the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Spatiotemporal heterogeneity in the increase of ocean acidity extremes in the Northeast Pacific
Matthias Münnich
Nicolas Gruber
Abstract. The acidification of the ocean (OA) increases the frequency and intensity of ocean acidity extreme events (OAXs), but this increase is not occurring homogeneously in time and space. Here we use daily output from a hindcast simulation with a high-resolution regional ocean model coupled to a biogeochemical-ecosystem model (ROMS-BEC) to investigate this heterogeneity in the progression of OAX in the upper 250 m of the Northeast Pacific from 1984 to 2019. We focus on the temporal and spatial changes in OAX using a relative threshold approach and using a fixed baseline reflecting the initial conditions. Concretely, conditions are considered extreme when the local hydrogen ion concentration ([H+]) exceeds the 99th percentile of the [H+] distribution of the baseline simulation where atmospheric CO2 was held at its 1979 level. Within the 36 years of our hindcast simulation, the increase in atmospheric CO2 causes a strong increase in OAX throughout the upper 250 m, but most accentuated near the surface. On average across the entire Northeast Pacific, for every additional 10 μatm of CO2 in the atmosphere, OAXs occupy an additional 6.3 % of the upper 250 m depth, last 7.6 days longer, and are 0.18 nmol L−1 (~ −0.006 pH units) more intense. This causes the OAXs to occupy at the end of the simulation a more than 10-times larger volume. The more than 11-fold increase in length, and the strong increase in the number of extreme days per year causes 88 % of the surface area in 2019 to experience near permanent extreme conditions. Finally, the model simulates a more than 6-fold intensification of the OAXs, causing also the intensity of the events with return periods of 10 years or more to increase by more than 80 %. Superimposed on these overall trends are very substantial spatial and temporal differences in these changes. The fraction of the volume identified as extreme across the top 250 m increases in the Central Northeast Pacific up to 160-times, while the deeper layers of the nearshore regions experience "only" a 4-fold increase. Throughout the upper 50 m of the Northeast Pacific, OAXs increase relatively linearly with time, but sudden rapid increases in yearly extreme days and OAX duration are simulated to occur in the thermocline of the Central Northeast Pacific. These differences largely emerge from the large spatial differences in the magnitude and nature of variability in [H+], with the transition between the rather variable thermocline waters of the Offshore Northeast Pacific and the very stable waters of the Central Northeast Pacific causing a very sharp transition in the occurrence of OAX. This transition is caused by the limited offshore reach of offshore propagating eddies that are the dominant driver of OAX in the Northeast Pacific. As the OAXs become more extreme, more of them also become undersaturated with respect to aragonite (ΩA < 1), i.e., become corrosive. In the final year of our hindcast, we find that below 100 m OAXs are characterized by corrosive conditions across a wide stretch of the region offshore of the U.S. and Canadian Coasts. The spatially and temporal heterogeneous increases in OAX, including the abrupt appearance of extremes, likely have negative effects on the ability of marine organisms to adapt to the progression of OA and its associated extremes.
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Flora Desmet et al.
Status: closed
-
RC1: 'Comment on bg-2023-60', Anonymous Referee #1, 26 Apr 2023
General Comments
The authors provide an informative and insightful analysis of ocean acidification extreme events (OAX) in the North Pacific. They demonstrate the skill of their model in simulating observed OA trends, and they have addressed its representation of mean conditions and variability in a previous study (Desmet et al, 2022). They highlight the regional heterogeneity in frequency and intensity of extreme conditions in a changing climate and point out the processes contributing to regional differences, such as the magnitude of regional variability relative to the long-term trend and the influence of off-shore propagating eddies on property distributions. Additionally, they attribute changing OAX conditions to CO2 increase by comparing to a control simulation based on 1979 conditions. Their analysis of the role of mesoscale eddies in driving regional OAX characteristics is particularly interesting.
Specific Comments
- It appears that the model-observation comparisons are made only with surface data. Since an analysis of conditions at depths of up to 250 m is one of the interesting aspects of this study, could the authors provide some evaluation/discussion of model skill at these depths?
- Please include some discussion of the altered southern boundary condition in the control run in the context of attribution of OAX events to the rise in atmospheric CO2. How large is the impact of the southern boundary DIC concentration relative to gas exchange within the domain? Is the removed trend in DIC fully attributable to atmospheric CO2 increase or could there be a contribution associated with interannual variability?
