the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 03 Jan 2024
Exploring the causes of multicentury pluvials in the Altiplano with a climate modelling experiment
Abstract. Proxy records have long documented the existence of multicentury hydroclimate anomalies in the Altiplano of South America. However, the causes and mechanisms of these events are still largely unknown. Here we present the results of an innovative climate modelling experiment that explores the oceanic drivers and atmospheric mechanisms conducive to long-term precipitation variability in the southern part of the Altiplano (18–25° S). For doing so, we performed 100-yr-long hydroclimate simulations using a regional climate model forced by climatological conditions that resulted in pluvial December-January-February (DJF) seasons during historical times. Our modelling simulations produce long-term negative DJF precipitation trends for the southern Altiplano, suggesting that the mechanisms leading to historical wet summers are unable to sustain century-scale pluvials such as the ones documented in paleoclimate records. Our simulations show that permanent La Niña conditions, as well as SSTs anomalies in the southern tropical Atlantic, progressively reinforce upper and lower-troposphere features that inhibit moisture transport towards the Altiplano. We further observed increases in March-April-May precipitation, suggesting the emergence of a long-term seasonal shift. Our simulations reproduce a sustained northward migration of the Atlantic trade winds, resulting in contrasting hydroclimate responses between the Altiplano and the tropical Andes. The atmospheric processes associated with these differences provide a useful analogue for explaining divergences in proxy records. Our study shows how regional climate modelling can be used to test paleoclimate hypothesis, emphasizing the necessity of combining proxy and modelling data to improve our understanding of long-term hydroclimate change.
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RC1: 'Comment on cp-2023-97', Anonymous Referee #1, 31 Jan 2024
The paper is interesting and worth publishing, especially since the application of higher resolution regional climate models in paleoclimate science is still underdeveloped.. However, I think the model setup is rather unrealistic to really be used for interpreting past climate conditions, as it only tests a perpetual La Nina climatological state without any interannual variability and it does not consider any changes in past boundary conditions or external forcing. I also think the authors missed an opportunity by not including a second, higher-resolution, domain over the zone of interest (the Altiplano). The current model configuration equates to a numerical upscaling rather than downscaling (the resulting WRF resolution is coarser than the driving ERA5 reanalysis data), and the resulting resolution over the Altiplano is very course. Nonetheless, the results are interesting as a sensitivity study, and worth publishing after some revisions.
Main comments
The abstract on its own is unclear. You should better clarify what type of model experiment(s) you perform (time period?, boundary conditions?) that lead to the changes in atmospheric circulation and sustained wet conditions (or not). Reading the abstract on its own, I was rather confused about what exactly was done in this study. For example, the result that persistent La Nina conditions inhibit moisture transport to the Altiplano is counterintuitive based on everything we know about ENSO impacts in this part of the world, and requires more context in the abstract.
Discussion lines 65-70: The issue of climate models failing to reproduce observed hydroclimate changes in the region was already documented by Rojas et al. in Climate of the Past (2016). Maybe add this study as an additional example to this discussion.
Fig. 1b. I think this Figure requires a better explanation. I can’t really see a clear pluvial event in these records. The actual proxies plotted should be discussed in the text or the Figure caption so it is clear what they show and how they should be interpreted.
The horizontal resolution of the simulation is quite course (55 km). In fact the WRF model run is coarser than the driving ERA5 dataset. Hence in effect you are upscaling and not downscaling. Why not use a 2nd embedded domain with higher horizontal resolution over the Altiplano region?
You force the WRF model with ERA5 boundary conditions and then validate your WRF results with the same ERA5 dataset. That seems like circular reasoning. While the validation with CHIRPS is independent, I would not include the ERA5 comparisons unless you aim to investigate the added value from downscaling.
I am a bit concerned about the long-term drift in your model simulations. But it may reflect the fact that the model was forced with boundary conditions that constitute an extreme wet-year outlier. Sustaining such extreme conditions for 100 years would likely require changes in the boundary conditions that favor maintaining massive moisture transport to the Altiplano year after year. In the current environment, recharging the atmosphere with sufficient moisture is unlikely without imposing some kind of interannual climate variability that was suppressed in these simulations.
