Impact of the Guinea Coast upwelling on atmospheric dynamics, precipitation and pollutant transport over Southern West Africa

de Coëtlogon, Gaëlle; Deroubaix, Adrien; Flamant, Cyrille; Menut, Laurent; Gaetani, Marco

doi:https://doi.org/10.5194/egusphere-2023-681

Preprints

https://doi.org/10.5194/egusphere-2023-681

Preprints

26 Apr 2023

| 26 Apr 2023

Impact of the Guinea Coast upwelling on atmospheric dynamics, precipitation and pollutant transport over Southern West Africa

Gaëlle de Coëtlogon, Adrien Deroubaix, Cyrille Flamant, Laurent Menut, and Marco Gaetani

Abstract. In West Africa, the zonal band of precipitation is generally located around the southern coast in June before migrating northward towards the Sahel in late June / early July. This gives way to a relative dry season for coastal regions from Côte d’Ivoire to Benin called "little dry season" which lasts until September–October. Previous studies have noted that the coastal rainfall cessation in early July seems to coincide with the emergence of an upwelling along the Guinea coast: the aim of this study is to investigate the mechanisms by which this upwelling would have an impact on precipitation, using a set of numerical simulations performed with the regional atmospheric model Weather Research and Forecasting (WRF v 3.7.1,). Sensitivity experiments highlight the response of the atmospheric circulation to an intensification, or conversely a reduction, of the strength of the coastal upwelling: they clearly show that the coastal upwelling emergence is responsible for the cessation of coastal precipitation by weakening the northward humidity transport, thus decreasing the coastal convergence of the humidity transport and inhibiting the deep atmospheric convection. In addition, the diurnal cycle of the low-level circulation plays a critical role: since the land breeze controls the seaward convergence of diurnal anomaly of humidity transport, explaining the late night / early morning peak observed in coastal precipitation, the emergence of the coastal upwelling strongly attenuates this peak because of a reduced land-sea temperature gradient in the night and a weaker land breeze. The impact on the inland transport of anthropogenic pollution is also shown with numerical simulations of aerosols with the CHIMERE chemistry-transport model: warmer (colder) SSTs increase (decrease) the inland transport of pollutants, especially during the night, suggesting an influence of the upwelling intensity on the coastal low-level jet. Important considerations for inland humidity transport and the predictability of the West African Monsoon precipitation in summer may arise from this work and motivate further research.

Received: 06 Apr 2023 – Discussion started: 26 Apr 2023

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Gaëlle de Coëtlogon et al.

Status: closed

RC1:
'Comment on egusphere-2023-681', Elsa Mohino, 25 Jul 2023

General comments:
This manuscript analyses the effect of coastal upwelling off the Guinean Coast in coastal precipitation and pollution transport over Souther West Africa in the transition period June – July by means of sensitivity experiments with the regional atmosphere WRF model and the transport CHIMERE model. The simulations consist in a reference experiment using as boundary conditions NOAA SSTs from 2016, and two altered experiments, namely a warm and a cold one, in which upwelling is damped and enhanced, respectively. All experiments provide 10 ensemble members. After analysing the model response, the authors conclude that a stronger upwelling weakens coastal precipitation by promoting less surface wind and moisture convergence locally, especially in the late night/early morning due to the reduction of land breeze. Through modifying winds, upwelling also reduces the inland transport of pollutants produced in major coastal cities, especially at night.
As acknowledged in the manuscript, the main hypothesis on the effect of upwelling on coastal precipitation is not novel. However, the authors provide further evidence on the mechanisms at play and their effect based on the sensitivity experiments performed. They also present novel results related to the effect on pollution transport inland and the conclusions are well supported by their results. I find, however, the manuscript would benefit from additional discussion on the limits of the experimental setup and on the results on pollutant transport (see specific comments section). The scientific approach is clearly outlined, though some clarifications are still needed on the experimental setup (see specific comment section). Formally, I find the manuscript clear, well structured and written, and overall easy to follow, just some minor corrections and clarifications are still needed (see technical corrections). The title and abstract are clear and present the main ideas and results of the manuscript. All in all, I support the publication of the manuscript after modifications following the comments below.
Specific comments:
1) In general, coastal upwelling is heavily influenced by surface winds. The modification of surface winds shown in their experiments could well feedback onto the upwelling altering it. However, the authors do not take into account this possible atmosphere-ocean coupling not only in designing the experiments but also on commenting the results and limits of their findings. I suggest to revise the introduction to take ocean-atmosphere coupling into account in the region and add a discussion in the last section on the limits of their experimental setup and possible impact on their interpretation of the results.
2) I wonder to what extent we can trust the results shown on the pollution transport. Are there any observations or different model results to compare with, even for the reference simulation?
3) The authors do not explain the choice of trend correction used in the warmES and coldES experiments. If I understand correctly, inside the delimited region of -0.052ºC/day linear trend, for the warmES experiment, the linear trend at all grid points is set at -0.052ºC/day. Why? Why not a stronger or weaker trend?
4) I couldn’t find what were the lateral atmospheric boundary conditions used in the experiment. Please clarify.

