Controls of the Latitudinal Migration of the Brazil-Malvinas Confluence described in MOM6-SWA14

Laureanti, Nicole C.; Curchitser, Enrique; Hedstrom, Katherine; Adcroft, Alistair; Hallberg, Robert; Harrison, Matthew J.; Dussin, Raphael; Chou, Sin Chan; Nobre, Paulo; Giarolla, Emanuel; Camayo, Rosio

doi:10.5194/egusphere-2025-3823

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https://doi.org/10.5194/egusphere-2025-3823

Preprints

23 Oct 2025

| 23 Oct 2025

Controls of the Latitudinal Migration of the Brazil-Malvinas Confluence described in MOM6-SWA14

Nicole C. Laureanti, Enrique Curchitser, Katherine Hedstrom, Alistair Adcroft, Robert Hallberg, Matthew J. Harrison, Raphael Dussin, Sin Chan Chou, Paulo Nobre, Emanuel Giarolla, and Rosio Camayo

Abstract. The distribution and productivity of nutrients, eddy formation, energy dissipation, and other ocean properties are influenced by the variability of Western Boundary Currents (WBCs). In the Southwestern Atlantic, the key features are the Brazil-Malvinas Confluence (BMC) and the North of Brazil Current (NBC). This work investigates them using a 20-year high-resolution ocean model simulation with the Modular Ocean Model version 6 (MOM6) 1/14° configuration of the Southwestern Atlantic (SWA). The results reveal a significant deviation in the path and trends of volume transport of the WBCs over the decades. The Brazil-Malvinas Confluence (BMC) region gets saltier and warmer, with increased kinetic energy and transport. Although transport trends in the NBC indicate reduced transport, this results from weaker wind forcing, which reduces the mixing layer depth in the simulation and the subsurface transport in the region. The warming in the Brazil Current region triggers a stronger southward flow, resulting in a southward shift of 0.93° ± 0.08 of latitude/decade in the BMC separation. Working against this flow, the propagation of the Kelvin Waves from the Eastern Pacific Ocean induces a northern shift of the BMC, revealed by topographic Kelvin waves in the spectral analysis. This Pacific-Atlantic inter-basin relation indicated here underscores the importance of propagating Pacific disturbances into the region to maintain the positioning of the BMC and its properties under a warming Atlantic Ocean.

Received: 06 Aug 2025 – Discussion started: 23 Oct 2025

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Nicole C. Laureanti, Enrique Curchitser, Katherine Hedstrom, Alistair Adcroft, Robert Hallberg, Matthew J. Harrison, Raphael Dussin, Sin Chan Chou, Paulo Nobre, Emanuel Giarolla, and Rosio Camayo

Status: final response (author comments only)

RC1:
'Comment on egusphere-2025-3823', Anonymous Referee #1, 28 Nov 2025
General comments:
In this paper, the authors investigate the variability of the Southwestern Atlantic circulation, focusing on the western boundary currents. Their analysis is based on a high-resolution simulation (1/14º) MOM6 of 20 years and validated. Their results include properties and transports, the shift of the Brazil-Malvinas Confluence separation, and the role of inter-basin interactions.
Overall, the paper is well-structured, well-written, and well-referenced, and I truly think this is a relevant study for GMD.
My comments are mainly oriented to improve the presentation of the results, particularly the figures, along with a few suggestions to improve the text and discussion of the results.
Specific comments on figures:
Figure 1. I suggest changing the blue color, as it makes the label “Amazon” difficult to read. Additionally, increasing the latitude and longitude numbers, as well as the colorbar numbers and title, would improve overall readability.

Figure 8. The years on the x-axis are hard to read. Rotating them 45º would enhance clarity. Consider including the BMC and trend within the corresponding subplot for easier interpretation.

Minor comments and typos:
Line 19. The acronym Brazil-Malvinas Confluence (BMC) is defined twice. I recommend removing the second occurrence in the abstract.

Line 21. I suggest changing “mixing layer depth” to Mixed Layer Depth (MLD) to maintain consistency with the rest of the manuscript.

The term Kelvin wave(s) appears with both uppercase and lowercase “w” throughout the text. I recommend choosing one for consistency.

Line 119. The acronym GLORYS is not defined. Moreover, the reanalysis is referred to inconsistently as Glorys, GLORYS12s, GLORYS12, GLORYS 12v1, GLORYS12 v1. I recommend standardizing the notation for consistency, while keeping distinctions when referring to different resolutions of the product.

Line 198. The sentence currently repeats “during winter”. I suggest rewriting it to avoid this redundancy.

In section 3.1.2. Are the authors referring to temperature? Or is it potential temperature?

Line 276. A hyphen is missing. It should be: meso- and large-scale circulation.

Line 303. The acronym SSH should be defined.

Line 340. The statement “Different Authors indicated that both currents can widely vary their transport from 5 to 88 Sv”. Should be reconsidered. According to Goni et al. (2011), “Most recently, it has been reported that the transport of this current varies widely between 10 and 88 Sv [Garzoli, 1993;Peterson, 1992].” Please verify and update citation.

Line 404. There is a missing space in 22Sv.

Lines 407 and 436. The authors refer to Figures 8a and 8b, but the subplots in Figure 8 are not labeled. I recommend either adding the letters to the subplots or removing the references to “a” and “b” in the text.

Line 409. The sentence “With maximum latitude at 5ºN”. I suggest adding “With a” to improve readability.

Lines 413-417. These sentences could benefit from improved clarity and consistency in terminology. Currently, wind speed and wind intensity are used interchangeably, which may confuse readers. Additionally, the discussion of reduced wind speed as a limiting factor is somewhat repetitive. I recommend combining the explanation of the cause (low wind speed) with its consequences (reduced wind stress, weaker eddy formation, and decreased NBC retroflection transport) into one sentence.

