the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 20 Nov 2024
Coastal circulation and eddies generation in the Southwest Mexican Pacific
Abstract. In this work, we investigate if it is possible to identify the Mexican coastal current using satellite data and how this coastal current interacts with the coastline. This study is carried out using the mean sea level anomaly and derived geostrophic velocities from the Copernicus Marine Service from 1993 to 2021. By computing variance ellipses and empirical orthogonal functions for the ocean velocity data, we were able to identify and delineate the coastal current and determine its main characteristics: an average width of 95 km and an average velocity of 0.3 m s-1. The preferred direction of this coastal current is parallel to the coast, with episodes towards the pole and towards the equators showing seasonal variability. Using a 3D numerical model, we found that the interaction of the coastal current with the coastline generates eddies near the coast in some places forming a wide concavity. The eddies are formed while the current is present (poleward or equatorward), become intense, and move away from the coast until the current weakens. The statistics of some physical variables of near-shore eddies show that they have a similar radius of 30 km and a vorticity of 0.5 X 10-5 s-1. We conclude that there is a large number of eddies in the coastal zone of the Mexican Tropical Pacific, and some of these eddies are formed by the interaction of the coastal current with the coast.
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RC1: 'Comment on egusphere-2024-3403', Anonymous Referee #1, 15 Jan 2025
This is the review of the paper entitled "Coastal circulation and eddies generation in the Southwest Mexican Pacific” by F. Velázquez-Muñoz, R. Cruz-Gómez, and Cesar Monzon. The paper uses satellite data and numerical modelling to study the variability of the Mexican coastal current and eddy generation in the Pacific Tropical Mexican. Although the characterization of Mexican coastal current is somewhat known with previous works, I believe that the novelty of this work is the generation of eddies in the bays driven by the interaction of the current and the geometry of the coast. I do not see what the new findings regarding the characteristics and variability is of the coastal current in this analysis that has not been shown before? The bimodal structure and variability are already known. However, I believe that this work could be still published if some additional work is performed. Specifically, the generation mechanism of the eddies is poorly supported and just described superficially. To provide insights on mechanism of generation, the authors could use the numerical outputs to compute the terms for the material rate of change of the vertical vorticity and inspect what are the dominant forcings that “inject” vorticity in the bays were the eddies are generated. I encourage the authors in do so this analysis to conclude about the generation mechanisms of the observed eddies.
The paper needs additional computations, and the English grammar also needs to be revisited. For all that I recommend a major revision. Please find detailed comments below.
Line 26: gom (2015) is not referenced in bibliography. Please include the reference.
Line 31: replace “it” by “is”
Line 25: current, temperature, and salinity (use commas)
Lines 38-39: “towards the interior of the ocean” --> westwards
Line 53: msla should be uppercase as was defined the acronym.
Line 57: “the current direction at a point”. Why choose a single location for polar histogram of current? The authors should average over the entire region of interest or over a larger area since the current at a single location can be very variable, for example, could influenced by the presence of an eddy or meandering jets.
Line 61: The second eddy is located between LC and Punta Maldonado (PM), Guerrero.
Line 65: Please define “potential vortex”??
Line 78: replace “is” by “are”
Line 79: “rmax”. Is this rmax a radius as defined by AMEDA or the semi-major axis of the ellipse? Please clarify.
Line 80 and throughout the paper: what the authors mean by “nodes”? Are “locations”?
Line 81: “greater than 0.3”. What is the reasoning to choose 0.3m/s? This selection would widen or shorten the region driven by along-coast "unidirectional" current . Please clarify.
Line 83: “main axis” is the semi-major axis.
Line 87: “in front of the coast” refers to offshore?
Line 94: Is this a time series of complex number? Define i=sqrt(-1) and rewrite the equation w(t)=u(t)' + i*v(t)'.
Line 107: “shows” --> “show” and “more small” --> “smaller”
Line 113: “clockwise eddy” --> “Anticyclone”
Line 121-122: This tabulation for colors is bit confussing. Typically, red is used for (warm) anticyclones and blue for (cold) cyclones...but ok
Line 126: “locations where the coastline has a concavity” --> bays.
Line 135: “For the remaining months (January, February, …)”. I do not see comparable number of cyclonic and anticyclonic eddies. Note that in January and December there are 100/50 anticyclonic/cyclonic eddies (2 times more). Or in November 20/40 anticyclonic/cyclonic (2 times less) but not comparable. What I see is a clear relationship between poleward current and anticyclone generation, equatorward current and cyclone generation.
