General comments:
The authors have partly incorporate my previous comments. However, the revised manuscript remains not entirely convincing and sometimes even more confusing. In particular, they still insist that “diverse aspects of the solar signal on the Earth’s surface can be explained solely by solar UV heating changes in the upper stratosphere which penetrate to the troposphere …” (lines 28-29, Page 16). However, their own analysis and other papers suggest that the ocean-air interaction and a change in ITCZ positions do contribute or even amplify the solar signal. This type of writing could cause a great amount of confusing in the literature. In addition, the discussion around the mechanisms remain very speculative but the speculative view is presented with even greater certainty. I also notice that the authors chose to use different periods of ERSST data to produce Figures 1, 7 and 8. This needs to be explained as why different periods were used for different diagnostics and sensitivity of the results in association with the periods chosen.
The clarity of paper must be improved by reducing potential confusion in terms of the responsible mechanisms. When come to the discussion, the authors need to take good care about what has been done previously and what kind of new insight is provided by their analyses here. Also, they need to carefully differentiate the cause and the response of the solar signals. I have several specific comments that I would like to see addressed before the paper is published.
Specific comments:
1. Lines 16, page 2. I think that CMD projection method is not as simple as “taking the composite difference between periods of high and low solar activity during the 11-year cycle”. Please clarify.
2. Line 25-26, page 2. The authors need to explain exactly what new knowledge this paper can add to the existing literature. For instance, please explain why investigating “processes producing solar signals on the Earth’s surface in a specific region” cannot lead to “the overall understanding of solar signals on the entire Earth’s surface”.
3. Lines 22, Page 2. “solar surface signal” is a strange phrase and potentially misleading. Consider improving. “solar signal near the surface”?
4. 2.1 Data section, page 4. The authors need to explain why three different time periods of ERSST data were used to produce Figures 1 (1854-2007), 7 (1880-2010) and 8 (1890-2012). How sensitive the results would be if a single period was used for those diagnostics?
5. Lines 10-13, page 4. It is better to state that the period used for the analysis is 1979-2010, rather than “1979 to the present”.
6. Lines 7-8, page 5. “Since the ERA-I dataset only starts in 1979, the QBO regressors are removed from the MLR when the long-term historical dataset (i.e. ERSST) is analysed.” This sentence does not make sense for the following reasons. 1) Do you get consistent result if you remove the QBO regressors from ERA-I analysis? Namely, when you use MLR for the 1979-2010, would the results, especially the tropical lower stratosphere warming signal, change if you remove the QBO regressors? 2) Note that the QBO records starts from 1953. See http://www.geo.fu-berlin.de/en/met/ag/strat/produkte/qbo/. 3) Would you get the same results if the MLR (with the QBO included) is applied to the period of 1953-2007?
7. Lines 10-11, page 5. It is stated that the solar regression coefficient is F10.7 solar fluxes for the stratospheric signal while sunspot number is used as the regressor for the analysis of the ERSST dataset. It is also stated that F10.7 is a better index to represent the solar UV. Admitting that there is a clear difference between the sunspot number and the F10.7 solar fluxes, on what ground the authors can claim that “diverse aspects of the solar signal on the Earth’s surface can be explained solely by solar UV heating changes in the upper stratosphere which penetrate to the troposphere (line 28-29, page 16)? Thus, it appears to me that the MLR analysis is applied rather arbitrarily with different number of regressors and different solar indices.
8. Line 16, page 5. “it is not relevant to include” -> “it is not appropriate to include”.
9. Line 22, page 5. Check “first-order?”
10. Line 4, page 7. “2-4 years” here. But “2-3 years” in other places (e.g. line 33, page 9; line 22, page 13; line 2, page 14).
11. Lines 23-31, Page 7. Would this NAM pattern of solar signal gets stronger or more significant if a time-lag of 2-4 years is applied to the MLR? Or the pattern becomes more NAO-like after a time-lag of 2-4 years? Note that the AO pattern cannot be identified in a physically consistent way in EOF analysis and the dynamical relevance of the AO/NAM has been questioned by many researchers (Deser 2000; Ambaum et al. 2001; Wallace and Thompson 2002; Christiansen 2002 etc). It has been suggested that the NAO and AO/NAM are indistinguishable in a statistical sense at the time scale of the life cycle of synoptic wave breaking (Feldstein and Franzke 2006). But contributions from stratospheric polar vortex occur at time scales of 30–60 days. It is not clear on which time-scale the solar UV in the upper stratosphere can produce the NAM-like pattern near the surface. Thus, it is not straight forward to conclude the signal definitely originates from the upper stratosphere.
12. Line 23, page 8. “very sharp zonal wind anomalies”. This is confusing, consider rephrasing. Perhaps “zonal winds anomalies with large meridional gradient”.
