Tropical Cyclones

One will be able to answer most of the questions related to Tropical Cyclones with this article-important topic for Prelims 2020.

Tropical cyclone, also called  typhoon or  hurricane, an intense circular storm that originates over warm tropical oceans and is characterized by low atmospheric pressure, high winds, and heavy rain. Drawing energy from the sea surface and maintaining its strength as long as it remains over warm water, a tropical cyclone generates winds that exceed 119 km (74 miles) per hour- Below this speed its called Tropical Storm.

Every year during the late  summer months (July– September in the Northern Hemisphere and January– March in the Southern Hemisphere), cyclones strike regions as far apart as the Gulf Coast of North America , northwestern Australia, and eastern India and Bangladesh.

Tropical cyclones are known by various names in different parts of the world. In the North Atlantic Ocean and the eastern North Pacific they are called hurricanes, and in the western North Pacific around the Philippines, Japan, and China the storms are referred to as typhoons. In the western South Pacific and Indian Ocean they are variously referred to as  tropical cyclones.

Anatomy of a Tropical Cyclone

Tropical cyclones are compact, circular storms, generally some 320 km (200 miles) in diameter, whose winds swirl around a central region of low atmospheric pressure. The winds are driven by this low-pressure core and by the rotation of Earth, which deflects the path of the wind through a phenomenon known as the Coriolis force. As a result, tropical cyclones rotate in a counterclockwise (or cyclonic) direction in the Northern Hemisphere and in a clockwise (or anticyclonic) direction in the Southern Hemisphere.

The wind field of a tropical cyclone may be divided into three regions. First is a ring-shaped outer region, typically having an outer radius of about 160 km (100 miles) and an inner radius of about 30 to 50 km (20 to 30 miles). In this region the winds increase uniformly in speed toward the centre. Wind speeds attain their maximum value at the second region, the eyewall, which is typically 15 to 30 km (10 to 20 miles) from the centre of the storm. The eyewall in turn surrounds the interior region, called the eye, where wind speeds decrease rapidly and the air is often calm.

Eye

A characteristic feature of tropical cyclones is the eye, a central region of clear skies, warm temperatures, and low atmospheric pressure. 

Eyewall

The most dangerous and destructive part of a tropical cyclone is the eyewall. Here winds are strongest, rainfall is heaviest, and deep convective clouds rise from close to Earth’s surface to a height of 15,000 metres .

When updrafts reach the stable tropopause (the upper boundary of the troposphere, some 16 km [10 miles] above the surface), the air flows outward. The Coriolis force deflects this outward flow, creating a broad anticyclonic circulation aloft. Therefore, horizontal circulation in the upper levels of a tropical cyclone is opposite to that near the surface.

Rainbands

In addition to deep convective cells (compact regions of vertical air movement) surrounding the eye, there are often secondary cells arranged in bands around the centre. These bands, commonly called rainbands, spiral into the centre of the storm.

There are six conditions favourable for formation of Tropical Cyclones :

1. The temperature of the surface layer of ocean water must be 26.5 °C (80 °F) or warmer, and this warm layer must be at least 50 metres (150 feet) deep.
2. A preexisting atmospheric circulation must be located near the surface warm layer.
3. The atmosphere must cool quickly enough with height to support the formation of deep convective clouds.
4. The middle atmosphere must be relatively humid at a height of about 5,000 metres (16,000 feet) above the surface.
5. The developing system must be at least 500 km (300 miles) away from the Equator.
6. The wind speed must change slowly with height through the troposphere—no more than 10 metres (33 feet) per second between the surface and an altitude of about 10,000 metres (33,000 feet).

Important Point :

For the development of the rapid rotation characteristic of tropical cyclones, the low-pressure centre must be located at least 500 km (300 miles) away from the Equator. If the initial disturbance is too close to the Equator, then the effect of the Coriolis force will be too small to provide the necessary spin. The Coriolis force deflects the air that is being drawn into the surface low-pressure centre, setting up a cyclonic rotation.

Storm Surge

A storm surge accompanying an intense tropical cyclone can be as high as 6 metres (20 feet). Most of the surge is caused by friction between the strong winds in the storm’s eyewall and the ocean surface, which piles water up in the direction that the wind is blowing. 

A small part of the total storm surge is due to the change in atmospheric pressure across the tropical cyclone. 

Flooding caused by the storm surge is responsible for most of the deaths associated with tropical cyclone landfalls.

Is climate change a contributor to formation of Tropical Cyclones?

Climate change would suggest warmer sea temperatures in some parts of the world, and warm sea temperatures create ideal conditions for cyclones to form.Hence climate change will lead to increase in occurrence of tropical cyclones.

Naming of Cyclones

(asked in UPSC Mains earlier)

The process of naming cyclones involves several countries in the region and is done under the aegis of the World Meteorological Organization.

The World Meteorological Organisation/Economic and Social Commission for Asia and the Pacific Panel on Tropical Cyclones, at its twenty-seventh session held in 2000 in Muscat, Oman, agreed to assign names to the tropical cyclones in the Bay of Bengal and Arabian Sea.

For the Indian Ocean region, deliberations for naming cyclones began in 2000 and a formula was agreed upon in 2004.

The eight countries along the Bay of Bengal and Arabian Sea suggest names that are sequentially listed. The nations suggest names alphabetically — Bangladesh, India, Maldives, Myanmar, Oman, Pakistan, Sri Lanka and Thailand.

The Regional Specialised Meteorological Centre (RSMC) based here gives a tropical cyclone an identification from the names list. The identification system covers both the Arabian Sea and the Bay of Bengal.

Why Bay of Bengal is the world’s hotbed of tropical cyclones?

Or

Why Bay of Bengal witnesses more Cyclones than Arabian Sea?

The worst places for storm surges tend to be shallow, concave bays where water, pushed by the strong winds of a tropical cyclone, gets concentrated or funnelled as the storm moves up the bay.

The Bay of Bengal is a textbook example of this type of geography.

High sea surface temperatures in the Bay of Bengal trigger extremely strong cyclones.

Tropical cyclones need a temperature of 25-27 degrees atleast. Arabian Sea is comparatively cooler compared to Bay of Bengal.

(Article will be updated and notified if needed)