QBO in ozone forcing in CMIP7

[JamesAnstey]

The CMIP6 historical experiment ozone forcing included a quasi-biennial oscillation (QBO) component in the stratosphere. This paper shows that the QBO (equatorial stratospheric zonal-mean zonal wind) tends to synchronize across realizations when a model uses this forcing, and doesn't synchronize for models using prognostic ozone.

The question for discussion here is: should the CMIP7 ozone forcing include imposed interannual variability in ozone (as the CMIP6 forcing did), or should the QBO signal in ozone be removed?

[znichollscr]

@hegglinm any thoughts here?

[sosprey]

This is an important issue of relevance to other groups (e.g. WCRP EPESC) where attribution of external forcing and internal variability is of key importance. Clearly, having an internal mode of variability synchronise ensemble members in attribution experiments (and other situations) is something to avoid if possible. A transparent discussion on this issue, involving relevant groups involved in the creation of ozone datasets for CMIPx and other MIPs would be timely.

[JamesAnstey]

According to the delivery summary table that I found linked from the docs, historical ozone is planned for the end of May. Is any appreciable QBO signal expected in the dataset, and if so is the intention to leave it in or to remove it? @hegglinm @znichollscr @ccmi1-test

[ccmi1-test]

A common dataset of tropical winds, quite similar to the usual FUB dataset derived from radiosondes but extended in time beyond the available observations by cutting and pasting sections, has been provided to the modelling groups that are running a simulation to contribute to the CMIP7 ozone dataset. Whether a particular model has an internally-generated QBO or not, the goal is that all models have a synchronized QBO by nudging to the provided wind dataset and then the QBO signal in ozone will be removed by linear regression - or some other statistical treatment. If I am not mistaken, the idea is to provide versions with and without a QBO signal.

[JamesAnstey]

Great, thanks for the update @ccmi1-test. How many models are contributing? (Sorry if this is in the docs and I missed it.)

[ccmi1-test]

There are three models, as I understand it. WACCM, EMAC and CMAM. We are considering a 25% retaliatory tariff against the American model though. ;-) More seriously, if you have any comments on the method let us know. Michaela is the one who will bring it all together and I am not sure if she is thinking about fitting the QBO individually for each model and then merging or fitting on the final dataset.