- In the context of relative threshold, fixed baseline stressor detection, the authors highlight the importance of the ratio of the long-term trend to local variability in determining OAX increase. Could they also shed light on whether temporal changes in the variability contribute? For instance, does the magnitude of variability increase in some regions? This is addressed to an extent by Figure B4, but I find the histograms difficult to interpret due to their small size.
Technical Corrections
line 49: please add citation.
Figure 1: Please consider rephrasing “[H+] intensity” to reflect that it is the intensity of high [H+] events. Perhaps something like “[H+] event intensity”
line 231: “this horizon shoals by 50 m on average”. Is this the average over the whole domain or the coastal areas?
line 532: Are the units on sigma degrees latitude?
Citation: https://doi.org/10.5194/bg-2023-60-RC1 -
AC1: 'Reply on RC1', Flora Desmet, 26 Oct 2023
Dear reviewer,
We thank you for the very supportive and insightful comments on our manuscript. We appreciate the valuable remarks that helped us to significantly improve the quality of this manuscript.
Attached you can find our reply to your comments. The document also includes the applied changes in the manuscript.
Best regards,Flora Desmet
-
RC2: 'Comment on bg-2023-60', Anonymous Referee #2, 16 Aug 2023
REFEREE REPORT(S):
Referee: 1
GENERAL COMMENTS
The manuscript titled “Spatiotemporal heterogeneity in the increase of ocean acidity extremes in the Northeast Pacific” undertook an assessment and quantification of ocean acidity extreme events (OAXs) in the Northeast Pacific region. This was accomplished using a high-resolution regional Earth System Model (ROMS-BEC) building upon prior research (Desmet et al. 2022). The authors comprehensively examined various properties of OAXs in the Northeast Pacific Ocean including their intensity, frequency, duration, and heterogeneities in time and space. By employing the return periods and time of emergence (ToE) framework, they substantially captured in the occurrences of OAXs in depths over the recent 30 years.
The paper is logically organized, and the figure are well-presented. The investigation into OAXs in the Northeast Pacific is particularly interesting and holds substantial value due to utilization of a high-resolution model validated against observations. Nonetheless, there remain certain questions and comments that require clarification prior to proceeding further consideration of publication.
MAJOR COMMENTS
- The authors underscored the significance of mesoscale eddies in shaping the spatial heterogeneities of OAXs throughout the manuscript. This phenomenon was enabled by the employment of the high-resolution model (ROMS-BEC model), which resolves the mesoscale eddies and associated processes. Would the distinctive pattern of maximum intensity persist under coarser resolutions? To elucidate the reason for the spatial heterogeneities, I propose conducting the additional simulations using the same model with a coarse resolution. While a comparison with Burger’s findings in Earth system model has been made, the models are different, and this evidence is indirect. By comparing outcomes from the coarse resolution experiment using the same simulation parameters, you could substantially bolster the argument for the role of mesoscale eddies in driving OAXs.
- While authors use various properties of OAXs, some of these properties require explicit definitions prior to their usage in the manuscript such as volume fraction of OAX, contribution in Fig.2, near-permanent OAX in Section 3.5, 4.2. Additionally, it’s unclear whether the subsurface layers encompass both 100m and 200m or solely the 200m depth. While these undefined properties are conceptually understandable, but it is needed to provide more precise definitions in an academic context. These specific properties would be recommended within the methods or results sections. Additionally, because the manuscript includes substantial number of acronyms employed throughout the manuscript, it could be beneficial to present a comprehensive table listing these acronyms for clarity.
- The manuscript employs substantial numbers to elucidate OAX properties. However, the manuscript is inconvenient for matching the numbers with the figures. Consequently, the manuscript is difficult to follow, and the messages of research can be hindered. For instance, the discussion of Time of Emergence (ToE) is described as 16 years, 25years, 30-32years (L400). However, the ToE years are represented by the year ranging from 1984 to 2020 in figure 7. Authors need to make consistency by providing supplementary for matching ToE explained in the discussion. Similarly in Section 3.2, matching the numbers with figure 3 is also difficult to follow. This issue of mismatching or uncomfortable explanations are abundant through the manuscript. So, the manuscript is needed to enhance its comprehensibility.
- The abstract, while comprehensive, stretches to approximately 550 words, which surpasses the typical length of around 300 words. To effectively encapsulate the core research messages, I recommend shortening the abstract to a more concise form.
