Sea ice reduction in the Barents-Kara Sea enhances June precipitation in the Yangtze River basin

Xie, Tianli; Zhou, Zhen-Qiang; Zhang, Renhe; Wu, Bingyi; Zhang, Peng

doi:https://doi.org/10.5194/egusphere-2024-2417

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https://doi.org/10.5194/egusphere-2024-2417

Preprints

18 Sep 2024

| 18 Sep 2024

Sea ice reduction in the Barents-Kara Sea enhances June precipitation in the Yangtze River basin

Tianli Xie, Zhen-Qiang Zhou, Renhe Zhang, Bingyi Wu, and Peng Zhang

Abstract. This study investigates the influence of June sea surface temperature (SST) and sea ice in the Barents-Kara Sea (BKS) on concurrent rainfall variability in the Yangtze River basin from 1982 to 2021 using both observational data and numerical experiments. The observed decrease in BKS sea ice and the corresponding increase in SST during June aligns with enhanced precipitation in the Yangtze River basin on the interannual timescale. The BKS thermal forcing induces an equivalent barotropic Rossby wave train in the middle and upper troposphere, which propagates southeastward to the Northwest Pacific (NWP). This Rossby wave train features two positive centers over the BKS and NWP, and one negative center above the Baikal Lake. The strengthened NWP subtropical high and upper-level westerly jet contribute to increased rainfall in the Yangtze River basin by enhancing moisture transport and anomalous ascending motions. These findings provide important implications for predicting summer rainfall in East Asia.

Received: 07 Aug 2024 – Discussion started: 18 Sep 2024

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Tianli Xie, Zhen-Qiang Zhou, Renhe Zhang, Bingyi Wu, and Peng Zhang

Status: final response (author comments only)

RC1:
'Comment on egusphere-2024-2417', Anonymous Referee #1, 01 Oct 2024
This study examines the relationship between June sea surface temperature (SST) and sea ice in the Barents-Kara Sea (BKS) and rainfall variability in the Yangtze River basin from 1982 to 2021. It finds that the observed decline in sea ice and rise in SST over the BKS region correlate with increased precipitation in the Yangtze River basin on an interannual timescale. The research identifies a barotropic Rossby wave train triggered by BKS thermal forcing, which propagates southeastward, contributing to enhanced rainfall through the strengthening of the subtropical high over the northwest Pacific region. These results have significant implications for summer rainfall predictions in East Asia. The paper is well organized and I have some comments and questions.

Review Comments:
Figure 1a displays regions of white within the SIC correlation coefficient map. These areas could either represent absence of sea ice in June or result from low standard deviation values. Given the potential for minimal or no sea ice presence, it would be helpful if the authors elucidated the rationale behind extending the study area beyond the Kara Sea. A more comprehensive explanation is warranted. In addition, delete either one colorbar in Fig.1a and Fig.1b since they are same.

The manuscript provides an extensive description of radiation flux; however, the definitions of the directional components within this context lacks clarity. It is advisable for the authors to incorporate precise definitions for these components to enhance understanding.

Figure 6 presents the results from the model simulations, utilizing the second principal component (PC2) of the EOF analysis, which indicates the presence of a wave-like pattern. This prompts two critical inquiries: Firstly, which EOF mode in the observations corresponds to the ‘+-+’ pattern? Secondly, do the model simulations align with the observations?

In Figure 5c, the units of the arrows should be specified as kg m⁻¹ s⁻¹. It is advisable for the authors to amend this notation in the figure accordingly.

