Variations of monsoon rainfall: A simple unified index

The monsoon onset is a critical event in the Bangladesh calendar, especially for the domestic agricultural sector. Providing information about the monsoon onset for the past, present, and future has potential benefit for a country so vulnerable to changes in climate. But, when does the monsoon start? To produce any scientific information about monsoon onsets, lengths, and withdrawals, we first need to apply a monsoon definition to our data. Choosing a scientific definition is not such a simple exercise in Bangladesh. Different definitions lead to different monsoon onsets and thereby also monsoon lengths. If a climate application aims to provide information about the monsoon onset, then we need to understand how the people who might use this information perceive the monsoon onset. We then need to understand how their perceptions compare with previous scientific work. In this study we carried out a structured questionnaire in six rural regions around Bangladesh and asked the local agriculturists how they defined the monsoon and when they thought it started. It turns out that the agriculturists and previous scientific publications do not necessarily agree. Our results do not undermine previous scientific work on the monsoon in Bangladesh, but they do carry an important message about how we should design, implement, and evaluate climate applications in Bangladesh that encompass the monsoon onset.

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“…Some define the monsoon essentially as a rainfall phenomenon (Dang-Quang et al 2014). Others describe it as a wind phenomenon (Ramage 1995).…”

Section: But Why Ask the Locals?mentioning

confidence: 99%

Complementing Scientific Monsoon Definitions with Social Perception in Bangladesh

Stiller-Reeve

Syed

Spengler

et al. 2015

Bulletin of the American Meteorological Society

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“…Omotosho (1992) and Omotosho et al (2000) considered that changes in both upper-air wind direction and surface equivalent potential temperature, which represents variations of moisture in the monsoon flow, herald rainy season onsets in West Africa. Other studies used either the change of prevailing winds (Holland, 1986), used combined wind-rainfall criteria (Hendon and Liebmann, 1990), or used combined wind-mean sea level pressure criteria (Nguyen et al, 2014b) to define monsoon onset dates. Li and Yanai (1996) and Mao et al (2004) have shown that the reversal of the meridional temperature gradient in the upper troposphere (200-500 mb) indicates the monsoon onset.…”

Section: Introductionmentioning

confidence: 99%

Predictability of the rainy season onset date in Central Highlands of Vietnam

Pham-Thanh

Linden

Ngo‐Duc

et al. 2019

Intl Journal of Climatology

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In this study, different criteria to determine the rainy season onset date (RSOD) and its predictability in the Central Highlands (CH) of Vietnam are investigated. Using daily rainfall amounts from 10 meteorological stations for the period 1981-2015, four criteria to determine the RSOD were tested in order to select the one that most reasonably depicts the climatology of the RSOD over the CH. Results show that the RSOD varies strongly from year to year. In the long-term mean, the onset starts first in southern parts of the CH and then progresses to the northern parts. The earliest onset date is around the beginning of April, and the latest in the second half of May. The average RSOD is on 28 April with a standard deviation of 14 days. Thus, the RSOD is distinct from the summer monsoon onset, namely, leading the mean summer monsoon onset by about 3 weeks in some years. In terms of remote influences, the RSOD in the CH has a high correlation with the El Niño-Southern Oscillation phenomenon, with most RSODs being later during El Niño years, while being earlier during La Niña years. The RSOD in the CH also shows high correlations with sea surface temperatures (SSTs), 850-hPa zonal winds (U850) and mean sea level pressures (PMSL) over certain regions of the Pacific and Indian Oceans. Building on these relationships, the RSOD can be successfully predicted using large-scale fields of SST, U850 and PMSL as predictors in two different approaches, namely the field mean and principal component analysis. This suggests that the overall approach can also be applied in predicting the RSOD in the CH on sub-seasonal to seasonal timescales. K E Y W O R D SCentral Highlands, predictability, rainy season onset, seasonal prediction, Vietnam

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“…The last category combines dry-wet alteration and prevailing wind reversal, such as from the studies of Webster (1987), Qian et al (2002a), , Zhang and Wang (2008), and Qian and Tang (2010). As indicated recently by Nguyen et al (2014), some "spots" could not be detected by some definitions used local rainfall or zonal wind threshold of local difference between two calendar seasons of summer and winter. A spot in southern China also exists in our previous study using a rainfall of local precipitation difference between two calendar pentads 8 and 44 (Qian and Tang 2010).…”

Section: Review Of Historymentioning

confidence: 91%

“…Although the term of global monsoon domain has been proposed in many previous works using different indices, debates on whether such a global domain can be objectively determined still remain, as summarized by Wang and Wu et al (2008) and indicated recently by Nguyen et al (2014). By an annual precipitation range between local summer and winter, depicted a global monsoon domain.…”

Section: Introductionmentioning

confidence: 99%

“…By an annual precipitation range between local summer and winter, depicted a global monsoon domain. However, as Nguyen et al (2014) indicated, some "spots" could not be detected by their definitions due to the use of a local calendar rainfall or zonal wind threshold with the difference between two calendar seasons of local summer and winter. This is a critical issue concerned by climatologists and short-term climate forecasters.…”

Section: Introductionmentioning

confidence: 99%

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The global monsoon definition using the difference of local minimum and maximum pentad precipitation rates associated with cross-equatorial flow reversal

Qian

Jiang

2015

Theor Appl Climatol

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Since most previous attempts to establish monsoon indices have been limited to specific regions, they have lacked the applicability to universally describe the global monsoon domain. In this paper, we first review the history of global monsoon study and then identify the climatology of global precipitation associated with major systems of the atmospheric general circulation. A new index, based on the annual and semiannual harmonic precipitation rate difference between two local calendar maximal and minimal precipitation pentads, is used to identify the global monsoon domain focusing on where experienced and what caused the climatic dry-wet alteration. The global monsoon domain is defined by the regions where two pentad-mean precipitation difference exceeds 4 mm·day −1 , which is also influenced by the low-level prevailing wind reversal associated with the cross-equatorial flow. This definition not only confirmed previous results of the classical global monsoon domain from the tropical Africa to Asia-Australia and nonclassical monsoon region in the tropical America but also solved an issue of missing local summer monsoon spots.

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Variations of monsoon rainfall: A simple unified index

Cited by 16 publications

References 30 publications

Complementing Scientific Monsoon Definitions with Social Perception in Bangladesh

Complementing Scientific Monsoon Definitions with Social Perception in Bangladesh

Predictability of the rainy season onset date in Central Highlands of Vietnam

The global monsoon definition using the difference of local minimum and maximum pentad precipitation rates associated with cross-equatorial flow reversal

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