Twin Cyclones

• Context (DTE | IE | IT): Twin cyclones Tej (over Arabian Sea) and Hamoon (over Bay of Bengal) developed over the north Indian ocean.
• Twin cyclones are two tropical cyclones that develop simultaneously in the same region.

Reasons for Formation of Twin Cyclones

Equatorial Rossby Waves

• Oceanic and atmospheric rossby waves (or Planetary waves) occur due to the Earth’s rotation (Coriolis Force). These waves occur in rotating fluids.
• Oceanic rossby waves are huge waves in the ocean with wavelengths of around 4,000–5,000 kms.
• This system consists of a vortex in the northern hemisphere mirrored by a corresponding one in the southern hemisphere.
• The northern vortex spins counterclockwise with a positive spin, while the southern vortex spins clockwise with a negative spin.
• Both have positive value of the vorticity which is a measure of the rotation.

Madden-Julian Oscillation

• MJO is an eastward moving trough (series of thunderstorms) in the tropics that recurs every 30 to 60 days.
• The MJO consists of enhanced rainfall convective phase and suppressed rainfall convective phase.
• During enhanced phase, increased cloud cover reduces solar radiation reaching the ocean surface, leading to cooling effects.
• Conversely, during the suppressed phase, reduced cloud cover allows more solar radiation to penetrate the ocean, leading to warming of SSTs.

Fujiwhara Effect When two tropical cyclones (or hurricanes) come in close proximity, they begin to rotate around a common midpoint (which looks like a dance). It is called the Fujiwhara Effect. Conditions necessary for the effect to occur: Two tropical cyclones formed around the same time in the same ocean region. The distance between the centres (or eyes) of the cyclones is less than 1,400 km. An intensity that could vary between a depression (wind speed under 63 km/hour) and a super typhoon (wind speed over 209 km/hour). For details on Fujiwhara Effect > PMF IAS August, 2023 CA