Line 144: “submesoscale eddies” --> You are not resolving the submesoscale eddies as observed in the PDF of the radius (Fig. 6a). There is a strong drop of the frequency below radius of 20km. This is because the resolution of the data is 25 x 25km. You need to use SWOT data (higher resolution ssh and geostrophic velocity data; however, the temporal resolution is of 15-20 days so not adequate for this kind of study). I would call mesoscale eddies.
Paragraph 156-161: Is this temporal behavior based on actual observations. The authors should compare with the temporal modulation of the satellite-derived geostrophic current.
Paragraph 163-171: The days selected in figures 8 and 9 are arbitrary. Why choose times where zero-acceleration? What occurs during acceleration or deceleration of the coastal current? Please discuss the differences between acceleration/deceleration and steady current (zero acceleration). Also, I am wondering whether this numerical analysis is showing the generation mechanism (?): (1) is the result of potential vorticity conservation for an equivalent barotropic flow? (2) is the result of propagation of low-frequency coastal waves, (3) current instabilities...The analysis of the terms of the vorticity equation would provide insights on generation mechanisms: vortex stretching, tilting, advection of vorticity, ...
Citation: https://doi.org/10.5194/egusphere-2024-3403-RC1 -
RC2: 'Comment on egusphere-2024-3403', Anonymous Referee #2, 17 Jan 2025
The article examines the Mexican coastal current, proposing a method to identify the current and its characteristics using variability ellipses and EOF analysis of satellite-derived velocity data. I was particularly interested in the dynamics of coastal eddies and their formation as presented in this work. The authors explore the mechanism of eddy formation through an idealized numerical model, highlighting how the interaction between the coastal current and concave sections of the coastline may generate eddies. While the model-based explanation might not be entirely comprehensive, it represents a promising start for further research. Overall, the article merits attention and could be published after certain revisions to clarify these points.
The article does not mention the role of baroclinic effects in either the formation of the current or the generation of eddies. I would have liked to see a discussion in the paper addressing this aspect—clarifying whether these effects are insignificant or, on the contrary, important should be noted. Additionally, the article does not mention tides at all. As far as I understand, the role of tides in this region may not be strong; however, they could play an important role in the mean current and in the formation of eddies.
I would appreciate more detailed explanations regarding the model domain and boundary conditions. The coastline is oriented at an angle (not strictly west–east), and the model domain shown in Figure 7 does not appear to be rotated. If the domain is indeed not rotated, the setup of the boundaries is somewhat clear, but it remains unclear how the Coriolis force was accounted for under these conditions. On the other hand, if the domain has been rotated, additional explanation is needed on how the open boundary conditions were specified to accommodate this rotation.
In Section 5, it is unclear whether eddies actually form in the model or if the discussion only points to the preconditions for their formation. If the model only suggests the possibility of eddy formation without actually resolving them, this should be clearly reflected and explained in the text. Given a model resolution of 2 km, one would reasonably expect to resolve eddy dynamics for eddies around 30 km in size. If eddies do not form in the model, a detailed explanation should be provided. Without such clarification, the conclusion “In these areas, the current separates from the coast, creating recirculation zones that become eddies.” appears speculative. I can only speculate that the very formulation of the modeling problem may not allow for the further development of vortex structures due to the characteristics of the idealized flow set by the boundary conditions or other model parameters. In my opinion, under such a formulation, the article can be published in its current version; however, this limitation should be clearly stated in the text and reflected in the conclusions.
Minor Points:
In the abstract, you mention that you demonstrated the presence and formation of eddies in the coastal zone using a model. However, throughout the article, some conclusions are based on assumptions derived partly from indirect observational data. It might be beneficial to strengthen the conclusions in the abstract by indicating that the findings are supported not only by the model but also by observations. This would provide a more comprehensive overview of the evidence backing your conclusions.
Line 96 and other places: “gom (2015)” is not in the reference list.
Line 108: Please extend with one sentence on what eddy formation has been described by Zamudio (2001).
Figure 4: Missing colorbar. Add to the caption what the eddies’ colors mean (red for counterclockwise rotation and blue for clockwise). Also extend the caption with the time periods used for averaging (years 1993 to 2021).
Figure 5: “frecuency” → “frequency”.
Line 153: There is an inaccuracy in terminology. For a clearer description, it should be specified that this refers to open boundary conditions. Since there are three open boundaries, it is necessary to clarify how the velocities were set on all boundaries.
Citation: https://doi.org/10.5194/egusphere-2024-3403-RC2
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