European summer weather linked to North Atlantic freshwater events in preceding years

Oltmanns, Marilena; Holliday, N. Penny; Screen, James; Moat, Ben I.; Josey, Simon A.; Evans, D. Gwyn; Bacon, Sheldon

doi:https://doi.org/10.5194/wcd-2023-1

Preprints

https://doi.org/10.5194/wcd-2023-1

Preprints

24 Jan 2023

| 24 Jan 2023

Status: a revised version of this preprint was accepted for the journal WCD and is expected to appear here in due course.

European summer weather linked to North Atlantic freshwater events in preceding years

Marilena Oltmanns, N. Penny Holliday, James Screen, Ben I. Moat, Simon A. Josey, D. Gwyn Evans, and Sheldon Bacon

Abstract. Amplified Arctic ice loss in recent decades has been linked to increased occurrence of extreme mid-latitude weather. The underlying mechanisms remain elusive, however. One potential link occurs through the ocean as the loss of sea ice and glacial ice leads to increased freshwater fluxes into the North Atlantic. Thus, in this study, we examine the extent to which North Atlantic freshwater anomalies constrain the subsequent ocean-atmosphere evolution and assess their implications for European summer weather. Combining remote sensing, atmospheric reanalyses and model simulations, we show that stronger freshwater anomalies are associated with sharper sea surface temperature gradients over the North Atlantic in winter, destabilising the overlying atmosphere and inducing a northward shift in the North Atlantic Current. In turn, the jet stream over the North Atlantic is deflected northward in the following summers, leading to warmer and drier weather over Europe. Our results suggest that growing freshwater fluxes into the North Atlantic will increase the risk of heat waves and droughts over the coming decades, and could yield enhanced predictability of European summer weather, months to years in advance.

Received: 16 Jan 2023 – Discussion started: 24 Jan 2023

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Marilena Oltmanns et al.

Status: closed

RC1:
'Comment on wcd-2023-1', Anonymous Referee #1, 20 Mar 2023

The authors use statistical analysis of observations and reanalysis data to support their hypothesis that warmer and drier summer weather in Europe can be linked to freshwater anomalies in the North Atlantic subpolar gyre region during the preceding year. The proposed mechanism for this link is a northward shift of the North Atlantic current leading to a similar deflection of the jet stream and therefore altering the advection pathway of maritime air masses. The foundation of the analysis are freshwater indices derived from a mass balance equation that are used to identify freshwater anomalies in relation to simultaneous sea surface temperature (SST) anomalies linked to the North Atlantic Oscillation (NAO).
I understand that this is a re-submission of an earlier version of the manuscript, but I was not involved in the previous review process. Therefore, I cannot assess how the manuscript has been improved, but rather provide a fresh pair of eyes. Please find my comments attached.

Citation: https://doi.org/10.5194/wcd-2023-1-RC1
- AC1: 'Reply on RC1', Marilena Oltmanns, 08 May 2023
  
  The comment was uploaded in the form of a supplement: https://wcd.copernicus.org/preprints/wcd-2023-1/wcd-2023-1-AC1-supplement.pdf
  
  Citation: https://doi.org/10.5194/wcd-2023-1-AC1
RC2:
'Comment on wcd-2023-1', Anonymous Referee #2, 31 Mar 2023
This paper documents an apparent impact of subpolar freshwater anomalies onto the North Atlantic sea surface temperatures (SSTs) in winter and the subsequent summer. These summer SSTs are then argued to drive changes in the atmospheric circulation that drive significant changes in summer temperatures though analysis of reanalysis data and model simulations. Overall, the authors argue that freshwater changes in the North Atlantic can drive a large amount of the variance in European summer temperatures and so argue that such a mechanism could provide significant extra skill in seasonal predictions.

Overall this is an interesting study and presents an exciting set of results. However, I have a number of issues with the manuscript in its current form, which I list below in my major comments. Therefore, I do not recommend publication at this time.

