the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 23 Jan 2023
Modelling the effect of aerosol and greenhouse gas forcing on the South & East Asian monsoons with an intermediate complexity climate model
Abstract. The South and East Asian summer monsoons are globally significant meteorological features, creating a strongly seasonal pattern of precipitation, with the majority of the annual precipitation falling between June and September. The stability of such a strongly seasonal hydrological cycle is of extreme importance for a vast range of ecosystems and for the livelihoods of a large share of the world's population. Simulations are performed with an intermediate complexity climate model, PLASIM, in order to assess the future response of the South and East Asian monsoons to changing concentrations of aerosols and greenhouse gases. The radiative forcing associated with aerosol loading consists of a mid-tropospheric warming and a compensating surface cooling, which is applied to India, Southeast Asia and East China, both concurrently and independently. The primary effect of increased aerosol loading is a decrease in summer precipitation in the vicinity of the applied forcing, although the regional responses vary significantly. The decrease in precipitation is only partially ascribable to a decrease in the precipitable water, and instead derives from a reduction of the precipitation efficiency, due to changes in the stratification of the atmosphere. When the aerosol loading is added in all regions simultaneously, precipitation in East China is most strongly affected, with a quite distinct transition to a low precipitation regime as the radiative forcing increases beyond 60 W/m2. The response is less abrupt as we move westward, with precipitation in South India being least affected. By applying the aerosol loading to each region individually, we are able to explain the mechanism behind the lower sensitivity observed in India, and attribute it to aerosol forcing over East China. Additionally, we note that the effect on precipitation is approximately linear with the forcing. The impact of doubling carbon dioxide levels is to increase precipitation over the region, whilst simultaneously weakening the circulation. When the carbon dioxide and aerosol forcings are applied at the same time, the carbon dioxide forcing partially offsets the surface cooling and reduction in precipitation associated with the aerosol response.
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Notice on discussion status
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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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
Journal article(s) based on this preprint
Interactive discussion
Status: closed
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RC1: 'Comment on egusphere-2023-12', Anonymous Referee #1, 14 Feb 2023
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-12/egusphere-2023-12-RC1-supplement.pdf
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AC1: 'Reply on RC1', Lucy Recchia, 05 Apr 2023
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-12/egusphere-2023-12-AC1-supplement.pdf
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AC1: 'Reply on RC1', Lucy Recchia, 05 Apr 2023
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RC2: 'Comment on egusphere-2023-12', Anonymous Referee #2, 26 Feb 2023
Review of the paper titled “Modelling the effect of aerosol and greenhouse gas forcing on the South & East Asian monsoons with an intermediate complexity climate model” by Recchia and Lucarini
The paper highlights the usage of an intermediate complexity climate model, PLASIM to examine the monsoon responses to aerosol and 2xCO2 forcings over South and East Asia. The chosen topic is interesting and could be useful for the scientific community working in this field. In this paper, the authors carried out sensitivity experiments by increasing the aerosol forcings over all three regions selected i.e., India, Southeast Asia, and eastern China. They found a consistent decrease in the monsoon precipitation due to aerosol forcings. Area-wise perturbed experiments suggested that increased aerosol forcing over east China increases monsoon precipitation over India. While including 2xCO2 forcings, the decrease in precipitation as noted due to aerosol forcings is partly compensated. The results are promising as it is performed by a non-expensive intermediate complexity climate model through idealized experiments. However, I have some concerns that the authors can address before accepting the paper for publication.
Specific comments:
- Model evaluation against the observations is missing in the manuscript. Some meteorological parameters like precipitation, winds, and surface temperature as shown are needed to be validated against the observations.
- The values selected for aerosol forcings are too high. The values ranging from 30 to 40 W m-2are valid over a local region and season depending upon the emission type but applying all over India or eastern China could have an overestimation of aerosol effects. What is the rational thinking behind increasing aerosol forcing to 150 Wm-2? If there is no interactive chemistry component in the model then these are highly idealized simulations. I suggest making it clear in the title.
