Large-scale transport into the Arctic: the roles of the midlatitude jet and the Hadley Cell

Yang, Huang; Waugh, Darryn W.; Orbe, Clara; Zeng, Guang; Morgenstern, Olaf; Kinnison, Douglas E.; Lamarque, Jean-Francois; Tilmes, Simone; Plummer, David A.; Jöckel, Patrick; Strahan, Susan E.; Stone, Kane A.; Schofield, Robyn

doi:https://doi.org/10.5194/acp-19-5511-2019

Articles | Volume 19, issue 8

https://doi.org/10.5194/acp-19-5511-2019

© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Special issue:

Chemistry–Climate Modelling Initiative (CCMI) (ACP/AMT/ESSD/GMD...

https://doi.org/10.5194/acp-19-5511-2019

© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 19, issue 8

Research article

|

26 Apr 2019

Research article |

| 26 Apr 2019

Large-scale transport into the Arctic: the roles of the midlatitude jet and the Hadley Cell

Huang Yang, Darryn W. Waugh, Clara Orbe, Guang Zeng, Olaf Morgenstern, Douglas E. Kinnison, Jean-Francois Lamarque, Simone Tilmes, David A. Plummer, Patrick Jöckel, Susan E. Strahan, Kane A. Stone, and Robyn Schofield

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Final revised paper (published on 26 Apr 2019)
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AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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RC1: 'Review of “Large-scale transport into the Arctic: the roles of the midlatitude jet and the Hadley cell” by Yang et al.', Anonymous Referee #1, 26 Sep 2018
- AC1: 'Response to Reviewer 1', Huang Yang, 07 Dec 2018
RC2: 'review of Large-scale transport into the Arctic: the roles of the midlatitude jet and the Hadley Cell', Anonymous Referee #2, 11 Oct 2018
- AC2: 'Response to Reviewer 2', Huang Yang, 07 Dec 2018

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

AR by Huang Yang on behalf of the Authors (05 Jan 2019) Manuscript

ED: Referee Nomination & Report Request started (08 Jan 2019) by Peter Hess

RR by Anonymous Referee #1 (17 Jan 2019)

Suggestions for revision or reasons for rejection

Review of "Large-scale transport into the Arctic: the roles of the midlatitude jet and the Hadley cell" by Yang et al.

This manuscript discusses the simulated tracer transport from midlatitude to the Arctic in CCMI models. The authors showed that for an idealized tracer CO50, large inter-model spread is found in its concentration over Arctic with or without specified dynamics. Unlike tracer NH5 reported in earlier studies, this inter-model difference cannot be explained by parameterized convection, but is found to be correlated with midlatitude jet position and Hadley cell boundary. Long-range transport of chemically and radiatively important species are important issues that lacks of full understanding. This manuscript provides a useful assessment of the CCMI models’ performance on this subject. In this revision, the authors have made some efforts to improve the robustness of their results. I have some remaining minor comments for the authors to consider.

The “20%-40%” inter-model spread of the Arctic CO50 concentration. This number is quoted in the abstract and the conclusion, but it is not very clear where this number comes from. From the paragraph on page 6 line 4-13, it seems this number is calculated as the range (max-min) of the Arctic mean CO50 concentration divided by its multi-model mean. In this paragraph, the range is quoted to be ~11ppbv in DJF. Estimated from Fig. 2, the multi-model mean Arctic CO50 concentration is between 20 to 25 ppmv below 400 hPa. Then the relative inter-model spread would be 44%-55%. Is it because the authors define the inter-model spread as something else than the range? Perhaps the standard deviation among models? In any case, the authors need to articulate how they calculate this inter-model spread.

Throughout the manuscript, the authors are doing averages over difference latitudes and pressure levels. It is not always clear why the authors made these choices. For example, CO50 concentration is averaged over 500-800 hPa, but the tracer flux is averaged over 700-1000 hPa, and the convective mass flux and the low level meridional winds are averaged over 800-950 hPa. The tracer mass flux is averaged over 40°N-60°N for JJA but 30°N-50°N for DJF. The authors need to justify their choices.

Page 5 Line 27: I don’t see how Fig. S1 support the argument in this sentence. Fig. S1 does not show the emission nor convection.

Page 5 Line 5-6: From Fig. 2, it is not obvious that the inter-model range in JJA peaks around 400 hPa. It seems the JJA inter-model range remains relative constant at all levels below 400 hPa.

Page 8 Line 4: why the word “strong” needs to be quoted?

Page 21 Table 2 caption: Why some variables are scaled? For the SD simulations without a superscript note, are they using a common nudging time scale? What is the difference between the “replay” simulation and the standard nudging simulation?

Page 22 Fig. 1 caption: panel (e) and (f) do not show the “meridional and vertical transport of CO50”. They only show the zonal mean CO50 concentration cross sections.

