Review of cpd-11-4003-2015

I feel this manuscript provides strong rationale and very helpful description of the PlioMIP2 experiment. I recommend that it is published subject to some revisions. I look forward to the actual experiment and hope that some interesting science will emerge from it. Below I’m suggesting some big revisions to the ensemble of simulations requested. I’m happy with the authors directly about whether these revisions truly represent better value for resources.

The vast majority of the simulations are required solely for the forcing factorisation. You might want to consider just important you feel this component of the research is. I worry that the amount of simulations required really justify the extra effort. They need 6x as much computation as just doing the PlioMIP2 entry card, but surely gain nothing like as much as six times the information (considering the fact all CMIP6 models must do the DECK, I’m not counting the preindustrial run). You may want to think of the factorisation as a sub-experiment, otherwise PlioMIP2 appears really daunting.

Topography

I did have one question about the scope of the manuscript. It wasn’t clear to whether it aims to serve to just as an experimental description, or will also act as the full description of the boundary condition datasets. I know that the previous experimental design (Haywood 2011) was complemented by a data description paper (Dowsett 2010). Whilst I think that most of datasets are adequately discussed in this manuscript (or prior publications), the topography feels under described. I hope that a separate manuscript is planned to describe all the underlying assumptions for this dataset and highlight the important changes. I would certainly like to see more discussion of the uncertainty inherent in the topography reconstruction. For example, a major change from PRISM3 is the closing of the Bering Strait. I remember seeing a poster at AGU 2014 by Dick Peltier presenting an alternate topography with it closed - this also included the novel scientific components described here. Whilst I’m happy with the reconstruction you present here, I don’t feel there is any acknowledgment that it may have uncertainties.

It isn’t clear to me how the ice sheets and topography are actually separated in the factorisation approach. I think you need to provide guidance in the manuscript. Does imposing ice-sheets also contain the topographic element associated with the ice-sheet or does that count in the topography? At its simplistic this could be ice-sheets could be thought of as white mountains, so the ‘i’ component only relates to the land surface specification. That doesn’t make much sense intuitively. The problem is however much more complicated than that, as you’ve taken account of the glacial isostatic adjustment in the your topographic reconstruction. At this point, I’m not sure it’s that important (it would need to be discussed seriously in the factorisation results). Here you do need to provide instructions to allow the runs to be performed.

I was also confused by the discussion of the standard experiment. Surely altering the Bering Strait is a change in the Land-Sea mask. If modelling groups have the ability to do this change (and in my experience making new land is more awkward than new ocean), then shouldn’t they be doing the other experiment. Incidentally, I would anticipate that this change is important for the AMOC, so well worth including if possible.