the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 13 Dec 2024
The future North Atlantic jet stream and storm track: relative contributions from sea ice and sea surface temperature changes
Abstract. Using a novel set of coordinated simulations with four different models, the response of the wintertime (December–February) North Atlantic jet stream and storm track to prescribed sea surface temperatures and sea-ice loss is analysed. Three out of the four models show a southward shift of the upper-level jet stream with an increase in jet speed over Europe, where the contribution of sea surface temperatures dominates over the effects of sea-ice loss. However, the remaining model lacks the increase in jet speed over Europe, which originates from opposite responses of similar magnitude due to the future sea surface temperatures and sea-ice cover. The jet stream responses are primarily driven by the change in the meridional temperature gradient and, as a consequence, baroclinicity. At the same time, momentum flux convergence acts as a secondary amplifying and dampening factor. The same three models see a significant eastward shift of the extratropical cyclone track density, which is equally driven by changes to sea surface temperatures and sea ice cover. A consistent feature across all models is a decrease in the frequency of extratropical cyclones in the Mediterranean. The responses of extratropical cyclones to future sea-ice cover and sea surface temperatures do not exceed the inter-model climatological differences. Notable differences in the future response occur, and thus considerable uncertainty remains in how the European climate will respond to a warmer climate.
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RC1: 'Comment on egusphere-2024-3713', Anonymous Referee #1, 17 Jan 2025
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Review of the manuscript
The future North Atlantic jet stream and storm track: relative
contributions from sea ice and sea surface temperature changes
Daniel Köhler, Petri Räisänen, Tuomas Naakka, Kalle Nordling, and Victoria A. Sinclair
submitted to WCD (WCD-2024-3713)
Using coordinated simulations with four different global atmospheric models,
the study explores the roles of future sea surface temperature changes and
sea-ice loss for changes of the wintertime North Atlantic jet streams and
storm tracks.
The simulated changes are extensively described with a focus on the
dynamical drivers of the North Atlantic jet stream changes, namely
the changes in the baroclinicity and momentum flux convergence.
However, differences between the model responses with respect to the
position and strength of jet stream and storm track shifts, point to
uncertainties and leaving the European climate's future response uncertain.Given that there is still a debate within the scientific community about
the impact of Arctic sea ice loss in the mid-latitude westerlies and storm
tracks (with the possibility that Arctic sea ice loss weakens the
North Atlantic jet stream favoring more severe cold winters), the topic
of this study is timely and relevant. As reasons for the disagreement in
the response to sea ice loss found in modelling studies include differences
in the forcing fields (pattern and magnitude of forcing), coordinated
experiments are an appropriate approach to tackle this question.
The Polar Amplification Model Intercomparison Project (PAMIP, Smith et al.,
2019) as part of CMIP6 provided a large set of coordinated experiments
which allows to study the relative roles of local sea ice and remote sea
surface temperature changes in response of the global climate system
to changes in Polar sea ice.
This study is in my view complementary using a smaller set of models and
different forcing fields.The manuscript is well-structured, but especially the part describing the results
is lengthy, and, at some places, provides too much details.
In comparison, the discussion and conclusion part is too short, and
misses links to relevant other studies.Overall, the submitted manuscript needs careful and major revision.
---------------------------------Major comments:
(1) I appreciate the careful description of the figures in the results sections.
Nonetheless, I recommend a shortening of the description part of the manuscript,
especially sections 4 to 6, to allow for clearer main messages. In addition,
I strongly recommend more discussions of the results in comparison to
other studies. This could be done either in the respective sections or
as an additional section.
As one example, I would like to mention the role of deep Arctic warming
for mid-latitude atmospheric circulation changes which is discussed e.g.
in Cohen et al. (2020), Xu et al. (2023), Kim et al. (2021).
Since the model results presented here do show significant differences
in the vertical extent of Arctic warming in response to se-ice loss (figure 10),
the results obtained here have to be discussed in the context of other
studies which will certainly provide interesting insights!
Such discussion are needed also for the other interesting results to place them
in the context of other studies.(2) Based on the above mentioned extended discussions, the conclusions in section 7
have to be improved to highlight the new insights from this study and to elaborate
on future research.(3) In the introduction, the benefits of coordinated model experiments should be
discussed more in depth, in particular the PAMIP approach (Smith et al., 2019),
and the differences to the approach applied in this study. It would be also good to
explain a bit more the role of the coordinated model experiments for the whole
EU project CriceS.(4) The relative small sizes of the model runs with 40 years and what does this
mean for the robustness of the results should be more critically discussed,
given that, e.g. Peings et al. (2021) showed that even 100 years might be not
enough to capture the remote atmospheric response to future Arctic sea ice loss
against the large internal variability.(5) The applied statistical testing has to be critically reviewed. For all testings,
a two-sided t test has been applied. For some of the shown distributions (e.g.