Major comments
My main concern with this manuscript is that the most important results are buried in the appendix - that is the definition of the freshwater budget method, and the results evaluating the impact of freshwater on the North Atlantic SSTs. I note that this is similar to the structure of Oltmanns et al, 2020, in GRL. This usage of the appendix meant that the paper was quite challenging to read as I had to keep flipping back and forth in the paper to try to understand the logic. Furthermore, I would argue that the most important result in the paper is the evaluation of impact of freshwater onto the resulting SST (e.g., section A3). Therefore, I would strongly recommend the authors to move more of the important background information into the main paper and systematically step the reader through the ideas.

Although I found the results exciting, ultimately the results on the potential impact of freshwater anomalies on Europe were much more uncertain than I feel the authors described. The uncertainty spawns, I think, from the following important reasons.
There is little physical understanding of how the summer NAO is leading large freshwater driven SST changes across the subpolar North Atlantic. For example, there is much discussion about how the sign of the relationship between summer NAO and the winter SST anomaly changes at ~0.5, but no reason for this is given. How can we be certain that the freshwater analysis is picking up real physical changes related to changes in freshwater?

Related to the above, the paper focuses on interannual changes - however, there is plenty of observed evidence that there is significant decadal time-scale variability in the subpolar North Atlantic, and so questions arise on what the freshwater analysis, which appears to assume independence between years, is picking up. For example, figure 2c shows that it is the decadal time-scales that dominate the summer NAO time-series, consistent with a southward shift of the North Atlantic Jet (e.g., Dong et al, 2013). Given the low-frequency changes, how can the authors be sure that they are seeing the interannual impact of freshwater changes as opposed to a different, or related, mechanism? For example, is the summer NAO and winter SST responding to another mechanism which is driving both (e.g., large-scale ocean circulation, external forcings etc)? As mentioned below, I think overlaying a timeseries of summer NAO with subpolar SSTs would be useful in seeing the potential importance of these longer timescales.

The model experiments are interesting, but they do not, of course, explore the impact of freshwater changes on European weather. Indeed, the models are feeling the influence of both SSTs and, presumably, external forcings, and the assumption is that the influence of freshwater can be isolated by focusing on the summer NAO analysis. However, and related to point b, this relies that there are no other mechanisms in play that could explain both the summer NAO and the winter SSTs. Indeed, I can’t help noticing that the SST patterns being explored in the models (figure 8a and figure 9a and b) look like the cold AMV pattern (e.g., Zhang et al, 2019). Therefore, how can the authors be confident that the atmospheric circulation patterns are being driven by just the subpolar North Atlantic, rather than the tropical North Atlantic?

Minor comments
Line 1 - The paper discusses here and in the introduction the possibility of long-term forced changes in the arctic having an impact on the mid-latitude weather, but then exclusively focuses on interannual variability? This left me slightly confused about what I was supposed to be taking from the analysis, so maybe it worth rethinking the motivation?
Line 63 - why not just use HadISST rather than merge the two datasets?
Section 2.2 - how do these simulations differ from CMIP style AMIP runs (e.g., Eyring et al, 2016)?
Section 2.3 - I am confused by the discussion of a trend, in part due to the framing of the paper on understanding the impact of long-term forced changes in the Arctic on the wider climate, but I think the point is that trends over land may alter the relationship with ocean SST anomalies? Please clarify.
Line 133 - You state that “we derive indices that exhibit a strong relationship to subpolar temperatures but not to the drivers of density anomalies”, but then argue that the summer NAO drives changes in freshwater (which will drive the density anomalies) - can you clarify your point, please?
Line 140 - you mention the potential impact for the summer NAO on freshwater, but won’t it also affect the heat fluxes too? How do you take account of these in your analysis?
Line 392 - “...we select all the years that lead to an increase in the slope of the regression line.” - I’m uncomfortable about this - it sounds a bit like cherry picking to get a stronger signal (which in turn would affect your conclusions about how important freshwater changes are). Please could the authors justify this choice physically?
Figure A1 - I can't help but to see that there is a long-term change in the summer NAO with most negative years occurring after 2000. This is consistent with the long-term trend in summer jet. The question this raises is how important is this longer term variability in the sNAO for your results? One thing that is missing is a time series of subpolar SST, which also shows significant decadal timescale variability. Please add this to figure A1 at least. However, following up on this, previous studies have linked these low frequency variability in the jet to the changes in the subpolar temperatures (e.g., Dong et al, 2016) - therefore, how certain are you that the changes in summer NAO are not reflecting decadal time-scale changes in the subpolar gyre?
Line 405 - It's not clear to me why the geostrophic currents are not important - there is nothing in the 95% confidence lines that are related to constant lines of density. Please can you clarify your reasoning here?
Section A3 - it is not clear why you are regressing the JFM heat fluxes onto your summer NAO? In order to rule out the impact of surface heat fluxes on the JFM SSTs it is the integrated heat fluxes between the summer and JFM that are important?
417 - the mean mixed layer is the annual mean mixed layer?
Line 440 - Please elaborate on the origin of the uncertainty numbers.
Figure 3 - what is the data used to plot the SSS anomalies, or are they implied from your mass balance analysis?
Section 4.2 - I think when you mean first summer, you mean the same summer as the summer NAO event? However, it was not clear as you’re talking about looking in subsequent summers which I took to be after the winter - please clarify. This is particularly important for figure 6, which is unclear what summers you are looking at.
Figure 7 - Are you computing the variance explained using all years, or are you only focusing on the years that you have large changes? Either way, how important is the long-term trend in atmospheric circulation shown in figure 2c for your analysis?
Figure 9 - Is the only difference between these figures and that shown in figure 8 (d and f) that they are split across model ensembles? Not clear to me why this is relevant - could you just include the ensemble spread in figure 8 ?

