It is the month of September and the Indian monsoon has been active across nearly the whole country over the last few months. The monsoon rain brings with it new hope to a parched land, agriculture activities are renewed and the entire country has the appearance of being washed over removing all traces of dust and dirt accumulated over the blistering summer. This most defining of Indian seasons directly determines the quality of life of more than a billion people, but its origin and inner workings are shrouded in mystery. How is the monsoon formed? How do scientists predict arrival of the monsoon? Why do some years have less rain while others have more? In this post we will be chasing the monsoon winds from the southern hemisphere right up to the Himalayas, and see how cold winds from the North Pole bring rain in Tamil Nadu.
Climatology Basics
As we embark on our journey as cloud chasers, let us start by understanding some of the key factors that influence climate, and in particular the monsoons themselves.
Master of the Game
Climate and weather on Earth is influenced directly by the amount of sunlight that a region receives. Earth, as we know, is spherical in shape, with the sphere bulging around the equator. In addition to revolving around the Sun, the Earth also rotates on its axes with one full rotation taking 24 hrs. The axis is in itself tilted about 23.5 degrees, so different places on the Earth receive sunlight at different angles. In general, regions around the equator receive the maximum sunlight – and nearly vertically – while regions towards the poles receive lesser sunlight and at an angle. This causes the equatorial regions to be hotter than the polar regions all throughout the year.
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| The angle at which the sun’s rays reach a point on Earth influences the temperature and climate of that region. As you move North/South from the equator, the Sun’s rays fall slanted. A slanted ray has to travel more through space before hitting the Earth and ends up losing more energy than a vertical ray that hits the equator. The Sun is the master of the game as far as Earth’s climate goes. |
Temperature, Pressure and Density
When air gets hot, it tends to rise. This is because its density and pressure drops. A low pressure zone is developed where the air is hot and is rising up. Conversely, when air gets cold, it gets denser and settles down. A high pressure zone is developed where the air is cold and settling down. This basic phenomenon drives all the wind flows around the Earth.
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| The terms ‘Air’ and ‘Wind’ are often interchangeably used, but they actually mean different things. Air is what we are enveloped in. It is a mixture of Nitrogen, Oxygen, Carbon-di-oxide and many other such gases. Wind, on the other hand, is the movement of air from a place of high pressure to a place of low pressure. |
Trade Winds
As the air around the equator heats up, it rises creating a low pressure zone. This hot air moves towards the poles, cools down and sinks around 30o north and south latitudes. As it sinks it gets drawn towards the low pressure zone around the equator. When the air travels towards the equator, it gradually gets heated up and once heated up rises creating a low pressure zone and the entire cycle repeats.
These wind flows are highly predictable and have been known to sailors from time immemorial. Trading ships of the past would rely on these winds to power their sail boats and cross vast ocean stretches. Trade and commerce in Europe, in particular, benefited from the mastery of these winds and over time they came to be called as ‘Trade Winds’ – the winds that aid trade.
Coriolis Effect
The Earth takes about 24 hours to complete one rotation on its axis. Having said that, not all points on Earth rotate at the same time, in fact there are two points on the earth that rotate at zero speed. Surprised? The mathematics behind it is actually pretty straight forward:
- The circumference of the Earth at the equator is about 40,000 kilometers. As it takes 24 hours to complete one rotation, the speed at the equator is nearly (40,000 km / 24 hours) = 1,667 km/hr
- As you go further away from the equator, the circumference reduces and with it the speed of rotation. So if we take the city of Bhopal which lies on the Tropic of Cancer, the circumference is around 36,768 kilometers and the speed of rotation drops to (36,768 km / 24 hours) = 1,532 km/hr
- And right at the poles (north or south), the circumference is zero. So the speed of rotation is (0 km / 24 hours) = 0 km/hr
Wind deflection
Impressive as this may sounds, how does this impact wind flow and climate? Imagine you have an object, say a ball, that you are throwing from Kanyakumari towards your friend standing at Bhopal (Kanyakumari is closer to equator than Bhopal). Due to the different speeds of rotation between the two places, the ball will land away from your friend and towards the right. This apparent deflection of the ball is what is called the Coriolis effect. And what happens to our imaginary ball happens to wind flows as well.
