| 15 Aug 2025
Spatiotemporal scales of mode water transformation in the Sea of Oman
Abstract. In the Sea of Oman, mode water forms at the surface and is trapped under a warm stratified layer in summer. This capped and well-mixed oxygenated layer decouples the oxygen minimum zone from ocean surface processes and provides a space for remineralisation, reducing oxygen demand in the deeper oxygen minimum zone. Several physical processes, from isopycnal and diapycnal mixing to advection, transform mode water and change its properties. Using monthly climatologies derived from profiling floats and high-resolution underwater glider observations, we perform a volume budget analysis to investigate the mechanisms driving mode water volume change in the Sea of Oman from monthly to 3-day temporal scales. Isopycnal and diapycnal water-mass transformations are estimated in a density-spice framework. Mode water predominantly transforms along isopycnals, yet strong but transient diapycnal transformation occurs at shorter timescales. Moreover, fluxes between the mode water layer and its surroundings are highly sensitive to the presence of mesoscale eddies. Across eddies, diapycnal and isopycnal transformations intensify by 61 % and 45 % respectively, compared to non-eddy conditions, indicating that eddies are drivers of both lateral and vertical water mass exchanges. This study provides a new methodological approach to understanding water mass transformation using high-resolution underwater gliders, and shows that this water mass transformation framework can be used at higher resolution than traditional climatological products or models. By comparing monthly climatological products to the high-resolution glider data, we estimate that the climatological estimates are outside of the high-resolution glider mean ± standard error 40 % of the time for diapycnal and 60 % of the time for isopycnal transformation. These results highlight the intense variability occurring at small scales and can serve to inform future estimates of water mass transformation uncertainty from coarser products.
General Comments
This is a well-written and scientifically rigorous manuscript that addresses the spatiotemporal scales of mode water transformation in the Sea of Oman. The authors combine Argo climatologies with high-resolution underwater glider observations to investigate the relative roles of isopycnal and diapycnal processes in modifying mode water volume and properties. The methodological framework, based on σ–τ coordinates, is innovative and provides new insight into how mesoscale eddies influence transformation rates.
I commend the authors for the clarity of their writing, the thoroughness of their analysis, and the careful integration of climatological and glider datasets. The paper convincingly demonstrates the added value of high-resolution glider observations for capturing episodic and small-scale processes that are missed in climatologies. The conclusions about eddy-driven intensification of both diapycnal and isopycnal transformations are compelling and make a valuable contribution to our understanding of mode water variability.
Overall, this is an excellent paper that makes a valuable contribution to the field. I recommend publication after the authors consider the following specific and technical comments.
- Joe Gradone
Specific Comments
Line 10: The second sentence of the abstract is a bit of a beast to read. I only became oriented once making it to the end where you state “deeper oxygen minimum zone”. I suggest moving the word “deeper” to the first mention of the oxygen minimum zone and then consider breaking this sentence up into two sentences.
Line 13-14: Initially, when I read the abstract, I was questioning how you could do this analysis on a 3-day temporal scale with a monthly climatology. The use of glider data in the analysis is clear in the main paper. I suggest maybe something like “higher resolution underwater glider observations” to distinguish the difference.
Line 81: Observations projected onto an orange line, not a blue line, correct?
Line 82: Is 2 km horizontally not a bit too fine for glider data?
Figure 1e: I recognize there is a lot of information on this plot but as someone who is colorblind, I cannot fully understand what is going on. The red dotted line is difficult to see and the white and pink lines blend together.
Line 184: Text says Figure 1e-g, but those subplots do not exist in Figure #1
Line 186: Define what mintier means here, not necessarily standard oceanographic knowledge. You define it well on line 201, so just consider some language to that effect here.
Figure 5: If this a pain, don’t sweat it, minor comment here. It would help to further orient the reader if you could make panels c, f, and i have the 24.5 isopycnal surface approximately in line with the depth where the hatching stops in the corresponding plots to the left.
Figure 5 cont.: I cannot tell the difference between the colored bars for “diapycnal”, “isopycnal”, and “exchange” in panels c, f, and i.
Figure 5 cont.: Helpful for the reader if you could put either a title or a small line of text, sort of like a legend, showing j-l correspond to diapycnal, isopycnal, and exchange, though the change in colors will also likely help this a ton.
Lines 316: Since the exchange term is computed as a residual, uncertainties in isopycnal and diapycnal terms will propagate directly into this estimate. Could you expand on how robust this separation between mixing and advective exchange is, and whether the relative magnitudes may be sensitive to error?
Line 365: Consider rewording/expanding to include a more general term, such as tracers. Maybe adopt the wording from line 398-399.
Technical Comments
Line 84: I would expect a 6 km running mean to filter out submesoscale variability. With the Rossby radius of deformation at this latitude (O) 20 km, can you comment on whether this reduces your ability to resolve the lower end of the mesoscale as well?
Line 87: An equation would be nice for both EKE calculations.
Line 88: Can you elaborate/clarify on what you mean re: “anomalies of the dive-averaged currents derived from the glider flight model”? Anomaly relative to what?
Line 100: A 200 km buffer from the across-Gulf transect for remapping seems very wide.
Line 122: The spice coordinate captures the isopycnal change more so than using potential density as a coordinate, no? Consider rewording this sentence.
Line 190: I am a little confused at how the thinning of mode waters results in a densification, but the signs of both the isopycnal and diapycnal transformation are negative (Line 192-193), implying a reduction in density.
Timescale of transformation of mode water section: I found the inclusion of both the climatological analysis and the higher resolution glider data on the same plots in Figure 2 to be a lot to unpack. Similarly, while I think the title of this section is a nice description, the first paragraph could use some additional language to highlight the time period it refers to. Similar to how the second paragraph highlights how the glider data allows for a higher resolution analysis. I don’t necessarily think the two paragraphs warrant their own section, but the differences in the findings are noteworthy enough to warrant additional descriptive text, at a minimum. Initially, I was going to suggest breaking Figure 2 up into two different figures, but I do find the comparison to the climatology to be helpful. The additional text in the results section will likely make the figure more digestible.
Line 368: While I understand a large aspect of the importance of Arabian Sea mode waters is their influence on subsurface oxygen concentration, I find the discussion around your results in the context of prior oxygen-focused literature to be too direct, as it does not actually utilize any oxygen data in your analysis. Simply, the last sentence of the first paragraph in the Discussion section can either be reworded or expanded to better reflect which aspect of Jutras et al. (2025)’s study your results expand. Then, more explicitly, how one might infer the resulting changes/implications in oxygen concentration from your findings. It is clear how your findings are focused on shorter timescale changes in mode waters, but I find this important paragraph in need of larger clarification.