Technical corrections:
- Line 42: two instances of “based” in the same sentence. Rephrase to avoid repetition.
- Line 43: why do you use bold here ?
- Line 72: remove excess of points.
- Lines 94-96: “In addition, east of …” I cannot follow this sentence. Could you rephrase, please?
- Lines 104-105: “… because the near-surface …” The higher content of water vapor on the continent than over ocean is not discussed later on, right? This is a bit confusing to me. Could you explain it more, please?
- Line 117: “These two convergence areaS ..”
- Line 148: time serie → time series (both instances)
- Line 149: remove “capped”. If I understood correctly, your methodology sets the linear trend in all points inside the modified region to -0.052ºC/day.
- Lines 153-154: I do not follow how you conclude that the damping effect on warmES is 1/3. If I understood correctly you methodology, in point A and B (an all points inside the modified area) the linear trend is set to -0.052ºC/day, so the total change between June 5^th to July 7^th would be: 34 days times -0.052ºC/day = -1.77 ºC, which is not 1/3 of 3ºC. Please clarify.
- Line 158-159: same comment as below regarding the coldES experiment and the 1/3 enhancement.
- Line 255: Why do you use “arbitrary units” in the concentrations? How can you be sure to add same contributions? Why not use some real unit?
- Line 265: “… negative anomalies to the southeast …” is this correct?
- Line 275: WarmES → ColdES
- Line 279: “cover(Schuster” → “cover (Schuster”
- Line 313: “which end” → “whose end” or “the end of which”
- Line 313: “During this period..” Which period? Two different have just been mentioned (the nighttime peak and its end).
- Line 314: “marked” → “markedly”
- Line 314: “may be related the” → “may be related to the”
- Line 314-315: “may be related ...warmer coastal SST.” I don’t follow this explanation. Coastal SSTs are colder in ColdES, right?
- Line 317-318: How does this possible strengthening of the low level jet in the warmES experiment related to the weaker transport in this simulation. Overall I find this paragraph confusing.
- Line 325: Perhaps change the title of the section to “Conclusions and discussion”?
- Figure 1: increase the size of numbers in colorbars
- Figure 2: increase size of colorbar and text fonts.
- Figure 3: add the coast line to the plots.
- Figure 4: what are the arrows in panels a and b representing? Is it meridional and vertical wind ? Please clarify in the caption and add reference arrow
- Figure 7: improve the quality of panels a and b. It is difficult to see the arrows, contours and labels due to the low resolution of the panels.
- Figure 8: Is the zonal wind represented in the arrows as the x component? Please add this info into the caption because it is weird to show the zonal wind in a projection of time of the day.
- Figure 9: add the letters to the plots. In plots a and b, add the location of the five major coastal cities. Enhance size of numbers to make them more consistent with other figures.
- Figure 10: enhance resolution (or better use vector plot). It is horizontal relative vorticity in shading, right? Could you clarify?

