the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 24 Jun 2021
Biological response to wind and terrestrial nitrate in the western and southern continental shelves of the Gulf of Mexico
Abstract. In Mexico, 16 rivers directly discharge into the Gulf of Mexico. The Mexican rivers and those coming from the United States generate large regions in which phytoplanktonic primary production possesses a seasonal component that is linked to these nutrient-rich freshwater inputs. In the present study, new flow and daily nutrient data were obtained for the largest Mexican rivers. These data were integrated as forcing factors in a configuration of the hydrodynamic Coastal and Regional Ocean COmmunity model coupled to an N2PZD2 biogeochemical model. To correctly represent biological processes in coastal regions, a biological bottom condition was implemented in the biogeochemical model. With this condition, it was possible to represent remineralization on the continental shelf of the Gulf of Mexico. We present a 21 year simulation using two different configurations. The first included river forcing, and the second did not consider their influence. The results were validated with satellite images of the surface concentration of chlorophyll and compared with data from previous studies. The coupled model was able to realistically reproduce the seasonal dynamics of primary production in the Gulf of Mexico based on the concentration and distribution of chlorophyll, both at the surface and in the water column. Finally, the physical processes that influence the dynamics of primary production in the deep region and continental shelf of the gulf were defined. In the deep region, primary production was dominated by vertical mixing induced by the passage of cold fronts during winter and mesoscale structures. On the continental shelf, such dynamics were dominated by coastal upwelling and fluvial nutrient contributions.
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Interactive discussion
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RC1: 'Comment on bg-2021-142', Anonymous Referee #1, 21 Jul 2021
Biological response to wind and terrestrial nitrate in the western and southern continental shelves of the Gulf of Mexico
Javier González-Ramírez et al.
General Comments
In this work, the authors coupled the CROCO model to a biogeochemical model to describe the physical processes that influence the dynamics of phytoplanktonic primary production in the Gulf of Mexico. To correctly represent biological processes in coastal regions of the Gulf of Mexico, the authors implemented a biological boundary condition at the bottom. They ran two configurations of the coupled model: “rivers off” and “rivers on.” The authors depicted the main underlying mechanisms (annual cycle of wind, nutrients contributions from rivers, among others) associated with the primary production dynamics in coastal regions and the deeper region of the Gulf of Mexico. The paper is interesting and deserves eventual publication on BG. However, before publication, there are some questions and clarifications that, in my opinion, have to be addressed.
Introduction section
1. L19. Authors defined the Gulf of Mexico as GoM, but in several parts of the text, they refer to the Gulf of Mexico as “gulf,” which is not previously defined. The authors should use the term GoM instead of the “gulf,” or they can clarify it in this way: Gulf of Mexico (GoM or gulf hereafter).
2. L20. Caption Figure 1: Change “The gray areas were …” to “The areas within gray boxes were ...”
3. L26. Change “Gulf of Mexico” to “GoM”
4. L39-40. To help the reader, the authors should mention the main findings or conclusions reported by González-Ramírez and Parés-Sierra (2019).
Model configuration section
5. L73-74. In Figures 2 and 3, it seems the Grijalva-Usumacinta is one river, but in line 74, it seems they are two different rivers (Grijalva and Usumacinta). In addition to the complete time series in Figures 2 and 3, the authors could add another subplot in each figure showing the daily annual cycle.
6. Figure 3. Why is only the Grijalva-Usumacinta river shown? Correct the spelling for Usumacinta.
Results section
7. L80-87. Why do the authors only show one configuration of the model in Figure 4? Is “rivers off” or “rivers on”? Please add the difference between the model and observations. The authors should show the observations in the first row then the simulations.
8. L85-87. Please give more analysis or citations to support what you said in these lines?
9. There is no mention of Figure 5 in the manuscript. Please add both configurations of the model in Figure 5. Are the correlations statistically significant? What is the confidence interval for r?
10. L118-124. The criterion used to locate the passage of anticyclonic eddies and identify the eddy type is not clear. If there are two types of eddies, please use a different color for each kind in Figure 6. Is the simulation shown in Figure 6 “river on” or “rivers off”? Why do the authors show only one configuration of the model? It is important to clarify this.
11. L128. To avoid confusion, please change: “… to the continental TAVE (Fig. 1, a) and BOC (Fig. 1, b) shelves.” to “… to the continental TAVE and BOC shelves (see Fig. 1).”
12. L128-130. More analysis is needed to see the differences between both configurations of the model. Please add the mean, the standard deviation of chl for each simulation, and the correlation between simulated chl and the corresponding wind component in each region.
13. L138-140. Is the simulated surface chlorophyll concentration [c(t)] from the “rivers off” or “rivers on” configuration? The authors could mention this in the manuscript or the caption of Figure 8.
