Tropical cyclones in Indian Ocean

Reasons For High frequency of cyclones in Bay of Bengal

• BOB receives higher rainfall and continuous influx of mighty rivers like Ganga, Brahmaputra etc. This provides abundant moisture for the cyclone formation.
• The wind velocity in BOB is lower as compared to Arabian Sea. Sluggish winds do not dissipate the heat and hence keep the temperature higher in BOB.
• BOB also receives cyclones from Pacific through narrow straights in SE Asia and gets reenergized on reaching BOB.
• Tropical cyclones formed in BOB move towards west under the influence of easterlies but dissipate on reaching the Indian landmass and hence do not reach Arabian Sea.

The post-monsoon period sees a higher number of cyclones than the pre-monsoon period. 

• This is because summers and pre-monsoons see dry and hot air moving from north-western India towards the Bay. This blocks the rise of air from the water, and the subsequent formation of clouds, preventing cyclone-friendly conditions. 
• But the absence of this air movement in the post-monsoon phase increases the chances of cyclones.
• All these factors make the Bay of Bengal the one of the most sensitive areas in the world when it comes to cyclones. It also explains why people in the coastal states along the Bay live in perpetual risk of this destructive weather phenomenon.

Reasons for high cyclonic activity in Arabian Sea in recent times 

• Due to global warming, sea surface temperatures in the Arabian Sea have risen substantially during the last century. The temperature is presently 1.2–1.4⁰C higher than it was four decades ago. Active convection, torrential rainfall, and strong cyclones are all supported by the warmer temperatures.
• Rising temperatures are allowing the Arabian Sea to provide adequate energy for storm intensification. 
• Arabian Sea provides favourable wind shear for cyclones. For example, Cyclone Ockhi’s depression was driven from the Bay of Bengal to the Arabian Sea by a stronger easterly wind.
• The occurrence of El Nino Modoki is increasing. It is a climate phenomenon known as ‘pseudo El Nino,’ which causes conditions in the Bay of Bengal that are unsuitable for cyclogenesis. In the Arabian Sea, however, this situation is favourable for the creation of cyclones.
• The warming of west wind drift is pushing the warm waters into the Arabian Sea and developing a positive feedback system.

When does a “Depression” becomes a “Cyclone”?

• World Meteorological Organisation uses the term ‘Tropical Cyclone’ to cover weather systems in which winds exceed ‘Gale Force’ (minimum of 34 knots or 63 kph). A gale is a strong wind, typically used as a descriptor in nautical contexts. The U.S. National Weather Service defines a gale as 34–47 knots (63–87 km/h, 17.5–24.2 m/s or 39–54 miles/hour) of sustained surface winds. 
• Further categories are determined similarly by wind speeds.

From where are the names derived? 

• Cyclones that form in every ocean basin across the world are named by the Regional Specialised Meteorological Centres (RSMCs) and Tropical Cyclone Warning Centres (TCWCs). There are six RSMCs in the world, including the India Meteorological Department (IMD), and five TCWCs.
• The RSMC New Delhi Tropical Cyclone Centre is responsible to name the tropical cyclones that have formed over the Bay of Bengal and the Arabian Sea when they have reached the relevant intensity.
• As an RSMC, the IMD names the cyclones developing over the north Indian Ocean, including the Bay of Bengal and Arabian Sea, after following a standard procedure. The IMD is also mandated to issue advisories to 12 other countries in the region on the development of cyclones and storms.

Rules of Nomenclature

• WMO maintains rotating lists of names which are appropriate for each Tropical Cyclone basin. The Panel Member’s names are listed alphabetically country wise.
• The names will be used sequentially column wise.
• The first name will start from the first row of column one and continue sequentially to the last row in the column thirteen.
• The names of tropical cyclones over the north Indian Ocean will not be repeated, once used it will cease to be used again. The name should be new. It should not be there in the already existing list of any of the RSMCs worldwide including RSMC New Delhi.
• The name of a tropical cyclone from South China Sea which crosses Thailand and emerge into the Bay of Bengal as a Tropical cyclone will not be changed.
• The proposed name should be provided with its pronunciation and voice over.
• The names of tropical cyclones over the north Indian Ocean will not be repeated. Once used, it will cease to be used again. Thus, the name should be new.