Geodiode

It’s the world’s largest weather system. Affecting almost half of the world’s population, its seasonal changes in winds and rain determine the cycles of nature and humankind, year after year. It is the story of the world’s largest continent, fighting with two oceans, played out in the atmosphere above. From the blistering heat of India, to the frigid north of Siberia, this is the Asian Monsoon.

No discussion of Earth’s climate can go without some mention of the largest single pattern of weather on the globe. Because the Asian Monsoon covers so many individual climate zones, there was no room to give it a good treatment in any of the various chapters that covered Asia in my Secrets of World Climate series. And so it gets an chapter of its own in my Climate Casebook.

Etymology

To begin our understanding of this enormous weather system, we should start at the beginning, and the word itself. Monsoon comes through Portuguese monção from the original Arabic word mawsim, which means “season / change in wind direction”. You see, the Asian Monsoon even affects Arabia, even though that change in wind direction results in nothing yielding rain. A monsoon is, in its simplest form, always a change in wind direction, but in the case of the Asian Monsoon, this change in wind direction occurs around the whole of one side of the world’s largest continent, producing a wet summer and dry winter across a huge area of Earth.

Geographic Extent

Now those of you familiar with tropical climates will know something of wet and dry seasons, and how the trade winds of the tropics determine these. I covered this in my chapter on Tropical Wet and Dry Climates in my other series. They always occur within the tropics of Cancer and Capricorn, from 23°N to 23°C south. So wet and dry seasons related to changing wind direction are not unique to Asia. What makes the Asian Monsoon stand out are two things – the great intensity of the wet season in the Indian Subcontinent and the extension of the wet and dry season pattern in Eastern Asia way beyond the tropics into the temperate, continental and subarctic latitudes.

So the Asian Monsoon has these two notable aspects, and each defines the two broad halves of the monsoon.

The intense monsoon that affects India and Bangladesh is the most well known, and it determines the lives of the one and a half billion people that live there, more heavily than any other weather system in the world. The economic fortunes, even the survival itself, of these populations depends upon the timing and severity of the wet season that washes over these lands between June and September.

Less well known is the East Asian Monsoon. Though less intense, its range is much more extensive, and as many people experience its annual rhythm, with Eastern Russia, Mongolia, Korea, Japan, Taiwan and most of China affected.

Dynamics – The Annual Movement of the ITCZ

Before we look at these areas in more detail, we should try to understand what dynamics are special to the Asian Monsoon. This weather system and its causes are complex and even to this day are not fully understood. However it is generally accepted that the two strongest influences are the seasonal march of the doldrums above and below the equator that affects all tropical wet and dry climates, magnified by the seasonal heating and cooling of the giant Asian landmass.

The doldrums, also called the Intertropical Convergence Zone, or ITCZ, is the band of low pressure around the tropics where heating from the sun causes upward convection of hot air leading to a drawing in of air from the surrounding land or sea in the form of trade winds. As the global seasons alternate summer between the northern and southern hemispheres, the ITCZ follows, with it tracking north in the northern summer, and then south in the southern summer. This band is almost always accompanied by heavy thunderstorms and consequential rainfall as the hot and moist air is unstably thrust into the cold upper atmosphere.

Summer Monsoon

In the northern summer the ITCZ moves up into India and China, bringing with it southerly trade winds which blow onto the continent from the surrounding seas. This is what brings the rain with the monsoon, as it does with any tropical wet season. But this effect is strengthened further by the presence of the large Tibetan plateau, the heating of which leads to a deepening of the low pressure, strengthening the monsoon over India far beyond a standard tropical wet season.

Additional summer heating of the central Asian land mass north of Tibet, far from the moderating ocean, is also strong, and extends as far as Siberia, leading to winds blowing in from the Pacific across all of Eastern Asia from Hong Kong to the Russian Pacific coast. This is the only part of the world where summer peaks in rainfall occur in the mid to high latitudes and special climate zones, the Continental and Subarctic Monsoon types, were designated by Vladimir Koppen to account for this extension into the far north of what is essentially a tropical weather system.

Koppen Climate Zones, Winter Monsoon, Effect of Australia and Ocean Pressure Zones

While we’re on the subject of Koppen, we shouldn’t confuse the Koppen Climate type Am – the Tropical Monsoon, with the Asian Monsoon, as this zone can occur all over the tropics, from Miami and Rio in the Americas, to Central Africa, Indonesia and the Philippines. The Asian Monsoon encompasses a multitude of Koppen Climate zones, all of which have peak rainfall during the summer. These are Am – Tropical Monsoon, Aw – Tropical Savannah, Cwa – Subtropical Monsoon, Cwb – Subtropical Highland, Dwa/Dwb – Continental Monsoon, and Dwc/Dwd – Subarctic Monsoon.

So that is summer. But what about winter? Well, the opposite occurs. The ITCZ retreats back to below the equator while temperatures over Central Asia plummet below zero, leading to sinking, dense air and high pressure. This high pressure is the strongest on Earth and the continental winds that blow out from it to the oceans lead to dry winters across most of Asia. Only a few places, where such seasonal monsoon winds blow over water in both directions, receive rain or snow year round.

So this, in essence, is the main mechanism driving the Asian Monsoon. There is more complexity to it of course. Australia is believed to have a strong influence on Asia, even though it is much smaller, because it acts as a pressure “counter-pole” – with Australian high pressure in the northern summer against the Asian low, and vice versa six months later. The Indian and Western Pacific oceans are the other “counter-poles” that either provide moist winds in the form of high pressure, or sink dry continental winds in the form of low pressure.