The manuscript states, "However, the intensified net heat flux from the atmosphere to the ocean in the BKS region is not a direct cause of sea ice melting, but rather a consequence of decreased sea ice concentration." It is recommended that the authors further elucidate the causal relationship between these phenomena.
Citation: https://doi.org/10.5194/egusphere-2024-2417-RC1
- AC1: 'Reply on RC1', Zhen-Qiang Zhou, 18 Oct 2024
  
  Reply to Reviewer #1:
  
  This study examines the relationship between June sea surface temperature (SST) and sea ice in the Barents-Kara Sea (BKS) and rainfall variability in the Yangtze River basin from 1982 to 2021. It finds that the observed decline in sea ice and rise in SST over the BKS region correlate with increased precipitation in the Yangtze River basin on an interannual timescale. The research identifies a barotropic Rossby wave train triggered by BKS thermal forcing, which propagates southeastward, contributing to enhanced rainfall through the strengthening of the subtropical high over the northwest Pacific region. These results have significant implications for summer rainfall predictions in East Asia. The paper is well organized and I have some comments and questions.
  Thank you for the comments. We have implemented all the suggestions as detailed below. The original comments are quoted in Italic.
  1. Figure 1a displays regions of white within the SIC correlation coefficient map. These areas could either represent absence of sea ice in June or result from low standard deviation values. Given the potential for minimal or no sea ice presence, it would be helpful if the authors elucidated the rationale behind extending the study area beyond the Kara Sea. A more comprehensive explanation is warranted. In addition, delete either one colorbar in Fig.1a and Fig.1b since they are same.
  Response: Following the suggestion, we have reviewed the SIC climatology for June (Fig. R1a), which shows that the southern Barents Sea exhibits minimal sea ice coverage and relatively low standard deviation (<0.11) in sea ice concentration (Fig. R1b). However, in this region, SST demonstrates significant variability (>0.36, Fig. R2d). Therefore, the selection of our study area is based on the combined variability of both SIC and SST. Additionally, the SIC and SST variations in the Kara Sea are consistent with those in the Barents Sea during the same period (Fig. R2). Based on these considerations, we have chosen to include both the Barents and Kara Seas as the study area.
  
  Furthermore, the figure has been revised as suggested. (Fig.1)
  
  2. The manuscript provides an extensive description of radiation flux; however, the definitions of the directional components within this context lacks clarity. It is advisable for the authors to incorporate precise definitions for these components to enhance understanding.
  Response: We have added detailed definitions of the directional components of radiation flux in the manuscript (Lines 100-101).
  3. Figure 6 presents the results from the model simulations, utilizing the second principal component (PC2) of the EOF analysis, which indicates the presence of a wave-like pattern. This prompts two critical inquiries: Firstly, which EOF mode in the observations corresponds to the ‘+-+’ pattern? Secondly, do the model simulations align with the observations?
  Response: Following the suggestion, we conducted a Singular Value Decomposition (SVD) analysis (Fig. R3), where the first mode explains 53% of the total covariance. The 250 hPa geopotential height field displays a wave-like pattern (Fig. R3a), originating from the BKS and propagating across the mid-latitude continents to East Asia, reflecting the observed ‘+-+’ pattern. Furthermore, the model simulations are consistent with the observational results, demonstrating a similar wave-like pattern.
  4. In Figure 5c, the units of the arrows should be specified as kg m⁻¹ s⁻¹. It is advisable for the authors to amend this notation in the figure accordingly.
  Response: We have revised the figure, as suggested (Fig.5).
  5. The manuscript states, "However, the intensified net heat flux from the atmosphere to the ocean in the BKS region is not a direct cause of sea ice melting, but rather a consequence of decreased sea ice concentration." It is recommended that the authors further elucidate the causal relationship between these phenomena.
  Response: To clarify the causal relationship, we initially considered the hypothesis that atmospheric heating could directly cause sea ice melting. However, Fig. 4b shows that shortwave radiation is higher over the ocean than land, while the center of the high-pressure system is located over the Siberian landmass, which contradicts this assumption. Instead, further analysis reveals that increased shortwave radiation is concentrated in the Kara Sea, where sea ice has significantly decreased, consistent with the ice-albedo feedback mechanism (Kellogg, 1975; Curry et al., 1995; Screen and Simmonds, 2012).
  Reference:
  
  Kellogg, W. W.: Climatic feedback mechanisms involving the polar regions, Climate of the Arctic, 111-116, 1975.
  