MINOR COMMENTS
L152: What about the mean biases in the spatial patterns of pH and ? Comparing these biases with data from the Global Ocean Data Analysis Project (GLODAP) could provide the better information for model validation.
L210: The authors mention the strong year-to-year variability and abrupt transition in the subsurface layer. However, there is no explanation of mechanisms for the strong variability and abrupt transition.
L315: It would be helpful to specify which figure corresponds to the statement being made.
L326: Also, there is no description of explanation in strongest step increases in subsurface of the CNP, not merely mentioning the occurrence of the strongest step.
L357-361: It is advisable to compare the results of Burger et al. 2020 and provide supplementary with similar analysis by using GFDL model results or other Earth system models. Alternatively, if the explanation relies on a specific figure, please indicate which figure elucidates the increase in the number of days as per Burger et al. 2020.
L388: It would be helpful to specify which figure corresponds to the statement being made.
L400, L411: Provide additional details regarding the marine environment, such as temperature, salinity, or other influential factors.
L412: How to suggest the emergence of near-permanent OAXs from this figure? A more comprehensive explanation is needed to clarify this point.
Citation: https://doi.org/10.5194/bg-2023-60-RC2 -
AC2: 'Reply on RC2', Flora Desmet, 26 Oct 2023
Dear reviewer,
We thank you for the insightful and encouraging feedback on our manuscript. We appreciate the valuable remarks that helped us to significantly improve the quality of this manuscript.
Attached you can find our reply to your comments. The document also includes the applied changes in the manuscript.
Best regards,Flora Desmet
Status: closed
-
RC1: 'Comment on bg-2023-60', Anonymous Referee #1, 26 Apr 2023
General Comments
The authors provide an informative and insightful analysis of ocean acidification extreme events (OAX) in the North Pacific. They demonstrate the skill of their model in simulating observed OA trends, and they have addressed its representation of mean conditions and variability in a previous study (Desmet et al, 2022). They highlight the regional heterogeneity in frequency and intensity of extreme conditions in a changing climate and point out the processes contributing to regional differences, such as the magnitude of regional variability relative to the long-term trend and the influence of off-shore propagating eddies on property distributions. Additionally, they attribute changing OAX conditions to CO2 increase by comparing to a control simulation based on 1979 conditions. Their analysis of the role of mesoscale eddies in driving regional OAX characteristics is particularly interesting.
Specific Comments
- It appears that the model-observation comparisons are made only with surface data. Since an analysis of conditions at depths of up to 250 m is one of the interesting aspects of this study, could the authors provide some evaluation/discussion of model skill at these depths?
- Please include some discussion of the altered southern boundary condition in the control run in the context of attribution of OAX events to the rise in atmospheric CO2. How large is the impact of the southern boundary DIC concentration relative to gas exchange within the domain? Is the removed trend in DIC fully attributable to atmospheric CO2 increase or could there be a contribution associated with interannual variability?
- In the context of relative threshold, fixed baseline stressor detection, the authors highlight the importance of the ratio of the long-term trend to local variability in determining OAX increase. Could they also shed light on whether temporal changes in the variability contribute? For instance, does the magnitude of variability increase in some regions? This is addressed to an extent by Figure B4, but I find the histograms difficult to interpret due to their small size.
Technical Corrections
line 49: please add citation.
Figure 1: Please consider rephrasing “[H+] intensity” to reflect that it is the intensity of high [H+] events. Perhaps something like “[H+] event intensity”
line 231: “this horizon shoals by 50 m on average”. Is this the average over the whole domain or the coastal areas?
line 532: Are the units on sigma degrees latitude?
Citation: https://doi.org/10.5194/bg-2023-60-RC1 -
AC1: 'Reply on RC1', Flora Desmet, 26 Oct 2023
Dear reviewer,
We thank you for the very supportive and insightful comments on our manuscript. We appreciate the valuable remarks that helped us to significantly improve the quality of this manuscript.
Attached you can find our reply to your comments. The document also includes the applied changes in the manuscript.
Best regards,Flora Desmet
-
RC2: 'Comment on bg-2023-60', Anonymous Referee #2, 16 Aug 2023
REFEREE REPORT(S):
Referee: 1
GENERAL COMMENTS
The manuscript titled “Spatiotemporal heterogeneity in the increase of ocean acidity extremes in the Northeast Pacific” undertook an assessment and quantification of ocean acidity extreme events (OAXs) in the Northeast Pacific region. This was accomplished using a high-resolution regional Earth System Model (ROMS-BEC) building upon prior research (Desmet et al. 2022). The authors comprehensively examined various properties of OAXs in the Northeast Pacific Ocean including their intensity, frequency, duration, and heterogeneities in time and space. By employing the return periods and time of emergence (ToE) framework, they substantially captured in the occurrences of OAXs in depths over the recent 30 years.