The monsoon’s growth in summer and retreat in winter can be seen in this composite of monthly average rainfall across the continent. It’s quite a mesmerizing animation, isn’t it? I could watch it for hours… Ok, moving on. Let’s now journey through the lands affected by the monsoon and examine how local topography and other factors leads to particular effects from one region to another.

Arabia

The journey starts in the Arabian peninsula, with the origin of the word itself, but it’s also the westernmost point under the direct monsoon influence. In summer, winds blow towards the intense low pressure over Central Asia, but because these winds have travelled over the continent of Africa, they bring little to no rain. In winter, the winds switch direction, blowing out from the central Asian high, and once again are dry, continental winds. This is one reason why the Arabian peninsula is a desert.

Indian Subcontinent

Moving east into India, and the same wind directions, SW in summer, NE in winter, produces a dramatically different result. Now the SW summer winds blow over the warm Arabian Sea throughout the summer months, bringing in storm after storm that lashes the western coast of India from Kerala in the south to Gujarat in the north. All along this coast sits one edge of the Deccan plateau, and these moist winds are pushed upward and cooled, releasing their moisture in spectacular fashion as this graph of Mumbai shows.

The monsoon progresses across the Indian Subcontinent in stages, with the south receiving rains as early as late May, and the north as late as mid-July. Protected somewhat by the Deccan, much of the country experiences less severe monsoon rain. But where these strong, moisture-laden winds hit the Himalayas, they are thrust upward in a similar fashion to the west coast and dump almost all their moisture on the windward slopes. And with this mechanism, known as orographic lift, we come to a global superlative. The Indian Monsoon produces the wettest places in the world.

Mawsynram and Cherrapunji, both in the Indian state of Meghalaya lie in the foothills of the Eastern Himalayas, and the world’s wettest title alternates from year to year between these two towns. Here’s the graph of Cherrapunji, with the scale kept at the standard used for all my charts, just so you can get your head around how much rain falls here. In one month alone at the peak of the summer monsoon, more rain falls here than in an entire year in Singapore, in the heart of the wet Tropical Rainforest, three times more rain than the annual total in rainy Vancouver, and six times more than the annual amount in supposedly rainy London. Such is the power of the monsoon.

This graph of Lhasa in Tibet shows the effect of the Himalayas. Being sheltered from the full brunt of the monsoon behind the highest peaks of those mountains, including Everest, comparatively little rain gets through, although the mark of the monsoon is still clearly visible in the difference between winter and summer.

In the Indian winter, the dry NE wind blows from Central Asia out into the Indian Ocean, leading to completely dry conditions for months at a time, first with cool temperatures, and then very hot temperatures, which leads to the unique three seasons pattern of much of that country. A hot and wet rainy season, warm and dry “winter”, then very hot and dry “summer”.

Such is the strength and consistency of the seasonal monsoon winds, that they are even able to change the direction of an ocean current. A current’s direction is primarily determined by the winds blowing over it, so in summer, the around India flows east from Arabia to Myanmar, and in winter it flows west. So the name of this current alternates between the SW Monsoon Drift and the NE Monsoon Drift. This is the only major ocean current where such a reversal occurs every year. Such is the power of the monsoon.

China & Korea

Continuing further east, and we pass over South-East Asia. This area does feel the influence of the monsoon, but is within the normal latitudes where tropical wet and dry seasons occur, with Koppen types Am and Aw predominating, so does not require special mention in this article.

As we continue north and east, however, we enter Southern China, and the beginning of the Eastern Asian Monsoon. The interplay between continent and ocean now switches to the Pacific, and so we get a different character to that of the Indian – less intense, but spread out over a much greater area. The monsoon pushes inland hundreds of miles into the heart of China, subtropical in the south, continental in the north, and as one travels further north, the amount of rain, and the number of months in which it falls reduces, as the Pacific moisture is lost over the land carried by southerly winds.

Korea, like Northern China experiences the monsoon as a continental climate, with dry and cold winters blowing in from the Siberian High. But, being on the coast, the monsoon brings very wet summers, as this graph of Seoul shows.

Japan

Because Japan is an island archipelago, the change in wind direction produces rain or snow across all seasons, blowing in from the Pacific in summer and in winter, across the Sea of Japan that lies between this country and Russia. So one gets a more even distribution of precipitation across the year throughout the islands, although along this mountainous set of islands, the main population centres along the east and southern coasts experience summer rainfall peaks, while the north and west facing coasts get noticeable winter peaks, usually in the form of snow. In fact Sapporo, the largest city on the northern island of Hokkaido, is the snowiest city in the world, thanks to the winter monsoon winds.

Typhoons in East Asia are another distinction between this part of the monsoon and that of the Indian part. These often ferocious storms account for a considerable proportion of total rainfall in the season as they lash cities from Taiwan and Hong Kong in the South to Japan in the north.

Siberia & Conclusion

The East Asian Monsoon’s influence reaches its end in the far north of Eastern Siberia, where the influence of the winter Siberian High leads to very dry winters. In summer, southerly monsoon winds still penetrate this vast area, bringing rain, but by this stage, are weakened considerably and the amounts are little compared to those further south. Still it is a remarkable fact that a singly connected weather system can affect such a large area, across so many lines of latitude, from 20°N in India, through to 70°N in Siberia. Such is the power of the monsoon.

The Asian Monsoon varies in intensity from one year to another, affecting both India and China, so these average rainfall graphs, as always, can be deceptive in hiding the true pattern of annual variability. Too little rain, and effective drought can occur, since the rest of the year in many places is very dry already. Too much rain, and flooding occurs across large areas, in addition to landslides along the southern and eastern edges of the Himalayas. Too little and too much rain can lead to many thousands of deaths in this way. The monsoon is a taker as well as a bringer of life.