  Curry, J. A., Schramm, J. L., and Ebert, E. E.: Sea ice-albedo climate feedback mechanism, Journal of Climate, 8, 240-247,
  
  1995.
  
  Screen, J. A. and Simmonds, I.: Declining summer snowfall in the Arctic: Causes, impacts and feedbacks, Climate dynamics,
  
  38, 2243-2256, 2012.
  
  Citation: https://doi.org/10.5194/egusphere-2024-2417-AC1
AC1:
'Reply on RC1', Zhen-Qiang Zhou, 18 Oct 2024

Reply to Reviewer #1:

This study examines the relationship between June sea surface temperature (SST) and sea ice in the Barents-Kara Sea (BKS) and rainfall variability in the Yangtze River basin from 1982 to 2021. It finds that the observed decline in sea ice and rise in SST over the BKS region correlate with increased precipitation in the Yangtze River basin on an interannual timescale. The research identifies a barotropic Rossby wave train triggered by BKS thermal forcing, which propagates southeastward, contributing to enhanced rainfall through the strengthening of the subtropical high over the northwest Pacific region. These results have significant implications for summer rainfall predictions in East Asia. The paper is well organized and I have some comments and questions.
Thank you for the comments. We have implemented all the suggestions as detailed below. The original comments are quoted in Italic.
1. Figure 1a displays regions of white within the SIC correlation coefficient map. These areas could either represent absence of sea ice in June or result from low standard deviation values. Given the potential for minimal or no sea ice presence, it would be helpful if the authors elucidated the rationale behind extending the study area beyond the Kara Sea. A more comprehensive explanation is warranted. In addition, delete either one colorbar in Fig.1a and Fig.1b since they are same.
Response: Following the suggestion, we have reviewed the SIC climatology for June (Fig. R1a), which shows that the southern Barents Sea exhibits minimal sea ice coverage and relatively low standard deviation (<0.11) in sea ice concentration (Fig. R1b). However, in this region, SST demonstrates significant variability (>0.36, Fig. R2d). Therefore, the selection of our study area is based on the combined variability of both SIC and SST. Additionally, the SIC and SST variations in the Kara Sea are consistent with those in the Barents Sea during the same period (Fig. R2). Based on these considerations, we have chosen to include both the Barents and Kara Seas as the study area.

Furthermore, the figure has been revised as suggested. (Fig.1)

2. The manuscript provides an extensive description of radiation flux; however, the definitions of the directional components within this context lacks clarity. It is advisable for the authors to incorporate precise definitions for these components to enhance understanding.
Response: We have added detailed definitions of the directional components of radiation flux in the manuscript (Lines 100-101).
3. Figure 6 presents the results from the model simulations, utilizing the second principal component (PC2) of the EOF analysis, which indicates the presence of a wave-like pattern. This prompts two critical inquiries: Firstly, which EOF mode in the observations corresponds to the ‘+-+’ pattern? Secondly, do the model simulations align with the observations?
Response: Following the suggestion, we conducted a Singular Value Decomposition (SVD) analysis (Fig. R3), where the first mode explains 53% of the total covariance. The 250 hPa geopotential height field displays a wave-like pattern (Fig. R3a), originating from the BKS and propagating across the mid-latitude continents to East Asia, reflecting the observed ‘+-+’ pattern. Furthermore, the model simulations are consistent with the observational results, demonstrating a similar wave-like pattern.
4. In Figure 5c, the units of the arrows should be specified as kg m⁻¹ s⁻¹. It is advisable for the authors to amend this notation in the figure accordingly.
Response: We have revised the figure, as suggested (Fig.5).
5. The manuscript states, "However, the intensified net heat flux from the atmosphere to the ocean in the BKS region is not a direct cause of sea ice melting, but rather a consequence of decreased sea ice concentration." It is recommended that the authors further elucidate the causal relationship between these phenomena.
Response: To clarify the causal relationship, we initially considered the hypothesis that atmospheric heating could directly cause sea ice melting. However, Fig. 4b shows that shortwave radiation is higher over the ocean than land, while the center of the high-pressure system is located over the Siberian landmass, which contradicts this assumption. Instead, further analysis reveals that increased shortwave radiation is concentrated in the Kara Sea, where sea ice has significantly decreased, consistent with the ice-albedo feedback mechanism (Kellogg, 1975; Curry et al., 1995; Screen and Simmonds, 2012).
Reference:

