Cyclone Dana | From Depression to Cyclone

Director General of Meteorology in the India Meteorological Department (IMD) Mrutyunjaya Mohapatra has told that a depression over Bay of Bengal is likely to form by 22^nd November 2024 and will transform into a cyclone by October 23.

This makes it important to understand the following core concepts:

What is a depression and how is it different from a cyclone?

Depression is basically a low pressure area characterised by unstable weather conditions. When these depressions are formed over the Tropical areas, they are called as ‘Tropical depressions’. Warm moist air rises due to heating by Solar insolation. This rising warm air leads to the formation of a low pressure condition on the surface. The rising warm air gradually cools down with altitude due to a drop in the temperature at the Normal Lapse Rate (NLR – 6.5 degrees/Celsius). Cooling of the warm air leads to the condensation and cloud formation which eventually leads to precipitation. Depressions can be identified by looking at the weather charts as the areas of closely spaced Isobars.
Tropical cyclones are a violently rotating vertical column of wind system formed in warm Tropical weathers. They are intensified form of depressions. Therefore, both of these are basically the low pressure systems.
There are certain criteria which are set by the Indian Meteorological Department (IMD) to differentiate between various types of such low pressure systems based on the pressure and wind conditions. Maximum sustained wind and pressure deficit/ number of closed isobars are used as the criteria. The system is called as low if there is one closed isobar in the interval of 2 hPa. It is called depression, if there are two closed isobars and so on.

Following chart represent those differences:

How does Tropical Cyclone form?

In the tropical latitudes, there is the presence of Easterly surface winds. Under favourable circumstances, low pressure system develops. This gives rise to surface level convergence of winds. Is the sea/ocean is warm enough, the upper level (middle Troposphere) divergence also forms. Low level convergence coupled with upper level divergence gives rise to vertical motion to the wind which takes the moist air upwards. With gradual cooling of the rising air, latent heat of condensation is released which further warms up the area, further leading to fall in pressure. This process continues and a low pressure system gradually intensifies into a cyclonic storm.

Tropical cyclone requires certain pre-conditions to form:

Warm surface waters (more than 26.5 degree Celsius) is required as warm waters are necessary to fuel the heat engine of the tropical cyclone.
Pre-existing near surface disturbance with sufficient vorticity (rotation) and convergence as the cyclones do not develop spontaneously.
Relative moist layer near the mid Troposphere and faster drop in temperature with height.
Presence of Coriolis force is required to produce a ‘spin effect’. Coriolis force is generated because of the rotational movement of the Earth along its axis. Coriolis force is directly proportional to the sine degree of the Latitude i.e. it is maximum at the poles (Sine 90 is maximum) and zero at the equator (Sine zero is 0). This is the reason that why Tropical cyclones are not formed at the Equator and there is no inter-hemispheric movement of the Tropical cyclones. A narrow corridor of width of about 300 km on either side of the equator is free from cyclones.
Weak vertical wind shear (magnitude of the wind change with height) is also required as the large wind shear will dissipate the energy/ heat and will disrupt the deep convection around the centre.

Why Bay of Bengal is more prone to the Tropical cyclones compared to the Arabian Sea?

The average annual frequency of tropical cyclones in the north Indian Ocean (Bay of Bengal and Arabian Sea) is about 5 (about 5-6 % of the Global annual average) and in the globe is about 80. The frequency is more in the Bay of Bengal than in the Arabian
Sea, the ratio being is 4:1.

Reasons responsible:

Bay of Bengal (BOB) is warmer than the Arabian sea. The average sea surface temperature (SST) of the BOB is around 28 degree Celsius while that of the AS is 24-25 degree Celsius. Relative shallowness and continuous influx of fresh water which reduces inter mixing of oceanic water makes the surface of the BOB warmer.
Arabian sea is relatively dry in middle Troposphere on account of surrounded by the desert areas of Arabian peninsula.
Arabian Sea also has higher wind shear because of the presence of heterogeneous topography and presence of dominating South-westerly winds.
BOB also receives the remnants of Typhoons generated over the Western Pacific Ocean. As the frequency of typhoons over northwest Pacific is quite high (about 35% of the global annual average), the Bay of Bengal also gets its increased quota. On the other hand, cyclones in BOB either die or lower down the intensity when they reach AS
The trough formation over the BOB is relatively early compared to the AS. This makes BOB more prone to intensification.

How is the naming of Tropical cyclones done?

There are different ocean basins and different Regional Specialised Meteorological Centres (RSMCs) for those basins. In total, there are Six Regional Specialised Meteorological Centres (RSMCs) and five regional Tropical Cyclone Warning Centres (TCWCs) who issue advisories and name the tropical cyclones.
IMD is one of the six RSMCs and provide advisories to 13 member countries under WMO/ESCAP Panel.
RSMC, New Delhi is also mandated to name the Tropical Cyclones developing over the north Indian Ocean (NIO) including the Bay of Bengal (BoB) and the Arabian Sea (AS).

Process followed:

Each regional body determines a pre-designated list of cyclones names for six years separately at its annual session.
For north Indian Ocean including Bay of Bengal and Arabian Sea, the RSMC, New Delhi assigns the name to tropical cyclones following a standard procedure.
The latest list of the names for the cyclones in North Indian Ocean has been made available in 2020. The Panel Member’s names are listed alphabetically country wise.

The proposed name should be neutral to (a) politics and political figures (b) religious believes, (c) cultures and (d) gender Name should be chosen in such a way that it does not hurt the sentiments of any group of population over the globe.
The names will be used sequentially column wise.
Maximum length of the name will be eight letters.
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 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.

Reference & credits