Answer:
- The Madden-Julian Oscillation (MJO) is a major intra-seasonal atmospheric disturbance/motion moving from west to east along the equator in the tropical atmosphere. Its cycle (periodicity) is typically 30 to 60 days. It is not a static weather system, but a moving band (pulse) of clouds, winds, and rain that progresses from the Indian Ocean toward the Pacific Ocean.
The MJO fundamentally operates on a cyclical pair of two contrasting atmospheric phases:
- Convective / Enhanced Phase: When the ocean surface temperature is high, the lower warm and moist air becomes lighter and rises rapidly. This vertical upward movement of air is called convection.
- Impact: Upon rising, condensation forms massive cumulonimbus clouds. This generates low pressure at the surface, increases convergence, and leads to intense heavy rainfall in this region.
- Suppressed / Subsidence Phase: Behind or ahead of the convective zone, dry and cold air from the upper atmospheric levels sinks downward. This process of downward settling of air is termed subsidence/descending.
- Impact: The sinking of air leads to high pressure at the surface and causes divergence. Subsidence suppresses cloud formation and vertical movement, keeping the skies clear and making the weather dry.
Impact on the Variability of Monsoonal Rainfall in the Indian Ocean Region:
- Active & Break Periods: When the convective phase of the MJO is positioned over the Indian Ocean, the Southwest monsoon strengthens and enters an 'Active Spell'. Conversely, upon the arrival of the subsidence phase, rainfall ceases, leading to a 'Monsoon Break' (Dry Spell).
- Formation of Depressions and Cyclones: The convective phase provides favorable heat and moisture for the development of low-pressure troughs, depressions, and tropical cyclones in the Indian Ocean (Bay of Bengal and Arabian Sea).
- Balancing the El Niño Effect: If the monsoon is weakening due to El Niño in the Pacific Ocean, but the favorable convective phase of the MJO becomes active over the Indian Ocean at the right time, it moderates the adverse impact of El Niño and helps sustain normal rainfall.
- Spatial and Temporal Distribution of Rainfall: It directly regulates the onset of the monsoon as well as the fluctuations (variability) in the timing and volume of seasonal precipitation.
- The convective and subsidence cycle of the MJO acts as the primary regulatory engine of intra-seasonal variability in the Indian Ocean monsoon.