| 18 Mar 2026
Bottom currents at the Arctic Mid Oceanic Ridge: insights from deployed sensors, biological indicators, and global models
Abstract. Characterizing near-bottom ocean currents at deep-sea topographic features requires observations across multiple scales, from basin-wide circulation models to local in situ measurements. This study addresses this challenge at DeepInsight Hill on the Mohns Ridge by combining eight months of in situ current measurements, global ocean model predictions, and novel ROV-based biological flow indicators. Our findings reveal a complex and highly variable flow regime with a striking decoupling between tidal pressure and near-bottom currents. Spectral analysis shows that the dominant semi-diurnal tidal pressure signal does not translate into coherent near-bottom currents, suggesting conversion of tidal energy into internal waves at the ridge topography. During winter, sustained directional transport occurs simultaneously in opposite directions at the summit and base of DeepInsight Hill (S-SE and N-NW respectively), accounting for most of the effective water displacement over the record. However, foliaceous sponges, whose fan-shaped morphology is known to develop perpendicular to the prevailing current, show consistent W-SW orientations at the base of the hill that do not match the long-term instrumental mean flow direction (N-NW). Instead, these orientations align with global model predictions and the direction of episodic peak currents recorded during summer, suggesting that sponge morphology records specific high-energy flow events rather than time-averaged conditions, possibly amplified by peak summer nutrient supply. Bedforms observed at the same location eight months apart show no significant change, indicating that sediment-mobilizing currents are episodic and may not recur annually. Further data and interdisciplinary research are essential to resolve the governing factors of these systems and to refine environmental impact assessments in topographically complex deep-sea settings.
Review of “Bottom currents at the Arctic Mid Oceanic Ridge: insights from deployed sensors, biological indicators, and global models” by Souche et al.
In this study, the authors show a collection of diverse ocean current data obtained over the course of several years at DeepInsight Hill at the Mohns Ridge. The idea is to combine all different indicators of ocean currents (including biological indicators, current meter records and output from a global ocean model) to obtain a holistic picture of ocean currents at this topographic feature.
In my view, the analyses fall short of this idea and appear to be rather superficial and incomplete (see below).
Unfortunately, even with more detailed and systematic analyses, I am unsure what we could learn from the collective data presented here. Instead, I would expect that more thorough analyses would result in a more targeted study (e.g. aiming at exploring the relationship between sponges and currents or focusing on dynamic processes relevant at DeepInsight Hill), substantially differing from the present manuscript. This is why I recommend to reject the manuscript at this time.
Major Concerns:
Unclear scientific scope
The title suggests general analyses of bottom currents at the Mid Oceanic Ridge. This is not done here.
According to the abstract, this study adresses the challenge that: ”Characterizing near-bottom ocean currents at deep-sea topographic features requires observations across multiple scales, from basin-wide circulation models to local in situ measurements.”
I am unsure what the authors mean by “characterizing”, but what I see is a descriptive study presenting different (sometimes anecdotal) indicators of currents (direction and/or speed) at a specific location (all observations are in-situ in a very small area, so "across scales" can refer to time scales only). The different indicators are (loosely) compared, but I miss a clear goal, thorough evaluation and systematic analyses.
Selection of data:
The authors aim to include diverse data, but I am unsure what purpose some of the data serve:
Superficial analyses:
The nature of most data presented here means that more thorough analyses are limited to the current meter records. In my view, the authors did not take full advantage of this dataset, with both analyses and presentation lacking.
In particular, the presentation in figure 6 is incomplete, showing an arbitrary subset in a), with an unexplained scaling on the y-axis. Similarly, the method behind the calculation of spectra in b) is not explained. There are many choices which can impact on the results. It also would be more useful/common to show rotary spectra for the velocity components.
It is unclear to me, why one time series from 2025 spanning 1.5 days is used to illustrate “short-term variability” (Fig. 7). The 10min resolution from the longer time series is ample to investigate variability within tidal cycles. If it is meant to convey spatial variability, it would be necessary to know where this specific instrument was deployed and how its records differ from the others.
The authors identify an apparent discrepancy between the tidal signal in pressure and currents as “striking", but no quantitative analysis is performed that could shed light onto whether this is indeed unexpected or not. For example, a more rigorous tidal analysis could entail the identification of the dominant tidal constituents using common tools (e.g. u_tide) and compare their observed amplitude to the expected value (e.g. from tidal models, both for pressure as well as currents). Investigation into different processes such as possible bottom boundary layer attenuation or topographic trapping could further help explaining the results.
In terms of the longer term variability, much space is devoted to figures 8 and 9, but again the analysis appears rather superficial and conclusions remain largely qualitative (i.e. describing what is seen in the progressive vector diagram).
Quality of figures
More attention should be paid to the figures, their readability and what they are meant to convey. My short notes:
Figure 3: Why doesn’t this figure contain mean current arrows from the two instruments?
Figure 6: The units on the y-axes do not make sense without explicitly stating what the scaling is. What is the motivation for the time period selection in a? Spectral calculations are not explained. There are many choices with impact on the results. It would be more useful to see rotary spectra for the velocity components.
Figure 7: What is "Tide pressure"? Isn't this just pressure? Units of dBar are more common…
Figures 8,9: a,b,c are largely identical between the two figures. d,e,f,g,h are very hard to interpret. What is the radial axis of f/h? It is unclear what the message is.
Figures 10,11: What is the contribution? They are not analysed beyond stating that nothing much changes.