the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 18 Sep 2024
Sea ice reduction in the Barents-Kara Sea enhances June precipitation in the Yangtze River basin
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.
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RC1: 'Comment on egusphere-2024-2417', Anonymous Referee #1, 01 Oct 2024
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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
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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.
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