| 17 Jun 2026
A synoptic-dynamic interpretation of the Southern Annular Mode
Abstract. In this study, we describe the Southern Annular Mode (SAM) as a function of regional extratropical variability in order to understand the synoptic-dynamic features associated with it. The SAM is often considered to be the leading mode of variability in the extratropics and is used to interpret changes and impacts in the large-scale mid-latitude circulation and surface weather in the Southern Hemisphere. This interpretation of the SAM has however been questioned in recent years, given the SAM's lack of annularity and weak correlation to surface weather and synoptic features. From this regional perspective, we once again highlight the non-annularity of the SAM and show explicitly how these asymmetries can result in the misinterpretation and misattribution of regional surface weather impacts. Through analysing a set of weather features including Rossby wave breaking (RWB) zones, cyclones and jets, we show the SAM can be interpreted as a function of the RWB characteristics across the hemisphere, in line with similar perspectives of the North Atlantic Oscillation. In fact, changes in the storm track and jets occur locally and non-annularly across the hemisphere in response to regional RWB, in contrast to the annular view assumed by the SAM. This interpretation provides opportunity for further understanding of present and future Southern Hemispheric extratropical variability and its associated surface weather impacts.
The study investigates the link between the commonly used and hemispheric-wide definition of the Southern Annular Mode (hSAM), and a very local definition of the Southern Annular Mode (rSAM). Both definitions include a zonal mean pressure difference between two latitudes 40°S and 65°S. hSAM is defined by a zonal mean over all longitudes while rSAM is defined by a zonal mean over a small sector of 10° longitude. One main result is to show that hSAM is dependent on a multi linear combinations of all rSAMs. The most intense hSAM phases correspond to rSAM phases of the same sign in most regions but moderate hSAM phases are less dominated by the equivalent phase in rSAM. This result is not surprising but is well quantified. As expected from the link between spatial and temporal time scales in the atmosphere, the study also shows that the relation between hSAM and the rSAM is closer at longer time scales (monthly, seasonal) than at daily time scales. Then the link between Rossby wave breaking, jets, cyclones and rSAM is further detailed in the rest of the paper. An illustration of the weak link between rSAMs in some regions and moderate hSAM phases is done by showing two examples at the end of the paper. The figures have high quality, the text is clear in most part of the paper, and the sections well organized. I am not convinced the paper clearly brings very new results, it rather illustrates with clean figures and diagnostics the relation between synoptic-scale variability and the hemispheric-wide SAM. The most important concern I have is that the whole paper is based on an instantaneous view of the relationship between rSAM and hSAM and there is no attempt to illustrate potential time-lags links. For instance, it is well-known that Rossby wave breakings are associated with a transfer of energy from synoptic to planetary scales and this transfer may take some days as illustrated in Benedict et al or Franzke et al papers mentioned in the study. Also I do not think rSAM is a well chosen acronym. It is confusing because A in SAM means annular but the diagnostic is local and not annular at all. I therefore recommend publication of the paper after a major revision.
Main comments:
1) About time-lags links.Â
a) In the composites of rSAM and RWBs of Figs. 6 and 8, sophisticated diagnostics based on the identification of jet streaks and cyclones are shown, but it would be important to simply investigate time lag composites of the zonal wind. Indeed one can expect that after synoptic events like rSAM or RWB, some important zonal wind changes may appear at larger scales. For instance, Benedict et al. (2004) show that following RWB, the anomalies get larger and larger and finally project onto NAO anomalies. One could think that RWB events in the Southern Hemisphere could lead to pronounced zonal wind anomalies at large scales. This question seems important when one wants to link rSAM, RWB and hSAM. I am not saying that a single RWB could affect the whole hemisphere of course but at least this could affect a sector of the Southern Hemisphere larger than a synoptic-scale event. So my suggestion is to analyze time-lag composites of zonal wind anomalies in Figs.3, 6 and 8
b) In the same vein, have the authors looked at time-lag correlations between rSAMs in different sectors ? Same question for RWB. In Riviere and Drouard (2015, JAS), the NAM was shown to be first characterized by anomalies in RWBs and zonal wind in the North Pacific before propagating in the North Atlantic. In other words, several RWB events of the same nature may happen in different regions but with a time lag of several days.
2) Figure 6. Figures 6b and d show that CWB and AWB generally occur at the same place in terms of longitude/latitude/isentropic level during negative coherent rSAM events. Since AWB and CWB cannot occur simultaneously at the same place, do you think the composites of Figures 6b and d reflect the occurrence of different events ? To check that, I would encourage the authors to look at the pdf of CWB minus AWB densities for the negative coherent SAM events. Maybe the distribution is not centered over 0.Â
3) do the authors think that "rSAM" is the most appropriate wording for their regional diagnostic of pressure difference ? the letter "r" and the letter "A" in SAM are not compatible. Maybe "rDeltap" or "rzi" for regional zonal index are better suited. See for instance Feldstein and Lee (1996) for the zonal index definition.