- I assume these values 30 to 150 Wm-2 depict the aerosol atmospheric forcings (Top of the atmosphere (TOA) – surface) right? then why only it is applied to 550-750 hPa? Please clarify.
- It is a bit confusing that if the mid-tropospheric heating is applied then the monsoon circulation at 850 hPa should have strengthened over India and southern China. In general, it should have created a mid-tropospheric temperature gradient but I see a consistent decrease in the precipitation. Please clarify.
- Could you please include some discussion on why there is an increase in precipitation over north India in 120 to 150 Wm-2 aerosols forcing?
- The responses in monsoon precipitation obtained in this paper could be possibly due to large amounts of scattering aerosols (anthropogenic sulfates) which seems consistent with the earlier published literature. Here, through absorbing aerosol forcings, a similar effect is obtained. Why? I am not sure whether the dynamics are correctly responding to aerosol forcing.
- Whether the surface temperature anomalies or the tropospheric temperature anomalies induced by aerosol/2xCO2 forcings are more sensitive in driving monsoon precipitation should be pointed out.
- It would be nice to check the latitudinal cross-section of changes in air temperature vertically due to aerosol forcings. Subsequently, then while including 2xCO2 forcings.
- Inspection of vertical velocity responses to aerosol and 2xCO2 forcings could be useful. Please check.
- “The intense surface cooling is primarily responsible for activating the ice-albedo effect..” on page 10. How does the ice-albedo affect the vertical distribution of temperature?
- I could not understand how the aerosol forcing applied to eastern China increases precipitation over India. Are the changes statistically significant? There is still a decrease in surface temperature over India (Figure 11) without aerosol forcings. What could be the potential reason for an increase in precipitation? Please provide a physical explanation.
- The concept of the advection of dry air from Siberia is not well interpreted. It would be better to have a look at the responses of specific humidity to aerosol forcings.
- There are some limitations and uncertainties associated with results presented through idealized simulations using PLASIM. The author could point out and discuss it in a separate section.
Technical comments:
- Maps of good quality are not well superimposed on the spatial figures. They seem at a very coarse resolution and distorted.
Citation: https://doi.org/10.5194/egusphere-2023-12-RC2 -
AC2: 'Reply on RC2', Lucy Recchia, 13 Apr 2023
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-12/egusphere-2023-12-AC2-supplement.pdf
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RC3: 'Comment on egusphere-2023-12', Anonymous Referee #3, 28 Feb 2023
The manuscript describes the use of an intermediate complexity model to perform sensitivity experiments to prescribed aerosols and CO2 forcing over Asia, in combination as well as separately, also distinguishing between sub-regional forcing and identifying non-linearities.
The topic has been widely studied using a variety of modelling tools and simulations. Needless to say, there are still uncertainties and varied responses across models. It is a very important topic as aerosols are still very high over the region studies, and there may be large differences in the spatio-temporal evolution of regional emissions in the coming decades. The manuscript is well written and clear. I have some concerns on some of the experimental set-up and analysis that need to be addressed before acceptance.
Major comments:
The model validation should be done by comparing the output to observations
The magnitude of radiative forcing is definitely too large. Available estimates provide values up to 20/30 W m2 at the surface, I think it is generally OK to use larger values than observed in idealised settings (e.g., PDRMIP) to highlight the signal, but the values used here are definitely too large. I think these simulations are unrealistic.
Along these lines, I am not sure the comparison with 2xCO2 is appropriate, and especially the identification of which of the two drivers dominates.
Also, the prescribed forcing simulates the effect of absorbing aerosols, rather than sulfate. In reality, the latter are found to dominate the aerosol-driven monsoon changes. I think it is important to further underscore these differences and make it clearer when drawing conclusions.
As there is not seasonality in the forcing here, can the authors speculate on what this means in terms of realism of summer anomalies, also in terms of preconditioned conditions through the previous winter and spring?
I suggest also making the text a bit more concise, as I think some parts are not needed. For example, all the section on tipping points in the introduction is not really necessary. Similarly, Section 4.1 is not contribution much to the overall discussion. There are also other parts throughout the manuscript that can be shortened.