Figure 4: The differences in circulation between the initial and final decade are rather hard to identify. I suggest you incorporate Fig. S9 which shows the difference between the two time periods directly into Fig. 4. It will help to highlight the actual changes. For the right column with the transect, it would make more sense to invert the color scale for humidity so more humid conditions are shown in blue and drier conditions are depicted in red, as this would be more consistent with the changes in vertical velocity depicted in the left column.
Figure 5: Same comment: it intuitively makes more sense to reserve blue colors for wetter and red colors for drier conditions. Hence I would suggest inverting the color scale.
The modeling setup is certainly interesting and the results are worth exploring. Nonetheless, it needs to be clearly stated that these are not realistic representations of boundary conditions that led to wetter conditions in the past, given that neither greenhouse gases, volcanic, solar or orbital conditions were changed. Hence the focus here is exclusively on sustained modern wet-year boundary conditions (with a perpetual climatology and zero interannual variability), while many of the sustained changes in the past were likely externally forced. On pages 12-13 there is a long discussion about the implications of this study for paleoclimatic interpretations, e.g. during Heinrich 1, the late Holocene and the Little Ice Age. Please add a statement at the start of this discussion regarding the lack of realistic past boundary conditions and external forcings, thus limiting the ability of these simulations to serve as analogues for paleoclimatic interpretations.
Lines 305-311: When discussing the 2nd mode of Campos et al. (2019), please note that this study has since been updated by Orrison et al. (2022) with newer proxy data and the inclusion of an isotope-enabled model. They were able to show that the 2nd mode effectively represent SACZ variability.
Minor edits:
Line 30: hypothesis => hypotheses
Line 45: pacific => Pacific
Line 53: year = years
Line 79: et al => et al.
Legend Figure 1b: polle => pollen
Legend Figure 1b: cell => cellulose
Line 101: degenerate => weaken
Figure S2: above panel a) you imply that q 800 hPa is plotted, yet in the legend you write q 500 hpa => please clarify. The same applies to panel b) but here it is unclear whether omega is plotted at 200 hPa (title) or 500 hPa (caption). Finally, for vertical velocity in c) you write the unit is also m/s. Should it not be Pa/s (since you are plotting the data as a function of pressure level)? Also, please add a unit vector for vertical motion in the same way as you did for u and v. Note that some of these comments also apply to Fig S5.
Caption Figure S4: Climate Hazards Center InfraRed Precipitation with Station data => Climate Hazards Group InfraRed Precipitation with Station data
Caption Figure S4: CHIRPS data is plotted in orange, not green color
Caption Figure S4: what do you mean with ‘grilled’ precipitation dataset? Maybe ‘gridded’?
Table 1 and throughout paper:The index you use is called the ‘Nino3.4’ index, not the ‘El Nino 3.4’ index. After all this index is also used to characterize La Nina and neutral conditions in the central equatorial Pacific.
Caption Figure S5: s-1 => superscript ‘-1’
Table 1: Souteastern => Southeastern
Line 175: Climate Center for Atmospheric Research => National Center for Atmospheric Research
Line 209: parametrization => parameterization
Line 212: intimal => initial
Figure S7: Octuber => October
Line 286: accumulative => cumulative
Line 302: dryer => drier
Line 334: indicate in which journal this article was published.
Line 376: delete: ‘J. o. G. R. A.:’
Line 377: reference is incomplete: where was this paper published?
Line 388: you forgot to list a co-author of this paper; J. Michaelsen
Line 389: reference is incomplete: where was this paper published?
Line 406: add volume or issue number or doi, so article can be traced.
Line 474: delete ‘J. E. R. L.:’
Line 475: reference is incomplete: where was this paper published?
