the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 13 Aug 2024
Relationship between intraseasonal oscillations and abnormal rainfall in Vietnam
Abstract. Vietnam's summer monsoon season are charaterised by intense rainfall, driven by dynamic intraseasonal oscillations such as the Madden-Julian Oscillation (MJO) and the Boreal Summer Intraseasonal Oscillation (BSISO). Located near the equator with diverse terrain, the country presents a unique case for studying how these atmospheric mechanisms interact with local geography, influencing both the timing and spatial distribution of extreme rainfall events. Despite this, gaps remain in understanding the detailed coupled impact mechanisms that hinder the accurate prediction of abnormal rainfall over the region. This study pioneers an exploration into the interconnected dynamics of abnormal rainfall occurrences and coupled activities of BSISO and MJO in Vietnam. Using association analysis of abnormal events, detected from remote sensing gridded rainfall database, within conditional probability analysis framework, our findings reveal distinct seasonal patterns: during summer, North and South Vietnam experience primary impacts, whereas Central Vietnam becomes more susceptible during autumn. Notably, BSISO phases 3 and 5 dominate the summer landscape, while MJO phases exhibit balanced occurrence frequencies throughout the season. Probability assessments highlight phase 7 of BSISO in July and phases 6–7 in August as periods of heightened extreme rainfall probability in North and South Vietnam, contrasting with phases 5–8 in Central Vietnam. Additionally, MJO phase 5 emerges as a focal point for intensified extreme rainfall in October, alongside notable increases in phases 3–4 during September. This comprehensive analysis enhances our understanding of the complex interactions shaping Vietnam's monsoonal rainfall dynamics, offering valuable insights for future studies levaraging the two intraseasonal osciliations mechanisms to explain and predict abnormal heavey rainfall in Vietnam.
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Status: open (until 24 Sep 2024)
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RC1: 'Comment on egusphere-2024-2219', Anonymous Referee #1, 09 Sep 2024
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Peer-review of the manuscript: egusphere-2024-2219
Relationship between intraseasonal oscillations and abnormal rainfall in Vietnam
General comments: In this manuscript, the authors calculated the probability of occurrence of extreme rainfall days in each phase of the Boreal Summer Intraseasonal Oscillation (BSISO) and Madden-Julian Oscillation (MJO). They found that the phase of BSISO and MJO that play more important role than other phases in causing the heavy rainfall events is seasonal and regional dependent. The idea of this study is simple but useful. However, the main data used in this study, Climate Prediction Center Morphing Technique (CMORPH) precipitation data, does not perform well in depicting the rainfall variation over Vietnam. This leads to untrue features of rainfall variation and extreme rainfall events in this country. In addition, the number of heavy rainy days was calculated incorrectly, which affects to calculation of the probability of occurrence of extreme rainfall days in each phase of BSISO and MJO, the latter analysis and conclusion. Furthermore, the underlying physical mechanism associated with rainfall variation and heavy rainfall days were not explained clearly. Based on these reasons, I cannot recommend the paper to be accepted in the current format. The following specific comments are given to improve this manuscript.
Specific Comments
- In the manuscript, the authors examined the rainfall variation for different regions in Vietnam (i.e., North, Central, South, Central Highlands). It is better if the authors demarcate these regions in one figure. This will help the readers know where the authors want to mention.
- The performance of CMORPH data in depicting the rainfall variation in Vietnam is not good. For example, May-July is rainy season in the central highlands but dry season in the north central Vietnam. However, CMORPH shows that the rainfall during May-July in the north central Vietnam is higher than that in the Central Highlands (Figs. 1a-f), which is because CMORPH significantly underestimates rainfall in the Central Highlands (Trinh-Tuan et al., 2019). Consequently, the rainfall variation in Vietnam observed by CMORPH may not be the true features. Thus, the authors should use other precipitation data which can capture better the true features of rainfall variation in Vietnam.
- The authors calculated 90th percentile of rainfall across all grid points (Figs. 1g-l), then used this threshold to determine the number of extreme rainfall days (Figs. 1m-r). Accordingly, the number of extreme rainfall days should be about 3 days/ month for all grid points. However, the authors showed that the number of extreme rainfall days of each month vary significantly from 0 to 6 days (Figs. 1m-r). The authors should check their calculation because it affects to all the analysis and conclusion in the latter parts.
- Line 83-85, The authors wrote “Van de Linden et al. (2016) demonstrated that the heavy rainfall in northern Vietnam in August 2015 was caused by a westward movement of a low-pressure system over the northern region combined with the strong activity of the southwest monsoon”. Is this statement true? Van de Linden et al. (2016) studied about rainfall in the southern VN, they did nothing about heavy rainfall in Northern Vietnam.
