the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Physical processes in the upwelling regions of the tropical Atlantic
Gaël Alory
Founi Mesmin Awo
Marcus Dengler
Sandrine Djakouré
Rodrigue Anicet Imbol Koungue
Julien Jouanno
Mareike Körner
Marisa Roch
Mathieu Rouault
Abstract. In this paper, we review observational and modelling results on the upwelling in the inner tropical Atlantic. We focus on the physical processes that drive the seasonal variability of surface cooling and upward nutrient flux required to explain the seasonality of primary productivity. We separately consider the equatorial upwelling system, the northern coastal upwelling system of the Gulf of Guinea and the tropical Angolan upwelling system. For the equatorial regime, we discuss the forcing of upwelling velocity and turbulent mixing as well as the underlying dynamics responsible for thermocline movements and current structure. The coastal upwelling system in the Gulf of Guinea is concentrated along northern boundary and is driven by both, local and remote forcing. The particular role of the Guinea Current, nonlinearity and the shape of the coastline are emphasized. For the tropical Angolan upwelling, we show that this system is not wind-driven, but instead results from the combined effect of coastally trapped waves, surface heat and freshwater fluxes, and turbulent mixing. Finally, we review recent changes in the upwelling systems associated with climate variability and global warming and address possible responses of upwelling systems in future scenarios.
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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
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Preprint
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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
- Preprint
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- Final revised paper
Journal article(s) based on this preprint
Peter Brandt et al.
Interactive discussion
Status: closed
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RC1: 'Comment on egusphere-2022-1354', Anonymous Referee #1, 13 Jan 2023
General comments:
This is an interesting, extensive, and relevant compilation of knowledge about (upwelling in) the tropical Atlantic climate system. I find the introduction to be a bit disconnected which at times makes it hard to follow, the reader could be helped by working on the flow of the text and explaining why specific parts of the system are being introduced. Additionally, later on in the text statements are made that are not easily verifiable by the reader, e.g. wrt figure 3 and 4 and 5. More direction as to where the reader should focus and more explanation in the text would be helpful, especially to make this work accessible to a wider audience than the established tropical Atlantic community. Similarly the text at times mentions specific terms without either showing the equations or explaining what the terms represent. Either would be helpful here for completeness. Specific incidents are indicated below.
I recommend adding a connection to primary productivity / nutrient supply to the title, since it is discussed a lot in the text.
Specific comments
L50: add citation Yang, Yun, et al. "Suppressed Atlantic Niño/Niña variability under greenhouse warming." Nature Climate Change 12.9 (2022): 814-821.
Ll 80-85 The areas GGUS tAUS and EAUS should be indicated in the figure, the focus here is said to be on inner upwelling and not coastal upwelling. Indication of the region would help the reader understand which areas are being discussed and which are not.
Ll 88- 103 The discussion on where the water masses are coming from and going to : please add a sentence or two to the relevance of this discussion, potentially relating to primary productivity, OMZ, etc. Since this also is discussed in the individual sections (e.g. Ll243-245) this discussion here might be removed in favor of flow of the paper.
L 104: connection between upwelling and ITCZ unclear, I recommend mentioning the relevance of this section to the upwelling in the tropical Atlantic in the beginning of this section
Ll 133-135 please mark the upwelling favoring easterly winds in the figure, it is hard to see the variability in strength from Figure 1. When the winds are purely easterly they seem very weak (compared to September / October), should the reader focus solely on the region west of 40W? Please indicate in the figure and / or describe in the text.
Ll 138-140 Is the region between Cuanza and Kunene meant here with particularly weak winds? The winds further south seem strong. Please indicate maybe with a different colour the arrows of the area under discussion, and / or add more description to the text.
L 144 I am confused by the mention of inner tropical Atlantic upwelling, while the GGUS seems to follow the coast, similar to the tAUS. Again, indication of the areas in Fig 1 would help, and maybe a sentence on the differentiation between inner and coastal upwelling as it is used in this study.
