the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 25 Feb 2026
A synoptic clustering-based definition of South China Sea summer monsoon onset and application to seasonal prediction
Abstract. Accurately predicting the South China Sea summer monsoon (SCSSM) onset date is of crucial importance for effective water resource management, agricultural planning, and disaster prevention across East Asia and the Western North Pacific. However, reliable predictions at seasonal timescales remain challenging. To address this, we propose a synoptic circulation–based approach that defines monsoon onset through persistent large-scale circulation regimes identified by clustering low-level atmospheric fields. Using ECMWF SEAS5 seasonal forecasts, we evaluate onset predictions derived from the proposed regime-based definition against those based on a conventional zonal wind criterion. The regime-based definition yields systematic improvements in deterministic correlations, potential predictability, and categorical and probabilistic skill metrics. Skill gains are evident during both a dependent training period and an independent forecast period, with enhanced performance persisting at long lead times of up to five months. The improved predictability reflects the multi-timescale controls on monsoon onset, wherein slowly varying boundary forcing modulates large-scale circulation and subseasonal variability triggers the onset transition. By emphasizing regime persistence and structural evolution, the proposed framework better isolates the predictable component of onset variability and enhances forecast robustness. These findings demonstrate that circulation-regime–based definitions offer a physically grounded and effective alternative for seasonal prediction of monsoon onset.
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Status: open (until 24 Apr 2026)
- RC1: 'Comment on egusphere-2026-358', Anonymous Referee #1, 19 Mar 2026 reply
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CC1: 'Comment on egusphere-2026-358', Peng Hu, 30 Mar 2026
reply
The manuscript presents a synoptic circulation–based clustering approach (NL26) for defining the South China Sea summer monsoon onset. Overall, the study is well-conceived and methodologically sound. The proposed approach demonstrates substantial improvements in deterministic, categorical, and probabilistic prediction skill relative to the conventional U850-based onset definition (Wang et al., 2004; W04). The analysis is comprehensive, and the findings are potentially valuable for improving seasonal forecasts. However, several aspects of the manuscript require further clarification and elaboration before the work can be considered for publication.
(1) Abbreviations and notation: This manuscript employs numerous abbreviations (e.g., OD, NL26, W04, RPC, ACC, HSS, RPSS, BSS), which may impede readability. It is strongly recommended that the authors provide a dedicated table summarizing all abbreviations and their definitions for clarity.
(2) Differences between NL26 and W04 onset dates: Figure 2 indicates that the NL26 onset dates differ from W04 in certain years, but the current explanation is rather generic (e.g., “reflecting the persistence and maturity criteria”). The authors should provide a more detailed, year-specific analysis of these discrepancies. For example, are delayed onsets due to brief westerly intrusions being excluded by the persistence criterion? Are earlier onsets associated with an early and coherent transition to monsoon-type circulation? A more mechanistic discussion would enhance the reader’s understanding of the advantages and physical basis of the NL26 definition.
(3) Limitations of the proposed method: The manuscript primarily emphasizes the benefits of the NL26 approach but does not adequately discuss its potential limitations, which may include: (a) applicability in real-time operational forecasting contexts, (b) complexity of implementing the SOM plus K-means workflow, which could pose challenges for replication, (c) sensitivity to extreme events or limited sample years. A discussion of these limitations would provide a more balanced and rigorous assessment of the method.
(4) Weakening relationship between SCSSM onset and ENSO: The manuscript discusses the modulation of SCSSM onset by ENSO. However, recent studies suggest a interdecadal weakening of the ENSO–SCSSM onset relationship (Hu et al. 2022; Hu et al. 2026). The authors should acknowledge this weakening relationship and discuss its potential implications for forecast skill in the independent verification period (2017–2024).
Hu P, Chen W, Chen S, et al. The weakening relationship between ENSO and the South China Sea summer monsoon onset in recent decades. Advances in Atmospheric Sciences, 2022, 39(3): 443-455.
