The shift of the South-west monsoon season in India has a ripple effect that influences Australia’s summer monsoon, as per data from the Australian Bureau of Meteorology. A clear example of how one geographical climatic event can drastically affect national and international weather patterns. Herein lies the importance of understanding the integral role of solar heat in dictating the fluctuation of monsoon systems globally.
Monsoon Systems and Solar Heat
Solar heating forms the very foundation of how monsoon systems function. During India’s summer period in specific, solar heating plays a pivotal role in creating a discernable land-sea temperature difference. Because land heats up faster than water, this triggers an atmospheric pressure disparity where the land’s atmospheric pressure declines due to the expansion of heated air.
This pressure difference generates wind movement from the seas towards the hotter land, as wind generally travels from areas with high pressure to low. Consequently, moisture is transferred from the sea via the wind and deposited as rain on the land. This process, known as convection, is most apparent in India and South-East Asia’s monsoonal rainfalls.
Shifting Seasons and The Sun’s Position
A change in the seasons, such as winter in the Northern Hemisphere, leads the sun towards the Tropic of Capricorn. Consequently, the Southern Hemisphere receives more sunlight, particularly, the adjacent oceans get heated more than the Asian land. As the sun alters its position, the winds reverse their course, and the monsoonal rainfall translocates to the opposite hemisphere during the Austral summer (occurring between December and February).
The natural flow of monsoonal wind, however, can be disrupted by the presence of the Indian Ocean Dipole (IOD), as observed in 2019 when it intensified the Indian monsoon. This resulted in a delayed withdrawal, starting on October 9 as opposed to its usual date of September 1, marking it as the most deferred recorded in history.
Understanding Indian Ocean Dipole (IOD)
The term “positive IOD” implies a warmer western basin of the Indian Ocean relative to its eastern counterpart. In 2019, over a month, robust easterly trade winds across the tropical Indian Ocean brought up cooler water from the ocean’s depth in the eastern region nearer to Australia. Simultaneously, the warm waters off the Horn of Africa added to the existing temperature discrepancy across the basin.
| Date | Temperature Difference East Vs West in IOD |
|---|---|
| 2019 | Increased Temperature Difference |
IOD’s Impact on Australia
A delayed monsoon can have serious consequences for Australia’s summer crops. In fact, due to the 2019 delay, there was a reported decrease in planted area by about 28% in 2019-20. A positive IOD usually results in below-average rainfall during winter-spring in southern and central Australia and a harsher fire season in South-Eastern Australia.
On another note, the El-Nino Southern Oscillation (ENSO) in the tropical Pacific Ocean remained unaffected. When ENSO is neutral, it does not significantly impact the Australian and global climate, making other influences like the IOD more dominant.