In this paper, the authors examine the role of in situ exited planetary waves (PWs) for the major sudden stratospheric warming in the Southern Hemisphere 2002. It demonstrates that westward propagating PWs with zonal wave number 2 (PW2) were amplified due to the barotropic-baroclinic instability in the stratosphere and split the polar vortex. The instability was formed by the breaking of zonal wave number 1 PWs and depositing westward momentum. Furthermore, the authors suggest that the over-reflection of upward propagating PW2 contributed to split the polar vortex.

The paper is well-written and is highly relevant to EGUsphere. While some minor revisions are needed, I recommend the paper for publication, pending a few minor revisions.

MINOR COMMENTS

1) Sec 3.5, L288: ... can be traced back ...

  It is hard to identify from Figure 7. Please specify more clearly.

2) Sec 3.5, from L286:  Two paragraphs describe the possibility of over-reflection, which amplified the incident PW2 from troposphere. But main component of incident PW2 was EPW2, although amplified PW2 was mainly WPW2, as shown in Fig.4. It is OK? Please clarify.

3) Sec 3.5, L320: Although ..., over-reflection appears to play an increasingly dominant role in amplifying PW2 as the onset approached.

What makes the authors say so? What is the evidence?

4) Sec 4, L345: ... than tropospheric forcing

  This study shows two mechanism as described in Abstract: 1) PW1 break made instability which amplified PW2; 2) over-reflection of PW2. The latter is also accompanied by stratospheric instability and positive EPFD. Is the latter classified as tropospheric forcing? These two mechanisms are completely independent phenomena? Please help readers to have clear images. Comments 3) and 4) are true for Abstract as well.

WORDING

1) Eq.(4) , L101 : X’*cosφ*

2) Section 3.2, L143: above *3* hPa

3) Figure3(c): the contour interval of EPFD (red contours) is different from 3(b) and Fig.8

4) Section 3.3, L206: “aligning with the range of easterlies present in the instability region”

Show the contour interval or contour labels of u in Fig.3

5) Section 3.4, L236: An abrupt development of easterlies *were preceded by* ?

Review of “Role of in situ-excited planetary waves in polar vortex splitting during the 2002 Southern Hemisphere sudden stratospheric warming event”, authored by Ji-Hee Yoo and Hye-Yeong Chun.

This manuscript presents a study on the dynamics of the only major sudden warming that has been recorded on the Southern Hemisphere, using the reanalysis MERRA2. The authors thoroughly analyze the 5-day period before the central date of the event in terms of (linear) wave-mean flow interaction. They conclude that the breaking of planetary-scale wavenumber 1 in the polar stratosphere destabilized the flow and contributed to the generation of smaller-scale, wave-2 wave activity.

The figures are clear and the paper is well-written, and is relevant to ACP. I just have one concern that would only require adding a little bit of extra discussion.

This concern has to do with the applicability of linear wave propagation and wave-mean flow interaction theory to understand the dynamics of a highly distorted vortex as it is the case. I basically refer to the ideas and results by O’Neil and Pope (1988), who argued that the separation of the flow into a slowly-evolving, zonally symmetric component and a zonal harmonics might be overreaching during the final stages of the development of an SSW. There is a clear example in Figs. 6 and 7. Lines 243-244 argue that there is wave-1 focusing on high latitudes on 20-21 Sep “guided by the poleward-displaced vortex”. The authors refer to Fig 6a, where we see that the zonal-mean zonal wind on Sep 20 is poleward of 60ºS and vertically aligned. However, the PV maps of Fig. 7 (or Z maps of Fig. 2) do not show a “poleward-displaced vortex”, but a cyclonic vortex severely displaced off the pole and elongated. O’Neill and Pope make the case that in such situations, non-linear PV advection and vortex-vortex interactions by inspection of Ertel’s PV maps might be a better suited framework to interpret the dynamics.

In this framework, it would be interesting to discuss the results of O’Neill et al (2017) on the same SSW in the Southern Hemisphere. They showed that the vortex split happened due to the interaction of a synoptic-scale cyclonic circulation in the upper troposphere barotropically aligned with one of the stratospheric vortex tips, and argued that the Eliassen-Palm fluxes cannot unequivocally be interpreted as indicating wave propagation (in this specific context of high non-zonal flows), since it is a non-local (zonally averaged) diagnostic.

Other comments:

- Line 121. The reversal of zonal-mean winds (..) propagated downward → progressed down to10 hPa

- Line 122. It is not apparent from visual inspection of Fig. 1 that the winds decelerated 100 m/s in one week. Please check.

- Line 125-126. What do the authors mean by “an upward-propagating signal from the troposphere”? The troposphere is not shown in Fig. 1, with the lower boundary at 200 hPa (i.e. lowermost stratosphere at high latitude). Besides, the polar warming seems to be confined above 100 hPa or so.

- Lines 204-216. I find this description of Fig. 4 not clear enough. For example: “During (…) 22–25 September ,PW2 at 1 hPa predominantly exhibited westward phase speeds of up to 30 m/s”. I guess a westward phase of 30m/s refers to -30m/s in the Fig. However, the amplitude of PW2 with -30m/s on 22-25 Sep is quite small, it is larger at lower phase speeds.

References:

O'Neill, A. and Pope, V.D. (1988), Simulations of linear and nonlinear disturbances in the stratosphere. Q.J.R. Meteorol. Soc., 114: 1063-1110. https://doi.org/10.1002/qj.49711448210

O'Neill, A., Oatley, C.L., Charlton-Perez, A.J., Mitchell, D.M. and Jung, T. (2017), Vortex splitting on a planetary scale in the stratosphere by cyclogenesis on a subplanetary scale in the troposphere. Q.J.R. Meteorol. Soc., 143: 691-705. https://doi.org/10.1002/qj.2957.