Supplement references: the Journal name where Hersbach et al (2020) was published is missing
References cited
Campos, J.L.P.S., et al. 2019: Coherent South American Monsoon variability during the Last Millennium revealed through high-resolution proxy records. Geophys. Res. Lett., 46. 8261–8270. https://doi.org/10.1029/2019GL082513
Orrison, R., et al. 2022: South American Monsoon variability over the last millennium in paleoclimate records and isotope-enabled climate models. Clim. Past, 18, 2045–2062. https://doi.org/10.5194/cp-18-2045-2022
Rojas, M., et al. 2016: The South American Monsoon variability over the last millennium in climate simulations. Clim. Past, 12, 1681-1691. https://doi.org/10.5194/cp-12-1681-2016
Citation: https://doi.org/10.5194/cp-2023-97-RC1 -
CC1: 'Reply on RC1', Orlando Astudillo, 09 Feb 2024
Dear editor and referee
First of all, we would like to thank reviewer #1 for her/his positive comments,
appreciation and guide. Please find below a detailed response to each of the comments.For any further information, please do not hesitate in contacting me
Thank you for your time and consideration.
Sincerely,
Ignacio A. Jara, PhD
Department of Historical and Geographic Sciences, Universidad de Tarapacá, Chile
ijarap@academicos.uta.cl -
AC2: 'Reply on RC1', Ignacio Jara, 19 Mar 2024
Dear editor and referee
First of all, we would like to thank reviewer #1 for her/his positive comments,
appreciation and guide. Please find below a detailed response to each of the comments.For any further information, please do not hesitate in contacting me
Thank you for your time and consideration.
Sincerely,
Ignacio A. Jara, PhD
Department of Historical and Geographic Sciences, Universidad de Tarapacá, Chile
ijarap@academicos.uta.clReply
-
CC1: 'Reply on RC1', Orlando Astudillo, 09 Feb 2024
-
RC2: 'Comment on cp-2023-97', Anonymous Referee #2, 04 Mar 2024
Review of the manuscript
Exploring the causes of multicentury pluvials in the Altiplano with a
climate modelling experiment
by Ignacio A. Jara et al.
Submitted for publication in Climate of the Past
Summary:
The manuscript investigates changes in precipitation over the Altiplano using a regional climate model (WRF) to test whether changes induced by ocean background conditions impact the state of the precipitation levels. The simulations are carried out with 100 year long perpetual simulations for two high precipitation years and one neutral precipitation event as control, based on the ERA5 reanalysis data set. The authors conclude that the ocean background in the eastern Pacific and southwestern Atlantic is not capable in generating multi-decadal above-normal precipitation events, because all 100 year-long experiments (including the control simulation) show negative precipitation trends. Therefore the authors suggest that the precipitation dynamics over longer periods are controlled by the internal variability within the domain of the regional climate model.
General:
The setup of the WRF experiments is very artificial, especially for translating the results into any real-world situation in the paleoclimatic context. Moreover, all experiments including the control experiment show very similar structure concerning the precipitation trends over the Altiplano. A third argument making the setup unrealistic for paleoclimatic inferences, relates to the fact that no changes in the orbital forcing are taken into account, that might also play an important role when comparing results and testing hypothesis with empirical evidence 2.000 years in the past.
For the manuscript I cannot suggest publication in its present form and propose that authors i) carefully check the robustness of their setup and ii) re-submit a considerably revised version of the manuscript to a journal that also considers more theoretical setups and experiments outside the paleoclimatic context.
Suggestions and open questions:
– The general setup is very artificial, since only three years (i.e. 1983, 2003 and 2011) are simulated in a perpetual mode (under present-day conditions). This setup complicates a translation back into real world conditions. Therefore statements like „To our understanding, this study presents the first regional climate modelling experiment of long-term hydroclimate responses in the southern Altiplano, yielding novel information regarding the mechanisms that govern precipitation variability at centennial timescales.“ is misleading, because the simulation itself is not representing a realstic 100 year period. It is just a repetition of particular years showing above normal (neutral) precipitation over the Altiplano. At least the authors should simulate a full ENSO cycle with 7-10 years and also include more information from the Atlantic Multidecadal Variability (AMV), for a more realistic simulation of the impacts of basic modes of natural climatic variability. The authors even indicate the importance of the eastern equatorial Pacific and southwestern Atlantic in their correlation map between the SSTs and the of DJF precipitation variability in the southern Altiplano (cf. Fig S3b).
