(4) Climate System

What is a Monsoon?

Monsoon

What is a monsoon?

CS4

Monsoons

The other important area for massive rainstorms is the monsoon belt. The name monsoon comes from the Arabic word ‘mausim’ which means ‘season’, as most of the rains that fall in Southeast Asia occur during the summer. In the tropics the sun’s energy is most intense as the sun is directly overhead. This heats up the land and sea and thus warms the air above. This warm, moist air rises, leaving an area of low pressure beneath it, which helps to suck in air from the surrounding area. This suction results in the Trade Winds, which can travel from much higher latitudes to this area of rising air. As the winds come from both the Northern and Southern Hemispheres this area is known as the ITCZ. As the air at the ITCZ rises, it forms huge towering clouds and produces large amounts of rain. The ITCZ moves north and south with the seasons as the position of the most intense sunlight shifts up and down across the Equator. It is also influenced strongly by the position of the continents. This is because the land heats up faster and to a greater extent than the ocean and thus it can pull the ITCZ even further north or south during that season. An example of this is the Asian summer monsoon, during the summer near the Himalayan Mountains and the low lands of India heat up.

This pulls the ITCZ across the Equator on to Asia. Because the Southern Hemisphere winds have been pulled across the warm Indian Ocean they are warm and full of moisture; when they are forced to rise and cool down over India they produce very heavy rainfall throughout Southeast Asia and as far north as Japan. During Northern Hemisphere winter the ITCZ moves south of the Equator, but in Southeast Asia it means warm, moist winds from the North Pacific are dragged southward across the continent into the Southern Hemisphere. This means that some areas such as Indonesia and Southern China get two monsoonal rainy seasons a year: one from the north and one from the south. No wonder this is the most fertile place on Earth, supporting over two-fifths of the world’s population. Despite being the bringers of life, the rains can cause catastrophic hazards, especially in the form of flooding. Examples of this are the terrible floods in 1998 in Bangladesh and China which caused over $30 billion of damage and thousands of deaths.

Amazon monsoon

During the Southern Hemisphere summers the continent of South America heats up. This rising air leaves an area of low pressure at ground level, which is filled by sucking in the surrounding air. This pulls the convergence zone between the North and Southern tropical air southward over Brazil. The southward shift of the ITCZ brings with it lots of rain as the air being pulled across the Equator from the north originates over the warm, tropical Atlantic Ocean. This produces the Amazon monsoon and results in the mightiest river in the world and the greatest extent of rainforest on the planet. The Amazon Basin covers an amazing 2.7 million miles2 much of which is covered with rainforest. The Amazon River delivers 20 per cent of all the freshwater that enters the world’s oceans. Without the monsoon rains the most diverse habitat in the world would not exist.

Living under the Asian monsoons

Bangladesh is a country literally built by the monsoons as over three-quarters of the country is a deltaic region formed by the sediments brought in by the Ganges, Brahmaputra, and Meghna rivers-all fed by the summer monsoons. Over half the country lies less than 5 metres above sea level, thus flooding is a common occurrence. During a normal summer monsoon a quarter of the country is flooded. Yet these floods, like those of the Nile, bring life with them as well as destruction. The water irrigates and the silt fertilizes the land.

The fertile Bengal Delta supports one of the world’s most dense populations, over 110 million people in 140,000 km2. But every so often the monsoon floods exceed what even Bangladesh can cope with. In 1998 three-quarters of the country was flooded for 2 months, causing billions of pounds worth of damage and thousands of deaths. Bangladesh also has to cope with tropical cyclones. If we take 3 of the worst years for tropical cyclones we can see the loss of life has dramatically dropped. In 1970 there were over 300,000 cyclone-related deaths, in 1991 there were 138,000, while in 2007 there were just 3,500 deaths.

This is not because the tropical cyclones have grown gentler, far from it. It is because of good governance. The Bangladesh government has, first, invested in excellent meteorological facilities to make as accurate a prediction of when and where the cyclones will make land fall; second, they have set up a communication network using cyclists, so that once a cyclone warning is given, the message is carried to all the towns and villages that will be affected. They have also built cyclone shelters, protected water and sanitation facilities, and encouraged floating agriculture, which can withstand the storms. These relatively simple changes have resulted in the saving of hundreds of thousands of lives.