14. L152-154. To support this idea, add some references.
15. L174. Please correct the caption of Figure 10. There is no left or right in the top panel.
16. What is sigma in Figures 9 and 10?
17. L175-176. To support these lines, do the correlation between the simulated first principal component and wind anomalies; also, authors must compare those results with the model configuration “rivers off.”
Discussion section.
18. L195-220. Please give more citations to support these paragraphs. If possible, discuss these findings with previous work for different regions in Mexico or the world; otherwise, the authors could highlight this novelty of their work.
19. Figures 11 and 12 are not mentioned in the manuscript.
20. Revise all the captions of figures with more details.
Conclusions section.
21. I believe the authors must show results (Coherence, EOFs, etcetera) from both model configurations, mainly in the TAVE and BOC regions. Since one conclusion of this work is that the nutrients provided by local rivers dominated the variation in the chlorophyll concentration over these regions.
Citation: https://doi.org/10.5194/bg-2021-142-RC1 -
RC2: 'Comment on bg-2021-142', Anonymous Referee #2, 24 Jul 2021
The manuscript submitted by González-Ramírez et al. deals with the biological response to wind and river forcing in the western and southern shelves of the Gulf of Mexico. It is based on 21 year numerical simulations using a couped physical-biogeochemical model. I found that the manuscript is globally well organized but would benefit an extensive round of edits. Some portions are quite confused and have to be improved or removed.
The river forcing published by the authors in a previous paper and used in this modeling study constitutes the most innovative part of the manuscript. The lack of published river data was a major limitation for biogeochemical modeling studies on the western and southern shelves. However, there are a number of issues, in particular in the numerical setup, that need to be addressed.
The first major concern is the biogeochemical model used and its description in section 2.2.
The first point is the bottom boundary condition implemented in the biogeochemical model described in Powell et al., (2006) and supposed to mimic remineralization processes in the sediment. The equation (1) implemented into the model is clearly wrong. A simple units check reveals it (left hand side in mol.m-3.s-1 and right hand side in mol.m-3). At least some sinking velocities and stochiometric ratios are missing. This error likely have serious impacts on the modeling results presented in the manuscript, especially on the shelves where the water column is relatively shallow.
The authors should also revised eq. (2). To which time the superscripts on the right hand side refer?
The physical model CROCO is well described and extensively used in a number of studies, but the biogeochemical model requires a more detailed description. Compared to the model described in Powell et al., (2006), the authors are using an additional nutrient and an additional detritus compartment. Could the author detail more why they found necessary to add a detritus group? How the mortality and grazing rates are routed into LDet or SDet? What are the sinking velocities used? The model is relatively simple and the parameters used could be reported in a small table, as table 1 in Powell et al. (2006). Moreover, comparing the equations of phytoplankton in Powell et al. (2006), eq. 2, and in Fennel at al., (2006), eq. 1, I note that phytoplankton does not have a vertical sinking velocity. In consequence, can the authors justify why they add phytoplankton in the remineralization from sediment in eq 1?
The second major concern deals with the validation of the circulation on the western and southern shelves. The authors say that the model manage to reproduce the circulation of the GoM, namely the loop current, loop current eddies, the cyclonic circulation in the Bay of Campeche and the circulation on the shelves (l. 80-84). Since the ocean circulation is an important driver of biology, it is necessary to prove it by showing some figures.
Considering the error in the biological model, it is difficult to trust the results exposed in the manuscript and propose a constructive review. I would suggest that the authors first revise this major issue and update their results. Here are a list of more minor comments and suggestions to help the author in this process:
- l.9: The authors performed a set of two runs, one with rivers forcing and one without. In my mind, this method using the river data already published is the stronger point and the most innovative part of the manuscript. It can provide solid argument on the river influence on the GoM biogeochemical cycles. I would suggest to emphasize more the method and propose some figures showing the difference between the two runs (the difference in chlorophyll at surface for example).
- l.27: How the advection of low salinity water is associated with winds?
- l.30-31 : “the water is advected over the continental shelf from the TAVE to LATEX region” but these two regions are both part of the shelf. I do not understand this sentence.
- l. 48 : the model has a horizontal resolution of 1/20 degree, which might be around 5 km in this region. However the shelf is particularly narrow (is it well resolved?) and the circulation on the shelf is often associated with small scale currents, especially in the presence of strong buoyancy gradients due to river plumes. The author should discuss which physical processes they expect to resolve.
- l.48 : Are the vertical levels equally distributed or is a stretching function used ?
- l. 53 : Why choosing 2 different set of atmospheric data with different temporal resolution to force the model? The authors are using monthly climatologies for heat and salt fluxes and a product at 6h time step for the wind stress.
- l. 78 : How the others biogeochemical variables are initialized ?
- l. 86 : I cannot see the eddy in the BOC region on figure 4.
- l. 91 : the sentences should be conjugated in the same the tenses.