- In Fig. 3 the authors present the summertime precipitation evolution in their 100 year perpetual model simulations. All experiments, including the control experiment show a very strong decline from approx. 700 mm to 200 mm for the high precipitation events and from 500 mm to again 200 mm for the control experiment. Here I wonder why all experiments converge to the same value of 200 mm. The control experiment exactly demonstrates that the results are based on some internal model dynamics. I would expect that when the external forcing is meteorological the same at the boundaries each year, also precipitation levels should stay at least at similar (initial) levels. Therefore the question is where does all the moisture end up ? How would an experiment with considerably lower initial summertime precipitation over the Altiplano look like ? Would this also happen in reality that the Altiplano summertime precipitation would end at 200 mm ? In this context also the question remains whether the geographical domain of the regional climate model around South America is too small to account for oceanic impact (ENSO/AMV) and how/if this information is still inherited via the borders from the driving ERA5 reanalysis over the entire 100 years cycles.
- All experiments are carried out without any consideration of changes in orbital forcings (cf. Berger 1978), but only external forcings representing conditions of the late 20th and early 21st century. Conditions 2,000 years very different, with lower northern winter insolation and higher late northern summer/early autumn insolation, accentuating the annual insolation cycle. This might also have had an effect on the precipitation dynamics but cannot be explored with the present setup. Therefore authors should use paleoclimatic Earth System model simulations representing the background conditions of their period of interest and then force their model either with a quasi-equilibrium simulation of the time period or a transient time slot. This would implicitly include i) a more realistic representation of the general external background conditions and ii) also warrant a more realistic simulation of natural climate variability that is important for comparisons with all kinds of empirical archives.
Citation: https://doi.org/10.5194/cp-2023-97-RC2 -
AC1: 'Reply on RC2', Ignacio Jara, 19 Mar 2024
We appreciate the suggestions and open comments raised by Referee 2. A complete one by one response to all comments is attached as a separate PDF file here.
In summary, we agree that our modeling experiment was not constructed with realistic paleoclimate forcing conditions, and that our paleoclimate suggestions should be taken carefully. Yet we provide compelling evidence to sustain that our model simulations generate realistic long-term climate dynamics in response to the Pacific and Atlantic SSTs conditions used as external forcing. We contend that, despite the idealized nature of our WRF simulations, they provide novel and interesting insights into the causes and mechanism of long-term precipitation change in the Altiplano --a region where proxy and modeling data is scarce. As stated in the discussion and conclusion of our submitted manuscript, such insights allowed us to assess the research questions raised in the introduction and they have the potential to contribute to the analysis and interpretation of regional paleoclimate series. Therefore, we consider our study to be worth publishing in Climate of the Past.
Best wishes,
Ignacio A. Jara
-
AC1: 'Reply on RC2', Ignacio Jara, 19 Mar 2024
Status: closed
-
RC1: 'Comment on cp-2023-97', Anonymous Referee #1, 31 Jan 2024
The paper is interesting and worth publishing, especially since the application of higher resolution regional climate models in paleoclimate science is still underdeveloped.. However, I think the model setup is rather unrealistic to really be used for interpreting past climate conditions, as it only tests a perpetual La Nina climatological state without any interannual variability and it does not consider any changes in past boundary conditions or external forcing. I also think the authors missed an opportunity by not including a second, higher-resolution, domain over the zone of interest (the Altiplano). The current model configuration equates to a numerical upscaling rather than downscaling (the resulting WRF resolution is coarser than the driving ERA5 reanalysis data), and the resulting resolution over the Altiplano is very course. Nonetheless, the results are interesting as a sensitivity study, and worth publishing after some revisions.
Main comments
The abstract on its own is unclear. You should better clarify what type of model experiment(s) you perform (time period?, boundary conditions?) that lead to the changes in atmospheric circulation and sustained wet conditions (or not). Reading the abstract on its own, I was rather confused about what exactly was done in this study. For example, the result that persistent La Nina conditions inhibit moisture transport to the Altiplano is counterintuitive based on everything we know about ENSO impacts in this part of the world, and requires more context in the abstract.