Why is relative humidity used instead of specific humidity? I do not think RH at 200 hPa (Fig. 3) is really useful. The way winds are plotted is rather unusual (scale the arrows by the magnitude instead of plotting magnitude and direction separately). Also, it may be useful to calculate vertical integrated moisture transport.
Differences should have plotted a corresponding statistical significance.
The analysis of regional responses and the reciprocal influence between India and China is, in my opinion, the most interesting part. A dynamical analysis of the upper level circulation will also be useful.
What are the limitation of this study and in particular of using PLASIM? This should be clearly stated in the conclusions.
Citation: https://doi.org/10.5194/egusphere-2023-12-RC3 -
AC3: 'Reply on RC3', Lucy Recchia, 13 Apr 2023
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-12/egusphere-2023-12-AC3-supplement.pdf
-
AC3: 'Reply on RC3', Lucy Recchia, 13 Apr 2023
Interactive discussion
Status: closed
-
RC1: 'Comment on egusphere-2023-12', Anonymous Referee #1, 14 Feb 2023
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-12/egusphere-2023-12-RC1-supplement.pdf
-
AC1: 'Reply on RC1', Lucy Recchia, 05 Apr 2023
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-12/egusphere-2023-12-AC1-supplement.pdf
-
AC1: 'Reply on RC1', Lucy Recchia, 05 Apr 2023
-
RC2: 'Comment on egusphere-2023-12', Anonymous Referee #2, 26 Feb 2023
Review of the paper titled “Modelling the effect of aerosol and greenhouse gas forcing on the South & East Asian monsoons with an intermediate complexity climate model” by Recchia and Lucarini
The paper highlights the usage of an intermediate complexity climate model, PLASIM to examine the monsoon responses to aerosol and 2xCO2 forcings over South and East Asia. The chosen topic is interesting and could be useful for the scientific community working in this field. In this paper, the authors carried out sensitivity experiments by increasing the aerosol forcings over all three regions selected i.e., India, Southeast Asia, and eastern China. They found a consistent decrease in the monsoon precipitation due to aerosol forcings. Area-wise perturbed experiments suggested that increased aerosol forcing over east China increases monsoon precipitation over India. While including 2xCO2 forcings, the decrease in precipitation as noted due to aerosol forcings is partly compensated. The results are promising as it is performed by a non-expensive intermediate complexity climate model through idealized experiments. However, I have some concerns that the authors can address before accepting the paper for publication.
Specific comments:
- Model evaluation against the observations is missing in the manuscript. Some meteorological parameters like precipitation, winds, and surface temperature as shown are needed to be validated against the observations.
- The values selected for aerosol forcings are too high. The values ranging from 30 to 40 W m-2are valid over a local region and season depending upon the emission type but applying all over India or eastern China could have an overestimation of aerosol effects. What is the rational thinking behind increasing aerosol forcing to 150 Wm-2? If there is no interactive chemistry component in the model then these are highly idealized simulations. I suggest making it clear in the title.
- I assume these values 30 to 150 Wm-2 depict the aerosol atmospheric forcings (Top of the atmosphere (TOA) – surface) right? then why only it is applied to 550-750 hPa? Please clarify.
- It is a bit confusing that if the mid-tropospheric heating is applied then the monsoon circulation at 850 hPa should have strengthened over India and southern China. In general, it should have created a mid-tropospheric temperature gradient but I see a consistent decrease in the precipitation. Please clarify.
- Could you please include some discussion on why there is an increase in precipitation over north India in 120 to 150 Wm-2 aerosols forcing?
- The responses in monsoon precipitation obtained in this paper could be possibly due to large amounts of scattering aerosols (anthropogenic sulfates) which seems consistent with the earlier published literature. Here, through absorbing aerosol forcings, a similar effect is obtained. Why? I am not sure whether the dynamics are correctly responding to aerosol forcing.
- Whether the surface temperature anomalies or the tropospheric temperature anomalies induced by aerosol/2xCO2 forcings are more sensitive in driving monsoon precipitation should be pointed out.