Trade wind direction in Northern & Southern Hemisphere. Click on image to see enlarged version. Copyright LetUsDiscoverIndia.com
In the Northern Hemisphere, Coriolis effect deflects wind flows towards the right. In the southern hemisphere, Coriolis effect deflects wind flows towards the left. So in the Northern Hemisphere, cold air settling in down from 30o north latitude is deflected towards the West, and this forms the Northeasterly Trade winds. For the southern hemisphere, the same effect creates the Southeasterly Trade winds.
National Geographic has an interesting video describing the Coriolis effect, highly recommend watching it.
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| You can identify the direction of a wind from its name. So if a wind is termed an Easterly, it implies it is flowing from East to West. Likewise a Westerly flows from West to East. A Southeasterly trade wind is flowing in from South-East. |
Convergence Zone
The Northeasterly and Southeasterly Trade winds blow from east to west and converge at the equator. This convergence zone is called the Inter Tropical Convergence Zone or the ITCZ. While the ITCZ is generally at or around the equator, it does not have a fixed position. It could move further north or south depending on the prevailing temperature and weather conditions.
Inter Tropical Convergence Zone along the equator. Click on image to see enlarged version. Copyright LetUsDiscoverIndia.com
Chasing the monsoon winds
It’s now time to put together all these climatology basics and trace the origins of the monsoon.
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| The Indian Monsoon season has two distinct periods: summer monsoon (or South West monsoon) and winter monsoon (or North East monsoon). Summer monsoon is experienced between June-September and winter monsoon is usually between October-January. |
Spring Equinox
After the spring equinox on March 21, the sun begins its northward journey from the equator. The landmass north of the equator starts receiving more sunlight and starts getting heated up. Consequently air temperature over the landmass increases, and as the hot air rises a low pressure zone is created over north and north-western India. This low pressure zone is the central engine that drives the Indian monsoon and is also called the monsoon trough. The ITCZ follows the sun and pulls north away from the equator. By June-July, the ITCZ would have shifted up to 25o – 30o North.
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| A trough is a low pressure zone that extends over a very large area. Since the low pressure zone over India is associated with the monsoon, it is called a monsoon trough. |
Basic laws of physics now dictates that cooler high pressure air needs to rush in and fill in the void left by the rising hot air. The most obvious source of high pressure air would be cold winds flowing down from the polar region. This is where the Himalayas act as the great barrier and protect India from turning into a dry wasteland. With its towering height, the Himalayas prevent any cold polar/sub-polar winds crossing over to peninsular India. We need to look towards an entirely different high pressure wind source to plug-in the low pressure zone over north/north-west India.
Mauritius Connection
The Mascarene Islands, part of the Republic of Mauritius, is located in the southern Indian ocean close to the island of Madagascar. While the islands of Mauritius are world famous as a pristine tourist destination, the climate around these islands are extremely significant for the Indian monsoon system. Mascarene island region is the location of a near permanent sub tropical high pressure zone. Nearly all throughout the year, there are high pressure winds that blow out from around the latitudes of these islands. This high pressure zone is called the Mascarene High (MH).
We have seen how Coriolis effect in the southern hemisphere bends winds towards the left – so the winds blowing out from MH bend westwards and flow towards the African mainland. As the wind blows over the coast of Kenya and Somalia, it gets drawn in by the monsoon trough over north western India. Wind that was flowing west up to that point, turn right and start flowing in a north easterly direction. As the wind blows over the open Indian Ocean, it picks up moisture and reaches the western shores of India by the end of May. One of nature’s greatest spectacle, the great Indian monsoon, is imminent now.
South West Monsoon
As the moisture laden winds blow in from the south-western Indian ocean, they split into two branches. One keeps to the west of the Indian subcontinent and lashes into the Malabar coast, while the other twists around the southern coast and enters the Bay of Bengal.
From coast to coast
Wind gusts that reach the Malabar coast immediately run into a big obstacle – the Western Ghats. This formidable barrier forces the clouds to rise, causing them to shed moisture and leads to torrential rainfall on the west coast of the country. The date, typically, is Jun 1st and the South West monsoon has formally arrived in India. The ‘Arabian Sea branch’ of the monsoon keeps pushing northwards and brings in rain all along the coast and eventually escapes around the Ghats and moves towards central and north west India.
