the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 25 Nov 2024
What advances monsoon onset over India?
Abstract. In the monsoon regions, atmospheric convection is typically stronger over the oceans than over land. Rainfall over land is potentially affected by the dynamic response of the atmosphere to deep convection over the adjacent oceans. Here, we show, in the case of the Indian summer monsoon, that enhanced atmospheric deep convection over the Bay of Bengal ∼2 weeks before onset, advances monsoon onset over India. Since the sea surface temperature of the Bay of Bengal is already hot during spring, warm anomalies further enhance convection that drives a convergence of low-level winds. A part of this circulation response blows from central India to the Bay of Bengal. It paves the way for monsoon circulation over India and advances the onset of monsoon. We tested this hypothesis using an atmospheric model forcing it by warm sea surface temperature anomalies over the Bay of Bengal 10–15 days before monsoon onset. Although the experiments with warm sea surface temperature anomalies over the Bay of Bengal does not reproduce earlier onset, which we attribute to uncertainties linked to the parameterization of convection, it does show a circulation response consistent with our hypothesis.
Status: final response (author comments only)
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RC1: 'Comment on egusphere-2024-3322', Anonymous Referee #1, 20 Feb 2025
The paper examines an interesting phenomenon of warming in the north Bay of Bengal region about 10-15 days prior to Indian summer monsoon rainfall onset and the authors have discussed the possible atmospheric forcing and response to the warming and its impact on monsoon onset. The manuscript is well written and the idea is interesting. I recommend that the manuscript be accepted pending some minor changes and improvements. Following are my detailed comments:
- The introduction section needs to focus only on the literature review without mixing it with the explanation of the hypothesis and the methodology used for conducting this study. There can be a separate paragraph for explaining the rationale of the study.
- Line 30: How does preparation of composites suppress interannual variability? The authors need to explain this.
- In figure 2: Were the onset dates cross verified with IMD onset dates? Is there any discrepancy?
- Line 34: Can the authors point to any specific index that seems to have arbitrary thresholds?
- Lines 173-175: How can a cold atmospheric column carry higher relative humidity? Or is that a typo. It needs to be corrected.
- The experiments using CAM model seems to add very little to the explanation of the dynamics of the phenomenon discussed in the paper. Is the idea of the paper to understand the dynamics of early and late onset features or to test the fidelity of the model? This needs clarification.
Citation: https://doi.org/10.5194/egusphere-2024-3322-RC1 - AC1: 'Reply on RC1', Bidyut Bikash Goswami, 15 Sep 2025
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RC2: 'Comment on egusphere-2024-3322', Anonymous Referee #2, 19 Aug 2025
The paper discusses the influence of sea surface temperature (SST) in the Bay of Bengal on the timing of the Indian monsoon onset. While I agree that SST in the Bay of Bengal is an important factor in determining onset timing, I recommend that the paper, in its present form, be rejected. Below are my major concerns:
- The authors completely neglect the role of atmospheric dynamics, such as intraseasonal oscillations, equatorial waves, and atmosphere–ocean coupling. The influence of the Madden-Julian Oscillation (MJO) phase on monsoon variability and onset has been discussed in many papers (see references below). MJO convection arriving over the Indian Ocean in late May can propagate northward, triggering the onset. On the other hand, dry intraseasonal conditions in late May can delay the onset. While SST anomalies in the Bay of Bengal can influence the northward propagation of convection, these SST anomalies are not independent of atmospheric circulation; rather, they are often responses to intraseasonal oscillations or tropical cyclones developing in early May. While I understand that the authors focus on the impact of SST, the role of atmospheric dynamics must be discussed. At a minimum, the MJO phase should be included in the analysis.
- In the SST, precipitation, and circulation differences between early and late onset cases shown in the paper, it is difficult to separate the effect of the seasonal cycle from the proposed influence of the SST anomaly. I suggest that, before calculating these differences, the authors first compute anomalies relative to the seasonal cycle.
- Because the observational cases used to formulate the hypothesis on SST influence included various intraseasonal dynamic states, while the model experiments were initialized with the same atmospheric conditions, the model experiments do not truly test the proposed hypothesis. In fact, the model results do not support the mechanisms of onset variability proposed by the authors. The inclusion of the SST warm patch does not lead to an earlier onset, even though precipitation is enhanced in the 5–10°N region of the Bay of Bengal. The authors suggest that this is because the monsoon index they use reflects a broader large-scale pattern. However, since they initially claim that this index provides a good characterization of the onset, this explanation is not sufficient to account for the negative result of the experiment. It is possible that, since the onset in CAM is already early, the model’s large-scale conditions (e.g., land temperatures) may not support an even earlier monsoon onset related to local SST conditions in the Bay of Bengal. However, I suspect that the inclusion of a cold patch, rather than a warm one, in the Bay of Bengal could delay the onset -such a result could to some extent confirm the authors hypothesis on SST influence on onset timing.
Because I am not convinced that the authors’ hypothesis is properly formulated or that the model simulations effectively evaluate the scenario they propose, I cannot recommend acceptance of the paper in its present form.
A sample of papers discussing the influence of the equatorial modes on the monsoon onset :
Bhatla, R., Singh, M. and Pattanaik, D.R., 2017. Impact of Madden-Julian oscillation on onset of summer monsoon over India. Theoretical and Applied Climatology, 128(1), pp.381-391.
Bombardi, R.J., Moron, V. and Goodnight, J.S., 2020. Detection, variability, and predictability of monsoon onset and withdrawal dates: A review. Int. J. Climatol, 40(2), pp.641-667.
Flatau, M.K., Flatau, P.J. and Rudnick, D., 2001. The dynamics of double monsoon onsets. Journal of climate, 14(21), pp.4130-4146.
Joseph, S., Sahai, A.K. and Goswami, B.N., 2009. Eastward propagating MJO during boreal summer and Indian monsoon droughts. Climate Dynamics, 32(7), pp.1139-1153.
Lenka S, Gouda KC, Devi R, Joseph CM. Dynamical influence of MJO phases on the onset of Indian monsoon. Environmental Research Communications. 2023 Jun 28;5(6):061006.
Shroyer E, Tandon A, Sengupta D, Fernando HJ, Lucas AJ, Farrar JT, Chattopadhyay R, de Szoeke S, Flatau M, Rydbeck A, Wijesekera H. Bay of Bengal intraseasonal oscillations and the 2018 monsoon onset. Bulletin of the American Meteorological Society. 2021 Oct;102(10):E1936-51.
Taraphdar, Sourav & Zhang, Fuqing & Leung, L. & Chen, Xingchao & Pauluis, Olivier. (2018). MJO Affects the Monsoon Onset Timing over the Indian Region. Geophysical Research Letters. 45. 10.1029/2018GL078804.
Tyagi, V., Das, S., Das, S.K. and Shukla, B.P., 2025. On double monsoon onset and its predictive signature. Meteorology and Atmospheric Physics, 137(2), p.18.
Citation: https://doi.org/10.5194/egusphere-2024-3322-RC2 - AC2: 'Reply on RC2', Bidyut Bikash Goswami, 15 Sep 2025
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