Line 436. The acronym North Brazil Current (NBC) does not need to be defined here, as it has already been introduced earlier.

Major comments:
Lines 215-217. The authors state that the unbalanced freshwater influence is primarily local and does not lead to a general bias. However, I observe differences in the mixed layer depth, particularly in the Brazil-Malvinas Confluence. I suggest including a plot of the MLD bias in Figure 2 to provide additional support for this claim. Furthermore, in the following section, the authors note critical differences in MLD compared to observational data.

Line 414. It would be interesting if the authors could demonstrate this: “This represents the lack of mixing, which can be either due to the influence of the wind speed (Figure 5c) or the use of a z-coordinate vertical structure.” It would improve clarity and support the conclusions (line 533) if the authors explicitly quantify or visually demonstrate the relative contributions of each factor.
Citation: https://doi.org/10.5194/egusphere-2025-3823-RC1
RC2: 'Comment on egusphere-2025-3823', Anonymous Referee #2, 09 Jan 2026

This study presents a high-resolution (1/14°) regional ocean simulation of the Southwestern Atlantic (SWA) using the Modular Ocean Model version 6 (MOM6) over a 20-year period (1997–2016). The primary objective is to investigate the variability and trends of the Western Boundary Currents (WBCs), specifically the Brazil-Malvinas Confluence (BMC) and the North Brazil Current (NBC).
The authors report a significant deviation in the volume transport trends over the decades. A key finding is the southward shift of the BMC separation at a rate of 0.93°±0.08 per decade, driven by warming in the Brazil Current region. Furthermore, the study identifies that the propagation of topographic Kelvin waves from the Eastern Pacific induces a northward shift in the BMC, acting as a counter-mechanism to the southward trend.
I support the publication with some comments (Minor/Major):
Text:
Lines 29 to 34: is not clear how the authors are presenting the work, I suggest reformulate these first sentences.
Line 46: I will suggest another term to replace “intercepted”, maybe just mention that the BC and the MC confluence at… and include some more citations of works that have study the confluence zone.
Line 47: include citations from Barré et al. 2006; Ferrari et al. 2017; Artana and Provost 2023, they have studied this region in terms of ocean dynamics and will give the authors some more ideas to explain clearer the confluence zone.
Line 50 and 51: Search for citations to support the sentence.
Line 193: did authors thought of performing and EOF analysis to find if there is a dipole?
Line 283 and lines 293 to 297: I will include more citations from recent works that also describe the MC and BC and their confluence using GLORYS12 and satellite data.
Lines 304 to 305: so then is the NBC the most prominent current? (lines 287). Explain clearer related to the preview statement in line 287.
Line 316: the variability depends on the dynamics the BMC is a confluence of 2 currents and the dynamic associated to this feature explains the high variability of the region. Search for more literature and explain better this concepts.
Line 346: support with citations related to bottom friction and bottom currents in the Argentine Basin, there are many works in the region.
Line 363: replace Glorys with GLORYS12 reanalysis.
Line 416: support with citations
Line 443 to 445: support with citations
Line 477: support with citations
Lines 482 and 483: This is an important result can you explain a bit more.
For the summary and conclusions if the authors accept the suggestions of new literature to be cited then this will enhance also the discussion.
Line 499: The maximum negative SST bias of 1.0°C is sufficiently good performance for forecasting ocean conditions. Why?
Figures:
Fig. 1: I think the river is in blue is Parana River and Rio de La Plata is the estuary in the discharge of Parana and Uruguay rivers.
Fig. 2: I suggest to use a thinner grid line is a bit to strong and difficult to appreciate the data, also will be good for the reader to plot more isolines to compare better the results from the model with the observation data.
Fig. 3: same grid problem as 2 and also some thinner isolines will help as well.
Fig. 4: I will suggest to remove the grid is making complicated to understand the results.
Fig. 6: maybe will be a good idea to show in the figures again the main currents and play a bit more with colors to enhance the MC and BC. Also grids thinner.
Fig. 8: for the years I suggest to put one year yes one year not they are too tight like it is.

Citation: https://doi.org/10.5194/egusphere-2025-3823-RC2

Nicole C. Laureanti, Enrique Curchitser, Katherine Hedstrom, Alistair Adcroft, Robert Hallberg, Matthew J. Harrison, Raphael Dussin, Sin Chan Chou, Paulo Nobre, Emanuel Giarolla, and Rosio Camayo

Data sets

MOM6-SWA14 Model outputs N. C. Laureanti et al. http://antares.esm.rutgers.edu:8080/thredds/catalog/MOM6/ESMG/SWA14/exp.010/catalog.html

Model code and software

MOM6 Ocean Model N. C. Laureanti et al. https://zenodo.org/records/17252994

Interactive computing environment

Model configuration and scripts N. C. Laureanti https://zenodo.org/records/17252554

Nicole C. Laureanti, Enrique Curchitser, Katherine Hedstrom, Alistair Adcroft, Robert Hallberg, Matthew J. Harrison, Raphael Dussin, Sin Chan Chou, Paulo Nobre, Emanuel Giarolla, and Rosio Camayo

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Short summary

This study investigates changes in the Southwestern Atlantic Ocean with a high-resolution ocean model. Particularly in the Brazil-Malvinas Confluence (BMC), it finds that the southward movement of the BMC, induced by the warming trends in the region, is balanced by northward flow from the Malvinas Current and Pacific Waves, affecting the Atlantic. The results also comment on disparities observed in the simulation, especially concerning the North Brazil Current, which impacts its evolution.


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