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RR by Anonymous Referee #3 (19 Feb 2019)

Suggestions for revision or reasons for rejection

Large-scale transport into the Arctic: the roles of the midlatitude jet and the Hadley Cell

H. Yang et al.

This is an interesting paper aimed at understanding the cross-model variation in the transport in to the Arctic of tracers with mid-latitude sources. Following several other works that have pointed out the diversity in Arctic abundances of these tracers, this work points out convincingly the importance of transport by the zonal mean, meridional circulation for understanding this diversity in the CCMI multi-model ensemble. An important wrinkle to this is that this meridional transport differs even across models that specify the meteorology in some way. Although the role of the mean meridional circulation does not explain all aspects of inter-model disagreement regarding large-scale transport into the Arctic, it seems to be a significant contributing factor, and well worth highlighting.

I find this to be an interesting study well worth publishing. I have some questions about the methodology and some suggestions about presentation, but if these are addressed I would recommend publication. I also found the writing to be a bit difficult to follow at times.

My only more significant comment is regarding the choice of flux decomposition into a zonal mean an eddy component. I was initially somewhat surprised at this choice, rather than decomposing into a stationary (say,monthly mean) and time-varying component, as it seems the latter would be better suited for understanding the local poleward transport in the Pacific sector. The time mean would filter out transport from synoptic scale eddies just as well. I would have thought the zonal mean meridional velocity would be just as, if not more, relevant to NH50 transport as to CO50, but that doesn't seem to be the case (I appreciate the CMF argument and this may well be the story). Perhaps the zonal decomposition is easier to study with the model output available (though this isn't so clear to me). Have you tried this decomposition?

On a related note, it seems that a second way to quantify the contribution of the meridional transport (beyond inter-model correlations) would be to directly calculate the tracer flux from the time mean (or zonal mean) meridional winds. I would expect this to have substantial variations even in CO and in NH50, even if there are confounding effects that reduce the correlations with overall Arctic burdens. This could be quite helpful for quantifying the importance of this effect for the transport of realistic tracers. Possibly one could even try isolating the role of inter-model differences in meridional velocity by computing the flux with a reference tracer distribution.

With regards to the presentation, I wonder if it would help to better distinguish between model integrations with constrained meteorology and those with free running dynamics. I have included specific suggestions below.

Specific comments

p3 l29 - Are the source data for this interpolation really on the model native grid? I would be surprised if any model runs in isobaric coordinates rather than hybrid pressure (this seems to be confirmed by Table 1). I suspect it's more likely that the source data have been interpolated by the modeling centers.

p6 l10-11: I was confused by the sentence starting 'However, it is difficult....' Is the point that these integrations are different because of how they implemented the source of CO50, but that they may also differ in their transport?

p6 l15: I can see how this source variability increases the model range in winter, but the EMAC models are right in the middle of the model distribution for summer Arctic burdens so the latter assertion wasn't so clear to me.

p7 l20-24: I wasn't so convinced by this argument - even if the transport into the Arctic is weaker in the summer, if the sources are at midlatitudes, this transport still seems essential for determining summer burdens, no? Unless a large fraction of the summer is left from winter-time transport, in which case this is probably worth discussing, since transport in the winter and spring would be more important than summer time transport.

p9 l12: This is inconsistent with the caption of Fig. 8 that gives an upper bound is 200 hPa.

p12 l15: By 'unchanged' do the authors mean that the model anomaly from the ensemble mean NH50 concentrations at high latitudes is consistent with its anomaly at midlatitudes?

For all figures I found myself wishing there was a clearer sorting in the legends by specified versus free dynamics. I don't think it is said anywhere explicitly that the open circles are specified dynamics integrations. Also, where correlation coefficients are given it would be informative to show coefficients for only the specified/free dynamics runs. Some of the regression lines (e.g. Fig. 10c) seemed to be obscuring clear differences in behavior; similarly in panels 12d,e,h, for instance, the two sets of integrations seem to follow rather different regression lines. This is discussed at some points in the text, but the presentation could be made clearer.

Fig. 2,4, and similar panels in the following: Would it be helpful to show ensemble means separate for specified dynamics and free running models?

Fig. 5: In this figure especially, it is hard to take in these 16 panels - sorting them by specified versus free dynamics would be more useful than by modeling center.

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ED: Publish subject to minor revisions (review by editor) (28 Feb 2019) by Peter Hess

AR by Huang Yang on behalf of the Authors (10 Mar 2019) Author's response Manuscript

ED: Publish subject to technical corrections (26 Mar 2019) by Peter Hess

AR by Huang Yang on behalf of the Authors (26 Mar 2019) Manuscript

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Short summary

We evaluate the performance of a suite of models in simulating the large-scale transport from the northern midlatitudes to the Arctic using a CO-like idealized tracer. We find a large multi-model spread of the Arctic concentration of this CO-like tracer that is well correlated with the differences in the location of the midlatitude jet as well as the northern Hadley Cell edge. Our results suggest the Hadley Cell is key and zonal-mean transport by surface meridional flow needs better constraint.