Review of the Manuscript: JCLI 3785

Dear Reviewer C:

Thank you very much for your nice and constructive comments. The author believes that the manuscript is much improved by your and other reviewers’ comments.

General comments:

This manuscript discusses physical and dynamical characteristics of the South Atlantic Convergence Zone from observational and modeling perspectives.

The most relevant contribution of this study is the regional modeling of the SACZ. This analysis is carried out by performing several sensitivity experiments with a regional model (TERC--‐RAMS) to test the importance of resolution, latent heat through changes in patterns of precipitation, topography and the extent of the coast line. Although this study certainly adds relevant insights to the present understanding of the dynamic and thermodynamic processes related to the establishment of the SACZ, it needs further clarifications and appropriate discussions (major revisions) before publication. Specific comments and suggestions are addressed below:

Major Comments:

Q1) Introduction, first paragraph:

“The SACZ develops in austral summer and characterizes the South American Summer Monsoon” should be “The SACZ develops in Austral summer and is an important component of the South American Monsoon System (SAMS) (as it is commonly referred in the specialized literature – e.g. Vera et al. 2006)

A1) We modified as you suggested. (P.4, L.4-7)

Q2) Introduction, pg 6 second paragraph.

It is not clear what the authors are trying to discuss in the second paragraph in the sentence starting with “In the Madden--‐Julian Oscillation, on submonthly time scales…”. It is certainly pertinent to review the literature on the role of intraseasonal variability and the Madden Julian Oscillation upon the variability of the SACZ. Authors should dedicate part of the introduction to that end and provide additional references, since several studies (observational and modeling) have explored these aspects before.

A2) We removed the description on MJO.(P.5 L.14-17)

Q3) In connection with (2), another important issue that is very controversial is this study is the interpretation that the SACZ has two “favored positions” (Second paragraph, pg 6). In none of the papers cited in this manuscript authors claim that there are two preferential positions of the SACZ and there is no evidence of a “jump” from one position to another. What has been shown in Nogues--‐Paegle and Mo, 1997, Liebmann et al. 1999; Carvalho et al. 2004 (among many others) is the existence of a seesaw associated with the propagation of a wave train from midlatitudes on submonthly (or intraseasonal) timescales. The SACZ has been historically identified with precipitation and cloudiness extending from east Amazon towards the Atlantic Ocean and not with the phase of the wave train that favors convection over Uruguay/Argentina. Moreover, what seems to be of common sense in the identification of the SACZ has been its persistence. Authors have omitted this discussion in this manuscript but I strongly recommend that these ideas be properly presented here.

A3) Thank you very much for your comment. We rewrite this part of introduction carefully considering your advice. (P.5 L.10-17) In this paper, we’d like to refer the precipitation zone over Uruguay and Argentine is also as SACZ, in spite of traditional name. (P.5 bottom and P.6 L.2)

Q4) Another important issue that has not been mentioned in this study and that would enrich very much the discussion of this manuscript is the identification of a continental versus oceanic activity of the SACZ( e.g, Carvalho et al. 2004, Grimm et al. 2007, Muza et al. 2009 –J. Climate, 22,1682--‐1699; Carvalho et al. 2010 –Climate Dynamics, DOI 10.1007/s00382-010-0806-2).

These papers suggest that the continental and oceanic activities of the SACZ are decoupled from each other. In a recent paper, Carvalho et al (2010) showed that the source of moisture for what they called “oceanic SACZ” is the Atlantic Ocean. On the other hand, when the SACZ is active over the continent, the Amazon basin becomes an important source of moisture, in addition to the Atlantic Ocean. I believe that the results shown in the present paper could complement these previous papers and I suggest that the case--‐studies presented here be analyzed within the context of these studies.

A4) We appreciate your good comment on recent studies on SACZ. We add Muza et al (2009) and Carvalho et al. 2010 as references. We refer these references in Introduction (P.6, 2nd para) and discuss on the results of “Modified coast run” emphasizing the difference of moisture source between continental and oceanic portions of the SACZ.(P.23 2nd para- P.24 1st para)

Q5) Pg 8. The comparison between the dipoles found in Grimm and Zill (2009) and Nogues--‐ Paegle and Mo (1997) does not seem appropriate. The former is concerned with seasonal variability whereas the latter is concerned with intraseasonal/submonthly variability. Therefore, these dipoles are not the manifestation of the same phenomena. The time scale of the dipole or seesaw found in Nogues--‐Paegle and Mo is of the order of days to weeks, whereas Grimm and Zilli show a dipole that occurs between spring and summer (longer time--‐scales). Please revisit these concepts.