Tectonics and climate

We saw before how climate is a function of how the sun’s energy falls on the Earth and is then redistributed around the globe. Both of these aspects are strongly influenced by plate tectonics. This is why 100 million years ago the Earth was much warmer and humid, and dinosaurs were happily living on Antarctica. Our modern climate system is a product of millions of years of plate tectonics, which have produced unique occurrences such as significant amounts of ice at both Poles. This produces a very strong Equator-pole temperature gradient and thus an extremely dynamic and energetic climate system.

Tectonics has two main effects on climate. First, there are direct effects, which include mountain and plateau uplift which changes atmospheric circulation and the hydrological cycle or ocean gateways, which change the way the oceans circulate. Second, there are indirect effects that affect the content of the atmosphere through subduction, volcanism, and consumption of gases by chemical weathering. One of the themes running through this book is the idea that nothing in climatology is complex. This is also true of the effects of tectonics on climate. In this text the influences are broken down into horizontal tectonics, which examines what happens if you simply move the continental plates around the globe. Next is vertical tectonics, which examines what happens if you create a mountain or a plateau. Last, we will look at the effects of volcanoes and super volcanoes on climate.

Horizontal tectonics

Latitudinal continents

The north–south position of the continents has a huge effect on the thermal gradient between the poles and the Equator. Geologists have run simple climate models to look at this effect. If you put all the continents around the Equator, the so-called ‘tropical ring world’, the temperature gradient between the poles and the Equator is about 30°C. This is because when the poles are covered with oceans they cannot go below freezing. This is due to a trick of both the atmosphere and the oceans. A fundamental rule of climate is that hot air rises and cold air drops. At the poles it is cold so the air falls and as it hits the ground it pushes outwards away from the pole. When sea water at the pole freezes it forms sea ice, and this ice is then blown away from the pole towards warmer water where it melts. This maintains the balance and prevents the temperature of the poles going below zero.

However, as soon as you introduce land onto the pole or even around the pole, ice can form permanently. If you do have a landmass like Antarctica over a pole with ice on it the Equator–pole temperature gradient is over 65°C; which is exactly what we have today. In contrast if you consider the Northern Hemisphere, the continents are not on the pole but surround it. So instead of one huge ice sheet, as we have in Antarctica, there is one smaller one on Greenland, and the continents act like a fence, keeping all the sea ice in the Arctic Ocean. So the Equator–pole temperature gradient of the Northern Hemisphere is somewhere between the extremes of the Antarctic and an ice-free continent, about 50°C.

The size of the Equator–pole temperature gradient is a fundamental driver of our climate. Because the main driver of ocean and atmospheric circulation is moving heat from the Equator to the poles. So this temperature gradient defines what sort of climate the world will have. A cold Earth has an extreme Equator–pole temperature gradient and thus a very dynamic climate. This is why we have strong hurricanes and winter storms: the climate system is trying to pump heat away from the hot tropics towards the cold poles.

Longitude continents

One of the key aspects of ocean circulation is how the oceans are contained. If there are no continents in the way then oceans will just continue to circulate around and around the globe. However, when an ocean current encounters a continent it is deflected both north and south. If we look at the modern configuration of the continents then there are three main longitudinal continents: (1) the Americas, (2) Europe down to southern Africa, and (3) Northeast Asia down to Australasia. A hundred million years ago the continents are still recognizable but they are in slightly different positions. The two striking features are, first, there was an ocean across the whole of the tropics through the Tethyan Sea and the Deep Central American passage. Second, there is no ocean circulating around Antarctica. These changes have huge effects on the circulation of the surface ocean and thus deep-ocean circulation and global climate. There are three main conceptual ways of understanding the effects of ocean gateways on ocean circulation. The first is a simple double-slice world with longitudinal continents on either side. Because ocean currents are driven by the surface winds in the tropics and poles the ocean currents are pushed to the west, while in the mid-latitudes they are pushed to the east. This produces the classic two-gyre solution in both hemispheres.