Ll 174-176 Which part of Figure 5 is indicated here? Maybe an extra figure with Thermocline movement in conjunction with temperature change should be shown here. Or alternatively Fig 3 / 4 are meant, using the SSH as proxy for thermocline? Related: the Figure caption of Figure 5 should mention the source of the data as do the other figures
Ll 188-201 Since the focus of this paper is seasonal variability, a note on what we (do not) know about the variability of the STCs and TCs as they relate to equatorial upwelling would be helpful.
L 200-202 “The different forcing terms..” it seems odd that in a review of the forcing terms of the tropical Atlantic upwelling the physical processes forcing the upwelling are only mentioned in a short sentence with a reference. I recommend expanding on this sentence and ideally drawing a connection to the next paragraph, turbulent mixing. Alternatively, a differentiation between the current work and Giordani and Caniaux 2011 would be helpful. Also since this review is on upwelling (and its impact on nutrient availability and primary production) the connection between upwelling and mixing could be explained.
L 223 Fig 3 or 4 can be referenced in addition to figure 5 since they show the surface and 5 the column, might be more intuitive for the reader
Ll223 Radenac reference, later on it is stated that the authors analysis PIRATA and models, please specify which dataset these results are based on as done later in e.g. L 241, 245
L 255 Fig 5 ; EUC and 20C are shown in all panels
L 277 December maximum is not clear in Fig 5d, looks similar throughout September - January. Fig 5c shows vertical advection maximum in November, how does this relate?
L 290 again confusion about inner vs coastal upwelling, explicit mention of coastal upwelling here (and throughout the text)
Ll 296-297 I suggest indicating the cells in Figure 1
Ll 313 “associated to the non-linear dynamics and its detachement..” Please add (half) a sentence on how this influences the upwelling
Ll 325 what do these non-linear terms represent? In this overview being more specific about the physical process would be helpful
Ll 331-332 this is a bit more explicit “when the nonlinear terms are removed and the Guinea Current is trapped” but more explanation would again be helpful. Since this paper summarizes the physical processes behind upwelling it should be explicit about these processes.
Ll 333-343 The discussion about the thermocline being closed to the surface in the simulation with least upwelling is difficult to follow. Earlier upwelling and upward movement of the thermocline have been positively correlated, how do they relate here? Seemingly the thermocline is shallower in the western upwelling cell while that cell has less upwelling (than the east), isn’t this counterintuitive?
L 358 “that is mostly wind driven” can this be seen in Fig 3? It would be good to refer back to the (relevant section of that) figure
L 397 again please indicate the tAUS in Fig 1
L 407 “are generally weak throughout the year” makes me think that it would also be good to indicate the tAUS region in Fig 4 or highlight the arrows in a different color (color coding arrows per upwelling zone might be a really good idea)
Ll 415-416 “..four remotely forced CTWs throughout the year (Fig 4b)” can these be indicated in the figure, as arrows or similar
Ll 448-449 Indicate tAUS in figure 2? Is the very very narrow coastal strip e.g. in 4b meant here, or solely Fig 4c where the colder coastal SSTs seem more obvious? Again how do the authors distinguish between coastal and interior upwelling?
Ll 451-455 description of coastal upwelling? It seems that the word inner in the beginning should be omitted or well defined.
Ll 489-490 “the spatially-averaged generation” of turbulence?
L 493 also evident in Figure 10?
L 496-497 related to increased mixing?
Ll 502-504 what is the causal relationship here? More mixing = more cooling and therefore less stratification, but here the argument seems to be more mixing => less stratification => more cooling, can you be more explicit about the suggested series of events?
L 514 suggest removing “it is”
L 522 additional forcing
Ll 522-524 this causality is not clear, please clarify
L538 what is the timescale of the AMOC weakening? Decadal?
L 540 add citation same as above Yun Yang
Ll 544 “or productivity” maybe better to phrase “also indicated by trends in productivity”
L 556 Please remind the reader how the influence of Ekman transport fits in with the seasonal modulation
L 586 what does inner mean here
Minor:
Some inconsistencies with the plural and singular in the text, e.g. Ll 241-242 ..waters… has ..
