What exactly caused the Chennai rains this december (2015)?

What exactly caused the Chennai rains this December (2015)?

                                    Article author: K Sreram

Heavy rain that hadn’t occurred in Chennai for over hundred years is something important to consider to understand our world’s climatic change. We have all heard about global warming, in which the global surface temperature gradually increase as a consequence of greenhouse effect. The global average temperature raised by 1˚C from 1880 to 2010. Does this value sound quite small? Well it's probably not. Because, this data is “average global surface temperature”. So on an average, all places on the map experience a temperature hike by 1˚C; obviously we have the net hike being directly proportional to the Earth’s surface area; which means that it eventually sums up to a huge value. Can the Chennai rains be related to this temperature hike? Yes it can! Global warming does not just cause glaciers melting in the Polar Regions, and the raising of the sea water level.

fig 1: This shows that the more hotter water is present in the surface while the colder water is present under the hotter region, as colder water generally have higher density.

The global climatic system periodically experiences changes in its pattern. The one that occurred in Chennai is a consequence of a climatic change. Before we go on to the scientific details, let’s recall some stuff are all familiar with. We are familiar with the cause of rain, and the water-cycle which forms as a consiquence. When the water gets heated up, it raises as vapour and the dust particles present in the atmosphere, which cool faster, reduce the temperature of these vapour molecules surrounding it eventually forming water molecules. As the density of these water molecules increase in the clouds, these water droplets fall to the earth, which is what we call rain. So obviously, this raise in temperature due to global warming might have caused a lot of water to raise up as vapour. But could it really be as simple as that? If then why didn’t heavy rains pour last year? Or why is this kind of rain so rare? The reason for this years' rain is the occurrence of a phenomena called El Niño. El Niño is a region of warm water that occurs between central and east-central region of the Pacific Ocean, also including the Pacific coast of South America. This warm water belt occurs periodically over time, causing heavy rains in most regions around the world and causing droughts in places which usually receive abundance of rain. This is actually a consequence of reverse in surface water’s flow pattern caused by the reverse in direction of the winds.

Fig 2: Shows the southern trade winds dragging the Warmer water towards the western region. 

Fig 2: Shows the southern trade winds dragging the Warmer water towards the western region.

Generally, southeast trade winds blow from east to west as a result of the Coriolis force caused by the rotation of the Earth. The earth rotates from west to east, so from Chennai coast, it is possible to see the sun rising from the sea; from newton’s first law which states that, an object that is at rest tends to be at rest and the object that is at motion tends to be at motion we can incur that the air which was initially assumed to be at rest in the Pacific region, relatively gets moved from east to west as the Earth moves from west to east. This is because of the air’s inertia (this causes our southeast trade winds). These southeast trade winds carry the warmer water, which is usually present in the surface of the Ocean (because of its lower pressure) towards the Asian and Australian continent. Thus lower pressure forms in the Asian region and higher pressure along the South American region. This causes a “water cycle” to arise, making the hot water to raise from the Pacific region which is near the Australian region, and this hot air enriched with vapour moves towards South American coast pouring rain along the way.

This forms a circulation process in which the hotter water from the places near South American coast move towards the Australian and South-East Asian coasts and the rain clouds move towards the South American region (pouring rain in the Western parts). So some amount of rain gets poured in the Australian and South-East Asian coast due to this effect. This circulation also encourages the cold water that lies under the warm water belt to raise up near the South American region. So we have a warm water region in the Western part and cold water region in the Eastern part.

Fig 3: Shows the case of El Nino, where the direction of the southern trade winds reverse direction and cause a larger portion of the water area to be warmer eventually causing larger amount of vapor to raise from the Ocean. This eventually causes more rainfall on many countries throughout the world. 

Fig 3: Shows the case of El Nino, where the direction of the southern trade winds reverse direction and cause a larger portion of the water area to be warmer eventually causing larger amount of vapor to raise from the Ocean. This eventually causes more rainfall on many countries throughout the world. 

But on some occasions, the southern trade winds die out. These trade winds are what cause the warmer water to move westward and the colder water to raise from under the Ocean in the eastern region. But in its absence, the cold water ceases to expand towards west thereby causing the warmer water to take the place of the colder water in the eastern parts. This is termed as the El Niño Southern Oscillation (ENSO). Over years, the warm water region and cold water region don’t show a constant and even occurrence. Depending on the change in trade winds, these regions also experience changes. At times the hotter water dominate (in case of ENSO) and at times the colder water dominate (in case of La Nina where the opposite happens) the Pacific Ocean, hence forming an oscillatory pattern. So when El Nino occurs, large rain clouds get formed above the Pacific Ocean, and cause heavy rains in many regions all around the world. This is an important cause of seasonal changes that greatly influence the global weather pattern.

What we just experienced in Chennai is the effect of an Extreme El Nino. This last occurred eighteen years back (from 2015). Though it wasn’t enough to break the hundred year record in Chennai, it still brought in a lot of rain. One of the major consequence of these frequent occurrences of El Nino is global warming. The overall increase in temperature of the water surface cause larger amount of water to raise into the atmosphere causing a lot of rain. Reports show a hike in the frequency of occurrences of this phenomena.

Though the great damage caused by this weather condition was blamed on the Chennai city’s poor planning, global climatic change is not going to just stop with this and we are bound to experience more changes in the weather pattern in the near future. The only permanent solution will be to control global warming.