EGU2020-10820
https://doi.org/10.5194/egusphere-egu2020-10820
EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Southern Hemisphere jet stream: emergent constraints on future shift in zonally varying framework

Camille Li1,2, Fumiaki Ogawa1,2, Martin King2,3, Jerry Tjiputra2,3, Bjørnar Jensen4, and Klaus Johannsen4
Camille Li et al.
  • 1University of Bergen, Geophysical Institute, Bergen, Norway (camille.li@uib.no)
  • 2Bjerknes Centre for Climate Research, Bergen, Norway
  • 3NORCE Climate, Bergen Norway
  • 4NORCE Computing, Bergen, Norway

IPCC climate models (CMIP3/5) predict a poleward shift of the Southern Hemisphere (SH) jet stream under global warming, with a large spread across the models. Efforts to find emergent constraints for the future jet shift (response) have relied on the simulated present-day jet position (observable). However, this has been investigated primarily in a zonal-mean framework, which averages out important zonal asymmetries. In this study, we revisit the problem allowing for variations in the longitude, height and season of the response to gain a better physical understanding of the nature of the future jet shift in individual models. Results from a manual data analysis will help guide an exploration of the problem using a big data approach, in particular, the application of a genetic algorithm that finds optimal solutions based on iterative random selection within large sample data spaces.

How to cite: Li, C., Ogawa, F., King, M., Tjiputra, J., Jensen, B., and Johannsen, K.: Southern Hemisphere jet stream: emergent constraints on future shift in zonally varying framework, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10820, https://doi.org/10.5194/egusphere-egu2020-10820, 2020

Comments on the presentation

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Presentation version 2 – uploaded on 30 Apr 2020
Session/presentation ID was corrected.
  • CC1: Comment on EGU2020-10820, Matthew Patterson, 04 May 2020

    Dear Dr Ogawa

    My name is Matt and I'm a PhD student with Tim Woollings at Oxford, particularly focusing on the wintertime split jet in the southern hemisphere. I really enjoyed watching your presentation and found your finding of tropical SST biases correlating with the future jet shift to be very interesting. 

    I was wondering, do you have a picture of what physically might be connecting SST biases to the jet shift? Looking at your figure on page 4, unlike the summer, the wind change in RCP8.5 runs is quite zonally symmetric with the response localised to the south of Australia. Do you think that stationary waves may play a role in mediating this interaction?  This would have to be indirect because the flow in the tropics is easterly, preventing  Rossby wave propagation, but perhaps the SST biases alter the Hadley cell enough to affect the Rossby wave source in the tropics?

    Best wishes

    Matt Patterson

    • CC2: Reply to CC1, Fumiaki Ogawa, 04 May 2020
      Dear Matt,  thank you for your comment.

      Unfortunately, we still do not manage to physically explain the connection from the Tropical SST bias to the future South Pacific jet.
      Given the statistical relation, the future change of South Pacific jet shift may be involved in a stationary Rossby wave train originated from the tropical latitudes. The future Pacific jet shift is indeed associated with geo-potential height changes (not shown), but it does not indicate a clear wave train like the PNA pattern. The future change of geo-potential height is not so similar to the Pacific South American pattern (PSA, see Mo 2000) either . Another possibility is a future polarity shift of an atmospheric low-frequency variability. I compared the meridional structure of the South Pacific future westerly changes with the split-flow index variability (SFI, see Bals-Elsholz 2001), but the distribution was quite different to each other. I am not focusing on the tropical climate changes in zonally varying framework (such as Hadley/Walker cells) which might influence the future Pacific jet shift.

      • CC3: Reply to CC2, Fumiaki Ogawa, 04 May 2020

        Typo in the last sentence: "not focusing" -> "now focusing".

      • CC4: Reply to CC2, Matthew Patterson, 04 May 2020

        Dear Fumiaki

        Thanks for your reply, that's interesting that there is a geopotential height response but it isn't particularly wavelike, nor does it look like the PSA, I wonder whether removing the zonal mean geopotential height might reveal wavelike behaviour? Freitas and Rao (2014) use CMIP3 model data and show (their figure 8) quite a wavelike change in a warming scenario, but is more akin to a weakening of the Inatsu and Hoskins (2004) stationary wave from the Indian Ocean than the PSA pattern.

        • CC5: Reply to CC4, Fumiaki Ogawa, 05 May 2020

          Dear Matt,

          Thank you for your comments!
          I agree that it is better to investigate the zonally asymmetric component to discuss stationary waves related to climate change.
          What I wrote above regarding the similarity to PSA was actually from my results on the zonally asymmetric component of the climate change.
          At 300hPa, our result using CMIP5 is indeed similar to Freitas & Rao (2014)’s Figure 8d. 
          Associated change of the wave activity flux vectors shown in the paper does not indicate an organized favorable orientation, which indicates that it is unlike a well-organized wave train.
          More comments/suggestions are welcome! 

Presentation version 1 – uploaded on 28 Apr 2020 , no comments