Hu P, Chen W, Cai Q, et al. Delayed tropical Asian summer monsoon onset in recent decades. Geophysical Research Letters, 2026, 53(1): e2025GL120825.
(5) Please delete the phrase “also known as the East Sea in Vietnam”, as this name is not widely recognized or accurate for the international audience.
Citation: https://doi.org/10.5194/egusphere-2026-358-CC1 -
CC2: 'Comment on egusphere-2026-358', Peng Hu, 30 Mar 2026
reply
The manuscript presents a synoptic circulation–based clustering approach (NL26) for defining the South China Sea summer monsoon onset. Overall, the study is well-conceived and methodologically sound. The proposed approach demonstrates substantial improvements in deterministic, categorical, and probabilistic prediction skill relative to the conventional U850-based onset definition (Wang et al., 2004; W04). The analysis is comprehensive, and the findings are potentially valuable for improving seasonal forecasts. However, several aspects of the manuscript require further clarification and elaboration before the work can be considered for publication.
(1) Abbreviations and notation: This manuscript employs numerous abbreviations (e.g., OD, NL26, W04, RPC, ACC, HSS, RPSS, BSS), which may impede readability. It is strongly recommended that the authors provide a dedicated table summarizing all abbreviations and their definitions for clarity.
(2) Differences between NL26 and W04 onset dates: Figure 2 indicates that the NL26 onset dates differ from W04 in certain years, but the current explanation is rather generic (e.g., “reflecting the persistence and maturity criteria”). The authors should provide a more detailed, year-specific analysis of these discrepancies. For example, are delayed onsets due to brief westerly intrusions being excluded by the persistence criterion? Are earlier onsets associated with an early and coherent transition to monsoon-type circulation? A more mechanistic discussion would enhance the reader’s understanding of the advantages and physical basis of the NL26 definition.
(3) Limitations of the proposed method: The manuscript primarily emphasizes the benefits of the NL26 approach but does not adequately discuss its potential limitations, which may include: (a) applicability in real-time operational forecasting contexts, (b) complexity of implementing the SOM plus K-means workflow, which could pose challenges for replication, (c) sensitivity to extreme events or limited sample years. A discussion of these limitations would provide a more balanced and rigorous assessment of the method.
(4) Weakening relationship between SCSSM onset and ENSO: The manuscript discusses the modulation of SCSSM onset by ENSO. However, recent studies suggest a interdecadal weakening of the ENSO–SCSSM onset relationship (Hu et al. 2022; Hu et al. 2026). The authors should acknowledge this weakening relationship and discuss its potential implications for forecast skill in the independent verification period (2017–2024).
Hu P, Chen W, Chen S, et al. The weakening relationship between ENSO and the South China Sea summer monsoon onset in recent decades. Advances in Atmospheric Sciences, 2022, 39(3): 443-455.
Hu P, Chen W, Cai Q, et al. Delayed tropical Asian summer monsoon onset in recent decades. Geophysical Research Letters, 2026, 53(1): e2025GL120825.
(5) Please delete the phrase “also known as the East Sea in Vietnam”, as this name is not widely recognized or accurate for the international audience.
Citation: https://doi.org/10.5194/egusphere-2026-358-CC2
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This study proposes a novel synoptic clustering-based approach (NL26) using Self-Organizing Maps (SOM) to define the South China Sea Summer Monsoon (SCSSM) onset based on persistent large-scale circulation regimes. Evaluated using the ECMWF SEAS5 seasonal hindcasts, the author concludes that this regime-based definition yields systematic improvements over the conventional zonal wind-based criterion (W04) in deterministic and probabilistic skill metrics up to a 5-month lead time.