Citation: https://doi.org/10.5194/egusphere-2022-1354-RC1 - AC1: 'Reply on RC1', Peter Brandt, 28 Mar 2023
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RC2: 'Comment on egusphere-2022-1354', Erik van Sebille, 28 Feb 2023
Full disclosure: I am writing this review as Topic Editor for this manuscript. It has proven very difficult to find a second independent and knowledgeable reviewer for this manuscript, as most experts are already involved in the manuscript as authors. After discussions with two of the Chief Editors, I decided to write this second review as Topic Editor; focussing mostly on the effectiveness of the manuscript as a review article - Erik van Sebille
The topic of the manuscript (physical processes in the upwelling regions of the tropical Atlantic) surely warrants an in-depth review, and Ocean Science is an appropriate journal for it. The authors are from a wide and diverse range of institutes and expertises. This is all very strong and supports the trust in the review as unbiased and complete.
However, I have a few suggestions that I think will make the manuscript stronger and more impactful as a review article:
1. I encourage the authors to think about a schematic or figure that summarizes the key processes that are discussed in the paper. Such a figure might be hugely impactful, as it illustrates what the review is about2. The meridional extent of the region is never defined. While the zonal extent is discussed (line 54 and further), the meridional extend stays vague. Is it the 20S to 10N of figure 1?
3. Some of the modeling results (e.g. Figs 7, 8, 9) miss a clear reference. It is unclear now whether these figures have been created specifically for this manuscript, or come from another paper. In the first case, there should be much more discussion of the model setup etc (and it might be doubtful whether new/unpublished results fit in a review article like this); in the second case the captions need a clear reference.
4. Section 5 misses figures. Whereas the other sections all have figures, this doesn't. Is that intentional? More generally, the style of the different sections varies quite a bit. I realize that each co-author was probably responsible for leading one section, but I would then still recommend carefully going through the text (and figures) to harmonize the style. That will help readers, and thus increase impact.
5. The manuscript tends to be fairly descriptive (answering what is happening) and relatively low on (physical) explanation. For example, line 601 states that "The zonal velocity field instead is dominated by the equatorial basin resonance of the 2nd and 4th baroclinic modes resulting in an EUC that vertically migrate largely independent of the thermocline (Brandt et al. 2016)" This sentence does not provide much information on _why_ it are the 2nd and 4th modes that are important here. That information may be in the reference, but the purpose of a review article is also to provide an accessible overview of the state-of-the-art; in this case (because of the title) also in terms of physical processes.
6. I somewhat missed a discussion of the similarities and differences with upwelling regions of the tropical Pacific and Indian Ocean. While the uniqueness of the basin is described well in the introduction; the extend to which the physical processes differ or agree in the other two basins is not very well discussed. I would expect that to be another purpose of such a review article?
Minor comments:
- line 14: 'inner' is a strange word here. 'Offshore'?
- line 21: misses 'the' before 'northern boundary'
- line 22: remove comma after both
- line 22: nonlinearity in what?
- Figure 1: I don't understand the units of mg m-3. Until what depth is this then? Would it not make more sense as mg m-2?
- line 82: the capitalization rules between tAUS and EAUS are not clear.
- line 145: is the ecosystem indeed nitrate-limited? Would be good to provide a reference for that implicit assumption
- Figure 3: why not use the same time period for all four datasets. Most of the period overlaps, but the start and end years are not consistent. It would be stronger to harmonize that
- line 194: is this the mean in time? Or the mean in space?
- line 203: 'helps to define' is vague wording here.
- line 210: 'seasonally varying'
- line 231-234: so what is the conclusion then?
- line 268: why use the fourth baroclinic mode? What is so special about the fourth one?
- line 309: which capes are meant with 'the capes'?
- line 329-330: the 25C threshold for upwelling seems fairly arbitrary. The paper that it is based on in almost 50 years old. Has no newer research been done on this?