A5) We modified description carefully following your comments. Grimm and Zilli (2009) is removed in this paragraph. (P.5, 2nd para, P6 1st para).

Q6) Pg. 9: not clear what is meant by “positive (cyclonic) anomalies induce… whereas negative (anticyclonic) anomalies induce to…”. Anomalies of what? This is confusing because cyclonic (anticyclonic) relative vorticity is negative (positive) in the SH. Please clarify which anomalies you are talking about.

A6) We modified as “positive (cyclonic ) anomalies” into “positive height (anticyclonic)” anomalies, etc. (P.8 L.15-17).

Q7) Pg. 16, first paragraph. “The statistical analysis… indicates that the SACZ intensified when it passed over the BP. Again, how is the SACZ defined? If persistence is implicit for the definition of this zone, then that interpretation is not appropriate. Based on previous papers, including Kodama et al. 1992, it seems that the most accurate interpretation should be “The SACZ intensifies as the extratropical disturbances pass over BP”. That would make much more sense, since the SACZ is not defined when the extratropical wave trains favor convection over Uruguay and Argentina.

A7) We refer the precipitation zones extending southeastward as SACZ in spite of traditional definition (P.5 bottom to P6. L.2, See also A3)

Q8) Designed experiments and conclusions: this is certainly the most relevant contribution of this manuscript and therefore needs attention. The experiments that deal with topography are clearly presented and conclusions are consistent with previous studies (e.g. Grimm et al.2007). The “stop rain” experiment seems to confirm what has been observed before in innumerous papers (e.g., Herdies et al 2002 JGR Atmos 107:8079; Rickenbach et al.2002 – JGR Atmos, 107,D20,doi:10.1029/2000JD000263, Carvalho et al. 2002; JGR Atmos, 29, 10.102/2001JD000745; Jones and Carvalho 2002 – J. Climate 15, 905--‐914). In the context of these previous studies, it is not clear the actual contribution of this experiment to the present knowledge about the SACZ. The removal of moisture from the domain in the “stop rain” experiment is balanced by an increase in subsidence. That is consistent with the intensification of the Atlantic high pressure system and its consequent displacement toward the continent. This displacement inhibits the cyclonic circulation to the west of the climatological position of the SACZ. Easterly winds become more intense as the result of this displacement. Easterly versus westerly regimes have been related to profound changes in moisture transport (e.g. Herdies et al. 2002), break and active phases of the monsoon (Jones and Carvalho 2002) and changes in the frequency and intensity of mesoscale systems (Carvalho et al. 2002). It seems somehow misleading to claim that the ‘stop rain’ experiment evaluated the importance of latent heat due to precipitation. Rather, this experiment showed that dry conditions over eastern coastal South America during the summer monsoon are indeed related to the anomalous displacement of the Atlantic high towards the continent. This displacement, according to Jones and Carvalho 2002 and Carvalho et al. 2002 can be forced by the propagation of midlatitude wave trains triggered (or not) by MJO events (e.g. Carvalho et al 2004). Based on these observations, it is not clear what can be learned from the “stop rain” experiment that has not been pointed out from those previous observational studies. A set of experiments in which precipitation is not totally stopped but continuously decreases would be more appropriate to provide a comprehensive analysis of the effects of moisture in triggering changes in circulation regimes in the region. That seems to be an important question that has not been answered yet. In this regard, it would be quite interesting to evaluate soil moisture, sensible and latent heat fluxes for different thresholds of moisture (and consequently precipitation). Otherwise, authors should put these results into the context of those observational studies and provide a more convincing and comprehensive discussion of the relevance of the present study that has not been shown before.