Kellogg, W. W.: Climatic feedback mechanisms involving the polar regions, Climate of the Arctic, 111-116, 1975.

Curry, J. A., Schramm, J. L., and Ebert, E. E.: Sea ice-albedo climate feedback mechanism, Journal of Climate, 8, 240-247,

1995.

Screen, J. A. and Simmonds, I.: Declining summer snowfall in the Arctic: Causes, impacts and feedbacks, Climate dynamics,

38, 2243-2256, 2012.

Citation: https://doi.org/10.5194/egusphere-2024-2417-AC1
- RC2:
  'Reply on AC1', Anonymous Referee #1, 21 Oct 2024
  
  I have no further comments and suggest accepting this manuscript
  
  Citation: https://doi.org/10.5194/egusphere-2024-2417-RC2
  - AC2: 'Reply on RC2', Zhen-Qiang Zhou, 06 Nov 2024
    
    Thank you for your insightful comments.
    
    Citation: https://doi.org/10.5194/egusphere-2024-2417-AC2
RC3:
'Comment on egusphere-2024-2417', Anonymous Referee #2, 30 Oct 2024

This study investigates the potential relation in variability between the Barents-Kara seaice and the June rainfall in Yangtze River Basin. They show that the observed decrease in BKS sea ice and the corresponding increase in SST during June aligns with enhanced precipitation in the Yangtze River at interannual timescale. The BKS thermal forcing induces a barotropic Rossby wave train that propagates southeastward to the Northwest Pacific (NWP), leading to an anomalous anticyclone that enhances rainfall in the Yangtze River Basin. The results are worth publishing. I recommend minor revision with the following comments.

Fig. 1:
Are the correlations in Fig. 1a and 1b computed based on detrended data, since we are talking about correlations at interannual time scale?
What is the correlation between the time series of Rainfall and SIC/SST shown in Fig. 1c?
I wonder if these correlations still hold for July. If not, what causes the differences between June and July?

Fig. 3c: no unit for y axis pressure cooredinate.
Fig. 5a is mentioned before Fig. 4 in the main text.
Fig. 5b: should the unit of specific humidity be g kg-1, considering the value can be 1?

Around Line 160: I understand how melting sea-ice warms the surface, but how does the warm surface leads to the high-pressure system? An explanation is needed for this step.

Line 175-178 versus Line193-196: I am a little confused. Are these two sentences describing the same information? If not, please explain more specifically. If so, the authors may consider remove such redundancy.

The authors mentioned the CAM5 simulations in the methodology. However, it is unclear to me where they present the CAM5 result. It should be stated more clearly in the main text and figure captions which is from observations, and which is from CAM5 simulations.

Citation: https://doi.org/10.5194/egusphere-2024-2417-RC3
- AC3: 'Reply on RC3', Zhen-Qiang Zhou, 12 Nov 2024
  
  Reply to Reviewer #3:
  This study investigates the potential relation in variability between the Barents-Kara seaice and the June rainfall in Yangtze River Basin. They show that the observed decrease in BKS sea ice and the corresponding increase in SST during June aligns with enhanced precipitation in the Yangtze River at interannual timescale. The BKS thermal forcing induces a barotropic Rossby wave train that propagates southeastward to the Northwest Pacific (NWP), leading to an anomalous anticyclone that enhances rainfall in the Yangtze River Basin. The results are worth publishing. I recommend minor revision with the following comments:
  Thank you for the comments. We have implemented all the suggestions as detailed below. The original comments are quoted in Italic.
  