- line 487: what type of model was used by Zeng et al (2021)? A bit more description might help readers gauge the applicability of these conclusions
- line 513: this first sentence of section 5 is very vague. Which part of the upwelling? Variability of what?What is meant with 'It is the area and season most impacted [...]'?
- line 519: 'boreal summer' is a biased and confusing term. While not simply write the names of the months?
- line 543: what is meant with 'first' here?
- line 575: also provide a reference for the event in 2021?Citation: https://doi.org/10.5194/egusphere-2022-1354-RC2 - AC2: 'Reply on RC2', Peter Brandt, 28 Mar 2023
Interactive discussion
Status: closed
-
RC1: 'Comment on egusphere-2022-1354', Anonymous Referee #1, 13 Jan 2023
General comments:
This is an interesting, extensive, and relevant compilation of knowledge about (upwelling in) the tropical Atlantic climate system. I find the introduction to be a bit disconnected which at times makes it hard to follow, the reader could be helped by working on the flow of the text and explaining why specific parts of the system are being introduced. Additionally, later on in the text statements are made that are not easily verifiable by the reader, e.g. wrt figure 3 and 4 and 5. More direction as to where the reader should focus and more explanation in the text would be helpful, especially to make this work accessible to a wider audience than the established tropical Atlantic community. Similarly the text at times mentions specific terms without either showing the equations or explaining what the terms represent. Either would be helpful here for completeness. Specific incidents are indicated below.
I recommend adding a connection to primary productivity / nutrient supply to the title, since it is discussed a lot in the text.
Specific comments
L50: add citation Yang, Yun, et al. "Suppressed Atlantic Niño/Niña variability under greenhouse warming." Nature Climate Change 12.9 (2022): 814-821.
Ll 80-85 The areas GGUS tAUS and EAUS should be indicated in the figure, the focus here is said to be on inner upwelling and not coastal upwelling. Indication of the region would help the reader understand which areas are being discussed and which are not.
Ll 88- 103 The discussion on where the water masses are coming from and going to : please add a sentence or two to the relevance of this discussion, potentially relating to primary productivity, OMZ, etc. Since this also is discussed in the individual sections (e.g. Ll243-245) this discussion here might be removed in favor of flow of the paper.
L 104: connection between upwelling and ITCZ unclear, I recommend mentioning the relevance of this section to the upwelling in the tropical Atlantic in the beginning of this section
Ll 133-135 please mark the upwelling favoring easterly winds in the figure, it is hard to see the variability in strength from Figure 1. When the winds are purely easterly they seem very weak (compared to September / October), should the reader focus solely on the region west of 40W? Please indicate in the figure and / or describe in the text.
Ll 138-140 Is the region between Cuanza and Kunene meant here with particularly weak winds? The winds further south seem strong. Please indicate maybe with a different colour the arrows of the area under discussion, and / or add more description to the text.
L 144 I am confused by the mention of inner tropical Atlantic upwelling, while the GGUS seems to follow the coast, similar to the tAUS. Again, indication of the areas in Fig 1 would help, and maybe a sentence on the differentiation between inner and coastal upwelling as it is used in this study.
Ll 174-176 Which part of Figure 5 is indicated here? Maybe an extra figure with Thermocline movement in conjunction with temperature change should be shown here. Or alternatively Fig 3 / 4 are meant, using the SSH as proxy for thermocline? Related: the Figure caption of Figure 5 should mention the source of the data as do the other figures
Ll 188-201 Since the focus of this paper is seasonal variability, a note on what we (do not) know about the variability of the STCs and TCs as they relate to equatorial upwelling would be helpful.
L 200-202 “The different forcing terms..” it seems odd that in a review of the forcing terms of the tropical Atlantic upwelling the physical processes forcing the upwelling are only mentioned in a short sentence with a reference. I recommend expanding on this sentence and ideally drawing a connection to the next paragraph, turbulent mixing. Alternatively, a differentiation between the current work and Giordani and Caniaux 2011 would be helpful. Also since this review is on upwelling (and its impact on nutrient availability and primary production) the connection between upwelling and mixing could be explained.
