Seasonal to Interannual Variability of Vertical Wind Shear and Its Relationship with Tropical Cyclogenesis in the Mozambique Channel

Round 1

Reviewer 1 Report

The authors have examined the role of vertical wind shear in tropical cyclogenesis within the Mozambique Channel. II have attached my line-by-line commentary as a PDF form; however, I have a number of concerns regarding the publishing of this paper. (1) I believe that the motivation for this paper is lacking. As mentioned in the introduction to the paper, the detrimental effects of vertical shear on TC genesis has been well-established. The authors have not provided any reason why our current understanding of TC genesis cannot be applied to the Mozambique Channel. (2) Since the authors examined bulk shear between 200 mb and 850 mb, the methods used in this study have not produced any interesting results. A more useful study would be to examine how the vertical distribution of shear affects TC genesis. (3) I also believe more work should be done in the paper. When one performs a climatology for any atmospheric variable, it is usually necessary to show how interannual, interseasonal, intraseasonal, and multidecadal oscillation affect the results. For example, in this paper, the authors note the following: "For the period considered here, the VWS average is 8.02 72 m/s with a maximum of 10.63 m/s in 1992 and a minimum of 5.36 m/s in 2017 (Figure 9a). 73 The number of TC genesis per season is between 0-3, the maximum number of cyclogenesis occurred in 1998, 1999, 2003, 2014, 2015 and 2019, there were twelve years in 75 which no TCs formed and an average of 1.2 TCs occurred per season." Very little explanation is given on the reason behind these results. A quick assessment shows that maximum number of cyclogenesis events are associated with well-known ENSO seasons (1998/1999; 2015/2016). I don't see evidence that the authors examined how these modes of climatic variation affects their results.

Comments for author File: Comments.pdf

Author Response

Please find attached our reply point by point to your comments and suggestions.

 

Thanks

Atanasio 

Author Response File: Author Response.pdf

Reviewer 2 Report

In this work the authors try to connect the vertical wind shear with the genesis of Tropical Cyclones (TCs) in the Mozambique Channel. To this end, they use remote observations of Sea Surface Temperature (SST) and numerical atmospheric reanalyses that cover the last 40 years.

I am sorry to say that, from my perspective, although the idea is promising extra-work is needed to enrich this research and deepen on the dynamical connection between TC genesis and vertical wind shear. In the following lines I make some suggestions that can contribute to improve this work. On the other hand, the article has many English mistakes and typos that need to get resolved (e.g. only in the abstract I have found eight typos or expressions that are not correct). Therefore I encourage a re-submission of the manuscript once all flaws indicated below are addressed.

 

Structure and form of the manuscript

In the title the word climatology is too generic. I recommend better Seasonal and Inter-annual variability of vertical wind shear ...

Please, check page numbers, they are wrong.

 

Data

I have missed a better description of datasets. At least: Why have the authors selected those datasets? What is their spatial and original time resolution? Is this data freely accessible? How can be accessed?

 

Methods

It is not totally clear to me the definition of TC genesis that the authors use. Do you pick up the first point within the TC database that has TC category in the Mozambique Channel? Or this point can also initially be a tropical storm or a tropical depression?

Results

Regarding the above point, I find interesting to show the maximum TC category that the studied tropical systems reach (e.g. with a table). Does the max. TC intensity and the time taken by the TC to intensify depend on the magnitude of the vertical wind shear? Do the locations where TCs make landfall depend on the TC genesis location and/or the vertical wind shear?

Also it is not clear neither how the authors perform the composites. Do they look at each TC genesis, retrieve the map of a given variable (e.g. SST), and then compute the mean for all collected maps? This point should be clearer.

How do the authors explain that TCs develop under very strong vertical wind shear (Fig. 5, April panel)?

I suggest to plot gray contours in Fig. 5 with larger labels instead of colored contours since the color does not add new information.

Moreover, I consider more useful to show maps of anomalies (e.g. removing the TC-season mean and the trend) instead of mean maps. Anomalies allow to compare the different variables of interest and monthly changes in a more direct way.

Indeed, have the authors removed the trend of all data before performing the composites? How trends look like?

Likely is interesting to analyse with scatter plots of TC genesis with which variable of interest TC genesis points are more linearly related. For instance, plot SST in x-axis and vertical wind shear in y-axis, averaged around the TC genesis point according to a certain fixed square, e.g. of size 2 deg x 2 deg.

To better separate seasonal and inter-annual signals in the vertical wind shear (e.g. Fig. 9a) the authors can use a spectral decomposition (e.g. simple Fourier seasonal and semi-annual components) to at least remove the seasonal signal.