One of nature’s greatest spectacle, the great Indian monsoon, is imminent now.
In the meanwhile, the ‘Bay of Bengal branch’ moves rapidly northwards bringing the rainy season to West Bengal and the north eastern states. These states usually receive rain by June 5th. Deccan and central India starts receiving rains by mid-June and Delhi finally gets relief from the scorching summer heat by the last week of June.
Himalayan blockade
Both branches of the South West monsoon now race North and slam into the tallest obstacle on the planet – the mighty Himalayas. The Himalayan foothills gets drenched in rains. Since the rain bearing clouds cannot rise over and above the Himalayas (and escape into China), they get deflected and move towards the north west bringing rain finally – by early July – to the states of Punjab and Jammu & Kashmir. The whole of the country is now covered by the monsoon, and the rainy season is well and truly upon us.
Shifting trough
The monsoon trough – the extreme low pressure zone that is at the heart of the monsoon phenomenon – does not always lie at one position. Normally, the monsoon trough passes – in a nearly straight line – from Sri Ganganagar in Rajasthan via Delhi, Allahabad and Kolkata into the Bay of Bengal. This trough could move further north or south depending on other events outside of the weather system that drives the monsoon winds. Weather disturbances, for example, further west of India could pull the trough up and towards the Himalayas.
In general, the area to the south of the monsoon trough experiences heavy rains
If the monsoon trough moves further northwards, it will lie along the Himalayan foothills. Intense rainfall will be experienced along the lower slopes of the Himalayas leading to floods and landslides. As all rainfall is now concentrated in that area, rest of peninsular India and most of the northern plains will see a break in the rains. This is termed as ‘break-monsoon‘: literally, the monsoon has taken a break across the country! You can easily see this occurring during each monsoon season – just when you thought that the rains are never going to end, there is a sudden break for a few weeks during which time you hear about landslides and flooding in the Himalayan region, but the weather in peninsular and southern India is clear and calm.
The trough could move down south too, in which case peninsular India receives very heavy rain. In general, the area to the south of the monsoon trough experiences heavy rains.
Retreating Monsoon
While we are enjoying the rains, the Earth’s elliptic orbit around the Sun has completed another milestone. On June 21, the Summer Solistice, the Sun stops its northward motion and starts moving south. It needs to be noted that in reality the Sun does not move north or south relative to the Earth, it is the Earth’s elliptic orbit and the tilt of its axis that creates the apparent north-south motion of the sun.
High pressure winds over the southern Indian ocean have no incentive to be pulled over India and the monsoon system weakens
With the sun moving south away from the Tropic of Cancer, lesser heat is received by the Indian landmass. By the Autumn equinox on September 22, this becomes even more pronounced. Additionally, the monsoon rain cools off the land rapidly. These effects, combined, result in the low pressure area over north western India reducing in intensity (there is no more hot air to rise to create a low pressure area). High pressure winds over the southern Indian ocean have no incentive to be pulled over India and the monsoon system weakens.
Starting mid-September, the monsoon withdraws from India. This withdrawal starts from the north west and by mid October the South West monsoon fully pulls out of peninsular India. But there’s more drama to follow, cyclone season is just about to start on the Coromandel coast.
North East Monsoon
It is mid-October and the low pressure zone over north India has more or less fully disappeared. Barring some scattered rainfall the weather is generally pleasant with clear skies. The west coast gets the occasional rain due to any low pressure zone along the coast line. Weather disturbances over the polar region near Siberia and over the Mediterranean sea are about to change this status quo.
Once again, the Himalayas comes in the way
Western Disturbances
By early October, a high pressure region develops over Siberia and this pushes cold dry air from the polar region towards the warmer Indian latitudes. Joining hands with this mass of cold air is high pressure winds from the Mediterranean and Black Sea. Unlike the polar winds, the winds from the Mediterranean pick up moisture as they flow over water and the combined wind stream moves eastwards towards India. In meteorological terms, this is called ‘Western Disturbances‘.