Citation: https://doi.org/10.5194/egusphere-2023-681-RC1
- AC1: 'Reply on RC1', Gaelle de Coetlogon, 31 Aug 2023
  
  We want to express our sincere gratitude to Dr. Mohino for her careful review of our study and her constructive and very helpful suggestions. In the attached file, you will find our responses to the questions in blue characters to facilitate distinction.
  
  Citation: https://doi.org/10.5194/egusphere-2023-681-AC1
RC2:
'Comment on egusphere-2023-681', Anonymous Referee #2, 04 Oct 2023

In this manuscript entitled ‘Impact of the Guinea Coast upwelling on atmospheric dynamics, precipitation and pollutant transport over Southern West Africa’ de Coëtlogon et al. analyse the mechanism by which a zonal band of precipitation over West Africa is pushed North over the Sahel starting in late June/early July. Their work and the simulations presented are based upon the proposition of Tanguy et al. (2022), based on satellite observations, that the main driver for pushing North the precipitation are the sea-surface temperatures of the coastal upwelling near the coast of Guinea.
In contrast to Tanguy et al. (2022) this work is based on the analysis of two ensemble of simulations, one with warmer SSTs near the Guinean coast and a second one with warmer SSTs than the ensemble that was run as the control. The SST anomaly created amounts to 0.5°C on June 15^th and 1.0°C on July 7^th, the variability is conserved by the method. The diagnostics presented in the manuscript show how the meridional circulation changes in response to these warmer and colder temperatures, entraining (respectively slowing down) the transport of water vapor and hence the precipitation inland (northward from the Gulf of Guinea). These SST anomalies dampens the precipitation in the case of the cold upwelling by decreasing the convergence of humidity whereas the reverse is true when SSTs are increased (warm upwelling). The analysis through the diagnostics presented in the paper is convincing and the two ensemble of simulations illustrate well the role played by the convergence of humidity in this coastal region.
I have two remarks that might improve the manuscript.
The introduction focuses solely on how the Sahel precipitation is affected by the mechanism at hand that is the variations of SST near the coast of West Africa and in particular near Guinea and Benin. It would be useful to remind the reader that other processes play a role in the position and the strength of the precipitation over the Sahel as described by the following authors: Haywood et al., 2016; Miller et al., 2014 and Balkanski et al. 2021.
The second remark concerns the analysis of the effect on pollution which is analysed for the five main cities affected by the changes in circulation brought about by the upwelling temperature (Abidjan, Ivory Coast; Accra, Ghana; Lom, Togo; Cotonou, Benin and Lagos, Nigeria). Using a generic tracer for pollution gives an information about the relative variations that pollutants will incur but it would be much more informative to have run a model with a full or a simplified chemistry to study how this translates into concentrations for the main pollutants that are: O₃, NO_x, SO_x.

All references in the manuscript need to be checked as some are incomplete.
For example:
Tanguy, M., De Coëtlogon, G., and Eymard, L.: Sea surface temperature impact on diurnal cycle and seasonal evolution of the Guinea coastrainfall in Boreal spring and summer, Monthly Weather Review, ?, ?, https://doi.org/?, 2022.

Taking into consideration the two remarks this manuscript is worthy to be published in Atmospheric Chemistry and Physics.

references cited:
Tanguy, M., De Coëtlogon, G., and Eymard, L.: Sea surface temperature impact on diurnal cycle and seasonal evolution of the Guinea coastrainfall in Boreal spring and summer, Monthly Weather Review, 150, 12, pp. 3175-3194, https://doi.org/10.1175/MWR-D-21-0155.1, 2022.

Haywood, J. M., Jones, A., Dunstone, N., Milton, S., Vellinga, M., Bodas-Salcedo, A., Hawcroft, M., Kravitz, B., Cole, J., Watanabe, S., and Stephens, G.: The impact of equilibrating hemispheric albedos on tropical performance in the HadGEM2-ES coupled climate model, Geophys. Res. Lett., 43, 395–403, https://doi.org/10.1002/2015GL066903, 2016.