- 2g-l, Figs. 6g-l, and Figs. 7g-l are redundant. The main goal of this study is investigating the relationship between ISOs and heavy rainfall. The authors should focus on the phase which induces heavy rainfall events instead of the phase whose highest occurrence frequency.
- In Fig. 3, why the anomalous rainfall over the central Highlands is very small at phase 4 but the probability of extreme rainfall is high? The authors should give detail about the time of this figure.
- Line 129-132, the authors wrote “This identification of rainfall events is conducted across all grid points from May to November, which is considered the rainy season in North and South Vietnam. The period from December to April is considered dry season in Vietnam (Nguyen-Le et al. 2014); therefore, it is excluded from our analysis”. However, in Fig. 6, Fig. 7, Fig. 9, and Fig. 10, the authors plotted and do analysis for December to April. Why they are inconsistent?
- Line 155-159, “It is important to note that, although the timing of rainy season onset is similar between the North and South Vietnam, the rainfall amount in the former is significantly higher than that in the later. This difference in rainfall amount is primarily caused by the extratropical factors effecting the North Vietnam in summer (Tuan et al. 2019)”. How is the role of tropical factors such as tropical cyclone, monsoon trough, and intertropical convergence zones which authors mentioned in Line 75-76?
- Line 164-166: “In another hand, heavy rainfall only begins to appear in Central Vietnam in September which is related to the intensification of cold surge, activities of tropical disturbances and orographic effects (Yokoi and Matsumoto 2007)”. In Yokoi et al. (2007), there is no information that the authors cited. The authors should double check this.
- Line 186-188: “Lastly, the number of extreme rainfall days in Central Vietnam reaches its peak in October-November, corresponding to the increase of rainfall in the subregion in the period”. How could the authors write that while Fig. 1 does not have November?
- In line 236-239, the authors wrote “While the probability of extreme rainfall tends to decrease over the North Vietnam, it is rapidly intensified significantly in the North Central and South Vietnam, consistent with the development of cold surge and activities of tropical disturbances during this period”. If the heavy rainfall events are induced by development of cold surge and activities of tropical disturbances, what is the contribution of BSISOs to these heavy rainfall events?
- Line 266-270, the authors wrote “While anomalous rainfall displays ….. to early autumn”. Figure 6 and 7 only show the rainfall in phase with highest occurrence frequency, which does not provide enough evidence to support the authors’ statement.
- Figure 8: Why the authors chose October to plot instead of other months?
- In Fig. 4, Fig. 5, Fig. 9, and Fig. 10: How did the authors use to determine which phase play the most important role in modulating the extreme rainfall?
- Conclusion: Line 343-345, this conclusion is not consistent with the result observed in Fig. 9 and Fig. 10 and what the authors wrote in Line 305-311.
Technical corrections:
- Line 39: typo “heavey”.
- Line 51: typo “theCentral”.
- Line 55: There are no Nguyen et al. 2023 and Nguyen et al. 2022 in references.
- Line 72-73: There are no (Khanh 1993; 1998a, 1998b, Lanh 2012) in references.
- Line 80: Wu et al. (2011) should be Wu et al. (2012).
- Line 96: There is no Wu et al. (2021) in the references.
- Line 97: There is no is Yen et al. 2011 in the references.
- Line 115: The authors should provide full name of CMORPH before abbreviation
- Line 121: The authors need to provide reference for BSISO and MJO index.
- Line 127-128: “In this study, a rainfall event is identified based on the 90th percentile of days with rainfall exceeding 0.3 mm in each respective month”. The authors should revise this sentence to make it clearer.
- Line 157: “differrence” should be difference .
- Line 197-198: “In June and July, phase 5 and phase 3 of the BSISO-2 exhibit the highest frequencies, respectively”. Phase 5 does not show the highest frequency of BISOS-2, the authors should check.
- Line 228: typo “of of ”.
- Line 233: the sentence “From July to August, …. rainfall” need to revise.
- Line 318: The authors should indicate where is Bien Dong on the map.
References
Trinh-Tuan, L., Matsumoto, J., Ngo-Duc, T. et al. Evaluation of satellite precipitation products over Central Vietnam. Prog Earth Planet Sci 6, 54 (2019). https://doi.org/10.1186/s40645-019-0297-7
Citation: https://doi.org/10.5194/egusphere-2024-2219-RC1
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