Once again, the Himalayas comes in the way. The clouds drop their moisture causing rains in the lower Himalayas and snow in the upper regions. India’s rabi season, where wheat is grown, is entirely dependent on this rainfall. As the cold clouds move over the northern plains, fog is formed. The western disturbances are behind sudden heavy fog in a wide region stretching from Amritsar to Delhi to Lucknow and along the Gangetic plains during months of October-November.
Winter Monsoon
While some of the cold wind from the polar region enter through north west India, the rest make its way through the north east. The Bay of Bengal is relatively at a lower pressure, and the adjoining regions of the Indian subcontinent is at a higher pressure. Wind is drawn from all around towards the Bay of Bengal where it draws up moisture from the water.
As more wind rushes out towards the Bay, Coriolis effect takes over. Remember that due to Coriolis effect, wind flows towards the right in the Northern Hemisphere. High intensity, low pressure, rain bearing wind forms all over the Bay of Bengal and cyclone storms originate. These storms slam into the Coromandel coast all along from southern West Bengal through Orissa, Andhra Pradesh and Tamil Nadu. In fact Andhra Pradesh and Tamil Nadu gets most of its rains through this cyclonic system. And since the winds are from the north east, this monsoon system is called the North East Monsoon. It is also called the Winter monsoon as the rains are all through the winter season in the northern hemisphere (November – January).
By early-mid January, the rain clouds have completely moved away from the Indian subcontinent. The skies are clear and the air is crisp. The cycle though never ends – in a few months the spring equinox will mark the northward movement of the sun and the entire process restarts.
Mastering the Winds
The Trade winds are generally easterly winds, i.e. they flow from east to west. At the onset of the monsoon though, the wind direction changes and they start blowing from west to east. This was known to traders in the past and the Arabs kept this information secretive from the Europeans and monopolized spice trade with the Malabar coast. Without a knowledge of the monsoon wind pattern, the European explorers were effectively barred from trading with India via a sea route.
The story goes that Vasco da Gama found a guide whom be bribed to shared this secret and riding on the western winds sailed from Kenya to Calicut in 20 odd days. The rest, as they say, is history – not only was a sea trading route to India opened but trade swiftly moved to colonialization and oppression. India’s wealth funded the Industrial Revolution in Europe and in many ways the knowledge of the monsoon winds helped build Europe as we know it today.
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| The story also goes that when Vasco da Gama returned back from Calicut, he did not know about the Winter monsoon (and their easterly winds) and started in early October. Travelling against a westerly wind, it took him more than a 100 days to cross the ocean and reach the African coast, with more than half of his crew dead. Had he waited for the Winter monsoon he would have benefited by the easterly wind and reached earlier. |
Conclusion
The monsoon wind system is one of the most complex weather patterns on the planet. As we have seen many factors have to line up properly for a good rainy season. It starts with the high pressure zone off the Mascarene islands, is driven by the intensity of the monsoon trough over north/north-west India and is maintained at its tail end by wind systems starting over the poles and the Mediterranean Sea. Recently other variables such as the Indian Ocean Dipole (IOD) and the El-Nino effect have been identified as impacting the monsoon. All of this makes monsoon prediction extremely challenging. The Indian Meteorological Department (IMD) is constantly upgrading its models and techniques to help the farmers, planners, industrialists and economists better plan for the monsoon – nature’s most bountiful gift to India.
Further Reading
A short but interesting read about the physics behind the Coriolis effect can be found here. I especially liked the comparison with moving trains. The website is hosted by the US National Oceanic and Atmospheric Administration (NOAA).
NOAA has another interesting article on ITCZ and how the ITCZ is related to rainy and dry seasons. Read this in context of the ITCZ moving North post the Spring Equinox causing the monsoon trough to form over India.
University of Wisconsin-Madison releases a nice image, updated daily, showing the low level cloud drift over southern Indian ocean and India.
If graphs and charts are your thing, then the air pressure chart published by the IMD is very insightful. This is published daily and shows a forecast for 10 days. These charts are especially useful for understanding the high pressure/low pressure systems over the country and the surrounding ocean/sea and gives a clear indication of what to anticipate over the next week.
What path does the returning wind take from Kashmir and Pakistan south to Mauritius, some 50 Degrees of Latitude, over 5,000 kilometers? Does that huge air mass stay in the Troposphere?
Very well explained 👍