Comments on “Vortex Preconditioning of the 2021 Sudden Stratospheric Warming: Barotropic/Baroclinic Instability Associated with the Double Westerly Jets” by Yoo et al.

Summary

In this study, the authors focus on the vortex preconditioning of the 2021 SSW. The paper emphasized the impacts of the double jet cores on the 2021 SSW. I reviewed this paper when it was submitted to another journal, and most of my concerns in previous reviews have been well addressed. Therefore, I have no major comments, and only several very minor revisions are required.

Specific Comments:

1.    L26: where => when

2.    L29: The tense in the first half sentence and the second half sentence is not consistent. Suggest to use the simple present tense.

3.    L38: focuses => focuses on

4.    L228-242: This paragraph should be moved to the “method” section.

5.    There are too many plots in one figure, and the readability is still not good.  Figure 1c can be further condensed and only typical days are shown. Many plots in Figures 3–5 are also duplicated from day to day.

This manuscript presents a study on the dynamical mechanisms behind the stratospheric vortex split during the stratospheric sudden warming (SSW) that took place on January 5^th, 2021 using the reanalysis MERRA2. The authors present strong pieces of evidence that support the interpretation of baroclinic/barotropic instability in the upper stratosphere as the cause of the explosive growth of the planetary wave-2 that led to the vortex split. Alternative dynamical mechanisms are also explored, such as wave propagation from the troposphere, wave growth by asymmetric gravity wave forcing, or resonance, but none of them were found to be consistent with the flow and wave fluxes evolution during the SSW onset.

I find that the analysis is thorough and the figures are clearly interpreted, and that the results make a strong case for internal stratospheric dynamics as a source of explosive wave growth during SSWs. I have only a few very minor comments, I recommend publication.

- Line 47. This needs to be changed, the latest major warming happened last winter, in February 2023.

- line 100: the level of 1 hPa is located around the stratopause, hence “upper stratosphere”.

- Lines 127-133. It seems to me that there is a bit of over-interpretation of Fig. 2a, the time-height evolution of EPFz is not entirely inconsistent with the theoretical vertical group velocity. Given the evidence provided in Figs. 2b and 3 on the decoupling between the wave-2 in the lower and upper stratosphere, there is no need to stretch the argument in Fig. 2a. Besides, I find it more compelling the fact that there appears a local EPFz maxima above 5hPa and lags-1 to 1, which cannot be explained by linear upward propagation.