Twin Cyclones

This section brings us to some of the more advanced and fascinating concepts in cyclone dynamics: twin cyclones, the Fujiwhara effect, hybrid cyclones, and the Madden-Julian Oscillation (MJO). These are not only high-value for UPSC (both Prelims & Mains) but also help you appreciate how global circulation patterns interact with local weather phenomena. Let’s understand this:

Twin Cyclones

• Definition: When two tropical cyclones form on opposite sides of the equator at roughly the same time, they are called twin cyclones.
• Example: May 2022 → Cyclone Asani (Northern Hemisphere, counter-clockwise spin) and Cyclone Karim (Southern Hemisphere, clockwise spin).

👉 Why opposite spins?
Because of the Coriolis effect:

• Northern Hemisphere → counter-clockwise circulation.
• Southern Hemisphere → clockwise circulation.

What causes twin cyclones?

• The Madden-Julian Oscillation (MJO) and Rossby waves together create enhanced convection across the equatorial region, giving rise to paired storms.

🌪️ Fujiwhara Effect

• Sometimes, two cyclones form close to each other in the same hemisphere.
• In such cases, they begin to orbit around a common centre, like a cosmic dance.
• Outcomes:
  1. They may merge into one stronger cyclone.
  2. Or, they may deflect each other and move away on separate tracks.

👉 This interaction is called the Fujiwhara Effect (named after Japanese meteorologist Sakuhei Fujiwhara).

🌪️ Hybrid Cyclones

• These are rare systems that combine features of both:
  • Tropical cyclones (thermal origin, warm-core).
  • Extratropical cyclones (frontal systems, cold-core).
• Example: They may start as tropical cyclones but interact with mid-latitude westerlies, acquiring extratropical traits.

👉 Hybrid cyclones remind us that nature does not follow rigid boundaries; systems can blend characteristics.

🌊 Madden-Julian Oscillation (MJO)

This is the key background player in many global weather changes, including cyclone formation.

What is MJO?

• An eastward-moving pulse of convection (thunderstorms) across the tropics.
• Recurs every 30–60 days.
• Difference from ENSO:
  • ENSO (El Niño–La Niña) is stationary and lasts for seasons (3–7 years).
  • MJO is moving and short-term (week-to-week).

Structure of MJO

• Two phases:
  1. Enhanced convective phase → Heavy rainfall, deep convection.
  2. Suppressed convective phase → Reduced rainfall, clear skies.
• The dipole (wet vs dry) keeps propagating eastward between 30°N and 30°S.

🌦️ Impacts of MJO on Weather

• Monsoons: Modulates onset, strength, and withdrawal of monsoons in India.
• Tropical Cyclones: Influences the number and intensity of cyclones across almost all ocean basins.
• Extreme Events: Can trigger jet stream changes, leading to cold waves, heatwaves, or floods in regions like North America.

🌪️ How MJO Helps Twin Cyclone Formation

• The enhanced phase of MJO brings strong thunderstorms → provides the moisture and convection needed for cyclone birth.
• MJO circulation also intensifies rotation.

👉 Mechanism:

1. Normally, in the tropics, trade winds blow from the east.
2. Behind the MJO, air is drawn inward at low levels → creating low-level westerly winds (called westerly wind bursts).
3. These westerlies near the equator interact with easterlies further away → producing enhanced cyclonic rotation on both sides of the equator.
4. Result = Twin cyclones spinning in opposite directions simultaneously.