Miller, R. L., Knippertz, P., Pérez García-Pando, C., Perlwitz, J. P., and Tegen, I.: Impact of Dust Radiative Forcing upon Climate, in: Mineral Dust: A Key Player in the Earth System, edited by: Knippertz, and Stuut, J.-B. W., Springer Netherlands, Dordrecht, 327–357, https://doi.org/10.1007/978-94-017-8978-3_13, 2014.
Balkanski, Y., Bonnet, R., Boucher, O., Checa-Garcia, R., and Servonnat, J.: Better representation of dust can improve climate models with too weak an African monsoon, Atmos. Chem. Phys., 21, 11423–11435, https://doi.org/10.5194/acp-21-11423-2021, 2021.

Citation: https://doi.org/10.5194/egusphere-2023-681-RC2
- AC2:
  'Reply on RC2', Gaelle de Coetlogon, 19 Oct 2023
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-681/egusphere-2023-681-AC2-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2023-681-AC2
  - AC3: 'Reply on AC2', Gaelle de Coetlogon, 19 Oct 2023
    
    Publisher’s note: this comment is a copy of AC2 and its content was therefore removed.
    
    Citation: https://doi.org/10.5194/egusphere-2023-681-AC3

Status: closed

RC1:
'Comment on egusphere-2023-681', Elsa Mohino, 25 Jul 2023

General comments:
This manuscript analyses the effect of coastal upwelling off the Guinean Coast in coastal precipitation and pollution transport over Souther West Africa in the transition period June – July by means of sensitivity experiments with the regional atmosphere WRF model and the transport CHIMERE model. The simulations consist in a reference experiment using as boundary conditions NOAA SSTs from 2016, and two altered experiments, namely a warm and a cold one, in which upwelling is damped and enhanced, respectively. All experiments provide 10 ensemble members. After analysing the model response, the authors conclude that a stronger upwelling weakens coastal precipitation by promoting less surface wind and moisture convergence locally, especially in the late night/early morning due to the reduction of land breeze. Through modifying winds, upwelling also reduces the inland transport of pollutants produced in major coastal cities, especially at night.
As acknowledged in the manuscript, the main hypothesis on the effect of upwelling on coastal precipitation is not novel. However, the authors provide further evidence on the mechanisms at play and their effect based on the sensitivity experiments performed. They also present novel results related to the effect on pollution transport inland and the conclusions are well supported by their results. I find, however, the manuscript would benefit from additional discussion on the limits of the experimental setup and on the results on pollutant transport (see specific comments section). The scientific approach is clearly outlined, though some clarifications are still needed on the experimental setup (see specific comment section). Formally, I find the manuscript clear, well structured and written, and overall easy to follow, just some minor corrections and clarifications are still needed (see technical corrections). The title and abstract are clear and present the main ideas and results of the manuscript. All in all, I support the publication of the manuscript after modifications following the comments below.
Specific comments:
1) In general, coastal upwelling is heavily influenced by surface winds. The modification of surface winds shown in their experiments could well feedback onto the upwelling altering it. However, the authors do not take into account this possible atmosphere-ocean coupling not only in designing the experiments but also on commenting the results and limits of their findings. I suggest to revise the introduction to take ocean-atmosphere coupling into account in the region and add a discussion in the last section on the limits of their experimental setup and possible impact on their interpretation of the results.
2) I wonder to what extent we can trust the results shown on the pollution transport. Are there any observations or different model results to compare with, even for the reference simulation?
3) The authors do not explain the choice of trend correction used in the warmES and coldES experiments. If I understand correctly, inside the delimited region of -0.052ºC/day linear trend, for the warmES experiment, the linear trend at all grid points is set at -0.052ºC/day. Why? Why not a stronger or weaker trend?
4) I couldn’t find what were the lateral atmospheric boundary conditions used in the experiment. Please clarify.

