|This paper is still misleading, and I recommend that it only be accepted if the title and abstract are changed. It only presents simulations with ENSO neutral conditions, but that is not mentioned in the title or abstract. Both I and the other reviewer mentioned this, but the authors have not responded to this issue clearly. The authors have to make clear that they ignored SST impacts on atmospheric circulation, and only present simulations with this restriction. Why not change the title to this? “Volcanic stratospheric injections up to 160 Tg(S) yield a Eurasian winter warming indistinguishable from internal variability with El Niño/Southern Oscillation neutral conditions” And this ENSO neutral limitation also needs to be mentioned in the abstract.|
Lines 7-8 say, “no such warming is seen in simulation with more recent climate models” First, shouldn’t it be “simulations?” Second, this is not true. Coupe and Robock (2019) do show such warming.
Line 402: The authors add a reference to Dee et al. (2020) claiming that they support the claim that volcanic eruptions do not produce El Niños. But Dee et al. made a fundamental error, looking at absolute SSTs for an El Niño signal, failing to account for volcanic cooling. This was pointed out by Robock (2020), so it is incumbent on the authors to point that out and not just reference the incorrect results from Dee et al.
Robock, Alan, 2020: Comment on “No consistent ENSO response to volcanic forcing over the last millennium.” Science, 369 (6509), eabc0502, doi:10.1126/science.abc0502.
The authors responded to my review:
“First, we note that in their pioneering study, Robock and Mao (1992) specifically subtracted out the ENSO signal to isolate the volcanic response.”
This is not correct. Robock and Mao removed the average surface temperature response from El Niños and La Niñas that occurred in the absence of volcanic eruptions, in the naïve assumption that the climate responses were independent and could just be added. But they did not consider the nonlinear atmospheric circulation response to a simultaneous El Niño and volcanic eruption. That was the state-of-the-are 30 years ago. But now it is incumbent to include atmospheric dynamics and not simply add two separate surface temperature patterns.
“Second, we agree with the referee that the question of whether El Niño conditions do or do not alter the Eurasian winter response to volcanic eruptions is potentially interesting. But, the referee will agree, it is a separate question from the one we are addressing in this paper. In fact, is [sic] is a secondary question. Before addressing that question, we need determine whether volcanic eruptions - over a much broader range of amplitudes than those analyzed in Coupe and Robock (2021) - are able to produce a surface winter response without additional factors. This is the primary question.
“The referee will recognize, we hope, that our paper is focused on answering that primary question. Therefore, we deliberately chose to start all runs in an ENSO-neutral state, to isolate the volcanic from the response to ENSO, as we explain in the paper. We plan to address the secondary question too, and a follow-up paper on that is in preparation.”
I do recognize that the authors are trying to understand how large the forcing has to be after a volcanic eruption in an ENSO-neutral situation to produce winter warming. They call this a “primary question.” But since the two last large volcanic eruptions, 1982 El Chichón and 1991 Pinatubo, were accompanied by El Niños, and even the third largest one, 1963 Agung, had a small El Niño. [Whether the volcanic eruptions produced or enhanced the El Niños is a separate, but interesting issue.] So why is the primary question one that does not apply to any of the volcanic eruptions in the past 60 years? The authors do not explain this.
“In the present manuscript, we explore a very wide range eruption magnitudes, from Pinatubo to Tambora to Samalas, and well beyond.”
Not really. The 1257 Samalas eruption was also accompanied by an El Niño. See Robock (2020, reference above).
There was probably an El Niño at the time of the Tambora eruption, too:
A wet and cold El Nino: The Tambora volcano’s impact in the Australian colonies, by Don Garden, Journal of the Royal Australian Historical Society, 01 December 2021
So ignoring a simultaneous El Niño with large volcanic eruptions makes the current work less relevant for what has actually happened. I think it is incumbent on the authors to make that clear in this paper.