| 08 May 2026
TOPAZ5: A high-resolution ocean and sea-ice model for the Arctic and North Atlantic
Abstract. Accurate simulation of coupled ocean-sea ice dynamics is important for understanding Arctic climate variability. This paper evaluates TOPAZ5, a novel high-resolution (6–10 km) hindcast configuration of the HYCOM-CICE model optimized for the Arctic and North Atlantic, over the period 2009–2019. TOPAZ5 is validated against satellite altimetry, in situ observations, and reanalysis products, assessing sea surface temperature (SST), sea surface salinity (SSS), sea level anomaly (SLA), eddy kinetic energy (EKE), mixed layer depth (MLD), volume transports, and sea ice properties. TOPAZ5 captures the large-scale spatial patterns of SST and SSS with strong agreement relative to the OSTIA dataset and WOA climatology; however, persistent warm and saline surface biases remain, particularly during summer and over Arctic shelf regions. SLA trends are generally consistent with satellite altimetry and reanalyses, though positive anomalies in the Norwegian and Barents Seas are underestimated. Compared to observations, the model reproduces similar spatial patterns of EKE but underestimates their amplitude, especially in the Lofoten Basin and Labrador Sea. MLD variability shows reasonable agreement with observed seasonal stratification, though the model tends to produce overly deep winter mixing in the Labrador Sea. Sea ice seasonality is well represented, but the model overestimates the central Arctic thickness and underrepresents spatial and temporal variability in marginal ice zones. While volume transports through the Bering Strait are realistically simulated, the flows through the Fram Strait and Barents Sea Opening are underestimated. Overall, TOPAZ5 demonstrates sufficient skill in simulating large-scale hydrography and sea ice dynamics to support operational forecasting applications in the North Atlantic and Arctic.
This paper presents the latest version of the TOPAZ modelling system, v5, and evaluates it in free running mode (i.e. no data assimilation) across the pan-Arctic region. The authors focus the evaluation on a 10 year period (2009-2019) and consider fields such as sea surface temperature and salinity, sea surface height, sea ice thickness and concentration, mixed layer depth, and eddy kinetic energy. The authors also examine the 3D structure at a number of key sections, where model transports are calculated and evaluated. The paper shows where the model compares well with the observations as well as where the biases are larger.
This type of work needs to be published, as it provides the baseline to be references in all future papers that use TOPAZ5. GMD is an appropriate venue. That said, I think there are ways the manuscript can be improved, to make it more valuable to the broader community (i.e. not just those using TOPAZ). Additionally, I think some additional technical details, and some additional observational comparisons would help in providing a more complete observational baseline for this TOPAZ evaluation. I would thus recommend major revisions, with the specific details provided below.
A model evaluation paper can, in principle, be used by two research communities. One is users of the given model/configuration, to be able to refer to in future studies to show that the model is a good tool for their specific paper. In general, the manuscript does this well (comments on potentially improving this aspect are given below). However, the other community that can benefit from a model evaluation paper are those setting up their own model simulations, who can learn from the development of this given system. And this aspect is completely missing here. Yet, I doubt the PIs pulled the model out of the box, and ran just one experiment to get the results presented here. I would suspect that they ran multiple sensitivity experiments, to get the set of ocean and sea-ice parameters that they use. At least a sub-section on the steps and development would allow others to learn from the development work done here and increase the value and utility of this manuscript.
For either purpose, the summary of the model and its parameters is also missing relevant information. There is nothing on mixing schemes, parameters for viscosity and diffusion, relevant sea-ice variables such as ice strength, albedo, drag coefficients, etc. If these are all the same as the previous version of TOPAZ, this needs to be stated. But at the very least a table with all the relevant information about the model specifics is needed. As well, does the model include tidal forcing? Icebergs? All these details are needed.
Missing evaluation that would help the manuscript:
I think some needed model evaluation is missing. In terms of looking at the exchange out of the Arctic, the authors only look at Fram Strait. Yet, one question for models is how well they partition the Arctic transport each side of Greenland, which can significantly impact the sub-polar North Atlantic. I think it would be good to add a comparison section west of Greenland, such as Davis Strait.
I think the statement about good representation of deep water formation based on the MLDs is a bit simplified. It looks like the model has too deep deep convection, occurring over too broad a region – which is common for models of this resolution. Some more discussion of the implications of those biases would help. Plotting maximum MLD might also help with the evaluation and show if the model has convection to the bottom in some regions like the Labrador Sea.
Why not freshwater transport as well as volume transport to help evaluate the properties of the water carried through the studied sections? As above, would like to see a section west of Greenland. Also, for Bering Strait, the authors imply a close comparison with the 0.8 Sv of Woodgate et al. Yet the Woodgate et al. papers show an increasing trend that isn’t discussed.
The authors state credible AMOC dynamics. Yet no estimates of the model AMOC is presented. It would be good to see the modelling overturning in depth and density space along the OSNAP section, to compare with those estimates.
Something more on the deeper circulation, such as Atlantic Water circulation within the Arctic Ocean, and/or the overflows from the Nordic Seas would be useful evaluation.
Additional Items:
Is it more appropriate to discuss this paper as a model validation, or a model evaluation? I feel this manuscript is more focused on an observational evaluation, rather than a detailed validation of the code and numerical schemes.
L35: Note sure what “use in their majority advanced…” is trying to say.
Given the comment “horizontal resolution varying from 6.4 km at the Pacific boundary to 10.2 km at the Atlantic boundary”, might it be worth adding a panel to figure 1 showing a spatial map of the model resolution?
The model uses ERA5 forcing. Yet ERA5 is known to have a warm bias in the Arctic as well as having issues like occasional extremely strong and unrealistic wind events. Are any corrections applied to the ERA5 forcing?
L97: The authors mention estuaries along the Greenland coast. Do they mean fjords? In any case, I think it would be good to provide more detail about how this freshwater is distributed? For example, is any of it moved to the shelf for fjord systems that are not well resolved?
Why was the length of the spin-up decided to be the first 17 years? Any objective measure used to decide on that length?
Is the summer SST bias related to sea-ice biases?
Looking at the large SSS biases along the Arctic shelves, it looks as if there might be a river runoff issue?
For the section figures, they use discrete color intervals, which I feel enhances the figure and makes them easier to follow. Yet the spatial contour plots usen discrete color intervals, which blend together and are harder to read. I would suggest changing all of them to discrete color intervals as well.
Figure 4: The fonts are too small (comparing with figure 8 for example), and the lines not thick enough.
L296: The text states “large-scale altimetry pattern with high fidelity”. Is that a surprise if GLORYS is assimilating altimetric fields?
Figure 8: Font size is good but lines could be thicker.
L344: Given the comment about the limitations with small scale eddies – did the PIs want to then consider some eddy parameterization?