the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Spatial Shift and Intensification of Compound Drought and Heatwaves (CDHW) towards Southern and Eastern Tropical Regions over India
Abstract. Quantifying the regional intensification of Compound Droughts and Heatwaves (CDHW) is necessary in the context of changing precipitation regimes. We investigated different characteristics of CDHW events over India, with an intent to unravel the regional shifts and intensification of these events, especially under the context of changed precipitation spells over the region. We deployed the Standardized Net-Precipitation and Distribution Index (SNEPI) to account for the impact of intramonthly precipitation spell variability for drought quantification and further to examine the spatiotemporal evolution of CDHWs from 1951 to 2016. There was a dramatic 86 % rise in the frequency of individual heatwaves and a significant intensification of individual droughts in historically humid zones over the past 6 decades. Whereas, the proportion of India affected by the compound (CDHW) occurrences expanded from 10 to 35 %, with hotspots transitioning from historically arid northwestern regions to historically humid eastern and southern regions. The Moderate, Severe, Elevated and Intense CDHW events spread by 3 to 5 times spatially, while the worst Extreme category experienced a 25-fold spatial increase. 46.4 % of Moderate and 43.3 % of Elevated events escalated to higher severities, suggesting a cascading amplification of extremes. Despite intervening wet periods, 10–16 % of CDHWs recurred, indicating that mitigation was unsuccessful, especially over the traditionally wet and resilient regions. The study emphasizes the necessity of reevaluating climate adaptation strategies across both traditionally arid and humid zones under evolving rainfall distributions as a result of a 4–10-fold increase in exposed urban populations across emergent hotspots (Kerala and Assam) in southern and eastern frontiers.
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RC1: 'Comment on egusphere-2025-1886', Anonymous Referee #1, 11 Aug 2025
Recommendation: Major RevisionMajor Comments:(1) The manuscript does not adequately acknowledge the body of research already established in the field of Compound Drought and Heatwaves (CDHW).Several references (e.g., Lines 62–64) are generic and not directly relevant to CDHW-specific studies.A comprehensive literature review is necessary, focusing on relevant and recent works in this domain. The current citations should be replaced or supplemented with more pertinent references that align with the topic and claims of this paper.Most of the references are vague and are cited from a general perspective.(2) The introduction is poorly organized. The first paragraph discussing various compound extremes is unnecessary; the authors should start directly with droughts, heatwaves, and their compound occurrences.The literature review is fragmented, and research gaps are not clearly identified.The claimed novelty—that CDHW evolution at intramonthly scale is unexplored—is incorrect, as there are several studies in the past 8 years addressing CDHW at weekly, biweekly, and monthly scalesLine 131: The stated hypothesis has already been demonstrated in prior literature; thus, its inclusion as a hypothesis here is unjustified. The study’s objectives and genuinely new contributions remain unclear.(4) Lines 175–178: The manuscript states that precipitation data covers 6,995 grids, while temperature data is available at 395 stations. How was the analysis conducted with such mismatched spatial coverage and resolution? This needs to be clearly explained.(5) Lines 196–206: It is unclear whether the heatwave definition used is adopted from existing literature or proposed by the authors. If novel, a rationale and validation are needed.The choice of the SNEPI drought index over widely used indices such as SPI, SPEI, or PDSI is not justified. The use of an uncommon index raises concerns about comparability and acceptance in the broader literature.The CDHW definition and framework appear ambiguous and lack physical and temporal coherence. Drought evolves at weekly to monthly scales, while heatwaves occur at daily scales—this mismatch is not adequately addressed.The “overlapping framework” used to define CDHW is mathematically unclear and appears impractical for scaling or deployment.(6) Poor figure quality: The quality of the presented figures is poor. Fonts are too small, making them difficult to read. Figures must be revised for clarity, resolution, and readability.General Comments:(1) LINE 93: The authors should be consistent when writing the key term CDHW. its compound drought and heatwave, dont write hot-dry event. Both have different meaning and different definitions and occur at different time scales. This hot-dry spell wording is repeated many places.(2) Line 77-83: most of the references presented are not pertinent to the compound drought and heatwave study. they are rather individual extremes such as either drought or heatwave or other extreme databases. The authors should pay attention to what they are citing.Minor Comments:(1) Remove the bracket (CDHW) from the title.(2) Line 17: Is it precipitation "spell" variability? The spell of precipitation could be dry/wet. Did the authorsexplored these various categorical spells?(3) Line 27: The 5th highlight is not actually potent. It should be removed.(4) Line 42: change from compound (CDHW) to only CDHW.(5) LINE 147: menion the sqmiles area of India.Citation: https://doi.org/
10.5194/egusphere-2025-1886-RC1 -
RC2: 'Comment on egusphere-2025-1886', Anonymous Referee #2, 10 Sep 2025
Apologies for the delay in reviewing the manuscript. I have now read the manuscript; my initial assessment is that the findings are important, but I am concerned about the novelty of the work, and the authors need to perform more work to make the work suitable for this journal:
1. The first question I have is whether the co-occurrences of drought and heatwave are due to any specific co-founders, or is it because of general increases in both events, resulting in more co-occurrences. This is the fundamental question the authors need to address, and I request the authors to please consider this.
2. My second question is what are the joint impacts of the co-occurrences, can they evaluate the impacts to a specific sector from observed data? Can they use historical agricultural yield to show how the co-occurrence impacts?
3. The physics component is fully missing in this work, and that is my biggest concern. The physical explanation is needed. For example, do heatwaves impact SM and then lead to land-atmosphere feedback, and then rainfall, or does rainfall impact SM and then lead to heatwaves through feedback? These are questions that need to be addressed. Claiming them to be out of scope will not make the present study novel.
4. The writing is loosely done, for example, the last few lines of the abstract, where terms like "mitigation", "adaptation" are a bit randomly used. the authors have to be careful.
Citation: https://doi.org/10.5194/egusphere-2025-1886-RC2
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