EGU2020-18294
https://doi.org/10.5194/egusphere-egu2020-18294
EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Northern Hemisphere atmospheric blocking simulation in present and future climate

Fabio D'Andrea1 and Paolo Davini2
Fabio D'Andrea and Paolo Davini
  • 1Laboratoire de Meteorologie Dynamique, Ecole Normale Superieure, Paris, France (dandrea@lmd.ens.fr)
  • 2Istituto di Scienze dell’Atmosfera e del Clima, Consiglio Nazionale delle Ricerche (CNR-ISAC), Torino, Italy

We present a comprehensive analysis of the representation of winter and summer Northern Hempishere atmospheric blocking in global climate simulations in both present and future climate. Three generations of climate models are considered: CMIP-3 (2007), CMIP-5 (2012) and CMIP-6 (2019).
All models show common and extended underestimation of blocking frequencies, but a reduction of the negative biases in successive model generations is observed. However, in some specific regions and seasons as the winter European sector, even CMIP-6 models are not yet able to achieve the observed blocking frequency. For future decades the vast majority of models simulates a decrease of blocking frequency in both winter and summer, with the exception of summer blocking over the Urals and winter blocking over Western North America. Winter predicted decreases may be even larger than currently estimated considering that models with larger blocking frequencies  hence generally smaller errors - show larger reduction. Nonetheless trends computed over the historical period are weak and often contrasts with observations: this is particularly worrisome for summer Greenland blocking where models and observation significantly disagree. Finally, the intensity of global warming is related to blocking changes: wintertime European blocking is expected to decrease following larger global mean temperatures, while Western Russia summer blocking is expected to increase.

How to cite: D'Andrea, F. and Davini, P.: Northern Hemisphere atmospheric blocking simulation in present and future climate, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18294, https://doi.org/10.5194/egusphere-egu2020-18294, 2020

Comments on the presentation

AC: Author Comment | CC: Community Comment | Report abuse

Presentation version 1 – uploaded on 04 May 2020
  • CC1: Comment on EGU2020-18294, Pedro M. Sousa, 07 May 2020

    Dear Fabio and Paolo,

    Very interesting work!
    What was the minimum detection latitude considered here?
    I am asking this, since I've looking at trends in high-latitude blocks and subtropical ridges, and we see overall contrasting trends between both, negative and positive, respectively.

    Besides your overall decreases in blocking agreeing with our high-latitude trends, it seems to me that some positive trends in summer might also be a reflection of enhanced warm season subtropical ridges activity. We found this particularly relevant over central Eurasia, with stronger ridges and Omega blocks (similar to those during the 2010 Russian HWs).

    Do you have any further comments on this dicotomy, taking into account distinct methodologies, and the fact that you exclude low-latitude blocks (as in Davini et al., 2012)?

    Best regards,

    Pedro Sousa

  • AC1: Comment on EGU2020-18294, Paolo Davini, 07 May 2020

    Hi Pedro, thanks a lot for your question: the minimum lat is 30N.

    When talking about blocking you always fall into the limitation of the index you adopt. We like the reversal based index because it detects eastward flow where it shouldn't be, so we look for condition where the jet stream is "blocked". Morover, it is a solid definition also in a model or climate change world. We need to exclude low latitude blocking otherwise in summer you detect too much climatological easterlies (i.e. trade winds) since the jet are poleward wrt winter. An anomaly based index will work better for stronger ridges: it is however tricky to use it in a climate change scenario. 

    Said so, I agree with you, in the sense that I am quite sure we are missing some of these omega-like events both in present and future climate. It makes sense that ridges events are increasing since the jets are moving poleward. However, the summer increase in blocking events we see over the Urals may fit with the analysis you carried out.  

    I think it all ends up to the fact that the definition of blocking is blurry. 

    Best,

    Paolo