Today both the North Pacific Ocean and the North Atlantic Ocean have this type of circulation. The second scenario is a double-sliced world with a low latitude seaway. This produces a large tropical ocean circulating continually westward around the world. There are then two smaller gyres in each hemisphere. This is the circulation seen during the Cretaceous period, with the two gyres in each hemisphere occurring in the Pacific Ocean. The third scenario is a double sliced world with high latitude seaways. This produces strong circumpolar ocean currents in each hemisphere and a single tropical gyre in each hemisphere. Today the Southern Hemisphere resembles this scenario with a circumpolar current around Antarctica. The Southern Ocean thus acts like a giant ocean heat extractor and was instrumental in the huge build up of ice on Antarctica.

Deep-ocean circulation

Deep-ocean circulation is also an important consideration as it influences the circulation of the surface ocean and the distribution between the hemispheres. The presence or absence of ocean gateways has a profound effect on the deep-ocean circulation. For example, our modern day North Atlantic Deep Water (NADW), which helps to pull the Gulf Stream northwards maintaining the mild European climate may be only 4 million years old. If we run computer simulations of ocean circulation with and without the Drake Passage and the Panama Gateway, only the modern day combination produces significant NADW. Hence our modern day deep-ocean circulation is due to an open Drake Passage from about 25 million years ago and the closure of the Panama Gateway from about 4 million years later. It is all due to salt. Because of the greater effect of evaporation in the North Atlantic region, the North Atlantic Ocean is saltier than the Pacific Ocean.

NADW forms today when the warm, salty water from the Caribbean travels across the Atlantic Ocean and cools down. The high salt load and colder temperature act together to increase the density of the water so it is able to sink north of Iceland. So when the Panama passage way is open then fresher Pacific Ocean water leaks in and reduces the overall salt content of the North Atlantic Ocean. The surface water even when it is cooled is thus not dense enough to sink and so not as much NADW can be formed compared to today. So, one of the fundamental elements of our modern climate system, the competition between the Antarctic Bottom Water and the North Atlantic Deep Water, turns out to be a very young feature.

Vertical tectonics

As the tectonic plates move around the surface of the Earth they frequently clash together, when this happens land is pushed upwards. In some cases chains of mountains are formed or when whole regions are uplifted plateaus are formed. These have a profound effect on the climate system. One of these effects is a rain shadow, which is a dry area on the leeward side of a mountain system. There is usually a corresponding area of increased precipitation on the forward side. As a weather system at ground level moves towards a mountain or plateau it is usually relatively warm and moist.

As the air encounters the mountain it is forced to move up and over it. Because of decreasing atmospheric pressure with increasing altitude, the air has to expand and as it does it cools down. Cool air can hold less moisture than warm air so the relative humidity rapidly rises until it hits 100 per cent and strong rainfall occurs. As the air descends on the other side of the mountain atmospheric pressure increases and the air temperature rises and the relative humidity drops very low as little or no moisture is left in the air. Hence on the descending side there is a rain shadow as there is no moisture left with which to form rain and this can lead to the creation of a desert. This simple process can control the wetness or dryness of whole continents. When huge mountains or plateaus are thrust high up in the sky they interfere with the circulation of the atmosphere. Not only do they force air up and over them but in many cases they deflect the weather system around them.

This effect is compounded as uplift areas also warm up in summer and cool down in winter more than the surrounding lowlands. However, if you put the two modern plateaus in place, in other words, the uplifted regions of the Tibetan-Himalayan and Sierran-Coloradan plateaus then there are huge changes in circulation. Both these plateaus are massive. The Tibetan plateau is the world’s highest and largest with an area of 2.5 million km2, which is about four times the size of France. While the Colorado Plateau covers an area of 337,000 km2 and is joined to numerous other plateaus which make up the Sierran-Coloradan uplift complex?

