A "substantial" error in GISS Model E
Jan 25, 2016
Bishop Hill in Climate: Models

Over the weekend Nic Lewis posted a brief update to his latest posting on the Marvel et al paper. In it, he described something he had unearthed in a paper by Chandler et al. I've reproduced it here.

I have just discovered (from Chandler et al 2013) that there was an error in the ocean model in the version of GISS-E2-R used to run the CMIP5 simulations. The single forcing simulations were part of the CMIP5 design, although it is possible that some or all of them were run after the correction was implemented.

Chandler et al write:

We discuss two versions of the Pliocene and Preindustrial simulations because one set of simulations includes a post-CMIP5 correction to the model’s Gent-McWilliams ocean mixing scheme that has a substantial impact on the results – and offers a substantial improvement, correcting some serious problems with the GISS ModelE2-R ocean.

explaining the problem as follows:

...the simulations described here as “GM-CORR” utilise a correction to the Gent- McWilliams parameterisation in the ocean component of the coupled climate model. In prior implementations of the mesoscale mixing parameterisation in GISS ModelE, which like many ocean models uses a unified Redi/GM scheme (Redi, 1982; Gent and McWilliams, 1990; Gent et al., 1995; Visbeck et al., 1997), a miscalculation in the isopycnal slopes led to spurious heat fluxes across the neutral surfaces, resulting in an ocean interior generally too warm, but with southern high latitudes that were too cold. A correction to resolve the problem was made for this study, and it will also be employed in all subsequent versions of ModelE2-R going forward.

Interestingly, they also write:

One of the most significant differences of the Pliocene GM CORR simulations, compared with those of the uncorrected model, is the characteristic of the meridional overturning in the Atlantic Ocean. In GM UNCOR the Atlantic Meridional Overturning Circulation (AMOC) collapsed and did not recover, something that was expected to be related to problems with the ocean mixing scheme. Although we hesitate to state that this is a clear improvement (little direct evidence from observations), it seems likely that the collapsed AMOC in the previous simulation was erroneous.

It occurs to me to wonder whether this error in the GISS-E2-R ocean mixing parameterisation may account for its behaviour in Land use change run 1. It looks like something goes seriously wrong with the AMOC in the middle of the 20th century in that run, with no subsequent recovery evident. I have been unable to ascertain as yet whether that simulation was run using the uncorrected or the corrected ocean module code.

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