  1. Fig. 1: Are the correlations in Fig. 1a and 1b computed based on detrended data, since we are talking about correlations at interannual time scale? What is the correlation between the time series of Rainfall and SIC/SST shown in Fig. 1c? I wonder if these correlations still hold for July. If not, what causes the differences between June and July?
  Response: Yes, the correlations in Fig.1a and 1b were computed using detrended data as introduced in the "Datasets and Methodology" section of the manuscript. Following your suggestion, we calculated the correlation between the time series of rainfall and SIC/SST shown in Fig. 1c, which are -0.37/0.44 during 1982-2021, both above the 0.1 significance level. For July, the correlations between BKS SST and rainfall in the Yangtze River Basin becomes insignificant (Fig. R1), which might due to the northward movement of mean rain belt. Additionally, according to Wu et al. (2023), the anomalous melting of Arctic sea ice primarily affects East Asian summer (JJA) precipitation in middle and high latitudes regions, with minimal impacts on rainfall in the Yangtze River basin. As a result, the August shows a similar relationship as in July, which might contributes to the seasonal mean anomalies being insignificant as in Wu et al. 2023. In contrast, our study highlights that changes in BKS SIC/SST in June significantly influence rainfall in the Yangtze River Basin as one of major findings in the present study. The reasons for the reduced impact in July and August need further investigation. We have included this discussion in the revised manuscript. (Lines 121-124, Lines 147-150)
  Reference:
  
  Wu, B., Li, Z., Zhang, X., Sha, Y., Duan, X., Pang, X., and Ding, S.: Has Arctic sea ice loss affected summer precipitation in North China?, International Journal of Climatology, 2023.
  
  2.Fig.3c, 5a, 5b:
  
  Fig. 3c: no unit for y axis pressure cooredinate.
  
  Fig. 5a is mentioned before Fig. 4 in the main text.
  
  Fig. 5b: should the unit of specific humidity be g kg-1, considering the value can be 1?
  Response: Modified as suggested. (Fig.3c and Fig.5)
  
  3. Around Line 160: I understand how melting sea-ice warms the surface, but how does the warm surface leads to the high-pressure system? An explanation is needed for this step.
  Response: Clarified as suggested. The melting sea-ice induced low-level warm can enhances the whole high-pressure system as which exhibiting a quasi-barotropic structure with upward wave activities (Fig.3c), as a result, the intensified high-pressure system leads to increased shortwave radiation (Fig. 4b) and further snow and sea ice reductions, as a positive feedback loop. We have added the above description in the revised manuscript. (Line 171-175)
  
  4. Line 175-178 versus Line193-196: I am a little confused. Are these two sentences describing the same information? If not, please explain more specifically. If so, the authors may consider remove such redundancy.
  Response: Modified as suggested. (Line 186-192 and Line 215-219)
  
  5. The authors mentioned the CAM5 simulations in the methodology. However, it is unclear to me where they present the CAM5 result. It should be stated more clearly in the main text and figure captions which is from observations, and which is from CAM5 simulations.
  Response: Modified as suggested. Fig. 6 is the only figure from CAM5 simulations. (Line 243 and Fig.6)
  
  Citation: https://doi.org/10.5194/egusphere-2024-2417-AC3
- AC4: 'Reply on RC3', Zhen-Qiang Zhou, 12 Nov 2024
  
  Reply to Reviewer #3:
  This study investigates the potential relation in variability between the Barents-Kara seaice and the June rainfall in Yangtze River Basin. They show that the observed decrease in BKS sea ice and the corresponding increase in SST during June aligns with enhanced precipitation in the Yangtze River at interannual timescale. The BKS thermal forcing induces a barotropic Rossby wave train that propagates southeastward to the Northwest Pacific (NWP), leading to an anomalous anticyclone that enhances rainfall in the Yangtze River Basin. The results are worth publishing. I recommend minor revision with the following comments:
  Thank you for the comments. We have implemented all the suggestions as detailed below. The original comments are quoted in Italic.
  
