM2 have been evaluated on annual and climate timescales, its performance on subseasonal timescales has not. We analyze CESM2's performance in representing subseasonal sea ice variability by comparing its temporal and spatial interannual variability to observations. We also evaluate CESM2's performance at capturing very rapid sea ice loss events (VRILEs). VRILEs are substantial sea ice loss events that occur on the timescale of days. We find that CESM2 does a poor job of representing VRILEs and has less interannual variability than observations.
Peer Review of “Evaluation of subseasonal sea ice changes in the Community Earth System Model version two”
This study compares the sea ice evolution from a long-running CESM2 LENS climate forecast to NSIDC ice observations during periods in the simulations when the overall ice extent is comparable to the observations. CESM2 LENS exhibits a strong negative bias in Arctic-wide SIE. Despite this negative bias in CESM2 LENS, shorter reforecasts from a different configuration of CESM2 designed for S2S-scale prediction exhibit reduced ice loss compared to observations in localized regions during rapid episodic ice loss events (termed VRILEs).
The manuscript is well written and logically organized. However, I have two major issues with this study. First, I found it lacks overall coherency because the first part of the analysis evaluates a long-running climate simulation from CESM2 LENS while the second part evaluates shorter, initialized reforecasts using a different configuration of CESM2 for S2S-scale prediction. Sea ice biases in CESM2 LENS and CESM S2S likely differ and arise for different reasons. This makes it difficult to draw overarching conclusions about the ability of CESM2 to make subseasonal sea ice predictions. In addition, the analysis of VRILEs in the S2S reforecasts does not offer physical insight into why the predicted sea ice losses during these events differ from observations or from reforecasts with the CESM2 S2S configuration that uses the WACCM atmospheric model instead of CAM6. Addressing these issues would require redesigning the analysis, which is certainly possible but beyond scope of major revisions. Therefore, I do not recommend accepting the manuscript in its present state.
Decision: Soft Reject
Major Comments:
Specific/Minor Comments:
Line 4: It is unclear what the authors mean by “climate models underrepresent sea ice processes”. Please be more specific about what processes this refers to and what “underrepresent” means here (does it refer to models not melting or growing sea ice fast enough?)
Abstract: A brief overview of results from the evaluations of sea ice variability unrelated to VRILES is missing from the abstract and should be included (i.e., only VRILE analysis results appear in the abstract).
Lines 22-24: The timing of the first ice-free summer should not be used to judge the skill of a given climate model since an ice-free summer has not yet occurred in reality. Projections of the timing of the first ice-free summer are produced from climate models and have uncertainty associated with them. Therefore, a model that has an earlier or later ice-free summer projection than another model (or consensus of models) is not necessarily deficient because we don’t know when the first ice-free summer will actually occur. Thus, I recommend rewording or clarifying the second part of this sentence accordingly.
Lines 31, 172-3, 188, 268: The citations on these lines should appear entirely within parentheses.
Lines 77-79: If there are daily values from each individual ensemble member, shouldn’t it be possible to average all the members together to obtain a daily ensemble mean?
Line 83: I had to read this sentence several times to fully grasp what is meant by the model year in CESM2 LENS not directly relating to the calendar year. I recommend rewording this part to say something like “Since the sea ice extent in CESM2 LENS diverges from the observed sea ice extent, we compare the observed sea ice extent from NSIDC between 1980 and 2020 to earlier periods in the CESM2 forecast when the modeled sea ice extent is more comparable to the observations during this period.” I also felt this sentence should be moved later in this paragraph, to just before the description of the specific periods in the model being compared to NSIDC (~Line 90).
Line 119: I would replace “dynamical cores” here with “atmospheric model configurations” because the differences described here at not actually differences in dynamical cores (dynamical core refers to the numerical solver of the Navier-Stokes equations, not the physics packages or model grid configuration).
Lines 122-123: Do we know why the deeper atmosphere and interactive chemistry in WACCM6 results in better ice forecasts?
Line 127: It’s unclear what is meant by “does not provide enough time to evaluate the VRILE that occurred on 3 July”. Please provide a clearer explanation of what is meant here.
Line 136: Please add the phrase “with differences” before “becoming” on this line.
Section 2: A description of the basic aspects of CESM2 LENS (akin to the description of CESM2 S2S on Lines 112-118) is missing from this section. At minimum, the text should describe when the CESM2 LENS simulation is initialized (I assume 1850?) and what anthropogenic forcing scenario is used.
Line 150 (and elsewhere): Please replace all instances of “inner-quartile range” with the corrected name: “interquartile range”
Lines 166-168/Figure 4: Could the reduced variability of SIC within the marginal ice zone regions in CESM2 compared to NSIDC simply be due to these regions being ice-free in CESM2? This seems plausible given the large negative ice extent bias in CESM2, which would likely result in more ice-free areas at these lower latitudes. I recommend checking what the actual SIC values are in these regions in CESM2. It may also help to also include the 0.15 SIC contour from CESM2 on the panels in Fig. 4 so the reader has a better sense for where the ice edge is in the simulation (not just in the observations).
Line 170: Change “have positive values” to “there are positive values” to make this sentence grammatically correct.
Fig. 5: The caption says CESM2 VRILE sizes are indicated by blues, but when I zoom in on the smaller bars that presumably correspond to CESM2 in this figure, I see black, purple, red, orange and yellow segments, but no blue segments. I recommend either adjusting the color guidance in the caption to better correspond to what is shown in the figure, or changing the figure colors to truly have shades of blue for CESM2.
Fig. 6: This figure should include another row showing the 5-day change in SIC in NSIDC to illustrate what the observed ice loss associated with the VRILE was.
Lines 200-201: It’s not clear to me that the ice biases in a long running CESM2 LENS forecast should have any relationship to biases in short-term forecasts from CESM2 S2S, given the significant differences between the two model configurations (free running climate simulation vs. initialized short forecast). This gets back to my major comment 1 above.
Lines 231-234: Have the authors checked if the areas corresponding to the VRILE are actually open water (ice-free) in CESM at the beginning of the reforecast? The negative differences in Fig. 6d, along with what were known to be low sea ice concentrations in the Beaufort Sea at this time, suggests CESM may not even have sea ice here, which would change the interpretation of the results for this case.
Line 245: Change “unseasonal” to “unseasonable”.