Discussion lines 65-70: The issue of climate models failing to reproduce observed hydroclimate changes in the region was already documented by Rojas et al. in Climate of the Past (2016). Maybe add this study as an additional example to this discussion.
Fig. 1b. I think this Figure requires a better explanation. I can’t really see a clear pluvial event in these records. The actual proxies plotted should be discussed in the text or the Figure caption so it is clear what they show and how they should be interpreted.
The horizontal resolution of the simulation is quite course (55 km). In fact the WRF model run is coarser than the driving ERA5 dataset. Hence in effect you are upscaling and not downscaling. Why not use a 2nd embedded domain with higher horizontal resolution over the Altiplano region?
You force the WRF model with ERA5 boundary conditions and then validate your WRF results with the same ERA5 dataset. That seems like circular reasoning. While the validation with CHIRPS is independent, I would not include the ERA5 comparisons unless you aim to investigate the added value from downscaling.
I am a bit concerned about the long-term drift in your model simulations. But it may reflect the fact that the model was forced with boundary conditions that constitute an extreme wet-year outlier. Sustaining such extreme conditions for 100 years would likely require changes in the boundary conditions that favor maintaining massive moisture transport to the Altiplano year after year. In the current environment, recharging the atmosphere with sufficient moisture is unlikely without imposing some kind of interannual climate variability that was suppressed in these simulations.
Figure 4: The differences in circulation between the initial and final decade are rather hard to identify. I suggest you incorporate Fig. S9 which shows the difference between the two time periods directly into Fig. 4. It will help to highlight the actual changes. For the right column with the transect, it would make more sense to invert the color scale for humidity so more humid conditions are shown in blue and drier conditions are depicted in red, as this would be more consistent with the changes in vertical velocity depicted in the left column.
Figure 5: Same comment: it intuitively makes more sense to reserve blue colors for wetter and red colors for drier conditions. Hence I would suggest inverting the color scale.
The modeling setup is certainly interesting and the results are worth exploring. Nonetheless, it needs to be clearly stated that these are not realistic representations of boundary conditions that led to wetter conditions in the past, given that neither greenhouse gases, volcanic, solar or orbital conditions were changed. Hence the focus here is exclusively on sustained modern wet-year boundary conditions (with a perpetual climatology and zero interannual variability), while many of the sustained changes in the past were likely externally forced. On pages 12-13 there is a long discussion about the implications of this study for paleoclimatic interpretations, e.g. during Heinrich 1, the late Holocene and the Little Ice Age. Please add a statement at the start of this discussion regarding the lack of realistic past boundary conditions and external forcings, thus limiting the ability of these simulations to serve as analogues for paleoclimatic interpretations.
Lines 305-311: When discussing the 2nd mode of Campos et al. (2019), please note that this study has since been updated by Orrison et al. (2022) with newer proxy data and the inclusion of an isotope-enabled model. They were able to show that the 2nd mode effectively represent SACZ variability.
Minor edits:
Line 30: hypothesis => hypotheses
Line 45: pacific => Pacific
Line 53: year = years
Line 79: et al => et al.
Legend Figure 1b: polle => pollen
Legend Figure 1b: cell => cellulose
Line 101: degenerate => weaken
Figure S2: above panel a) you imply that q 800 hPa is plotted, yet in the legend you write q 500 hpa => please clarify. The same applies to panel b) but here it is unclear whether omega is plotted at 200 hPa (title) or 500 hPa (caption). Finally, for vertical velocity in c) you write the unit is also m/s. Should it not be Pa/s (since you are plotting the data as a function of pressure level)? Also, please add a unit vector for vertical motion in the same way as you did for u and v. Note that some of these comments also apply to Fig S5.
Caption Figure S4: Climate Hazards Center InfraRed Precipitation with Station data => Climate Hazards Group InfraRed Precipitation with Station data
Caption Figure S4: CHIRPS data is plotted in orange, not green color
Caption Figure S4: what do you mean with ‘grilled’ precipitation dataset? Maybe ‘gridded’?