In Northern Hemisphere summers these two major plateaus heat up more than the surrounding areas and thus the air above them rises creating a low-pressure zone. This sucks in surrounding air creating a cyclonic circulation deflecting weather system much further north and south. In Northern Hemisphere winters these highlands are much colder than the surrounding areas creating a high-pressure system and out-blowing anti-cyclonic circulation. This deflects Arctic air northwards and keeps the middle of the Asian and North American continents warmer than they would otherwise be. The atmospheric circulation becomes even more complicated when large ice sheets are present on Greenland, North America, and Europe. Because ice sheets are always cold they produce permanent high-pressure systems with out-blowing anti-cyclonic circulation, which is discussed later.

The summer cyclonic circulation around the Tibetan-Himalayan plateau also creates the Southeast Monsoonal system. Because part of the air that is pulled towards the Himalayas comes from the Indian Ocean it brings with it a lot of moisture. The resultant rainfall is essential for the well-being of two-fifths of the world’s population. Occur on the western or eastern boundary of a continent. As we saw before there are three main rainfall belts in the world, one in the tropics and one in the mid-latitudes in each hemisphere. Air in the tropics moves from east to west, while in the mid-latitudes it moves west to east.

So having mountains on the western side produces more rainfall on land and produces a wetter continent overall. By coincidence at the moment we have western mountain ranges running down the west coast of North America, the Rockies, and the west coast of South America, the Andes. These mountains not only produce significant wet areas but also famous deserts like the Atacama Desert in Chile and Death Valley in the United States, which are two of the driest deserts on Earth. The contrast between wet and dry regions is even sharper if the uplift produces a plateau.

Plate tectonics control the development of volcanoes, which have an important influence on climate through the introduction of gases and dust into the atmosphere. Normal sized volcanoes inject sulphur dioxide, carbon dioxide, and dust into the troposphere and can have a considerable effect on our weather. For example in 1883 Krakatoa erupted, killing 36,417 people. The eruption is considered to be the loudest sound ever heard in modern history, with reports of it being heard nearly 3,000 miles away. It was equivalent to 200 megatons of TNT, which is about 13,000 times the nuclear yield of the Little Boy bomb that devastated Hiroshima, Japan, during World War II. The sulphur dioxide and dust injected into the atmosphere increased the amount of sunlight reflected back into space and average global temperatures fell by as much as 1.2°C in the year following the eruption. Weather patterns continued to be chaotic for years and temperatures did not return to normal until 1888.

On the 15 June 1991 Mount Pinatubo erupted sending 20,000,000 tonnes of sulphur dioxide into the atmosphere. The sulphur dioxide oxidized in the atmosphere to produce a haze of sulfuric acid droplets, which gradually spread throughout the lower stratosphere over the year following the eruption. This time modern instruments were able to measure its effects, which included a 10 per cent reduction in the normal amount of sunlight reaching the Earth’s surface. This led to a decrease in Northern Hemisphere average temperatures of 0.5-0.6°C and a global decrease in temperature of about 0.4°C.

Both Krakatoa and Pinatubo had a short-term transient effect on climate. This is because the sulphur dioxide and dust were injected relatively low in the atmosphere and the amount of water also injected meant much of the material was washed out of the atmosphere within a few years.

However these two eruptions are very small compared to eruptions from super volcanoes. These are thousands of times larger than Krakatoa. They can occur when magma in the Earth rises into the crust from a hotspot but is unable to break through the crust. Pressure builds in a large and growing magma pool until the crust is unable to contain the pressure. They can also form at convergent plate boundaries, for example Toba, which last erupted about 74,000 years ago and ejected about 2,800 km3 of material into the atmosphere. They can also form in continental hotspot locations, for example Yellowstone, which last erupted 2.1 million years ago and ejected 2,500 km3 of material. Because of the scale of these events the sulphur dioxide and dust are injected much higher in the atmosphere and therefore the effects on the global climate can be much longer. Modeling work by the UK Meteorological Office suggested a tropical super volcano eruption would cause a drop in global temperatures of at least 6°C, with up to 15°C in the tropics for at least 3 years. Then over a decade the climate would slowly came back to within 1°C of normal. The final effects would take up to a hundred years to get rid of and would be devastating for us if it ever happened. However, in geological terms it is a very short-term event with no significant long-term effect on the climate system.

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