The “Impact of ITCA width on global climate: ITCZ-MIP” outlines an experimental protocol for a MIP, and presents some preliminary statistics of circulation shifts, ITCZ width and strength changes, and radiative budgets. The authors then pose three questions which the MIP seems well-suited to address.

I think this paper is a fantastic opening discussion piece for the ITCZ-MIP, that the writing is mostly-well-organized, easy to follow, and well-cited, and that the figure production is exceptionally clean and consistent. I think if I were to contribute to the MIP, this paper would give me a good template to follow. I recommend publication pending minor revisions. I have a couple of minor points about scientific clarity and reproducibility and some similarly minor points about the text itself.

# Questions/comments/suggestions

I’ve been around some of the discussion of this MIP as a possibility, but I haven’t been active in this community lately. So my main question may be one that is obvious to anyone focusing on ITCZ widening: what is it about the proposed and prototyped experimental protocol in particular that makes it well-suited in the authors’ opinion to shedding light on the mechanisms of ITCZ? The introduction mentioned work by co-author Byrne with Schneider and others, and it sounded like the MIP was going to address possible mechanisms for ITCZ width – such as they are. The two questions posed on lines 40–45. But it seems like it will do so indirectly; most of the discussion concerns inter-model differences, etc., rather than by directly addressing the relevance of proposed mechanisms. While the protocol seems to get good bang for the buck as far as strongly shifting the ITCZ while maintaining a recognizably earth-like climate, I can’t think of anything in the text that explains why the protocol is precisely what it is—or what alternatives were considered (and ultimately rejected for the prototype). It would have been nice to say something about how sensitive the ITCZ is to heat fluxes, and how a MIP design that forced ITCZ movement some other way may be less effective because of damping from SST changes (in a coupled ocean simulation). Q-fluxes seem to be the way to go, but I recommend explaining this more clearly in the manuscript.

I think the discussion of the confounding effect of global mean temperature change was very good.

It’s interesting to me that the MIP is named ITCZ-MIP, but it is so squarely focused on ITCZ width and strength, when there are other important aspects that could be addressed in an ITCZ-MIP. For example, the protocol in the submission is intentionally designed to keep the ITCZ at the equator, so another ITCZ-MIP might be necessary to analyze changes in ITCZ location. That said, the ITCZ location has been the subject of considerable work lately, so I don’t see the same urgent need to focus on it. But maybe this is really an ITCZW-MIP?

# A few line-by-line items

Line 17: “width of the ITCZ and its changes” is syntactically ambiguous. Context helps here, but maybe re-word this sentence.

Line 26: “since” suggests that the following cited papers would be after Byrne et al. (2018), but Harrop and Hartmann (2016) is obviously not. I wonder if the citation on line 27 should be written as (Dixit et al., 2018; see also Harrop and Hartmann, 2016).

Line 63: “Most simulations have a slab ocean with a 10-m mixed layer depth.” I wonder whether more needs to be said about this. There’s no seasonal cycle, so maybe it’s not an issue. Is this something that future MIP contributors need to get right?

Lines 42–45, 315–330: There are five big questions in this paper. The first two, which I call A1 and A2, are said to be addressed by this paper:

A1 - What mechanisms connect the ITCZ width to the width of the Hadley cell and the location of the storm tracks?

A2 – does equilibrium climate sensitivity depend on the ITCZ width?

A1 is addressed a little bit, as the Webb and Lock (2020) hypothesis is not rejected. But no alternative hypothesis is suggested.

The other three questions, which I call B1–B3, are written as follow-ups to be addressed by contributors and users of MIP simulation data:

B1 – Something about the robustness of the strength and width of the ITCZ.

B2 – Why do models disagree on “the relationship of climate sensitivity to base-state ITCZ width, even in sign.”?

B3 – What is the mechanism “…of action for the consistent changes in ITCZ width that are generated by the addition of heat via surface fluxes?”

I think the paragraphs about these questions should be revised to make the questions clear—and the sentences themselves should be stated plainly as sentences. Maybe even express them in a bulleted list.

Also, maybe bullet the first two questions, and discuss them in a way that the reader can’t mistake them for the three follow-on questions.

Figure 7: The “1e10” at the top of the colorbar would be clearer as “×10^10” (only formatted, without the ^).

Review of “Impact of ITCZ width on global climate: ITCZ-MIP”

This manuscript outlines the protocol for a proposed model intercomparison project (MIP) to study the Intertropical Convergence Zone (ITCZ).  The proposed ITCZ-MIP has a series of experiments (separated into two tiers) as well as a concrete list of requested output from each simulation.  The other portion of the manuscript is a pilot study using four models that run the ITCZ-MIP experiments.  The authors demonstrate their methodology achieves the desired result of predictably altering the ITCZ width in their simulations.  Their methodology does, however, create a significant change in global mean temperature, despite imposing zero global surface flux, which they show to result from changes in outgoing longwave radiation.  The final portion of the manuscript covers the role of the ITCZ in a changing climate.  Here, the models are far less consistent, and the authors use this inconsistency to motivate a need for more models to contribute to ITCZ-MIP. 

My primary concern has to do with how this study relates to the previous tropical rain belts with an annual cycle and a continent model intercomparison project (TRACMIP).  The experiment design of TRACMIP was built on the premise that the seasonality is important to understanding the ITCZ.  Specifically, TRACMIP was meant to address the finding that sensitivity of the ITCZ to model changes in equinoctial aquaplanet simulations can be very different from simulations with a seasonal cycle (see Figure 1 of Voigt et al., 2016).  I worry that by using equinoctial conditions for this study, the methods and conclusions drawn from ITCZ-MIP might have limited applicability to the ITCZ biases commonly found in fully coupled earth system models.  Similarly, Zhou et al. (2020) showed that changes in the annual mean ITCZ width can be complicated by its seasonal nature.  They found the ITCZ changes both its position and width within each season in such a way that the annual mean ITCZ appears to narrow, when the ITCZ actually widens at any time within the seasonal cycle.  While there are opportunities for scientific understanding with simpler experiment designs, I would have liked to have seen more discussion about the potential limitations of the experimental design considering the lack of seasonality.

While the above concern is one I believe requires a major revision to address fully, the rest of the manuscript is well written and suitable for publication in GMD.  I have a few additional minor comments below.

Minor comments

L82 “This narrower peak in SST…” should this reference Figure 4?

L298 “…its response is nonetheless similar to CESM1.”  Did you mean CESM2?

L301 “This is consistent with the their relationship…” Typo

References

Voigt, A., et al. (2016). The tropical rain belts with an annual cycle and a continent model intercomparison project: TRACMIP. Journal of Advances in Modeling Earth Systems. 8: 1868-1891.

Zhou et al. (2020) Contrasting Recent and Future ITCZ Changes From Distinct Tropical Warming Patterns. Geophysical Research Letters doi: 10.1029/2020GL089846