Table 1 and throughout paper:The index you use is called the ‘Nino3.4’ index, not the ‘El Nino 3.4’ index. After all this index is also used to characterize La Nina and neutral conditions in the central equatorial Pacific.
Caption Figure S5: s-1 => superscript ‘-1’
Table 1: Souteastern => Southeastern
Line 175: Climate Center for Atmospheric Research => National Center for Atmospheric Research
Line 209: parametrization => parameterization
Line 212: intimal => initial
Figure S7: Octuber => October
Line 286: accumulative => cumulative
Line 302: dryer => drier
Line 334: indicate in which journal this article was published.
Line 376: delete: ‘J. o. G. R. A.:’
Line 377: reference is incomplete: where was this paper published?
Line 388: you forgot to list a co-author of this paper; J. Michaelsen
Line 389: reference is incomplete: where was this paper published?
Line 406: add volume or issue number or doi, so article can be traced.
Line 474: delete ‘J. E. R. L.:’
Line 475: reference is incomplete: where was this paper published?
Supplement references: the Journal name where Hersbach et al (2020) was published is missing
References cited
Campos, J.L.P.S., et al. 2019: Coherent South American Monsoon variability during the Last Millennium revealed through high-resolution proxy records. Geophys. Res. Lett., 46. 8261–8270. https://doi.org/10.1029/2019GL082513
Orrison, R., et al. 2022: South American Monsoon variability over the last millennium in paleoclimate records and isotope-enabled climate models. Clim. Past, 18, 2045–2062. https://doi.org/10.5194/cp-18-2045-2022
Rojas, M., et al. 2016: The South American Monsoon variability over the last millennium in climate simulations. Clim. Past, 12, 1681-1691. https://doi.org/10.5194/cp-12-1681-2016
Citation: https://doi.org/10.5194/cp-2023-97-RC1 -
CC1: 'Reply on RC1', Orlando Astudillo, 09 Feb 2024
Dear editor and referee
First of all, we would like to thank reviewer #1 for her/his positive comments,
appreciation and guide. Please find below a detailed response to each of the comments.For any further information, please do not hesitate in contacting me
Thank you for your time and consideration.
Sincerely,
Ignacio A. Jara, PhD
Department of Historical and Geographic Sciences, Universidad de Tarapacá, Chile
ijarap@academicos.uta.cl -
AC2: 'Reply on RC1', Ignacio Jara, 19 Mar 2024
Dear editor and referee
First of all, we would like to thank reviewer #1 for her/his positive comments,
appreciation and guide. Please find below a detailed response to each of the comments.For any further information, please do not hesitate in contacting me
Thank you for your time and consideration.
Sincerely,
Ignacio A. Jara, PhD
Department of Historical and Geographic Sciences, Universidad de Tarapacá, Chile
ijarap@academicos.uta.clReply
-
CC1: 'Reply on RC1', Orlando Astudillo, 09 Feb 2024
-
RC2: 'Comment on cp-2023-97', Anonymous Referee #2, 04 Mar 2024
Review of the manuscript
Exploring the causes of multicentury pluvials in the Altiplano with a
climate modelling experiment
by Ignacio A. Jara et al.
Submitted for publication in Climate of the Past
Summary:
The manuscript investigates changes in precipitation over the Altiplano using a regional climate model (WRF) to test whether changes induced by ocean background conditions impact the state of the precipitation levels. The simulations are carried out with 100 year long perpetual simulations for two high precipitation years and one neutral precipitation event as control, based on the ERA5 reanalysis data set. The authors conclude that the ocean background in the eastern Pacific and southwestern Atlantic is not capable in generating multi-decadal above-normal precipitation events, because all 100 year-long experiments (including the control simulation) show negative precipitation trends. Therefore the authors suggest that the precipitation dynamics over longer periods are controlled by the internal variability within the domain of the regional climate model.
General:
The setup of the WRF experiments is very artificial, especially for translating the results into any real-world situation in the paleoclimatic context. Moreover, all experiments including the control experiment show very similar structure concerning the precipitation trends over the Altiplano. A third argument making the setup unrealistic for paleoclimatic inferences, relates to the fact that no changes in the orbital forcing are taken into account, that might also play an important role when comparing results and testing hypothesis with empirical evidence 2.000 years in the past.