Eyring, V., S. Bony, G. A. Meehl, C. A. Senior, B. Stevens, R. J. Stouffer, and K. E. Taylor, 2016: Overview of the Coupled Model Intercomparison Project phase 6 (CMIP6) experimental design and organization. Geosci. Model Dev., 9, 1937–1958

Dong, B., Sutton, R. T., Woollings, T. and Hodges, K. (2013) Variability of the North Atlantic summer storm track: mechanisms and impacts on European climate. Environmental Research Letters, 8 (3). 034037. ISSN 1748-9326 doi: https://doi.org/10.1088/1748-9326/8/3/034037

Zhang, R., Sutton, R., Danabasoglu, G., Kwon, Y.‐O., Marsh, R., Yeager, S. G., Amrhein, D. E. and Little, C. M. (2019) A review of the role of the Atlantic meridional overturning circulation in Atlantic multidecadal variability and associated climate impacts. Reviews of Geophysics, 57 (2). pp. 316-375. ISSN 8755-1209 doi: https://doi.org/10.1029/2019RG000644
Citation: https://doi.org/10.5194/wcd-2023-1-RC2
- AC2: 'Reply on RC2', Marilena Oltmanns, 08 May 2023
  
  The comment was uploaded in the form of a supplement: https://wcd.copernicus.org/preprints/wcd-2023-1/wcd-2023-1-AC2-supplement.pdf
  
  Citation: https://doi.org/10.5194/wcd-2023-1-AC2

Status: closed

RC1:
'Comment on wcd-2023-1', Anonymous Referee #1, 20 Mar 2023

The authors use statistical analysis of observations and reanalysis data to support their hypothesis that warmer and drier summer weather in Europe can be linked to freshwater anomalies in the North Atlantic subpolar gyre region during the preceding year. The proposed mechanism for this link is a northward shift of the North Atlantic current leading to a similar deflection of the jet stream and therefore altering the advection pathway of maritime air masses. The foundation of the analysis are freshwater indices derived from a mass balance equation that are used to identify freshwater anomalies in relation to simultaneous sea surface temperature (SST) anomalies linked to the North Atlantic Oscillation (NAO).
I understand that this is a re-submission of an earlier version of the manuscript, but I was not involved in the previous review process. Therefore, I cannot assess how the manuscript has been improved, but rather provide a fresh pair of eyes. Please find my comments attached.