L 223 Fig 3 or 4 can be referenced in addition to figure 5 since they show the surface and 5 the column, might be more intuitive for the reader
Ll223 Radenac reference, later on it is stated that the authors analysis PIRATA and models, please specify which dataset these results are based on as done later in e.g. L 241, 245
L 255 Fig 5 ; EUC and 20C are shown in all panels
L 277 December maximum is not clear in Fig 5d, looks similar throughout September - January. Fig 5c shows vertical advection maximum in November, how does this relate?
L 290 again confusion about inner vs coastal upwelling, explicit mention of coastal upwelling here (and throughout the text)
Ll 296-297 I suggest indicating the cells in Figure 1
Ll 313 “associated to the non-linear dynamics and its detachement..” Please add (half) a sentence on how this influences the upwelling
Ll 325 what do these non-linear terms represent? In this overview being more specific about the physical process would be helpful
Ll 331-332 this is a bit more explicit “when the nonlinear terms are removed and the Guinea Current is trapped” but more explanation would again be helpful. Since this paper summarizes the physical processes behind upwelling it should be explicit about these processes.
Ll 333-343 The discussion about the thermocline being closed to the surface in the simulation with least upwelling is difficult to follow. Earlier upwelling and upward movement of the thermocline have been positively correlated, how do they relate here? Seemingly the thermocline is shallower in the western upwelling cell while that cell has less upwelling (than the east), isn’t this counterintuitive?
L 358 “that is mostly wind driven” can this be seen in Fig 3? It would be good to refer back to the (relevant section of that) figure
L 397 again please indicate the tAUS in Fig 1
L 407 “are generally weak throughout the year” makes me think that it would also be good to indicate the tAUS region in Fig 4 or highlight the arrows in a different color (color coding arrows per upwelling zone might be a really good idea)
Ll 415-416 “..four remotely forced CTWs throughout the year (Fig 4b)” can these be indicated in the figure, as arrows or similar
Ll 448-449 Indicate tAUS in figure 2? Is the very very narrow coastal strip e.g. in 4b meant here, or solely Fig 4c where the colder coastal SSTs seem more obvious? Again how do the authors distinguish between coastal and interior upwelling?
Ll 451-455 description of coastal upwelling? It seems that the word inner in the beginning should be omitted or well defined.
Ll 489-490 “the spatially-averaged generation” of turbulence?
L 493 also evident in Figure 10?
L 496-497 related to increased mixing?
Ll 502-504 what is the causal relationship here? More mixing = more cooling and therefore less stratification, but here the argument seems to be more mixing => less stratification => more cooling, can you be more explicit about the suggested series of events?
L 514 suggest removing “it is”
L 522 additional forcing
Ll 522-524 this causality is not clear, please clarify
L538 what is the timescale of the AMOC weakening? Decadal?
L 540 add citation same as above Yun Yang
Ll 544 “or productivity” maybe better to phrase “also indicated by trends in productivity”
L 556 Please remind the reader how the influence of Ekman transport fits in with the seasonal modulation
L 586 what does inner mean here
Minor:
Some inconsistencies with the plural and singular in the text, e.g. Ll 241-242 ..waters… has ..
Citation: https://doi.org/10.5194/egusphere-2022-1354-RC1 - AC1: 'Reply on RC1', Peter Brandt, 28 Mar 2023
-
RC2: 'Comment on egusphere-2022-1354', Erik van Sebille, 28 Feb 2023
Full disclosure: I am writing this review as Topic Editor for this manuscript. It has proven very difficult to find a second independent and knowledgeable reviewer for this manuscript, as most experts are already involved in the manuscript as authors. After discussions with two of the Chief Editors, I decided to write this second review as Topic Editor; focussing mostly on the effectiveness of the manuscript as a review article - Erik van Sebille
The topic of the manuscript (physical processes in the upwelling regions of the tropical Atlantic) surely warrants an in-depth review, and Ocean Science is an appropriate journal for it. The authors are from a wide and diverse range of institutes and expertises. This is all very strong and supports the trust in the review as unbiased and complete.
