What is a monsoon?

Note: This post is written primarily for an Indian audience.

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What leads to a strong or a weak monsoon? What will happen to the monsoon as the world warms? What happens to the monsoon in an El Niño year? -- These are popular and practical questions that the reader has probably heard, read or thought about. This post has no answers for any of these questions, but instead focuses on a more foundational and theoretical question -- What is a monsoon?

Easy! Monsoon means the rainy season, right? This first order approximation is generally accepted as universal truth, since it works for a large part of the country. People equate monsoon with rains, especially summer rains. But what about winter rains? They are usually labelled as "post monsoon", and anything else can be put under "pre-monsoon". If you're a bit more nuanced, you may even label some non-summer rains as "non-monsoon". But the general discussion around rains in India tends to revolve around, and be labelled according to where it falls relative to the central axis of, "monsoon".

The "rain = monsoon" approximation is not entirely wrong, it's just an approximation. Most parts of the country do receive most of their annual rainfall during the summer months. This is but the first and easiest thing to know and observe about the monsoon, so let's try and go a bit deeper and understand a bit better.

First things first, summer is not the only rainy season in India and the Southwest / Summer Monsoon is not the only system that brings rain to India. There are also the Westerlies, but their influence is limited to the northern regions. While they do operate during the summer too, they are stronger during winter. And the Westerlies are not the only source of rain over India during the winter. The southeastern coast of India also receives "monsoon rain" during winter months. This is also referred to as the Northeast / Winter Monsoon. Now we have a preliminary categorization of precipitation systems:

(i) Summer / Southwest Monsoon -- brings rain during summer over most parts of the country
(ii) Winter / Northeast Monsoon -- brings rain during winter over southeast India
(iii) Westerlies -- bring rain during winter over northern India

For now, let's focus on the Monsoon bit. Which brings us back to our original question -- What is a monsoon? A monsoon is a seasonal reversal of surface winds. Surprised? What about rain?? Well, this reversal of surface winds tends to bring rain to the monsoonal regions i.e. the regions experiencing the monsoon. Further, the Indian monsoon is not the only monsoon in the world, though it is the best example of a monsoon. Apart from India, monsoons operate over northern Africa, South America and Australia too. The unifying factor in all the monsoons is that there is a seasonal reversal of surface winds which leads to moisture delivery to the monsoonal regions.

Here, I've plotted surface winds for the months of January and July. This is based on long term means provided in the NCEP-NCAR Reanalysis R1 dataset. A lot of information is lost in averaging things, but it also leads to stable patterns standing out. Here, we can see the reversal of surface winds, which is a stable and prominent climate pattern.

 

Note the July winds over central and peninsular India. These bring moisture from the Arabian Sea during summer as part of the Southwest / Summer Monsoon we discussed above. But there are no such southwesterly winds over the same region in January. Also note the January winds over southeastern coast of India. These bring moisture to those regions from the Bay of Bengal, as part of the Northeast / Winter Monsoon we discussed above. But during July, the wind direction is reversed.

As mentioned earlier, India is not the only monsoonal region in the world. Here is the same plot as above, but this time focused over northern Africa.

 

Do you notice the similar seasonal reversal of winds? That is the West African Monsoon.

Coming back to India: we discussed two different monsoons above - the Southwest / Summer Monsoon and the Northeast / Winter Monsoon. Actually, both of them arise from the same underlying phenomenon and just happen to bring rain to different parts of the country in different seasons. The basic "monsoon system" remains the same.

The monsoon is not a giant land-sea breeze

This is a popular misconception about the monsoon -- that it is similar to a land-sea breeze but simply on a very large scale. Land-sea breezes develop due to land-sea temperature contrasts. And so, if we try to model and predict the monsoon as a giant land-sea breeze, we should see stronger monsoons occuring due to stronger land-sea temperature contrasts. This is not true, neither on an inter-annual time scale (i.e. years with greater land-sea temperature contrasts do not see stronger monsoons), nor on an intra-annual time scale (i.e. within the same year, months with greater land-sea temperature contrasts do not see stronger monsoons). Land-sea temperature contrasts are somewhat important, but certainly not the cause of the monsoons.

So what causes these monsoons? I.e. what causes these seasonal reversals of winds? The same mechanism ties together the whole world's monsoonal regions: the movement of the ITCZ. The ITCZ stands for the Inter Tropical Convergence Zone. Understanding, modeling and predicting the monsoons has much to do with how well we understand, model and predict the movement of the ITCZ. But what is the ITCZ and why does it move? That shall have to be a discussion for another day. Till then, this is a good source to read more about the ITCZ.

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