the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Increasing frequency and lengthening season of western disturbances is linked to increasing strength and delayed northward migration of the subtropical jet
Kieran M. R. Hunt
Abstract. Western disturbances (WDs) are cyclonic storms that travel along the subtropical jet, bringing the majority of seasonal and extreme precipitation to mountainous South Asia in the winter months. They are a vital component of the region's water security. Although typically most common in the winter, WDs can also occur during the summer monsoon with catastrophic consequences. This happened earlier this year, leading to fatal floods across North India, including Delhi. Preceded by an unusually harsh winter season, questions are now being asked about how climate change is affecting WD frequency and intensity in both summer and winter seasons.
An analysis of 17 previous studies assessing trends in WD frequency revealed no consensus, at least in part because they quantified trends in different regions, seasons, and time periods. In this study, a more robust approach is used, quantifying trends in WD frequency and intensity by region and month, using a track catalogue derived from seventy years of ERA5 reanalysis data. Winter WDs have increased significantly over the Western and Central Himalaya and Hindu Kush in the last 70 years. This trend is attributed to a strengthening of the subtropical jet. The WD season has also significantly lengthened with WDs becoming far more common in May, June and July. For example, WDs have been twice as common in June in the last twenty years than during the previous fifty. This is attributed to delayed northward retreat of the subtropical jet, which historically has occurred before the onset of the summer monsoon. The most important implication is that the frequency of `monsoonal' WDs is increasing significantly, and therefore, due to climate change, catastrophic events like the 2013 Uttarakhand floods and the 2023 North India floods are becoming much more frequent.
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Kieran M. R. Hunt
Status: final response (author comments only)
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RC1: 'Comment on egusphere-2023-1778', A. P. Dimri, 02 Sep 2023
Review comments on ‘’Increasing frequency and lengthening season of western disturbances is linked to increasing strength and delayed northward migration of the subtropical jet” by Kieran M. R. Hunt
‘’mountainous South Asia’’ should be “Hindukush Himalayas” ONLY. Else mountainous South Asia also includes Khasi Jantia Hills in far east and in Burma too where WDs do not reach.
Change “…..WDs can also occur during the summer monsoon with catastrophic consequence…’’
To ‘….WDs can also occur during the summer as well interacting with monsoon leading to catastrophic 5 consequence
Change “…WD season has also significantly lengthened with WDs becoming far more common in May, June and July.” to “….WD season has also significantly lengthened with WDs freaking in May, June and July”
Pls see following as well.
Western Disturbances: A review. A. P. Dimri, D. Niyogi, A. P. Barros, J. Ridley, U. C. Mohanty, T. Yasunari, D. R. Sikka. Reviews of GeophysicsVolume 53, Issue 2 p. 225-246. https://doi.org/10.1002/2014RG000460
This is exceptionally good paper and work. Kieran has proposed a new dimension on SWJ dynamics leading to determine the WDs’.
I strongly recommend this paper to accept.
Dimri
Citation: https://doi.org/10.5194/egusphere-2023-1778-RC1 -
RC2: 'Comment on egusphere-2023-1778', Jean-Philippe Baudouin, 20 Sep 2023
The study discusses trends in mid-latitude upper tropospheric disturbances reaching Pakistan and NW India, also known as Western Disturbances (WD). As the author pointed out, the review of the literature is quite inconclusive, and a new study on the topic is welcome. The study uses a well-proofed detection technic that has been used to answer many scientific questions regarding WD in the past years. The observational dataset (ERA5 reanalysis) used for the analysis is also one of the best available for that kind of study, and its recent extension to dates prior to 1979 also enables potentially more robust trend analyses. In addition to these sound context review and framework, the study also brings clear and exciting new results on WD characteristic trends depending on the seasons. Yet, I would suggest clarifying the following points, in order to further improve the quality of the paper:
- I am wondering how robust the results are with respect to the dataset. The extension of ERA5 before 1979 is relatively new. It could be suffering from the assimilation of far fewer observations than the rest of the time period, and presenting spurious trends. In particular, a drop in the number of upper troposphere observations could be particularly detrimental to the analysis. For example, could it be possible that the fewer assimilated data in the early part of the reanalysis results in a blurring of the field used for WD detection, and therefore decrease the detection rate / intensity of the WDs? Has any study yet tried to validate upper-level tropospheric fields in ERA5? Could observations from radiosondes be used to validate some results of the study? (e.g. from https://www.ncei.noaa.gov/products/weather-balloon/integrated-global-radiosonde-archive)
- I am concerned that the paper could be seen as "yet another study with a different conclusion on WD activity trend". Could the author further analyse the differences between the new analysis compared to the previous studies? What should the reader do with all these seemingly contradicting studies? Even if the author cannot explain or suggest the reasons behind the differences with all the studies in Fig.1, particular attention should be put on Hunt et al. 2018 and Nischal et al. 2021. Those two studies are based on the very same algorithm and dataset as the current paper, and yet present respectively no trend, or a weak positive one. Are the results from this detection algorithm actually robust enough?
- I haven't really understood the purpose/design/discussion of the data from the right panel in figure 4, and panel a/b in figure 5, regarding jet characteristics. The author rightly notices that changes in the mean wind could be related to either jet speed or jet variation around its main position (L. 137-138). However, to distinguish between the two, the use of daily data (or higher frequency) would be needed. In fact, the algorithm from Schiemann et al. 2009 could be used to investigate trends in the entire distribution of jet latitudinal position and maximum intensity and would potentially give better insights into the jet behaviour.