A8) In our idea, large-scale circulation over South America is determined by monsoon circulation derived by latent heating and land-sea heat contrast and extra forcing from outside like as wave train. I believe the “Stop rain run” provides information how local latent heating over the BP modify the circulation around the heat source. The “Stop rain run” is an idealized experiment and it is difficult to be compared with phenomena in the real atmosphere. As you suggested, relationship between our results and previous studies on soil moisture and easterly wind is interesting theme for future. To examine these problems, another experiments in which rainfall is moderately suppressed are necessary as you suggested. These are future tasks.

Q9) Pg. 20, about the sentence “thermodynamic process of precipitation over the BP, rather than the dynamic effect of topography was important for maintaining the BPC along with the SACZ”. It is unclear what is actually meant here. Subsidence is coherent with the inhibiting of precipitation in the model, which is achieved by the westward displacement of the Atlantic high. Therefore, this comparison does not make much sense, I believe. The SACZ was ‘removed’ in the experiment and therefore it is unclear how it would be possible to evaluate or compare the effects of the BP in that case.

A9) I understand your comments. We thus remove this sentence. We emphases contribution of latent heating to form BPC to the southwest of the BP. And we remark that this result supports CISK mechanisms proposed by Grimm et al. (2007) between formation of BPC by the precipitation over the BP and moisture transport by the BPC to the BP, which intensify the precipitation over the BP. (P.27)

Q10) Pg 21: The “modified coast run” is a very interesting experiment and authors should explore it further. Any explanations for the differences between 1985--‐1995?

A10) Thank you for your encouragement. We add discussion on change in low-level moist flow observed in “modified coast run”, to be compared to the studies on the difference between the continental and oceanic portions of the SACZ (Muza et al. 2009, Carvalho et al. 2009). Especially we remarked the extension of northwesterly from Amazon changed between the “Control run” and “Modified coast run”. (P.23 2nd para to P.24 1st para)

Q11) Summary and discussions:

Pg 22: “this indicated that climatic position of the SACZ passed over the BP” does not make sense and should be removed.

A11) We removed.

Minor suggestions/comments:

Q1) Abstract is too long. It should be rewritten to present only a summary of the main results. Part of what is in the abstract could be moved to the introduction.

A1) We shortened abstract. (P.2-3)

Q2) Abstract, sentence starting with “Our results indicated that the cyclonic circulation is maintained not by the dynamic effect of the BP topography, but by the thermodynamic effect of the precipitation due to the BP influence on the topography” does not make much sense and should be rephrased.

A2) We removed these sentences. (P.2-3)

Q3) Introduction: First paragraph, sentence starting with “The SACZ is one of the significant subtropical …austral summer”. This sentence is too long and does not read very well.

A3) Thank you but I’d like not to change.(P.4 L.7-12)

Q4) Pg.6, second paragraph: Seasonal changes in the climatic position”… should be Seasonal changes in the climatological position.

A4) We changed as you suggested. (P.5 L.4-5)

Q5) Pg. 7, sentence starting with “They noted a large…” not sure what is meant here. Please revise this sentence.

A5) We removed this sentence.

Q6) Pg. 9 Suggestion for the text: second paragraph: “however, many problems remain “ --‐ However, many problems remain unsolved.

A6) We changed (P.9, L.7)

Q7) Pg. 11: “SST information” --‐ SST data.

A7) We changed (P.11, L.15)

Q8) Pg 12. “Statistical analysis”. This title is too vague. It would be better if authors use a title that reveals the objective of this session. Moreover, this session should start with a short summary of its goals.

A8) Corrected (P.12 L.1)

Q9) Please improve the sentence that starts with “95% significant levels for significant…” it does not make much sense.

A9) We corrected (P.13, L14-16)

Q10) Pg 16, first paragraph: “using the TERC--‐RAMS” --‐>” using TERC--‐RAMS”

A10) Corrected (P17,L6, and P17, L.12)

Q11) Pg. 17: “complicated topography” should be “complex topography”

A11) Corrected (P.18, L.8)

Q12) Pg. 17: “heating by precipitation” --‐>” heating due to precipitation”

A12) Corrected (P.18,L17)

Q13) Figures 9--‐12: there are too many labels and it is hard to see the patterns shown in these figures. Since shading represents precipitation, labels are not necessary and could be omitted for clarification

A13) We improve these figures using color print and omit of contour labels.