A Brief Summary of ACCESS-CM2 Development post CMIP6

Earth System Working Group
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Motivations

  • Improve the model performance in climate simulation: Reduce model biases seen in the ACCESS-CM2 historical simulation for CMIP6
  • Comprehensive understanding of the ACCESS-CM2 ESNO simulation in comparison with other CMIP5/6 models around the world

Efforts and Results

  • Implementation of the FIO MASNUM Wave Model (MWM) into MOM5. Conduct PD simulations – for better comparison against observations.
    • enhance vertical mixing
    • deeper thermocline in the tropical ocean
    • better ENSO lifecycle
  • Applying Stochastic Forcing in the Ocean Interior (based on the work of Terry O’kane et al. with ACCESS-OM2). Various scaling factors (0.1 - 1.0)
  • Tuning Tropical Ocean Background Diffusivity and Viscosity to enhance vertical mixing for deeper thermocline
  • Higher horizontal resolution for the ocean (1 degree → 0.25 degree)
  • Current work: Back to 1-deg ocean using the same parameters for Gent-McWilliams eddy parameterisation (inspired by the “good ENSO simulation” in the 0.25 deg ocean runs)

Slides

Dave_Bi_Summary–CM2 development post CMIP6 .pptx (4.2 MB)

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@dhb599 in your talk you said some post-CMIP6 modifications reduced the AABW. My understanding is that AABW in these coarse models is not produced via the dominant observed mechanism (near-coast sea-ice formation and subsequent brine rejection)

If the AABW is decreasing, but is formed by an incorrect mechanism, maybe this is a good thing? The best performing of the CMIP6 models (CESM) used an overflow parameterisation, perhaps this is a better target for improvement of AABW in CM2?

Maybe @adele157 's latest work on CDW and DSW exports could inform the required improvements in parameterisations identified by Kate

https://www.sciencedirect.com/science/article/pii/S1463500315000517

Or just copy what CESM does?

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Hi Aiden,

Thanks a lot for your insightful comments.

Yes, I think you are right that the AABW is not produced in coarse resolution models through “appropriate” physical processes such as the overflow. We know that in most, if not every, ocean models, the AABW formation is just one of the diagnostical fields – the maximum intensity of the upwelling cell transport off Antarctica, which is a combination of large scale (Euler), parameterized eddy (GM) and parameterized submesoscale transports of the water volume. However, I do think the thermohaline factors (e.g, ice formation associated vertical mixing enhancement, and deep convections) have been well (or say reasonably?) presented and have their impacts on the meridional circulation in models, although the AABWF may be poorly represented due to lack of realistic physical process.

Our recent tuning (i.e., increasing) of the two GM parameters was aimed to improve the model’s simulation of ENSO, and the resultant reduction of the AABW formation (and ACC transport) is kind of “side effect” and we don’t really appreciate such change, particularly the AABW response, so I won’t say it is a good thing, considering we don’t yet have other way to “control” the AABWF. Your suggestion of trying Kate’s overflow parameterization or the approach of CESM is interesting – actually, a similar idea had crossed our mind many years ago when we saw somebody’s paper on this subject (under review then) but didn’t do it for some (good) reason. At this stage we might not have strong motivation to take on this work, considering the effort we need to put in while focusing on other objectives of high priorities. However, it would be super nice and useful if our COSIMA colleagues would consider developing such a version of MOM5 for the ACCESS community.

Best regards,

Dave

1 Like
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Thank you for the detailed and thoughtful answer @dhb599.