On the Seasonal Western Boundary Current System of the Weddell Gyre

Pereira-Vázquez, Tania; Aguiar-González, Borja; Marrero-Díaz, Ángeles; Veny, Marta; Rodríguez-Santana, Ángel

doi:https://doi.org/10.5194/egusphere-2024-1166

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https://doi.org/10.5194/egusphere-2024-1166

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03 May 2024

| 03 May 2024

Status: this preprint is open for discussion.

On the Seasonal Western Boundary Current System of the Weddell Gyre

Tania Pereira-Vázquez, Borja Aguiar-González, Ángeles Marrero-Díaz, Marta Veny, and Ángel Rodríguez-Santana

Abstract. We investigate the seasonal Western Boundary Current System (WBCS) of the Weddell Sea Gyre using two open-access global ocean circulation reanalysis products (NEMO and HYCOM at different resolutions), direct velocity measurements and altimetry data across an extended version of the historical ADELIE transect, hereafter E-ADELIE. The NEMO-based products are GLORYS2V4 and GLORYS12V1, provided daily with 0.25º and 0.08° of horizontal resolution, respectively. The HYCOM-based product is GLBv0.08, provided daily with 0.08º of horizontal resolution between 40° S and 40° N, and 0.04º beyond these latitudes. The ADELIE extension is made to include a novel, persistent current, previously unreported, which we name as the Inner Weddell Current (IWC). With this approach, we aim to assess whether these open-access products capture properly the dynamics, natural mode of variability and spatio-temporal scales of the WBCS so that we can set the groundwork for future interannual variability studies. E-ADELIE is located at a key location of the WBCS, before it splits into different branches that redistribute Weddell Sea waters either leaving the basin towards Bransfield Strait, towards the South Atlantic Ocean, or recirculating within the gyre. The analyses include the characterization of the horizontal and vertical structure of the WBCS and its volume transport. Results show that both reanalysis products agree on key dynamics features only at high model resolutions; NEMO at the lower-resolution version lacks the typical multi-jet structure of the WBCS. The altimetry data is also in agreement in showing this year-round multi-jet structure but in winter, when data gaps prevent us from a comprehensive view. The cross-transect volume transport variability from the reanalysis products is consistent with the seasonality of the basin scale wind forcing in all model cases, with minimum values (25 ± 5 Sv) from September to December and maximum values (33 ± 5 Sv) from March to July. Also, the time-averaged transport aligns at approximately 30 ± 5 Sv in all products. Major discrepancies exist towards the interior of the gyre, where the IWC differs strongly among reanalysis products. The IWC is found about 50 km east of the Weddell Front extending up to 600 Km offshore, standing out as a bottom-reaching, broad current transporting ~14±3 Sv in NEMO and ~39±5 Sv in HYCOM. According to available Shipboard Acoustic Doppler Current Profiler (SADCP) measurements from two different surveys, LG0003a and NBP0106, we find the IWC surface volume transport (0–300 m depth) is about 6.67 Sv and 7.64 Sv, respectively between 400 and 720 km offshore the Antarctic Peninsula. When computing the same estimates from the reanalysis products, we obtain lower volume transports values ranging from 2–4 Sv. Results from this study suggest that the high-resolution version of NEMO (GLORYS12V1) approaches the real ocean in the western Weddell Sea the closest when compared to observations and literature. These results open the avenue for future research investigating the variability of the WBCS of the Weddell Gyre at interannual scales at a key location for water mass exchange between ocean basins.

