| 12 Jun 2026
Evaluating Transport Observations of the Atlantic Meridional Overturning Circulation at 11°S Using an Ocean Model
Abstract. The impact of the Atlantic Meridional Overturning Circulation (AMOC) on weather and climate, both regionally and globally, motivated the installation of several observational arrays. It is vital to not only derive transport time series from these arrays but to also quantify the associated uncertainties. Here, an observing system simulation experiment is performed to assess the uncertainty and potential of the TRACOS (Tropical Atlantic Circulation and Overturning at 11°S) array to calculate the geostrophic AMOC transport (AMOCg) from it. Accordingly, the observational setup is subsampled in a high-resolution ocean model and various approaches to derive AMOCg are tested. We find that the currently used approach based on bottom pressure recorders (BPRs) can explain 56 % of the short-term (seasonal to interannual) AMOCg variability, though overestimating the seasonal amplitude. Observations of longer-term variability are limited due to the pressure sensor drift. Currently, long-term (decadal to multi-decadal) variability is only captured by boundary current measurements which explain 62 % of the basin-wide AMOCg long-term variability, though with high root mean squared errors. Regarding potential improvements of the current approaches, we find: 1) The nominal drift rates of the reference sensors currently installed in self-calibrating BPRs are still too high to reliably detect a linear AMOCg trend of the magnitude presently considered realistic, namely about 1 Sv per decade. 2) Acoustic round-trip travel times are limited in use for AMOCg computation at 11°S. 3) Combining BPRs with moored temperature and salinity measurements is a promising approach that can improve AMOCg estimates of both short-term variability (to 79 % explained) and long-term variability (to 61 % explained). Overall, we find that, despite its relatively sparse instrumentation, the TRACOS array is capable of capturing AMOC signals, while we also highlight areas where uncertainties could be reduced.
This is my review of the paper "Evaluating Transport Observations of the Atlantic Meridional Overturning Circulation at 11°S Using an Ocean Model" submitted to Ocean Science. This paper uses a high resolution ocean model to test the observational techniques used to derive the AMOC at 11S from the TRACOS ocean observation array. The paper tests various methodologies by simulating real bottom pressure (PIES) and hydrographic (Tall mooring) data in the model, and divides the results into two categories: low (>5 years) and high (<2 years) frequency variability. Some of the tests performed here are the vertical interpolation, vertical resolution of the profiles, bottom pressure drifts and gaps, and reference depths for velocity/transport. In the end, all data from tall moorings and bottom pressure are used to simulate an optimal setting of the 11S array, and recommendations on how to improve the AMOC observations are given.
The paper is well written and design. It is a bit too long to be able to read it in a at once, and the tables are sometimes difficult to understand. I would suggest to leave part of the analysis for an appendix, and just give a summary in the text, as for example the part of the vertical interpolation, such as consolidating sections 3.13, 3.2, and 3.2.1 into one or leaving part of it in the appendix. I leave this to the discretion of the authors.
In general the paper is very informative, and builds on previous papers on the inclusion hydrographic and bottom pressure data analysis and the simulations of an enhanced method using all information available. The paper should be published after minor suggestions listed point-by-point below:
Point-by-point Comments:
L. 43 Strictly speaking, this is not an OSSE experiment since it does not test data assimilated in the model nature run. I would put a disclaimer in the abstract, introduction or methodology this work, just for accuracy. Maybe just call it a data impact study.
Abstract: L 10-13 The items (1) and (2) are not really "potential improvements of current approaches" but rather "misrepresentations of the current approach"
L. 133: "global mean SSH, which is derived from the local divergence of the velocity field and the balance of all local surface freshwater fluxes". Maybe you want to say "the component of the global SSH that is derived from the local ...."
L.140-154 This paragraph is long to not have any figure associated to it. Is there a way to include some model validation in the supplementary?
L.189 Since there are many authors, it would be appropriate to write the text in the third person.
L. 236 When pressure "profiles" ... - to be more specific.
L.249-250 - By choosing SSH right at the coasts, wouldn't it be detrimental to the signal-to-noise ratio due to coastal waves and costal wind variability? Maybe a disclaimer should be added about this negative side as well, at least in the observational sense.
Equation 7: Tg was previously defined as the zonal integration of velocity, calculated as the pressure difference, but here it is defines as the depth integration (Compare Eq. 7 with Eq 3). Should this be AMOC_g instead. This definition should be consistent.
Section 2.4.3 - I am not sure I understood from the text how Tau_ies is derived in the model. Since this is calculated from the sound speed, and this is not an output variable of the model, how is the time travel calculated?
L.359 The comparison between the rmse and standard deviation of the AMOC is using the same filtering, or it compares the rmse of the filtered data with the standard deviation of the full spectrum of the AMOC_g?
L.403-405 Before the trend removal every few years, was any trend added to the BPR data similar to what was described in section 3.1.2? If not, please explain there the difference in AMOC_g would come from?
L.416 Can you state in parenthesis the value of the correlation and rms.
L. 618 I do not understand why this statement was made that IES are not suitable for the AMOC_g if in the next sentence it will be used for it. It seems contradictory.
L. 698: "refined to enhanced" vertical resolution?
L. 724-725 These papers are for the North Atlantic. Is there any example for the South Atlantic. As a suggestion, a recent paper (Pita et al., 2024; https://doi.org/10.1029/2023JC020010) shows it in their Figure 11.