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- The 2019 June-September monsoon in India started its withdrawal on October 9, against the normal date of September 1, making it the most delayed in recorded history.
- It was also the strongest in recent years with a surplus of 10% in 2019 — both attributed in part due to the positive phase of the Indian Ocean Dipole (IOD).
- During a positive IOD phase, the west Indian Ocean warms up anomalously (creating lower pressure and wet climates) relative to the east (higher pressure and a dry phase, towards the Australian side).
- Climate models had indicated that the positive IOD may persist longer than typical events do.
- This could have contributed to a delay in transition of the monsoon trough from the Northern Hemisphere to the Southern Hemisphere and onset of the Australian monsoon.
- The IOD breakdown occurs when the monsoon trough moves into the southern hemisphere in early December (this is when the Northeast monsoon in India draws to a close).
- In 2019, the resulting drought and heat have seen Australia grapple with fire over many parts of the country from September.
- New South Wales (NSW), the country’s most populated state, alone reported loss of more than 2,000 homes and over 650 damaged.
- The southeastern state of NSWalone saw more than 4.9 million hectares being scorched.
Troughs and Ridges:
- Troughs – areas of low pressure featuring ascending motion of air and its cooling, clouds and rain
- Ridges (the reverse) – high pressure from descending motion of air, dryness, heat, no clouds or rain.
- Every trough is matched by a crest somewhere
- Playing out to the north and south of the equator, the progression of monsoon winds more or less corresponds to the natural occurrence of troughs and ridges in the same pressure field.