For the manuscript I cannot suggest publication in its present form and propose that authors i) carefully check the robustness of their setup and ii) re-submit a considerably revised version of the manuscript to a journal that also considers more theoretical setups and experiments outside the paleoclimatic context.
Suggestions and open questions:
– The general setup is very artificial, since only three years (i.e. 1983, 2003 and 2011) are simulated in a perpetual mode (under present-day conditions). This setup complicates a translation back into real world conditions. Therefore statements like „To our understanding, this study presents the first regional climate modelling experiment of long-term hydroclimate responses in the southern Altiplano, yielding novel information regarding the mechanisms that govern precipitation variability at centennial timescales.“ is misleading, because the simulation itself is not representing a realstic 100 year period. It is just a repetition of particular years showing above normal (neutral) precipitation over the Altiplano. At least the authors should simulate a full ENSO cycle with 7-10 years and also include more information from the Atlantic Multidecadal Variability (AMV), for a more realistic simulation of the impacts of basic modes of natural climatic variability. The authors even indicate the importance of the eastern equatorial Pacific and southwestern Atlantic in their correlation map between the SSTs and the of DJF precipitation variability in the southern Altiplano (cf. Fig S3b).
- In Fig. 3 the authors present the summertime precipitation evolution in their 100 year perpetual model simulations. All experiments, including the control experiment show a very strong decline from approx. 700 mm to 200 mm for the high precipitation events and from 500 mm to again 200 mm for the control experiment. Here I wonder why all experiments converge to the same value of 200 mm. The control experiment exactly demonstrates that the results are based on some internal model dynamics. I would expect that when the external forcing is meteorological the same at the boundaries each year, also precipitation levels should stay at least at similar (initial) levels. Therefore the question is where does all the moisture end up ? How would an experiment with considerably lower initial summertime precipitation over the Altiplano look like ? Would this also happen in reality that the Altiplano summertime precipitation would end at 200 mm ? In this context also the question remains whether the geographical domain of the regional climate model around South America is too small to account for oceanic impact (ENSO/AMV) and how/if this information is still inherited via the borders from the driving ERA5 reanalysis over the entire 100 years cycles.
- All experiments are carried out without any consideration of changes in orbital forcings (cf. Berger 1978), but only external forcings representing conditions of the late 20th and early 21st century. Conditions 2,000 years very different, with lower northern winter insolation and higher late northern summer/early autumn insolation, accentuating the annual insolation cycle. This might also have had an effect on the precipitation dynamics but cannot be explored with the present setup. Therefore authors should use paleoclimatic Earth System model simulations representing the background conditions of their period of interest and then force their model either with a quasi-equilibrium simulation of the time period or a transient time slot. This would implicitly include i) a more realistic representation of the general external background conditions and ii) also warrant a more realistic simulation of natural climate variability that is important for comparisons with all kinds of empirical archives.
Citation: https://doi.org/10.5194/cp-2023-97-RC2 -
AC1: 'Reply on RC2', Ignacio Jara, 19 Mar 2024
We appreciate the suggestions and open comments raised by Referee 2. A complete one by one response to all comments is attached as a separate PDF file here.
In summary, we agree that our modeling experiment was not constructed with realistic paleoclimate forcing conditions, and that our paleoclimate suggestions should be taken carefully. Yet we provide compelling evidence to sustain that our model simulations generate realistic long-term climate dynamics in response to the Pacific and Atlantic SSTs conditions used as external forcing. We contend that, despite the idealized nature of our WRF simulations, they provide novel and interesting insights into the causes and mechanism of long-term precipitation change in the Altiplano --a region where proxy and modeling data is scarce. As stated in the discussion and conclusion of our submitted manuscript, such insights allowed us to assess the research questions raised in the introduction and they have the potential to contribute to the analysis and interpretation of regional paleoclimate series. Therefore, we consider our study to be worth publishing in Climate of the Past.
Best wishes,
Ignacio A. Jara
-
AC1: 'Reply on RC2', Ignacio Jara, 19 Mar 2024
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