Received: 17 Apr 2024 – Discussion started: 03 May 2024

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Tania Pereira-Vázquez, Borja Aguiar-González, Ángeles Marrero-Díaz, Marta Veny, and Ángel Rodríguez-Santana

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RC1: 'Comment on egusphere-2024-1166', Anonymous Referee #1, 15 May 2024 reply

In this study seasonal variations of the Weddell Gyre’s Western Boundary Current System are investigated. Due to the lack of continuous observational data in this harsh and remote environment, the authors primarily rely on three open-access reanalysis products (GLORYS12V1, GLORYS12V4 and GLBv0.08) for their transport analysis across the historical ADELIE transect and beyond (Expanded ADELIE transect). The reanalysis products are provided in different resolutions (horizontally and vertically), time spans and different forcing mechanisms incorporated. The authors main conclusions include seasonal variations in cross-transect transport of the WBCS significantly correlated to seasonal variations in wind stress.
I have several major concerns with this manuscript, but what really concerns me the most is the total negligence of the different bathymetry outputs that vary quite significantly for each of the reanalysis products. For example in Fig. 5 at 350 km off the continental shelf the difference in the vertical extent of the water column between GLORYS12V1 and GLBv0.08 is 1000 m. Specifically in the Southern Ocean the bathymetry products incorporated in the reanalysis products aren’t reliable and do not represent troughs on the continental shelves, shelf break and slope sufficiently. Thus, the results of the cross-transect transports (top to bottom) calculations across multiple reanalysis products with different bathymetries can be impacted significantly, purely by different vertical extents. Thus, a direct comparison to observations is difficult without comprehensive discussion. This discussion and the caveats with respect to transport calculations that the different bathymetries bring have been left out entirely. Furthermore, it has been reported previously and also mentioned by the authors that some currents in the WBCS of the Weddell Gyre are bottom-intensified specifically at the continental slope, but bottom-triangles have not been included to improve transport calculations (as done by Thompson and Heywood (2008)).
Moreover, I would be hesitant to include GLBv0.08 at all (as there is no motivation given of why using it to begin with). To my understanding it does not provide good results or correlations (or at least it is not explained very well) and the time span and vertical resolution differ quite significantly from the other two products. Thus, I find a comparison of time-averaged parameters difficult if the time span differs between the different outputs. In that context, transports from time-averaged velocities of more than 20 years with observational data are not in the slightest comparable. By time-averaging, a lot of the vertical and horizontal structure is smoothed out and also seasonal and interannual variations (which are also quite strong in the Southern Ocean) are completely ignored. This comparison stands in contrast to your main result, where strongest transports occur during winter months. In my opinion, the authors have tried to cover to many questions at once without providing a detailed response to any of those questions. Thus, the ‘storyline’ is not clear to me.
Furthermore, I am missing a lot more detailed discussion on results with respect to differences between the reanalysis products, their caveats, sufficient reasoning for differences in results (other than resolution) and detailed comparison with previous studies. Specifically for differences in results a lot of the explanation provided is speculation and no evidence on the robustness of the results is provided. As you have nearly unlimited options when using reanalysis products, your study would really benefit from additional and detailed analysis and extensive discussion of caveats of the models and their variability in its entirety.
Please find all major and minor comments in the attached pfd.
In general, I think this study is interesting. It does have potential for a future publication. However, there are many aspects with respect to methodology and caveats of the reanalysis products, which have not been addressed sufficiently. Plenty of open questions remain. In all fairness, I doubt that including an extensive additional analysis and discussion in the time frame given for major corrections would be possible. Therefore, I suggest to reject the manuscript at this stage, but encourage the authors to resubmit once major issues have been addressed in detail.

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Citation: https://doi.org/10.5194/egusphere-2024-1166-RC1

Tania Pereira-Vázquez, Borja Aguiar-González, Ángeles Marrero-Díaz, Marta Veny, and Ángel Rodríguez-Santana

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Short summary

We explore the seasonal dynamics of the Western Boundary Current System of the Weddell Sea Gyre using two ocean circulation models and observational data. Our analysis aims to evaluate the goodness of ocean reanalysis products in capturing these dynamics and to lay the groundwork for future interannual variability studies. We also report a previously unknown current, the Inner Weddell Current. Our results provide valuable insights for understanding a key region for the global ocean circulation.


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