Technical corrections:
- Line 42: two instances of “based” in the same sentence. Rephrase to avoid repetition.
- Line 43: why do you use bold here ?
- Line 72: remove excess of points.
- Lines 94-96: “In addition, east of …” I cannot follow this sentence. Could you rephrase, please?
- Lines 104-105: “… because the near-surface …” The higher content of water vapor on the continent than over ocean is not discussed later on, right? This is a bit confusing to me. Could you explain it more, please?
- Line 117: “These two convergence areaS ..”
- Line 148: time serie → time series (both instances)
- Line 149: remove “capped”. If I understood correctly, your methodology sets the linear trend in all points inside the modified region to -0.052ºC/day.
- Lines 153-154: I do not follow how you conclude that the damping effect on warmES is 1/3. If I understood correctly you methodology, in point A and B (an all points inside the modified area) the linear trend is set to -0.052ºC/day, so the total change between June 5^th to July 7^th would be: 34 days times -0.052ºC/day = -1.77 ºC, which is not 1/3 of 3ºC. Please clarify.
- Line 158-159: same comment as below regarding the coldES experiment and the 1/3 enhancement.
- Line 255: Why do you use “arbitrary units” in the concentrations? How can you be sure to add same contributions? Why not use some real unit?
- Line 265: “… negative anomalies to the southeast …” is this correct?
- Line 275: WarmES → ColdES
- Line 279: “cover(Schuster” → “cover (Schuster”
- Line 313: “which end” → “whose end” or “the end of which”
- Line 313: “During this period..” Which period? Two different have just been mentioned (the nighttime peak and its end).
- Line 314: “marked” → “markedly”
- Line 314: “may be related the” → “may be related to the”
- Line 314-315: “may be related ...warmer coastal SST.” I don’t follow this explanation. Coastal SSTs are colder in ColdES, right?
- Line 317-318: How does this possible strengthening of the low level jet in the warmES experiment related to the weaker transport in this simulation. Overall I find this paragraph confusing.
- Line 325: Perhaps change the title of the section to “Conclusions and discussion”?
- Figure 1: increase the size of numbers in colorbars
- Figure 2: increase size of colorbar and text fonts.
- Figure 3: add the coast line to the plots.
- Figure 4: what are the arrows in panels a and b representing? Is it meridional and vertical wind ? Please clarify in the caption and add reference arrow
- Figure 7: improve the quality of panels a and b. It is difficult to see the arrows, contours and labels due to the low resolution of the panels.
- Figure 8: Is the zonal wind represented in the arrows as the x component? Please add this info into the caption because it is weird to show the zonal wind in a projection of time of the day.
- Figure 9: add the letters to the plots. In plots a and b, add the location of the five major coastal cities. Enhance size of numbers to make them more consistent with other figures.
- Figure 10: enhance resolution (or better use vector plot). It is horizontal relative vorticity in shading, right? Could you clarify?

Citation: https://doi.org/10.5194/egusphere-2023-681-RC1
- AC1: 'Reply on RC1', Gaelle de Coetlogon, 31 Aug 2023
  
  We want to express our sincere gratitude to Dr. Mohino for her careful review of our study and her constructive and very helpful suggestions. In the attached file, you will find our responses to the questions in blue characters to facilitate distinction.
  
  Citation: https://doi.org/10.5194/egusphere-2023-681-AC1
RC2:
'Comment on egusphere-2023-681', Anonymous Referee #2, 04 Oct 2023

