| 26 Jun 2026
Eulerian and Lagrangian Assessment of Arctic Surface Current Products Using Drifter Observations
Abstract. Accurate Arctic surface current estimates are needed to quantify freshwater redistribution, upper-ocean connectivity, and Arctic–North Atlantic exchanges, but validating them is difficult given sparse observations, sea-ice cover, and overlapping dynamical processes. This study evaluates six widely used products against drogued drifters from the Global Drifter Program (2011–2021): AVISO altimetry-derived geostrophic currents, NeurOST machine-learning reconstructions, GlobCurrent and OSCAR multi-observational/mixed-layer total currents, and the GLORYS and TOPAZ reanalyses. Skill is assessed with complementary Eulerian point-wise metrics and Lagrangian trajectory diagnostics, including a Lagrangian Uncertainty Quantification (LUQ) framework. Eulerian results show marked regional and seasonal contrasts. NeurOST and OSCAR agree best with drifter velocities, GlobCurrent and GLORYS show intermediate skill, TOPAZ has weaker near-surface speed agreement, and AVISO underestimates current intensity and variability. Products incorporating additional observational constraints, wind-driven contributions, or mixed-layer dynamics outperform altimetry-only geostrophic estimates, especially in regions shaped by mesoscale variability, bathymetry, sea ice, and narrow gateways. The Lagrangian analysis shows that good local velocity agreement does not guarantee realistic transport: small velocity differences accumulate along trajectories, producing large deviations in pathways, retention, and connectivity. LUQ diagnostics indicate part of this mismatch reflects intrinsic transport variability and initial-condition sensitivity rather than product error alone. The authors conclude that Eulerian validation must be complemented by Lagrangian, uncertainty-aware diagnostics for studies of freshwater pathways, tracer transport, and Arctic connectivity.
Review of the manuscript EGUSPHERE-2026-3629
In this manuscript, the authors compare the observational, quality-controlled data gathered by the ocean surface drifters from the well-known ‘Global Drifter Program’ with the output from various ocean current gridded products: AVISO altimetry-derived geostrophic currents, NeurOST machine-learning reconstructions, GlobCurrent and OSCAR multi-observational/mixed-layer total currents, and the GLORYS and TOPAZ reanalyses. The data comparison spans over a decade: years 2011-2021. In addition to the usual Eulerian evaluation of point-to-point similarity of current magnitude and direction, the authors use a Lagrangian method to show both discrepancy and coherence of the ocean surface’s circulation patterns. The identification of Lagrangian Coherent Structures (LCSs) as well as the use of the Lagrangian Uncertainty Quantification (LUQ) framework not only allows for trajectory-to-trajectory comparison between the observations and the chosen product but also permits transport assessment over a wider area (in the vicinity of the trajectory), organised by transport barriers and coherent structures.
The authors present and thoroughly analyse the statistical diagnostics, providing regional and seasonal differences between observations and the products as well. The presented case studies cover various dynamical regimes: from more energetic ones, where mesoscale processes have a major impact on the flow structure, to more coherent, large-scale, more stable patterns in the gateways to the Arctic Ocean. This leads the authors to two major conclusions: 1) Products incorporating observational constraints and wind effects outperform altimetry-only estimates; 2) good local velocity agreement does not guarantee realistic transport; and 3) no one product would provide the best results for all types of circulation patterns.
General assessment:
The manuscript's topic aligns well with the scope of Ocean Science and addresses the importance of validating publicly available gridded ocean current products. Despite the somewhat arbitrary choice of regional areas, the authors provide a logical explanation of the large number of metrics. Thanks to the application of two different methods, the authors successfully convinced the reader of their complementarity.
My bigger concern is the authors’ choice of the analysed case studies: why add these non-Arctic ones? How do they relate to the title of the article itself? I have a feeling that some of the cases were added not necessarily because they relate to the topic but rather because they are good to illustrate the differences between the products analysed. While the Lofoten Gyre and the Labrador Current are more closely connected to the Arctic (outflow and inflow areas), I would consider the Gulf Stream almost entirely out of scope of the paper. Perhaps adding ‘and sub-Arctic’ to the title would solve this issue?
Among others, the conclusion about the usefulness of the TOPAZ product for the ice-influenced cases, despite its weaker agreement in the global speed-based Eulerian statistics, is promising, especially due to the lack of observational data there, as illustrated by sparse GDP drifter coverage.
In my opinion, the manuscript is well written and illustrated and makes a proper research statement on the covered scientific topic. However, I also found it a bit too long and too repetitive in some parts. As mentioned above, some case choices seem slightly misleading. Therefore, I suggest that it can be published after some minor corrections.
Minor comments and remarks:
Title: ‘Eulerian and Lagrangian Assessment of Arctic Surface Current Products Using Drifter Observations’ – consider adding ‘and sub-Arctic’.
This section clearly describes the research background, motivation, data and methods and briefly introduces the next sections. The bibliography used is up to date and demonstrates a knowledge of the subject matter.
In this section, more information about the observations and gridded products is provided.
Line 182: ’This choice is made to ensure methodological consistency across all datasets’- Are all the other products similar? From their description, one may understand that they differ in the layers analysed.
In this section, the processing of the GDP drifters, Eulerian metrics, Langrangian validation framework and sensitivity analysis are described in detail. Furthermore, reasoning behind the chosen case studies is given, as well.
Line 272: ‘In ocean transport problems, relatively small velocity differences can accumulate over time and generate large deviations in particle trajectories.’ - This sentence is like the sentence in the previous section. Please consider rewriting/shortening it.
Line 334: ‘This analysis supported the use of τ = 5 days and R = 10 km’. – Which other values were considered in your sensitivity analysis?
This section provides a number of good illustrations of the differences in the seasonal, temporal and regional variabilities emerging from all products after the methodology has been used. However, the time series of the normalised LUQ values do not cover the same periods for all cases but are scattered among several years (in subplots of one figure). Is there a reason for this?
Line 439: ‘narrow Arctic gateways’ – I think you may add a sentence or two about the worst results that were obtained for S9 (the Canadian Archipelago); it really stands out in Figure 9.
I would also advise the authors to consider removing some repetitions that protract the text in this section as well: for example, these two sentences below are very similar:
Line 477: ‘Rather than evolving as a random diffusive cloud around the ground-truth trajectory, transport deviations are organized by the transport geometry of the underlying velocity field and by the Lagrangian coherent structures governing particle evolution.’
Line 506: ‘Accordingly, transport deviations are not uniformly distributed around the observed drifter position, but are organized by the underlying recirculation and its associated transport barriers.’
Line 574: ‘that products with with similar local velocity’ – remove repetition
In this part of the manuscript, the results are thoroughly discussed, together with the added value and limitations of the performed analysis. There are some repetitions from the previous parts, but this is probably justified and aims to highlight the importance of proposals.
This section lacks a broader context and a comparison of the results obtained with similar studies and some implications for future work. This might be helpful for the readers conducting similar data experiments or using similar methods.
Line 668: Section title - make it a verbless sentence to be consistent with previous subsection titles
This part is a summary of the obtained results; here the repetitions are unavoidable.
The manuscript contains 17 figures and 2 tables, and they are correctly prepared for publication. The fonts are large enough, the lines are not too thin, and the colours might probably be better in some of them (Figures 6, 7, 14-16). Please check the best colour palettes for scientific figures and data; this may help in your future submissions.