13. Line 24, page 8. “such strong meridional gradients of the zonal winds could not be produced by latitudinal difference of the radiative heating rate which mainly depends on the solar zenith angle.” Such a sentence requires quantitative support.
14. The text around figure 4, lines 4-19, page 9. Here, stratospheric subtropical jet and polar-night jet are mentioned. The definition of these two jets are introduced later in Fig. 13 and page 14. Consider placing Fig. 13 before or with Fig. 4 for clarity.
15. Line 24, page 9. “In spring, stratospheric circulation anomalies vanish and therefore, temperature anomalies over the continents weaken.” There are two problems with this sentence. Firstly, stratospheric circulation anomalies do not vanish in spring (MAM). In March at least, there is significant signal in both lower stratosphere and the troposphere. The authors can check this fact using ERA-I. Secondly, the word “therefore” has no scientific basis. The authors have not yet provided any evidence to suggest that the stratospheric effect is strong over the continents.
16. Figs. 3 and 7. To be consistent, both 90% and 95% confidence levels are required, same as you have done for Figs. 4, 5, 11 and 12 etc.
17. Results presented in fig. 7 are very hard to understand as it contains too much information, including DJF, MAM mean based on ERA-I and then DJF mean based on ERSST with different time lags. Naturally, the readers would ask why you don’t use the same data set for consistency? Why different seasonal mean is needed? How panel (b) is related to panel d)? Especially, you use sunspot number for ERSST analysis and F10.7 for the ERA-I and you have stated that these two solar indices represent different solar inputs.
18. Line 31, page 9 “temperature anomalies in the Pacific sector are created”. The word “created” needs changing. Perhaps -> “found”?
19. Section 3.4 tropical solar signals, page 10. To what extent and by which pathway can these tropical solar signals be explained by the stratospheric effect as it is claimed in the summary?
20. Line 23, page 13 “the calculated delays from a mechanistic model are too small to explain the observations.” This requires a citation.
21. Lines 21-33, page 13 and lines 1-3, page 14. I find that this part of discussion is very confusing and it is hard to understand what exactly the authors want to say. First, does the ocean heat contain play a role or not? Second, which results have suggested that the solar signal over the Pacific ocean PERSISTS in the ocean frontal zone is able to induce large atmospheric impact? Where do these Pacific warming signal come from? In fact, fig. 3 shows that larger solar signal over the Europe and smaller solar signal over the Pacific sector. Fig. 1 shows that the solar signal is very complex over the two ocean basins. Also note that the results may contain both initial response and amplification. How can you tell which is which from reanalysis data? Thirdly, the study of Frankignoul and Sennechael (2007) has nothing to do with the 11-year solar cycle. Fourthly, I do not understand as why “the warming in the Atlantic sector at lower latitudes … is not well positioned to produce large atmospheric effects”. These sentences are particularly confusing as the readers would think that the surface warming or cooling signals represent response to the 11-year solar cycle. How could the authors discuss them in terms of how they can produce large or small atmospheric effect? We need to know where these surface responses come from first and this is hard to tell at the moment either via modelling or observations.
22. Lines 9, page 14. “The mid-latitude warming on the Earth’s surface through the solar signal is produced in association with the downward penetration of zonal mean wind anomalies from the upper stratosphere during winter to spring in both hemispheres.” I disagree. The wind anomalies are not continuously significant from the upper stratosphere to the troposphere. In NH spring, the stratospheric and tropospheric signal reverses sign as well. Quantitatively, the authors are yet to show that temperature anomalies shown in Fig. 6 do match the required number (0.25 degree in the NH) in the annual mean field (Fig. 4).
23. Lines 16-21, page 14 and fig. 13. misplaced. See comment 14 above.
24. Line 31, page 14. In fact, planetary wave forcing in the NH winter is strongest during January rather than in late winter to spring.
25. Line 8, page 15. creates -> is associated with.
26. Line 8, page 15. “In the SH the planetary wave forcing is smaller, meaning the radiatively controlled stage lasts longer”. This is misleading. Climatologically speaking, this is true. In terms of solar effect, it is the relative effect matters and we do not really know.
27. Lines 9-10, page 15. This provides no explanation about the high latitude easterly anomalies and no explanation as why the anomalies extend from the upper levels to the lower levels. Does any model experiment suggest that a high UV input produces similar anomalies in the SH stratosphere? If so, cite the relevant study so that the text won’t sound too speculative.
28. Line 21, page 16. “a primary factor causing solar cycle variations in tropical SST”. This is too strong an expression. Regression or correlation cannot tell cause-response especially when the system involves amplification.
29. Line 28-29, page 16. I have major concern about this sentence. Given the authors have extensively discussed the roles of ocean amplification, wind-evaporation-SST feedbacks and northward-displaced ITCZ, this sentence is misleading and confusing. |
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