The manuscript addresses a highly relevant and challenging topic in seasonal monsoon prediction. The methodology is interesting, and the motivation aligns well with the scope of Weather and Climate Dynamics. However, before the manuscript can be recommended for publication, there are several major scientific concerns that need to be addressed. These primarily relate to the physical consistency of the new index, the validity of the "improved predictability" during extreme delayed years (e.g., 2018), and a lack of diagnostic evidence supporting the claims regarding subseasonal-to-interannual timescale interactions. I recommend a Major Revision.
Major Comments
1. Physical Consistency with Thermodynamic Metrics. Figure 2 shows a correlation of 0.74 between the NL26 and W04 indices. While the SOM clustering is designed to capture large-scale circulation regimes, the abrupt onset of deep convection and precipitation remains the most critical thermodynamic characteristic of the SCSSM onset. Since precipitation was not selected as an input variable for the clustering, it is vital to verify whether this pure circulation-based index remains physically consistent with actual convective activities. The author should include a comparative analysis between the NL26 index and key thermodynamic/convective indicators, specifically the Meridional Temperature Gradient (MTG) and Outgoing Longwave Radiation (OLR), to ensure that the identified circulation regimes robustly correspond to the onset of the monsoon rainfall.
2. Logical Transition from Climatology to Interannual Forcing. There is a noticeable logical gap between the climatological evolution presented in Figure 3 and the interannual Sea Surface Temperature (SST) patterns associated with early/late onset years shown in Figure 4. The transition from mean state characteristics to specific interannual boundary forcings feels abrupt and lacks sufficient diagnostic bridging in the text. Consider moving Figure 4 to the Supplementary Material, or significantly expand the dynamical explanation in the text to justify the transition to SST forcing patterns at this stage of the manuscript.
3. Cross-Validation against Operational Benchmarks. In Section 4.1.1 (Forecast skill assessment), Figure 5 validates the W04 model forecasts against W04 observations, while Figure 6 validates the NL26 forecasts against NL26 observations. Although internally consistent, this comparison is insufficient to demonstrate the practical forecasting value of the new method. Given that W04 is widely applied as a standard benchmark in operational forecasting services, please add an evaluation comparing the NL26 model forecasts directly against the W04 observational values. This cross-index validation is necessary to quantify the actual added value of the NL26 approach in real-world operational contexts.
4. Forecast Skill Evaluation in Extreme Years (e.g., 2018). In Section 4.2.1, the manuscript highlights that the correlation coefficient for the NL26 prediction is significantly higher than that of W04. However, a detailed comparison of Figures 8 and 9, alongside the lead-time performance (December to April), reveals a critical issue. The apparent improvement in NL26 is largely driven by its performance in specific years, such as 2018. In reality, 2018 featured an extremely late SCSSM onset. The NL26 index defines the "true" onset date for 2018 as May 8, which allows the model to register a "hit" at all lead times. However, this entirely misses the actual physical delay of the monsoon onset that year. The author must provide an in-depth discussion on extreme years where traditional ENSO-based seasonal forecasts typically fail (e.g., 2018 and 2019). It is essential to clarify whether the claimed "enhanced forecast robustness" is a genuine improvement in capturing anomalous monsoon dynamics or merely a mathematical artifact resulting from redefining the onset target.
5. Evidence for Subseasonal Variability Claims. The abstract and discussion state that the improved predictability reflects "multi-timescale controls" and that "subseasonal variability triggers the onset transition." The author suggests that the NL26 index better isolates the predictable component when the ENSO-monsoon relationship is weak. However, the main text lacks concrete case studies or dynamical diagnostics to substantiate this. Existing literature shows that predicting the onset is more difficult in years dominated by subseasonal signals. To support the current claims, the author should include specific case studies [such as 2019 or other years with strong intraseasonal oscillation but weak ENSO forcing] to explicitly demonstrate how the NL26 index outperforms traditional indices in capturing the superimposition of subseasonal signals onto the large-scale circulation.
Other suggestion
1. Typographical Error in Figure 2: In the final row of Figure 2, the subplot labels should be corrected to C1, C3, and C6 to align with the SOM clustering nomenclature used throughout the manuscript.