| 23 Oct 2025
Mesoscale Variability and Water Mass Transport of the Caribbean Current Revealed by High-Resolution Glider Observations
Abstract. The Caribbean Through-Flow (CTF) provides a key pathway linking the North Atlantic Subtropical Gyre and the upper limb of the Atlantic Meridional Overturning Circulation. Yet, its internal structure and variability remain poorly resolved. Autonomous underwater gliders offer a unique capability to address this gap by collecting high-resolution hydrographic and velocity observations in regions where sampling is sparse. Here, data from a glider that operated for >90 days along 69° W in summer 2024 were analyzed to investigate mesoscale-driven variability in the CTF. Two consecutive occupations of this ~600 km trans-Caribbean section revealed a sharp decline in zonal transport from -17.64 Sv to -9.22 Sv, coinciding with a shift in mesoscale activity. Rossby number and dynamic height anomaly calculations from the glider data showed a shift from flow largely in geostrophic balance during Transect #1 to increased mesoscale influence during Transect #2. Satellite altimetry spanning the full deployment suggested this shift was driven by a cyclonic eddy that passed through the northern half of the section between the timing of the two transects. Despite the large changes in transport between transect occupations, water mass analysis showed that the relative contributions from North and South Atlantic water masses remained nearly constant. Direct sampling of an anticyclonic eddy during a partial Transect #3 revealed strong temperature and salinity anomalies in the upper 200 m. These findings highlight how glider observations can resolve key features and processes governing variability in this critical inter-basin pathway and improve understanding of mesoscale influences on large-scale circulation.
This is a well-written and rigorous manuscript that addresses water mass transport of the Caribbean Current and how it is affected by mesoscale variability. The authors use high-resolution underwater glider observations to quantify zonal water mass transport across a 600 km transect. Using satellite altimetry, they relate changes in transport between three transects to the presence and passage of mesoscale eddies. The study provides new insight into how mesoscale eddies influence transport in this critical inter-basin pathway.
The manuscript is clear and convincingly demonstrates the added value of high-resolution glider observations for capturing mesoscale variability and constraining the transport of water masses in a region characterized by sparse hydrographic and velocity observations. This study makes a valuable contribution to our understanding of circulation and variability in the Caribbean Throughflow, showing the necessity of resolving eddy-driven processes in both modeling and observational frameworks.
I recommend publication after the authors address the following comments.
Comments
L47: Clarify the direction of the transport (negative vs. positive). This applies throughout the manuscript, as the direction of transport is sometimes unclear.
L48–50: The sentence “To close the budget with... Windward Passage (refs)” is convoluted. One unclear aspect is that the observed transport is negative (previous sentence), but the total estimated transport is positive; then you state it should be augmented by 6–9 Sv. In addition to rephrasing for clarity, it may be useful to add a schematic of the general/mean circulation pathway in Figure 1 for non-regional experts.
Figure 1: Keep color consistency for transects 1, 2, and 3 as in Figure 6 (Transect 1: blue; Transect 2: orange; Transect 3: light gray).
L73: The study compares transport among transects that each take almost three weeks to complete. This means that the southernmost values of transects 1 and 2 are closer in time (similarly for the northernmost values of transects 2 and 3). Please include a statement acknowledging and justifying this limitation, and discuss how it may affect comparisons of transport magnitude across transects.
L81: Define v as “... integrating the specific volume anomaly (v) relative to...”.
L100: How do the results change if you consider the total eastward and total westward transport separately? These values might be important, as the net transport may be near zero while substantial eastward and westward flows still occur. Showing only the total (net) transport could mask significant exchanges.
L104: The authors eliminate dv/dx for simplification. Can you justify that dv/dx is negligible, using satellite data or by estimating the potential error introduced by this assumption?
L111: Please provide an estimate of the residuals or uncertainties associated with the least-squares fitting in the water mass analysis.
L122: Is the surface layer (σ < 24.5) below the surface mixed layer? Please clarify.
Figures 2–5: I recommend adding relevant isopycnal contours (e.g., in Figures 2 and 5) to improve clarity in the results and discussion. This would also help clarify the depth range of the different water masses analyzed.
L131: You mention that “closed contours were analyzed at 1 cm intervals,” but the AVISO product has a 0.25° resolution. Please clarify this apparent discrepancy.
Table 1: How were the eddy characteristics (swirl velocity, translation velocity, depth scales, etc.) determined? If these are outputs from a py-eddy-tracker, please state so and provide a reference describing how these parameters are defined.
L160: The sentence “The total zonal transport... Transect #2” follows immediately after the meridional transport description. I suggest moving it to follow the description of E–W transport for Transects 1 and 2 (around line 154).
Figures 2–5e,f: Clarify the sign convention for transport. In Figure 2, do red or blue areas indicate eastward/westward or northward/southward transport? The color bar is reversed in Figure 5e,f—please clarify and ensure consistency between figures.
L165–174: The definitions of water masses lack references and density ranges. Figure 3 shows core densities but not references. Relatedly, how is the transport of each water mass (Figure 4) computed? If by density range, please specify; if by another method, please clarify.
L184: The sentence “Though other water masses... linear mixing of two water masses” could be expanded. Which other water masses could alter this mixture or contribute to transport? Can you estimate the potential uncertainty or overestimation in your transport estimates due to this simplification?
L188: uCW is not defined, nor is its density range provided.
Figure 3: CW is missing in panel c. Consider making the scattered dots semi-transparent; currently, the yellow points appear to overlay and obscure variability.
Figure 4: Are the transport estimates per water mass an average or the total transport per water mass? Please clarify and provide associated variability.
L201: Does “15 km of an anticyclone” refer to the distance from the eddy border or from its center?
L206: The use of multiple distance thresholds (15 km inside, 45 km inside, 50 km buffer outside) is confusing. Please clarify all definitions and distances. I suggest using a logarithmic scale in Figure 5b and marking these thresholds as horizontal lines to help the reader identify when the glider was inside or outside eddy boundaries.
Figure 5: Add markers indicating transect limits to help relate temporal and spatial components of the time series. In panel d, clarify the label “Eddy Absolute Salinity Anomaly”—what does “absolute” mean here? Also, in the figure caption, you refer to “glider RU29.” Since only one glider is used, simply state “glider” for consistency with other figures.
L250: Based on your argument, I would expect a difference between DHA and AVISO ADT for Transect 2 vs. 1. Can you show this to strengthen the argument?
Figure 8: Maintain color consistency (blue for Transect 1, orange for Transect 2) and adjust line styles for variable differentiation.
L262: Please state the transport values after interpolating the glider data to a coarser resolution—this would support your statement quantitatively.
L305: The study underscores the importance of eddies in modulating the Caribbean Current. Can you suggest what is needed to better constrain and quantify the variability these structures induce in the mean flow?
L316: Define NASTG.
L317–end of conclusions: This section introduces a new topic not discussed elsewhere in the paper, though it is important for future work. I suggest moving it to the Discussion section.
Additional comment:
Have you compared your estimates to surface geostrophic transport derived from altimetry? Discussing what the glider observations add to those estimates would strengthen the paper, particularly if you can show that altimetry alone cannot (or can) capture the observed transport.