Past and future response of the North Atlantic warming hole to anthropogenic forcings
Abstract. Most of the North Atlantic ocean has warmed over the last decades, except a region located over the subpolar gyre, known as the North Atlantic warming hole, where sea surface temperature has in contrast decreased. Previous assessments have attributed part of this cooling to the anthropogenic forcings (aerosols and greenhouse gases) modulated by decadal internal variability. Here I use an innovative and proven statistical method which combines climate models and observations to confirm the anthropogenic role in the WH cooling, and to provide estimates of the contribution of a set of given external forcings. Furthermore, the method is able to reduce the uncertainty in the WH temperature over the historical period, but also in the future, with a decrease of 65 % in the short term, up to 50 % in the long term. A model evaluation validates the reliability of the obtained projections. In particular, the projections associated with a strong temperature increase over the warming hole are now excluded from the likely range obtained after applying the method.
Status: final response (author comments only)
RC1: 'Comment on esd-2022-40', Anonymous Referee #1, 14 Oct 2022
- AC1: 'Reply on RC1', Saïd Qasmi, 19 Dec 2022
RC2: 'Comment on esd-2022-40', Anonymous Referee #2, 02 Nov 2022
- AC2: 'Reply on RC2', Saïd Qasmi, 19 Dec 2022
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I find this a very interesting and valuable statistical study comparing observed northern Atlantic sea surface temperature (SST) changes and model-simulated changes.
I am not a statistics expert and recommend that the statistical methodology should be reviewed by a suitable colleague, but I think I understand the basic approach. A key point of this is that no particular physical mechanism is studied (such as ocean circulation change), but rather simply the statistical relation of SST fields from various model runs (with different forcings) to the observed SST field. In this way we can find how various forcings relate to SST changes (though not by what mechanism) in the models, and whether that resembles what is observed in the northern Atlantic. I think it would be useful to make this a bit more clear right at the outset, because it confused me a bit while reading the introductory part of the paper, until I got to the Methods.
Important findings of this study in my view are:
- that aerosol forcing leads to the opposite SST response compared to observed. This should lay to rest the previous discussions of whether the Atlantic warming hole is caused by aerosol forcing.
- that the historical runs of CMIP6 with all forcings do not get the observed warming hole.
- that increasing greenhouse gases are the main reason for the observed warming hole.
If the author agrees with my assessment, I suggest bringing these conclusions out in somewhat clearer language in the paper, including the abstract.
I recommend using fewer acronyms (like WH for warming hole) because it makes the paper hard to read.