When is the monsoon the monsoon?

What comes to your mind when you hear the word monsoon?


The BCAS team carrying out the survey during the 2011 monsoon season (BCAS).

If one searches for a definition of the monsoon, you soon realises that there are many different definitions of the monsoon, some relying on, rainfall, outgoing longwave radiation or wind. Despite all the parameters used, we cannot escape the fact that the monsoon in its purest form is the reversal of the atmospheric circulation in accordance with the seasons. Therefore, one might argue that a researcher using a definition based on atmospheric circulation holds a lot of weight in a debate about the appropriateness of different definitions. The first thing that springs to everyone’s mind however, is extensive rainfall. In fact both definitions are useful. It is when we consider what the results are applied to, that one moves from being useful to actually being usable. In fact, it could be argued that an inappropriate definition becomes completely useless and actually potentially dangerous! Of course, the benefits of each well-formulated monsoon definition cannot be understated with regards to our understanding of the general circulation system, climate dynamics and teleconnections. However, we should also take a step back and ask ourselves whether the monsoon research carried out can be applied responsibly to the people who will be most affected by this phenomenon and its anticipated alterations related to climate change.

The inevitable need for an appropriate choice is evident by comparing a wind and rainfall based definition (two of many) of the monsoon over Bangladesh. The figure shows that the climatologically mean onset differs by up to 50 days over some parts of the country. This has far-reaching implications. For example, the timing of transplantation of rice saplings from nurseries to paddies depends on when the monsoon rains start. There is an obvious dilemma: If a famer or decision-maker is told that the monsoon next year or in the future is likely to start in the middle of June, this could imply a one-week difference with regard to the wind-based definition, but a six-week difference with regard to the rain-based definition. The importance of mitigation varies therefore dramatically depending on which definition you relate to at a certain time.

It follows, that in order to draw concrete conclusions about which monsoon definition is most appropriate to use in relation to the way rural Bangladeshis live and plan their lives, we need to go out in the field and ask them directly. Nobody has done this before and one might argue that it may already seem intuitive what the results will show. However, if we are going to refine research in order to better advise decision makers, we are in need of a more fundamental basis.

In the summer of 2011 the field teams at the Bangladesh Center for Advanced Studies carried out a survey in 6 locations spread all over Bangladesh. Over 1200 people from rural communities responded to questions regarding the monsoon definition, its onset and withdrawal. The photographs show the BCAS team members conducting the survey with a farmer in a village in central Bangladesh. The results revealed some compelling findings. Certainly, the most peculiar result was that some would use the song of the frogs as a sign that the monsoon had begun. Besides the frogs, 20% of all the respondents defined the monsoon as starting on the first day of the month of Ashar (roughly 15th June). This is equivalent to the western idea that spring starts on the 21st March for example. By far, the majority of respondents, a total of 65%, defined the monsoon with reference to rainfall. Some said it was the first day of rain, but most said it was the first prolonged period of rainfall. We also asked the farmers whether they consider there to be a difference between pre-monsoon rains and the monsoon itself; 99.6% responded with yes.


Figure showing the average ‘monsoon’ progression across Bangladesh according to a wind deifntion (top), a rainfall defintion (middle) and the community perception (below).

The key result is apparent when we look at the results from the question about when the Bangladeshi farmers perceived the monsoon to start. The average perceptions of the monsoon onset date across the country are shown in the figure. By disregarding the result in the northeastern part of the country, the monsoon seems to progress across the country from the southeast to the northwest, which agrees well with most published research. However, the perception of the famers in the northeast gives an onset date one whole month before the rest of the country. Comparing these results with wind- and rain-based definitions, it becomes obvious which of the two resembles the famer’s perceptions best. There are vast differences between the wind definition and what the farmers in northeast Bangladesh perceive.

It follows, that wind-based definitions of the monsoon over Bangladesh simply do not represent the perceptions of the rural communities. There appears to be a dichotomy between scientists using wind-based monsoon definitions and subsequently applying these definitions to socio-economic studies on the impact of the monsoon, and the rural communities and millions of people affected by the monsoon, who cannot directly relate their way of life to a definition that does not correspond with their perception and needs.

So why should their perceptions guide scientific definitions? There is an obvious and immediately clear reason for this: Vulnerability! The people of Bangladesh don’t care where the wind is coming from, they care whether and when it is going to rain or not. If it does not rain, this can have far-reaching and dramatic consequences.

If scientists want their research to make a responsible impact on the decision-making processes in Bangladesh, then they have to make an effort to see the problem through the eyes of the local communities and the affected farmers. The results from our survey give monsoon science the opportunity to verify their results against the perceptions of the people who are most vulnerable to climate change and the people who are affected most by just small inter-annual variations in the present climate.

[This is the English version of an article, which appeared in the Cicero publication Klima in 2012.  This was written with Thomas Spengler (Univeristy of Bergen) and Md. Abu Syed (Bangladesh Center for Advanced Studies; BCAS).]


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Mathew Stiller-Reeve

I am a postdoc researcher at NORCE Climate and the Bjerknes Centre for Climate Research. I research the monsoon in Bangladesh and I am the founder of ClimateSnack; a community that hopes to give all young and early career climate scientists an opportunity to practice and improve their scientific communication skills.

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