ETC life time in fig. 4b) it is obvious that the assumptions for the t-test, in
particular normally distributed samples, are not fulfilled.
For testing differences in fields, the tested fields are highly
spatio-temporally correlated. Thus the t-test has to be controlled for
field significance (e.g. by applying the false
discovery rate (FDR; Wilks, 2016).(6) I recommend an extended descriptions of the model experiments. I recognized that
more information is given in Naakka et al. (2024), but it would help the reader to have
more information also in this manuscript. For the description of the experiments, a table
would be good. Since the model simulations are atmosphere-only simulations, I suggest to
mention the similarities in the models, too:
e.g. NorESM atmospheric component is based on CESM atmospheric component, and EC-Earth
atmospheric component is based on an earlier version of OpenIFS. This should be also
included into the discussion of the results.----------------------------------------------------
Minor comments:(1) Introduction, L36-37: References are missing.
(2) Section 2, L112-115: Beside the E-vector, the Plumb flux (Plumb, 1985)and the
localized EP flux (Trenberth, 1986) allows local diagnosis
of the three-dimensional propagation of wave activity. Could you, please comment, why
you have chosen the E-vectors?(3) Section 2, L123-124: For frictionless motion!
(4) Section 3.1, L164-165: I would appreciate to see a corresponding figure to show the
differences in zonal wind and baroclinicity to ERA5, e.g. in the appendix.(5) Section 3.2, L199-200: Please explain, why the focus is on the jet exit region?
(6) Section 3.2, L220: Isn't the North-Atlantic jet an eddy-driven jet in all models?
(7) Section 3.3, L223: "ETCs affecting Europe (30° N - 70° N, 15° W - 35° E)," Does that
mean those ETC's which have their endpoint in that area?(8) Section 5.1, L353: "to increase the jet speed located above the maximum in the Baseline simulation"
For CESM, the increase in the jet speed is clearly southward of the maximum in the Baseline simulation,
which is not so obvious in the other models (Fig. 8g). Could you, please comment on this?
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Kim, D., et al. (2021). Atmospheric circulation sensitivity to changes in the vertical
structure of polar warming. GRL, 48, e2021GL094726.
https://doi.org/10.1029/2021GL094726Xu, M., et al. (2023). Important role of stratosphere-troposphere coupling in the
Arctic mid-to-upper tropospheric warming in response to sea-ice loss. npj Clim Atmos Sci 6.
https://doi.org/10.1038/s41612-023-00333-2Cohen, J., et al, (2020). Divergent consensuses on Arctic amplification influence on midlatitude
severe winter weather. Nat. Clim. Chang. 10, 20-29 (2020).
https://doi.org/10.1038/s41558-019-0662-ySmith, D. M., et al. (2019). The Polar Amplification Model Intercomparison Project (PAMIP)
contribution to CMIP6: investigating the causes and consequences of polar amplification.
Geosci. Model Dev., 12, 1139-1164.
https://doi.org/10.5194/gmd-12-1139-2019
Wilks,D. (2016). The stippling shows statistically significant grid points-
How Research Results are Routinely Overstated and Overinterpreted, and What to Do about It.
BAMS 97, 2263-2273.
https://doi.org/10.1175/BAMS-D-15-00267.1Peings, Y., et al. (2021). Are 100 ensemble members enough to capture the remote atmospheric
response to+ 2° C Arctic sea ice loss?. Journal of Climate, 34, 3751-3769.
https://doi.org/10.1175/JCLI-D-20-0613.1Plumb, R. A. (1985). On the three-dimensional propagation of stationary waves.
J. Stm. Sci., 42, 217-229.
https://doi.org/10.1175/1520-0469(1985)042<0217:OTTDPO>2.0.CO;2Trenberth, K. E. (1986). An assessment of the impact of transient eddies on the zonal
flow during a blocking episode using localized Eliassen-Palm flux diagnostics.
J. Atm. Sci., 43, 2070-2087.
https://doi.org/10.1175/1520-0469(1986)043<2070:AAOTIO>2.0.CO;2Citation: https://doi.org/10.5194/egusphere-2024-3713-RC1
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