- l. 91-95 : Where can I see this on the figures ?
- Figure 4: some biaises between model and observations need to be discussed. The model shows much more chlorophyll at the north of Yucatan. Is it due to an intense upwelling circulation lasting throughout the year? Then, the model show very low chlorophyll concentration on the shelf located at the west of Yucatan while satellite images reveal relatively high concentration.
- section 3.2 : I do not really understand why the authors discuss the passage of Loop current eddies. This is quite out of the scope of this manuscript. I appreciate the validation of the vertical chlorophyll distribution but I do not think figure 6 is well suited for this purpose. Why not showing mean seasonal profiles and compared them with observations ?
Citation: https://doi.org/10.5194/bg-2021-142-RC2
Interactive discussion
Status: closed
-
RC1: 'Comment on bg-2021-142', Anonymous Referee #1, 21 Jul 2021
Biological response to wind and terrestrial nitrate in the western and southern continental shelves of the Gulf of Mexico
Javier González-Ramírez et al.
General Comments
In this work, the authors coupled the CROCO model to a biogeochemical model to describe the physical processes that influence the dynamics of phytoplanktonic primary production in the Gulf of Mexico. To correctly represent biological processes in coastal regions of the Gulf of Mexico, the authors implemented a biological boundary condition at the bottom. They ran two configurations of the coupled model: “rivers off” and “rivers on.” The authors depicted the main underlying mechanisms (annual cycle of wind, nutrients contributions from rivers, among others) associated with the primary production dynamics in coastal regions and the deeper region of the Gulf of Mexico. The paper is interesting and deserves eventual publication on BG. However, before publication, there are some questions and clarifications that, in my opinion, have to be addressed.
Introduction section
1. L19. Authors defined the Gulf of Mexico as GoM, but in several parts of the text, they refer to the Gulf of Mexico as “gulf,” which is not previously defined. The authors should use the term GoM instead of the “gulf,” or they can clarify it in this way: Gulf of Mexico (GoM or gulf hereafter).
2. L20. Caption Figure 1: Change “The gray areas were …” to “The areas within gray boxes were ...”
3. L26. Change “Gulf of Mexico” to “GoM”
4. L39-40. To help the reader, the authors should mention the main findings or conclusions reported by González-Ramírez and Parés-Sierra (2019).
Model configuration section
5. L73-74. In Figures 2 and 3, it seems the Grijalva-Usumacinta is one river, but in line 74, it seems they are two different rivers (Grijalva and Usumacinta). In addition to the complete time series in Figures 2 and 3, the authors could add another subplot in each figure showing the daily annual cycle.
6. Figure 3. Why is only the Grijalva-Usumacinta river shown? Correct the spelling for Usumacinta.
Results section
7. L80-87. Why do the authors only show one configuration of the model in Figure 4? Is “rivers off” or “rivers on”? Please add the difference between the model and observations. The authors should show the observations in the first row then the simulations.
8. L85-87. Please give more analysis or citations to support what you said in these lines?
9. There is no mention of Figure 5 in the manuscript. Please add both configurations of the model in Figure 5. Are the correlations statistically significant? What is the confidence interval for r?
10. L118-124. The criterion used to locate the passage of anticyclonic eddies and identify the eddy type is not clear. If there are two types of eddies, please use a different color for each kind in Figure 6. Is the simulation shown in Figure 6 “river on” or “rivers off”? Why do the authors show only one configuration of the model? It is important to clarify this.
11. L128. To avoid confusion, please change: “… to the continental TAVE (Fig. 1, a) and BOC (Fig. 1, b) shelves.” to “… to the continental TAVE and BOC shelves (see Fig. 1).”
12. L128-130. More analysis is needed to see the differences between both configurations of the model. Please add the mean, the standard deviation of chl for each simulation, and the correlation between simulated chl and the corresponding wind component in each region.
13. L138-140. Is the simulated surface chlorophyll concentration [c(t)] from the “rivers off” or “rivers on” configuration? The authors could mention this in the manuscript or the caption of Figure 8.
14. L152-154. To support this idea, add some references.
15. L174. Please correct the caption of Figure 10. There is no left or right in the top panel.
16. What is sigma in Figures 9 and 10?
17. L175-176. To support these lines, do the correlation between the simulated first principal component and wind anomalies; also, authors must compare those results with the model configuration “rivers off.”
Discussion section.
18. L195-220. Please give more citations to support these paragraphs. If possible, discuss these findings with previous work for different regions in Mexico or the world; otherwise, the authors could highlight this novelty of their work.
19. Figures 11 and 12 are not mentioned in the manuscript.
20. Revise all the captions of figures with more details.
Conclusions section.
21. I believe the authors must show results (Coherence, EOFs, etcetera) from both model configurations, mainly in the TAVE and BOC regions. Since one conclusion of this work is that the nutrients provided by local rivers dominated the variation in the chlorophyll concentration over these regions.