  1. Fig. 1: Are the correlations in Fig. 1a and 1b computed based on detrended data, since we are talking about correlations at interannual time scale? What is the correlation between the time series of Rainfall and SIC/SST shown in Fig. 1c? I wonder if these correlations still hold for July. If not, what causes the differences between June and July?
  Response: Yes, the correlations in Fig.1a and 1b were computed using detrended data as introduced in the "Datasets and Methodology" section of the manuscript. Following your suggestion, we calculated the correlation between the time series of rainfall and SIC/SST shown in Fig. 1c, which are -0.37/0.44 during 1982-2021, both above the 0.1 significance level. For July, the correlations between BKS SST and rainfall in the Yangtze River Basin becomes insignificant (Fig. R1), which might due to the northward movement of mean rain belt. Additionally, according to Wu et al. (2023), the anomalous melting of Arctic sea ice primarily affects East Asian summer (JJA) precipitation in middle and high latitudes regions, with minimal impacts on rainfall in the Yangtze River basin. As a result, the August shows a similar relationship as in July, which might contributes to the seasonal mean anomalies being insignificant as in Wu et al. 2023. In contrast, our study highlights that changes in BKS SIC/SST in June significantly influence rainfall in the Yangtze River Basin as one of major findings in the present study. The reasons for the reduced impact in July and August need further investigation. We have included this discussion in the revised manuscript. (Lines 121-124, Lines 147-150)
  Reference:
  
  Wu, B., Li, Z., Zhang, X., Sha, Y., Duan, X., Pang, X., and Ding, S.: Has Arctic sea ice loss affected summer precipitation in North China?, International Journal of Climatology, 2023.
  
  2.Fig.3c, 5a, 5b:
  
  Fig. 3c: no unit for y axis pressure cooredinate.
  
  Fig. 5a is mentioned before Fig. 4 in the main text.
  
  Fig. 5b: should the unit of specific humidity be g kg-1, considering the value can be 1?
  Response: Modified as suggested. (Fig.3c and Fig.5)
  
  3. Around Line 160: I understand how melting sea-ice warms the surface, but how does the warm surface leads to the high-pressure system? An explanation is needed for this step.
  Response: Clarified as suggested. The melting sea-ice induced low-level warm can enhances the whole high-pressure system as which exhibiting a quasi-barotropic structure with upward wave activities (Fig.3c), as a result, the intensified high-pressure system leads to increased shortwave radiation (Fig. 4b) and further snow and sea ice reductions, as a positive feedback loop. We have added the above description in the revised manuscript. (Line 171-175)
  
  4. Line 175-178 versus Line193-196: I am a little confused. Are these two sentences describing the same information? If not, please explain more specifically. If so, the authors may consider remove such redundancy.
  Response: Modified as suggested. (Line 186-192 and Line 215-219)
  
  5. The authors mentioned the CAM5 simulations in the methodology. However, it is unclear to me where they present the CAM5 result. It should be stated more clearly in the main text and figure captions which is from observations, and which is from CAM5 simulations.
  Response: Modified as suggested. Fig. 6 is the only figure from CAM5 simulations. (Line 243 and Fig.6)
  
  Citation: https://doi.org/10.5194/egusphere-2024-2417-AC4

Tianli Xie, Zhen-Qiang Zhou, Renhe Zhang, Bingyi Wu, and Peng Zhang

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Short summary

As the Arctic warms, changes in sea ice and sea surface temperatures are altering global climate patterns. Our study links Barents-Kara Sea ice melt and rising sea surface temperatures to increased summer rainfall in the Yangtze River Basin. Thermal forcing from the BKS induces Rossby wave trains that enhance moisture transport and anomalous ascending motion, leading to increased rainfall. Understanding this connection is critical for predicting summer rainfall in East Asia.


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