Are trends in Fig. 9 statistically significant? What is the correlation between vertical wind shear

One way to analyse with more detail to what extent the vertical wind shear is correlated with the southwestern Indian Ocean SST warm pool is by computing the SST mean over a rectangle in the warm pool to get the timeseries. This timeseries can then be compared against vertical wind shear timeseries averaged over the Mozambique Channel. And then repeat it with other variables of interest (relative humidity, geopotential, relative vorticity, etc).

Regarding the inter-annual variability of TC-genesis, how does this variability depend on the dominant regional climate signals (e.g. the Indian Ocean Dipole)? Can you find any lagged correlation?

 

Reference

Barimalala, R., Blamey, R. C., Desbiolles, F., & Reason, C. J. C. (2020). Variability in the Mozambique Channel Trough and Impacts on Southeast African Rainfall, Journal of Climate, 33(2), 749-765. https://journals.ametsoc.org/view/journals/clim/33/2/jcli-d-19-0267.1.xml

 

 

 

 

 

 

Author Response

Please find attached the reply to your comments point by point.

 

Thanks !

Author Response File: Author Response.pdf

Reviewer 3 Report

In this study authors try to show that the high frequency of genesis of tropical cyclones in the Mozambique channel in the DFJM season is due to the climatologically favorable low vertical wind shear values over the region though other factors may also contribute to the cyclogenesis. The work needs a major revision and I have provided a few recommendations in the followings.

The vertical shear is not the only factor for the cyclogenesis. For example, Fig. 9 clearly shows the shear has low magnitude in 2006 but no cyclones are observed. Though authors have acknowledged the role of other factors, no comparative analyses were carried to show the real importance of the vertical shear compared to those other factors. I recommend authors to show the relative importance those other factors such as SST etc. in comparison to the vertical shear. This would help the readers better understand the importance of the shear in the cyclogenesis in the Mozambique channel. 

Figure 7 is referred before Fig 6 in the manuscript. Authors need to change the order of the figures.

In page 5: VWS < 11 m/s should be VWS > 11 m/s

Author Response

Please find the reply of your valuable comments in the attachment,

 

Best Regards,

 

Atanasio 

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

I appreciate the effort made by the authors. However, I still see two issues to be improved:

     - There are many English typos and errors that must be corrected (e.g. in the first line of the abstract, explores).

     -  I am not totally satisfied by the author's reply. Time should not be a constraint to perform an excellent academic work. Please, take your time to complete your analysis. If necessary, resubmit this work again.

 

 

 

Author Response

Dear reviewer,

The authors of the manuscript entitled “Seasonal to interannual Vertical Wind Shear and its Relationship with Tropical Cyclogenesis over the Mozambique Channel”  would like to thank you for the additional comments. Please find below the reply line by line to your comments.

The remaining items suggested to look at were analysed and the findings incorporated to the manuscript, namely:

1. There are many English typos and errors that must be corrected (e.g. in the first line of the abstract, explores).

An effort was done by the authors to correct all English typos and errors identified

2. I am not totally satisfied by the author's reply. Time should not be a constraint to perform an excellent academic work. Please, take your time to complete your analysis. If necessary, resubmit this work again.

The items that were not included in the previous version are now included to the manuscript, namely:

Do the locations where TCs make landfall depend on the TC genesis location and/or the vertical wind shear?

The influence of the VWS to TCs landfall was explored based on composite analysis and the results and conclusions included in the manuscript.  

Likely is interesting to analyse with scatter plots of TC genesis with which variable of interest TC genesis points are more linearly related. For instance, plot SST in x-axis and vertical wind shear in y-axis, averaged around the TC genesis point according to a certain fixed square, e.g. of size 2 deg x 2 deg.

 Scatter plots analyses were done based on the main cyclogenesis region, monthly data and for the pick cyclogenesis period

Regarding the inter-annual variability of TC-genesis, how does this variability depend on the dominant regional climate signals (e.g. the Indian Ocean Dipole)? Can you find any lagged correlation?

Additional correlations were done between the VWS and SIOD and significant correlations were found and included in the manuscript 

Reviewer 3 Report

The revised version needs a lot language corrections. I have suggested a few in the attached file but the authors need to take the help of a native speaker or a professional service for a careful language correction.

Comments for author File: Comments.pdf

Author Response

Dear reviewer,

The authors of the manuscript entitled “Seasonal to interannual Vertical Wind Shear and its Relationship with Tropical Cyclogenesis over the Mozambique Channel”  would like to thank you for the additional comments. An effort was done by the authors to correct all English typos and errors identified.

 

Best Regards,

Atanasio