The paper is logically organized, and the figure are well-presented. The investigation into OAXs in the Northeast Pacific is particularly interesting and holds substantial value due to utilization of a high-resolution model validated against observations. Nonetheless, there remain certain questions and comments that require clarification prior to proceeding further consideration of publication.
MAJOR COMMENTS
- The authors underscored the significance of mesoscale eddies in shaping the spatial heterogeneities of OAXs throughout the manuscript. This phenomenon was enabled by the employment of the high-resolution model (ROMS-BEC model), which resolves the mesoscale eddies and associated processes. Would the distinctive pattern of maximum intensity persist under coarser resolutions? To elucidate the reason for the spatial heterogeneities, I propose conducting the additional simulations using the same model with a coarse resolution. While a comparison with Burger’s findings in Earth system model has been made, the models are different, and this evidence is indirect. By comparing outcomes from the coarse resolution experiment using the same simulation parameters, you could substantially bolster the argument for the role of mesoscale eddies in driving OAXs.
- While authors use various properties of OAXs, some of these properties require explicit definitions prior to their usage in the manuscript such as volume fraction of OAX, contribution in Fig.2, near-permanent OAX in Section 3.5, 4.2. Additionally, it’s unclear whether the subsurface layers encompass both 100m and 200m or solely the 200m depth. While these undefined properties are conceptually understandable, but it is needed to provide more precise definitions in an academic context. These specific properties would be recommended within the methods or results sections. Additionally, because the manuscript includes substantial number of acronyms employed throughout the manuscript, it could be beneficial to present a comprehensive table listing these acronyms for clarity.
- The manuscript employs substantial numbers to elucidate OAX properties. However, the manuscript is inconvenient for matching the numbers with the figures. Consequently, the manuscript is difficult to follow, and the messages of research can be hindered. For instance, the discussion of Time of Emergence (ToE) is described as 16 years, 25years, 30-32years (L400). However, the ToE years are represented by the year ranging from 1984 to 2020 in figure 7. Authors need to make consistency by providing supplementary for matching ToE explained in the discussion. Similarly in Section 3.2, matching the numbers with figure 3 is also difficult to follow. This issue of mismatching or uncomfortable explanations are abundant through the manuscript. So, the manuscript is needed to enhance its comprehensibility.
- The abstract, while comprehensive, stretches to approximately 550 words, which surpasses the typical length of around 300 words. To effectively encapsulate the core research messages, I recommend shortening the abstract to a more concise form.
MINOR COMMENTS
L152: What about the mean biases in the spatial patterns of pH and ? Comparing these biases with data from the Global Ocean Data Analysis Project (GLODAP) could provide the better information for model validation.
L210: The authors mention the strong year-to-year variability and abrupt transition in the subsurface layer. However, there is no explanation of mechanisms for the strong variability and abrupt transition.
L315: It would be helpful to specify which figure corresponds to the statement being made.
L326: Also, there is no description of explanation in strongest step increases in subsurface of the CNP, not merely mentioning the occurrence of the strongest step.
L357-361: It is advisable to compare the results of Burger et al. 2020 and provide supplementary with similar analysis by using GFDL model results or other Earth system models. Alternatively, if the explanation relies on a specific figure, please indicate which figure elucidates the increase in the number of days as per Burger et al. 2020.
L388: It would be helpful to specify which figure corresponds to the statement being made.
L400, L411: Provide additional details regarding the marine environment, such as temperature, salinity, or other influential factors.
L412: How to suggest the emergence of near-permanent OAXs from this figure? A more comprehensive explanation is needed to clarify this point.
Citation: https://doi.org/10.5194/bg-2023-60-RC2 -
AC2: 'Reply on RC2', Flora Desmet, 26 Oct 2023
Dear reviewer,
We thank you for the insightful and encouraging feedback on our manuscript. We appreciate the valuable remarks that helped us to significantly improve the quality of this manuscript.
Attached you can find our reply to your comments. The document also includes the applied changes in the manuscript.
Best regards,Flora Desmet
Flora Desmet et al.
Flora Desmet et al.
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