In a groundbreaking research conducted by the Centre for Atmospheric and Oceanic Sciences (CAOS) at the Indian Institute of Science (IISc), it was discovered that El Niño, a recurrent climate phenomenon, is not the sole cause of droughts during the Indian summer monsoon. Typically, El Niño is characterized by unusually warm Pacific waters that attract moisture-laden clouds away from the Indian subcontinent, often resulting in disappointing monsoons from June to September.
Key Discoveries from the Research
The study revealed that 43% of the droughts that transpired during the Indian summer monsoon season over the past hundred years might have been triggered by atmospheric disturbances originating from the North Atlantic region. Intriguingly, these drought events in India took place in years when El Niño was not present.
The Driving Force behind these Droughts
The culprits behind these droughts were identified as abrupt and significant decreases in rainfall towards the end of August. This drop was connected to an atmospheric disruption in the mid-latitude region above the North Atlantic Ocean. This disturbance generated a pattern of atmospheric currents that swept over the Indian subcontinent and disturbed the course of the monsoon.
The Shift in Drought Patterns
According to the research, there was a notable variation in the pattern of droughts. During El Niño years, the shortage of rainfall began in mid-June and progressed across the nation. Contrastingly, under normal circumstances, the monsoon season would typically see regular rainfall until a sudden and sharp decline was observed in August.
The Cause of the August Decline
The drop in August rainfall was attributed to an uncommon atmospheric disturbance in the mid-latitudes, an area on Earth located between the latitudes 23° and 66° north. This anomaly arose from the interaction of winds in the upper atmosphere with a cyclonic circulation above unusually cold North Atlantic waters. The ensuing wave of air currents, known as a Rossby wave, journeyed from the North Atlantic towards the Tibetan plateau, reaching the Indian subcontinent around mid-August. This led to suppressed rainfall and resulted in drought-like conditions.
Understanding Other Atmospheric Circulations Impacting the Monsoon
In addition to the disturbances from the North Atlantic, the Indian Ocean Dipole (IOD) and the Madden-Julian Oscillation (MJO) were also identified as potential influences on the monsoon season.
The IOD, marked by the difference in sea surface temperature between the Arabian Sea and the eastern Indian Ocean, develops in the equatorial region of the Indian Ocean between April and May and peaks in October. Depending on its stage – positive or negative – the IOD can cause warmer or colder weather, and wetter or drier conditions in certain regions.
On the other hand, the MJO is an eastward moving band of rain clouds that circles the globe, returning to its starting point in 30 to 60 days. With two phases – the enhanced rainfall phase and the suppressed rainfall phase – strong MJO activity can divide the planet into halves, producing changes in cloud and rainfall patterns. As this entire dipole moves eastwards, it can influence the timing and strength of monsoons and impact weather phenomena globally.
These findings underline the intricate nature of the factors influencing India’s summer monsoons, shedding light on the important role of atmospheric disturbances from the North Atlantic, the IOD, and the MJO.