The Great Withdrawal
The southwest monsoon, which drenches the subcontinent from June to September, doesn't just switch off. Its retreat is a gradual, staggered process that typically begins in early September from the northwestern parts of the country, like Rajasthan. This
withdrawal is a slow march southeast, taking over six weeks to fully exit the peninsula. The India Meteorological Department (IMD) has specific criteria for declaring the withdrawal, including a cessation of rain for five continuous days and a significant drop in atmospheric moisture. As the landmass begins to cool faster than the ocean, the pressure systems that pull moisture-laden winds onto the subcontinent weaken, heralding the end of the season. This means that in mid-September, Delhi could be experiencing dry, post-monsoon air while Kerala is still receiving heavy monsoon rain.
Understanding the 'Monsoon Lull'
The term 'monsoon lull' or 'break' refers to a temporary but significant reduction in rainfall activity over the core monsoon zone of central and western India. It's a natural part of the monsoon cycle, not a sign that the season has failed. During a break, the monsoon trough—an elongated low-pressure area that is the main rain-bearing system—shifts northward towards the foothills of the Himalayas. This causes rainfall to decrease dramatically across the plains but can lead to heavy showers in the northeastern states and along the Himalayan belt. These breaks can last from a few days to a few weeks and are a primary reason why a season with 'normal' total rainfall can still feature damaging dry spells for agriculture.
The Bay of Bengal's Decisive Role
While the monsoon may be in a 'lull' over one part of India, the Bay of Bengal often has other plans. This warm body of water is a prolific engine for low-pressure systems, especially in September. These systems are essentially cyclonic circulations that draw in immense moisture and can move west or northwestward over land. They are the principal rain-bearers of the monsoon season, responsible for a significant portion of India's total rainfall. Even as the broader monsoon flow weakens, a potent low-pressure area forming in the Bay can reactivate the monsoon, triggering intense rainfall spells across eastern, central, and even northeastern India. These systems are why states like Odisha and West Bengal often face their most active weather in September, sometimes leading to floods even as other regions are drying out.
A Tug-of-War in the Skies
September's weather is best understood as a dynamic contest. On one side, you have the retreating southwest monsoon, gradually losing its grip on the subcontinent. On the other, you have the active and often aggressive weather systems forming in the Bay of Bengal. The result is a highly variable and often unpredictable pattern. A 'lull' can be abruptly broken by the arrival of a low-pressure system from the Bay. Meteorologists closely watch the Bay for the formation of these systems, as they can quickly intensify into depressions, bringing widespread, heavy rain to coastal and inland areas. It is this interplay that defines the character of the month—not quite monsoon, but not yet post-monsoon either.