In this manuscript entitled ‘Impact of the Guinea Coast upwelling on atmospheric dynamics, precipitation and pollutant transport over Southern West Africa’ de Coëtlogon et al. analyse the mechanism by which a zonal band of precipitation over West Africa is pushed North over the Sahel starting in late June/early July. Their work and the simulations presented are based upon the proposition of Tanguy et al. (2022), based on satellite observations, that the main driver for pushing North the precipitation are the sea-surface temperatures of the coastal upwelling near the coast of Guinea.
In contrast to Tanguy et al. (2022) this work is based on the analysis of two ensemble of simulations, one with warmer SSTs near the Guinean coast and a second one with warmer SSTs than the ensemble that was run as the control. The SST anomaly created amounts to 0.5°C on June 15^th and 1.0°C on July 7^th, the variability is conserved by the method. The diagnostics presented in the manuscript show how the meridional circulation changes in response to these warmer and colder temperatures, entraining (respectively slowing down) the transport of water vapor and hence the precipitation inland (northward from the Gulf of Guinea). These SST anomalies dampens the precipitation in the case of the cold upwelling by decreasing the convergence of humidity whereas the reverse is true when SSTs are increased (warm upwelling). The analysis through the diagnostics presented in the paper is convincing and the two ensemble of simulations illustrate well the role played by the convergence of humidity in this coastal region.
I have two remarks that might improve the manuscript.
The introduction focuses solely on how the Sahel precipitation is affected by the mechanism at hand that is the variations of SST near the coast of West Africa and in particular near Guinea and Benin. It would be useful to remind the reader that other processes play a role in the position and the strength of the precipitation over the Sahel as described by the following authors: Haywood et al., 2016; Miller et al., 2014 and Balkanski et al. 2021.
The second remark concerns the analysis of the effect on pollution which is analysed for the five main cities affected by the changes in circulation brought about by the upwelling temperature (Abidjan, Ivory Coast; Accra, Ghana; Lom, Togo; Cotonou, Benin and Lagos, Nigeria). Using a generic tracer for pollution gives an information about the relative variations that pollutants will incur but it would be much more informative to have run a model with a full or a simplified chemistry to study how this translates into concentrations for the main pollutants that are: O₃, NO_x, SO_x.

All references in the manuscript need to be checked as some are incomplete.
For example:
Tanguy, M., De Coëtlogon, G., and Eymard, L.: Sea surface temperature impact on diurnal cycle and seasonal evolution of the Guinea coastrainfall in Boreal spring and summer, Monthly Weather Review, ?, ?, https://doi.org/?, 2022.

Taking into consideration the two remarks this manuscript is worthy to be published in Atmospheric Chemistry and Physics.

references cited:
Tanguy, M., De Coëtlogon, G., and Eymard, L.: Sea surface temperature impact on diurnal cycle and seasonal evolution of the Guinea coastrainfall in Boreal spring and summer, Monthly Weather Review, 150, 12, pp. 3175-3194, https://doi.org/10.1175/MWR-D-21-0155.1, 2022.

Haywood, J. M., Jones, A., Dunstone, N., Milton, S., Vellinga, M., Bodas-Salcedo, A., Hawcroft, M., Kravitz, B., Cole, J., Watanabe, S., and Stephens, G.: The impact of equilibrating hemispheric albedos on tropical performance in the HadGEM2-ES coupled climate model, Geophys. Res. Lett., 43, 395–403, https://doi.org/10.1002/2015GL066903, 2016.

Miller, R. L., Knippertz, P., Pérez García-Pando, C., Perlwitz, J. P., and Tegen, I.: Impact of Dust Radiative Forcing upon Climate, in: Mineral Dust: A Key Player in the Earth System, edited by: Knippertz, and Stuut, J.-B. W., Springer Netherlands, Dordrecht, 327–357, https://doi.org/10.1007/978-94-017-8978-3_13, 2014.
Balkanski, Y., Bonnet, R., Boucher, O., Checa-Garcia, R., and Servonnat, J.: Better representation of dust can improve climate models with too weak an African monsoon, Atmos. Chem. Phys., 21, 11423–11435, https://doi.org/10.5194/acp-21-11423-2021, 2021.

Citation: https://doi.org/10.5194/egusphere-2023-681-RC2
- AC2:
  'Reply on RC2', Gaelle de Coetlogon, 19 Oct 2023
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-681/egusphere-2023-681-AC2-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2023-681-AC2
  - AC3: 'Reply on AC2', Gaelle de Coetlogon, 19 Oct 2023
    
    Publisher’s note: this comment is a copy of AC2 and its content was therefore removed.
    
    Citation: https://doi.org/10.5194/egusphere-2023-681-AC3

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Short summary

We test the hypothesis that sea temperature cooling along the southern coast of West Africa (coastal upwelling) plays an active role in the July rainfall cessation, using a numerical atmospheric model where the upwelling is dampened or intensified. The results clearly indicate that the upwelling strongly inhibits precipitation and reduces the transport of moisture and pollutants inland, which could contribute significantly to improving synoptic and seasonal forecasts in West Africa.


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