Citation: https://doi.org/10.5194/wcd-2023-1-RC1
- AC1: 'Reply on RC1', Marilena Oltmanns, 08 May 2023
  
  The comment was uploaded in the form of a supplement: https://wcd.copernicus.org/preprints/wcd-2023-1/wcd-2023-1-AC1-supplement.pdf
  
  Citation: https://doi.org/10.5194/wcd-2023-1-AC1
RC2:
'Comment on wcd-2023-1', Anonymous Referee #2, 31 Mar 2023
This paper documents an apparent impact of subpolar freshwater anomalies onto the North Atlantic sea surface temperatures (SSTs) in winter and the subsequent summer. These summer SSTs are then argued to drive changes in the atmospheric circulation that drive significant changes in summer temperatures though analysis of reanalysis data and model simulations. Overall, the authors argue that freshwater changes in the North Atlantic can drive a large amount of the variance in European summer temperatures and so argue that such a mechanism could provide significant extra skill in seasonal predictions.

Overall this is an interesting study and presents an exciting set of results. However, I have a number of issues with the manuscript in its current form, which I list below in my major comments. Therefore, I do not recommend publication at this time.

Major comments
My main concern with this manuscript is that the most important results are buried in the appendix - that is the definition of the freshwater budget method, and the results evaluating the impact of freshwater on the North Atlantic SSTs. I note that this is similar to the structure of Oltmanns et al, 2020, in GRL. This usage of the appendix meant that the paper was quite challenging to read as I had to keep flipping back and forth in the paper to try to understand the logic. Furthermore, I would argue that the most important result in the paper is the evaluation of impact of freshwater onto the resulting SST (e.g., section A3). Therefore, I would strongly recommend the authors to move more of the important background information into the main paper and systematically step the reader through the ideas.

Although I found the results exciting, ultimately the results on the potential impact of freshwater anomalies on Europe were much more uncertain than I feel the authors described. The uncertainty spawns, I think, from the following important reasons.
There is little physical understanding of how the summer NAO is leading large freshwater driven SST changes across the subpolar North Atlantic. For example, there is much discussion about how the sign of the relationship between summer NAO and the winter SST anomaly changes at ~0.5, but no reason for this is given. How can we be certain that the freshwater analysis is picking up real physical changes related to changes in freshwater?

Related to the above, the paper focuses on interannual changes - however, there is plenty of observed evidence that there is significant decadal time-scale variability in the subpolar North Atlantic, and so questions arise on what the freshwater analysis, which appears to assume independence between years, is picking up. For example, figure 2c shows that it is the decadal time-scales that dominate the summer NAO time-series, consistent with a southward shift of the North Atlantic Jet (e.g., Dong et al, 2013). Given the low-frequency changes, how can the authors be sure that they are seeing the interannual impact of freshwater changes as opposed to a different, or related, mechanism? For example, is the summer NAO and winter SST responding to another mechanism which is driving both (e.g., large-scale ocean circulation, external forcings etc)? As mentioned below, I think overlaying a timeseries of summer NAO with subpolar SSTs would be useful in seeing the potential importance of these longer timescales.

The model experiments are interesting, but they do not, of course, explore the impact of freshwater changes on European weather. Indeed, the models are feeling the influence of both SSTs and, presumably, external forcings, and the assumption is that the influence of freshwater can be isolated by focusing on the summer NAO analysis. However, and related to point b, this relies that there are no other mechanisms in play that could explain both the summer NAO and the winter SSTs. Indeed, I can’t help noticing that the SST patterns being explored in the models (figure 8a and figure 9a and b) look like the cold AMV pattern (e.g., Zhang et al, 2019). Therefore, how can the authors be confident that the atmospheric circulation patterns are being driven by just the subpolar North Atlantic, rather than the tropical North Atlantic?