Minor comments:
1) line 45: in Codron (2007, JAS), the JJA SAM is characterized by jet pulsing in the Pacific and jet shifting in the Indian Ocean. I would suggest to mention it.
2) Line 50: Please refer also to Ambaum et al (2001) for the discussion about NAM vs NAO
3) Lines 67-70: The NAM / RWB relationship has been investigated by Drouard et al (2015, J Clim) and Riviere and Drouard (2015, JAS). In particular, the latter paper shows that RWB signal in the North Pacific precedes the one in the North Atlantic (see my main comment above on the time lag)
4) Lines 76-80 please mention that the study considers all the seasons. Since the SAM is quite different between JJA and DJF, I am surprised there is no difference noted in the present study.
5) Line 97: 10° in longitude is a very small distance in mid-latitudes and even smaller than the synoptic scales. Why did the authors choose such a limited sector ? Please mention how the results would change by increasing the size of that sector.
6) line 103: "where" to be suppressed
7) Line 107 "predictor"
8) What do you mean by "generalized least squares solutions" ? Is it the same thing as multiple least squares regression models ?
9) Line 111-114: Where is such a framework of autocorrelation coefficient used in the paper ? Could the authors be more precise in this paragraph.Â
10) Lines 123-125: it would be nice to visualise the RWB zone in a schematic because the RWB zone varies from one RWB detection algorithm to another, and it is not clear how the present one works.
11) Line 156: additional figures are usually put in a supplementary file in EGUsphere journals and not in appendixes. Appendixes are more used to provide details on the methods. So please shift the current additional figures in a supplement information file.
12) Line 199: "highlights the asymmetry and lack of annularity of the SAM". Even though I understand what the authors want to say I am not sure this sentence is adequate. Figure 5 shows that the more rSAM of the same sign we have the stronger the hSAM phase is. So in my mind, hSAM has an annular property and extreme hSAMs occur when similar rSAMs events occur in very different sectors.
13) Line 232. The results of table 1 are consistent with the usual correspondence between spatial and temporal scales. The longer the time scale the larger the spatial scale.Â
14) Line 239: "consecutive" --> "contiguous" ? I think "contiguous" is more appropriate since we are talking about neighbouring regions. Also please specify that the coherence is spatial and not temporal
15) Figure 6: as previously said, please composite the zonal wind anomalies and look at different time lags. Also how do the composite fluctuate by looking at summer and winter separately ? Color curves in e and f are not described in the captionÂ
16) Line 255-261. Some of the observations can be commented at the light of previous studies. The fact that cyclonic RWB occurs on the poleward side and anticyclonic RWB on the equatorward side is logical with usual baroclinic life cycles (Thorncroft et al. 1993, QJRMS). The fact that cyclonic RWB occurs at a lower vertical levels than anticyclonic RWB has been explained by Riviere (2009) in terms of refractive index properties.
17) Figure 7b. Do negative rSAMs systematically occur with both CWB and AWB at the same time, or do the statistics hide different types of events, some of them being dominated by CWB some others by AWB ?
18) Lines 320-327: this paragraph was a bit difficult to read. Are we talking about positive coherent rSAM only in the whole paragraph ?
19) Figure 8, please provide composite of the zonal wind anomalies at different time lags
20) Line 331: are you sure this is Figure 4 you want to refer to ?
21) Line 346: replace "SAM-like" by "rSAM"
22) Lines 377-378: in my opinion, it is less important to discuss jet streaks or cyclones densities in relation with rSAM than zonal wind anomalies at different lags that can be more easily compared with hSAM properties.
23) Lines 401-402. I am not convinced by that statement. Following the present study, we should now consider that there are different ways to talk about SAM (hSAM/rSAM) so saying "this interpretation is inconsistent with the SAM" is misleading because we do not know which SAM we are talking about. If we talk about hSAM, maybe the relation between the different orientations of RWB and the hSAM phases exist. As mentioned above, the NAM was shown to be closely related to the nature of wave breaking in Drouard et al (2015) and Riviere and Drouard (2015). So we cannot avoid the possibility that hSAM is also related to the nature of wave breaking. This could be even a test to be made computing CWB/AWB frequencies for positive and negative hSAM.
24) Conclusion. I think the text should be clearer. In many places, the text should mention "rSAM" (or "rzi" if the acronym is changed) or "hSAM" rather than simply "SAM".
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