The Arabian Sea arm of the south-west monsoon is relying on the transient Madden-Julian Oscillation (MJO) wave for a normal monsoon, as reported by the India Met Department (IMD). The MJO, an oceanic-atmospheric phenomenon, significantly influences weather activities worldwide. It is renowned for causing significant fluctuation in tropical weather on weekly to monthly timescales. The MJO moves eastward in the form of a ‘pulse’ of clouds, rainfall, winds, and pressure around the equator, generally recurring every 30 to 60 days. This traversing phenomenon is most noticeable over the Indian and Pacific Oceans.
Phases of Madden-Julian Oscillation
The MJO comprises two phases or parts, with strong MJO activity frequently dividing the planet into two halves. One side falls within the enhanced convective phase, and the other side is in the suppressed convective phase.
In the enhanced rainfall phase, surface winds converge, pushing air upwards throughout the atmosphere, where winds then reverse and diverge. Such rising air movement tends to increase condensation and, thus, rainfall.
On the other hand, in the suppressed rainfall phase, the winds at the top of the atmosphere converge, which forces the air downwards and subsequently diverge at the surface. As air descends from high altitudes, it warms and dries, suppressing rainfall.
This dipole structure moves from west to east over time in the Tropics, leading to more cloud cover, rainfall, and even storms in the enhanced convective phase, and more sunshine and dryness in the suppressed convective phase.
Impact of MJO on Indian Monsoon
The Indian Ocean Dipole (IOD), El Nino, and MJO are all oceanic and atmospheric phenomena impacting weather on a substantial scale. While IOD pertains only to the Indian Ocean, the latter two affect global weather up to the mid-latitudes. Only MJO moves, while IOD and El Nino stay over their respective positions.
The journey of MJO spans eight phases. When it is over the Indian Ocean during the Monsoon season, it tends to bring good rainfall over the Indian subcontinent. However, when it experiences a longer cycle and stays over the Pacific Ocean, MJO can negatively impact the Indian Monsoon. Therefore, MJO’s presence, which links to enhanced and suppressed rainfall activity in the tropics, is very important for the Indian monsoonal rainfall.
| Phases | Description |
|---|---|
| Enhanced Rainfall Phase | In this phase, surface winds converge, pushing air upwards throughout the atmosphere, where winds then reverse and diverge. |
| Suppressed Rainfall Phase | In this phase, the winds at the top of the atmosphere converge, forcing the air downwards and subsequently diverge at the surface. |
Periodicity of MJO and Its Effects on Indian Monsoon
When MJO’s periodicity is nearly 30 days, it brings good rainfall during the Monsoon season. But if it exceeds 40 days, MJO doesn’t result in good showers and might even lead to a dry Monsoon. Hence, the shorter the cycle of MJO, the better the Indian Monsoon. This is because it then visits the Indian Ocean more frequently during the four-month-long period. The presence of MJO over the Pacific Ocean, along with an El Nino, can be detrimental for Monsoon rains.
