the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
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: open (extended)
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RC1: 'Comment on egusphere-2024-3322', Anonymous Referee #1, 20 Feb 2025
reply
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
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