Minor comments
Line 1 - The paper discusses here and in the introduction the possibility of long-term forced changes in the arctic having an impact on the mid-latitude weather, but then exclusively focuses on interannual variability? This left me slightly confused about what I was supposed to be taking from the analysis, so maybe it worth rethinking the motivation?
Line 63 - why not just use HadISST rather than merge the two datasets?
Section 2.2 - how do these simulations differ from CMIP style AMIP runs (e.g., Eyring et al, 2016)?
Section 2.3 - I am confused by the discussion of a trend, in part due to the framing of the paper on understanding the impact of long-term forced changes in the Arctic on the wider climate, but I think the point is that trends over land may alter the relationship with ocean SST anomalies? Please clarify.
Line 133 - You state that “we derive indices that exhibit a strong relationship to subpolar temperatures but not to the drivers of density anomalies”, but then argue that the summer NAO drives changes in freshwater (which will drive the density anomalies) - can you clarify your point, please?
Line 140 - you mention the potential impact for the summer NAO on freshwater, but won’t it also affect the heat fluxes too? How do you take account of these in your analysis?
Line 392 - “...we select all the years that lead to an increase in the slope of the regression line.” - I’m uncomfortable about this - it sounds a bit like cherry picking to get a stronger signal (which in turn would affect your conclusions about how important freshwater changes are). Please could the authors justify this choice physically?
Figure A1 - I can't help but to see that there is a long-term change in the summer NAO with most negative years occurring after 2000. This is consistent with the long-term trend in summer jet. The question this raises is how important is this longer term variability in the sNAO for your results? One thing that is missing is a time series of subpolar SST, which also shows significant decadal timescale variability. Please add this to figure A1 at least. However, following up on this, previous studies have linked these low frequency variability in the jet to the changes in the subpolar temperatures (e.g., Dong et al, 2016) - therefore, how certain are you that the changes in summer NAO are not reflecting decadal time-scale changes in the subpolar gyre?
Line 405 - It's not clear to me why the geostrophic currents are not important - there is nothing in the 95% confidence lines that are related to constant lines of density. Please can you clarify your reasoning here?
Section A3 - it is not clear why you are regressing the JFM heat fluxes onto your summer NAO? In order to rule out the impact of surface heat fluxes on the JFM SSTs it is the integrated heat fluxes between the summer and JFM that are important?
417 - the mean mixed layer is the annual mean mixed layer?
Line 440 - Please elaborate on the origin of the uncertainty numbers.
Figure 3 - what is the data used to plot the SSS anomalies, or are they implied from your mass balance analysis?
Section 4.2 - I think when you mean first summer, you mean the same summer as the summer NAO event? However, it was not clear as you’re talking about looking in subsequent summers which I took to be after the winter - please clarify. This is particularly important for figure 6, which is unclear what summers you are looking at.
Figure 7 - Are you computing the variance explained using all years, or are you only focusing on the years that you have large changes? Either way, how important is the long-term trend in atmospheric circulation shown in figure 2c for your analysis?
Figure 9 - Is the only difference between these figures and that shown in figure 8 (d and f) that they are split across model ensembles? Not clear to me why this is relevant - could you just include the ensemble spread in figure 8 ?

Eyring, V., S. Bony, G. A. Meehl, C. A. Senior, B. Stevens, R. J. Stouffer, and K. E. Taylor, 2016: Overview of the Coupled Model Intercomparison Project phase 6 (CMIP6) experimental design and organization. Geosci. Model Dev., 9, 1937–1958

Dong, B., Sutton, R. T., Woollings, T. and Hodges, K. (2013) Variability of the North Atlantic summer storm track: mechanisms and impacts on European climate. Environmental Research Letters, 8 (3). 034037. ISSN 1748-9326 doi: https://doi.org/10.1088/1748-9326/8/3/034037

Zhang, R., Sutton, R., Danabasoglu, G., Kwon, Y.‐O., Marsh, R., Yeager, S. G., Amrhein, D. E. and Little, C. M. (2019) A review of the role of the Atlantic meridional overturning circulation in Atlantic multidecadal variability and associated climate impacts. Reviews of Geophysics, 57 (2). pp. 316-375. ISSN 8755-1209 doi: https://doi.org/10.1029/2019RG000644
Citation: https://doi.org/10.5194/wcd-2023-1-RC2
- AC2: 'Reply on RC2', Marilena Oltmanns, 08 May 2023
  
  The comment was uploaded in the form of a supplement: https://wcd.copernicus.org/preprints/wcd-2023-1/wcd-2023-1-AC2-supplement.pdf
  
  Citation: https://doi.org/10.5194/wcd-2023-1-AC2

Marilena Oltmanns et al.

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Short summary

The Arctic is currently warming twice as fast as the global average. This results in enhanced melting and thus freshwater releases into the North Atlantic. Using a combination of observations and models, we show that atmosphere-ocean feedbacks initiated by freshwater releases into the North Atlantic lead to warmer and drier weather over Europe in subsequent summers. The existence of this dynamical link suggests that European summer weather can be predicted months to years in advance.


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