However, I have a few suggestions that I think will make the manuscript stronger and more impactful as a review article:
1. I encourage the authors to think about a schematic or figure that summarizes the key processes that are discussed in the paper. Such a figure might be hugely impactful, as it illustrates what the review is about2. The meridional extent of the region is never defined. While the zonal extent is discussed (line 54 and further), the meridional extend stays vague. Is it the 20S to 10N of figure 1?
3. Some of the modeling results (e.g. Figs 7, 8, 9) miss a clear reference. It is unclear now whether these figures have been created specifically for this manuscript, or come from another paper. In the first case, there should be much more discussion of the model setup etc (and it might be doubtful whether new/unpublished results fit in a review article like this); in the second case the captions need a clear reference.
4. Section 5 misses figures. Whereas the other sections all have figures, this doesn't. Is that intentional? More generally, the style of the different sections varies quite a bit. I realize that each co-author was probably responsible for leading one section, but I would then still recommend carefully going through the text (and figures) to harmonize the style. That will help readers, and thus increase impact.
5. The manuscript tends to be fairly descriptive (answering what is happening) and relatively low on (physical) explanation. For example, line 601 states that "The zonal velocity field instead is dominated by the equatorial basin resonance of the 2nd and 4th baroclinic modes resulting in an EUC that vertically migrate largely independent of the thermocline (Brandt et al. 2016)" This sentence does not provide much information on _why_ it are the 2nd and 4th modes that are important here. That information may be in the reference, but the purpose of a review article is also to provide an accessible overview of the state-of-the-art; in this case (because of the title) also in terms of physical processes.
6. I somewhat missed a discussion of the similarities and differences with upwelling regions of the tropical Pacific and Indian Ocean. While the uniqueness of the basin is described well in the introduction; the extend to which the physical processes differ or agree in the other two basins is not very well discussed. I would expect that to be another purpose of such a review article?
Minor comments:
- line 14: 'inner' is a strange word here. 'Offshore'?
- line 21: misses 'the' before 'northern boundary'
- line 22: remove comma after both
- line 22: nonlinearity in what?
- Figure 1: I don't understand the units of mg m-3. Until what depth is this then? Would it not make more sense as mg m-2?
- line 82: the capitalization rules between tAUS and EAUS are not clear.
- line 145: is the ecosystem indeed nitrate-limited? Would be good to provide a reference for that implicit assumption
- Figure 3: why not use the same time period for all four datasets. Most of the period overlaps, but the start and end years are not consistent. It would be stronger to harmonize that
- line 194: is this the mean in time? Or the mean in space?
- line 203: 'helps to define' is vague wording here.
- line 210: 'seasonally varying'
- line 231-234: so what is the conclusion then?
- line 268: why use the fourth baroclinic mode? What is so special about the fourth one?
- line 309: which capes are meant with 'the capes'?
- line 329-330: the 25C threshold for upwelling seems fairly arbitrary. The paper that it is based on in almost 50 years old. Has no newer research been done on this?
- line 487: what type of model was used by Zeng et al (2021)? A bit more description might help readers gauge the applicability of these conclusions
- line 513: this first sentence of section 5 is very vague. Which part of the upwelling? Variability of what?What is meant with 'It is the area and season most impacted [...]'?
- line 519: 'boreal summer' is a biased and confusing term. While not simply write the names of the months?
- line 543: what is meant with 'first' here?
- line 575: also provide a reference for the event in 2021?Citation: https://doi.org/10.5194/egusphere-2022-1354-RC2 - AC2: 'Reply on RC2', Peter Brandt, 28 Mar 2023
Peer review completion
Journal article(s) based on this preprint
Peter Brandt et al.
Peter Brandt et al.
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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
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