- Finally, there was little attempt to discuss the reason for the jet shift, and whether one should expect an amplification of this phenomenon in a warming world. The argument on TP high warming rate would rather explain an earlier jet shift to the North, rather than a delay (cf. Krishnan et al. 2018). And despite citing several studies that have investigated jet trends, I don't see in the paper a comparison between their results to the ones presented here. In addition, from Figure 4, I see that a dipole of wind speed anomaly is evident, with the negative anomaly over the tropics being the strongest. I am wondering whether this suggests a tropical origin to the trend, e.g. shift of the Walker circulation, stronger subsidence of the Hadley cell branch over N India, increased convection over Indonesia?
I also have a few minor comments and questions along the text, including a few typos:
L.5 "This happened ...": This sentence sounds off in the abstract, it's oddly contextual, as summer WDs are not the main point of the paper. It should rather go in the introduction.
L. 53: "such as trends ...": these trends are rather potential causes of WD change rather than sources of confusion, or I am missing the author's point.
Fig 1. and 5: I didn't notice before reading the legend that some signs were grey, and others black. Maybe increase the difference?
L. 61: "Can differences in trends by explained" -> Can differences in trends be explained
L.61. "using different intensity thresholds": despite this being one of the main questions of the author, this is only very quickly investigated in Figure 3, and the text does away with the difference as fast. This question should either be further explored or not be one of the main questions.
L. 79 "[20-42.5°N, 60-80°E]": does the size of the box impact the robustness of the trends (for example, wrt Nischal et al 2022 who used a smaller box). Also, this removes most of the mid-latitude disturbances that pass over Central Asia. It seems from Figure 3c that those are rarer. Does this characterise a southern shift of the spring storm track, alongside the jet?
L. 79 "1950-2022": any reason for not using the 1940-1949 time period of ERA5?
Fig 2 (legend) : "for April and May": the panel c says "April to June"
Still Fig.2: Why does it seem that the track density is higher in c) than in b), when WD frequency is higher in the winter period overall?
L. 97: "reflecting the greater variability in the subtropical jet behaviour after the winter season": More specifically in April/May and October/November, as the jet is instead more stable in summer than in winter
L. 98: "WD box": why not take the same box as for the detection algorithm? That would be more consistent.
L. 102: "significantly": it should be specified at least once in the text that this corresponds to a statistical test.
L. 105 "significant" : substantial (because it's not about the statistical test)
L. 105 "snowfall": but precipitation is not really co-located with the WD centre. Trends in precipitation in relation to WD would be a whole new paper I guess.
L. 121: "not been a significant shift in regions impacted by WDs": This is very interesting, but I am a bit puzzled. Since the jet has shifted, why hasn't the WD track too? Is it because of some intensity threshold? For example, the meanders in the jet, when it is south enough, do not intensify enough (because of the lack of relief interaction) to be detected by the algorithm? Do these immature WDs still have an impact on the ground though?
L. 139 "resolve this": Besides my disagreeing (See main point 3), this seems to be contradicted a few lines after: "requires more in-depth statistics" (L.144)
L. 140 "This coincides with the region of increasing u": Though the main anomaly is located to the North of the distribution's mode.
L. 145: "in different seasons" -> depending on the season (?)
L. 156: "important" -> impactful, consequential?
L.156: "during June, as typically the jet starts to migrate northwards ...": It starts to move in April / May. It's pretty much already established North of the TP in June (Schiemann et al 2009)
L. 157: "The negative correlation": what correlation? isn't it rather the negative trend (in intensity)?
Fig. 5c/d: cutting the WD detection at 40°N removes the possibility to see how the mid-latitude disturbances over Central Asia behave.
Fig.5e: I suggest having the same colour for the two "WD *, jet strength", and "WD *, jet latitude" respectively. Also, do WD frequency and intensity correlate?
L. 167 (and later) "historically": it's vague, and it suggests that WD frequency normally doesn't vary, which is untrue since at least decadal variability is presented. Which time period do you consider here?
L. 175 "take WDs away": maybe clarify that the WD would still be detected, but wouldn't intensify as much?
L. 177 "A stronger jet offers deeper vertical wind shear". The vertical structure of the wind change is not investigated here, a stronger jet could also only increase the upper troposphere wind shear.
L. 178: "Sankar and Babu (2021); Nischal et al. (2023)": formatting
L. 178: "in contrast": I was a bit confused about where the contrast was. There could be a sentence: "WD frequency predominantly relates to jet strength, whereas WD intensity predominantly relates to jet position, (although both jet statistics remain significantly correlate with each of the WD statistics)"
Also, the reasons for the difference in correlation could be clarified more explicitly:
A stronger jet either meanders more or simply advect more quickly the meanders (actually, which is it?), resulting in a higher number of meanders (and thus WD) at a specific location. (This is a large-scale process)
By contrast, the precise positioning of the jet is really what enables the meanders to grow into powerful WDs, through the relief interaction. (it is a more local scale process)
L. 182: "the sharp increase in jet strength": the jet latitude has actually a slightly higher absolute correlation. In any case, both jet strength and intensity seem important.
L. 183: "We can probably rule out jet strength as a confounding factor": Should we? The correlation is significant, why not try to understand why? Could it be that the jet is more wavy, and therefore the temporal averaging makes the jet appear weak?
L. 185: "between these two effects." which effects? The effect of jet latitude is the same between April and June, and the effect of jet strength is indeed the opposite between the two months but discarded...
L. 201: "are much stronger in winter months": This is not a result of this study. Hunt et al, 2018a already established it.
L. 228: "There is has been" -> There has been
Citation: https://doi.org/10.5194/egusphere-2023-1778-RC2
Kieran M. R. Hunt
Data sets
Tracks of western disturbances (1950-2022) impacting South Asia Kieran M. R. Hunt https://zenodo.org/record/8208019
Kieran M. R. Hunt
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