- It would be nice to check the latitudinal cross-section of changes in air temperature vertically due to aerosol forcings. Subsequently, then while including 2xCO2 forcings.
- Inspection of vertical velocity responses to aerosol and 2xCO2 forcings could be useful. Please check.
- “The intense surface cooling is primarily responsible for activating the ice-albedo effect..” on page 10. How does the ice-albedo affect the vertical distribution of temperature?
- I could not understand how the aerosol forcing applied to eastern China increases precipitation over India. Are the changes statistically significant? There is still a decrease in surface temperature over India (Figure 11) without aerosol forcings. What could be the potential reason for an increase in precipitation? Please provide a physical explanation.
- The concept of the advection of dry air from Siberia is not well interpreted. It would be better to have a look at the responses of specific humidity to aerosol forcings.
- There are some limitations and uncertainties associated with results presented through idealized simulations using PLASIM. The author could point out and discuss it in a separate section.
Technical comments:
- Maps of good quality are not well superimposed on the spatial figures. They seem at a very coarse resolution and distorted.
Citation: https://doi.org/10.5194/egusphere-2023-12-RC2 -
AC2: 'Reply on RC2', Lucy Recchia, 13 Apr 2023
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-12/egusphere-2023-12-AC2-supplement.pdf
-
RC3: 'Comment on egusphere-2023-12', Anonymous Referee #3, 28 Feb 2023
The manuscript describes the use of an intermediate complexity model to perform sensitivity experiments to prescribed aerosols and CO2 forcing over Asia, in combination as well as separately, also distinguishing between sub-regional forcing and identifying non-linearities.
The topic has been widely studied using a variety of modelling tools and simulations. Needless to say, there are still uncertainties and varied responses across models. It is a very important topic as aerosols are still very high over the region studies, and there may be large differences in the spatio-temporal evolution of regional emissions in the coming decades. The manuscript is well written and clear. I have some concerns on some of the experimental set-up and analysis that need to be addressed before acceptance.
Major comments:
The model validation should be done by comparing the output to observations
The magnitude of radiative forcing is definitely too large. Available estimates provide values up to 20/30 W m2 at the surface, I think it is generally OK to use larger values than observed in idealised settings (e.g., PDRMIP) to highlight the signal, but the values used here are definitely too large. I think these simulations are unrealistic.
Along these lines, I am not sure the comparison with 2xCO2 is appropriate, and especially the identification of which of the two drivers dominates.
Also, the prescribed forcing simulates the effect of absorbing aerosols, rather than sulfate. In reality, the latter are found to dominate the aerosol-driven monsoon changes. I think it is important to further underscore these differences and make it clearer when drawing conclusions.
As there is not seasonality in the forcing here, can the authors speculate on what this means in terms of realism of summer anomalies, also in terms of preconditioned conditions through the previous winter and spring?
I suggest also making the text a bit more concise, as I think some parts are not needed. For example, all the section on tipping points in the introduction is not really necessary. Similarly, Section 4.1 is not contribution much to the overall discussion. There are also other parts throughout the manuscript that can be shortened.
Why is relative humidity used instead of specific humidity? I do not think RH at 200 hPa (Fig. 3) is really useful. The way winds are plotted is rather unusual (scale the arrows by the magnitude instead of plotting magnitude and direction separately). Also, it may be useful to calculate vertical integrated moisture transport.
Differences should have plotted a corresponding statistical significance.
The analysis of regional responses and the reciprocal influence between India and China is, in my opinion, the most interesting part. A dynamical analysis of the upper level circulation will also be useful.
What are the limitation of this study and in particular of using PLASIM? This should be clearly stated in the conclusions.
Citation: https://doi.org/10.5194/egusphere-2023-12-RC3 -
AC3: 'Reply on RC3', Lucy Recchia, 13 Apr 2023
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-12/egusphere-2023-12-AC3-supplement.pdf
-
AC3: 'Reply on RC3', Lucy Recchia, 13 Apr 2023
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Lucy G. Recchia
Valerio Lucarini
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.