Citation: https://doi.org/10.5194/bg-2021-142-RC1 -
RC2: 'Comment on bg-2021-142', Anonymous Referee #2, 24 Jul 2021
The manuscript submitted by González-Ramírez et al. deals with the biological response to wind and river forcing in the western and southern shelves of the Gulf of Mexico. It is based on 21 year numerical simulations using a couped physical-biogeochemical model. I found that the manuscript is globally well organized but would benefit an extensive round of edits. Some portions are quite confused and have to be improved or removed.
The river forcing published by the authors in a previous paper and used in this modeling study constitutes the most innovative part of the manuscript. The lack of published river data was a major limitation for biogeochemical modeling studies on the western and southern shelves. However, there are a number of issues, in particular in the numerical setup, that need to be addressed.
The first major concern is the biogeochemical model used and its description in section 2.2.
The first point is the bottom boundary condition implemented in the biogeochemical model described in Powell et al., (2006) and supposed to mimic remineralization processes in the sediment. The equation (1) implemented into the model is clearly wrong. A simple units check reveals it (left hand side in mol.m-3.s-1 and right hand side in mol.m-3). At least some sinking velocities and stochiometric ratios are missing. This error likely have serious impacts on the modeling results presented in the manuscript, especially on the shelves where the water column is relatively shallow.
The authors should also revised eq. (2). To which time the superscripts on the right hand side refer?
The physical model CROCO is well described and extensively used in a number of studies, but the biogeochemical model requires a more detailed description. Compared to the model described in Powell et al., (2006), the authors are using an additional nutrient and an additional detritus compartment. Could the author detail more why they found necessary to add a detritus group? How the mortality and grazing rates are routed into LDet or SDet? What are the sinking velocities used? The model is relatively simple and the parameters used could be reported in a small table, as table 1 in Powell et al. (2006). Moreover, comparing the equations of phytoplankton in Powell et al. (2006), eq. 2, and in Fennel at al., (2006), eq. 1, I note that phytoplankton does not have a vertical sinking velocity. In consequence, can the authors justify why they add phytoplankton in the remineralization from sediment in eq 1?
The second major concern deals with the validation of the circulation on the western and southern shelves. The authors say that the model manage to reproduce the circulation of the GoM, namely the loop current, loop current eddies, the cyclonic circulation in the Bay of Campeche and the circulation on the shelves (l. 80-84). Since the ocean circulation is an important driver of biology, it is necessary to prove it by showing some figures.
Considering the error in the biological model, it is difficult to trust the results exposed in the manuscript and propose a constructive review. I would suggest that the authors first revise this major issue and update their results. Here are a list of more minor comments and suggestions to help the author in this process:
- l.9: The authors performed a set of two runs, one with rivers forcing and one without. In my mind, this method using the river data already published is the stronger point and the most innovative part of the manuscript. It can provide solid argument on the river influence on the GoM biogeochemical cycles. I would suggest to emphasize more the method and propose some figures showing the difference between the two runs (the difference in chlorophyll at surface for example).
- l.27: How the advection of low salinity water is associated with winds?
- l.30-31 : “the water is advected over the continental shelf from the TAVE to LATEX region” but these two regions are both part of the shelf. I do not understand this sentence.
- l. 48 : the model has a horizontal resolution of 1/20 degree, which might be around 5 km in this region. However the shelf is particularly narrow (is it well resolved?) and the circulation on the shelf is often associated with small scale currents, especially in the presence of strong buoyancy gradients due to river plumes. The author should discuss which physical processes they expect to resolve.
- l.48 : Are the vertical levels equally distributed or is a stretching function used ?
- l. 53 : Why choosing 2 different set of atmospheric data with different temporal resolution to force the model? The authors are using monthly climatologies for heat and salt fluxes and a product at 6h time step for the wind stress.
- l. 78 : How the others biogeochemical variables are initialized ?
- l. 86 : I cannot see the eddy in the BOC region on figure 4.
- l. 91 : the sentences should be conjugated in the same the tenses.
- l. 91-95 : Where can I see this on the figures ?
- Figure 4: some biaises between model and observations need to be discussed. The model shows much more chlorophyll at the north of Yucatan. Is it due to an intense upwelling circulation lasting throughout the year? Then, the model show very low chlorophyll concentration on the shelf located at the west of Yucatan while satellite images reveal relatively high concentration.
- section 3.2 : I do not really understand why the authors discuss the passage of Loop current eddies. This is quite out of the scope of this manuscript. I appreciate the validation of the vertical chlorophyll distribution but I do not think figure 6 is well suited for this purpose. Why not showing mean seasonal profiles and compared them with observations ?
Citation: https://doi.org/10.5194/bg-2021-142-RC2
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