Skymet forecasts a ‘healthy normal’ monsoon

Topics Covered: Important Geophysical phenomena such as earthquakes, Tsunami, Volcanic activity, cyclone etc.

Skymet forecasts a ‘healthy normal’ monsoon:

Context:

Skymet (a private weather forecast company) Weather report:

The monsoon is likely to be 103% of the long period average (LPA) this year. The LPA refers to the average all-India monsoon rainfall of 88 cm, which is a 50-year mean.
The odds of an El Nino, characterised by a heating of the equatorial central Pacific over half a degree, are low this year. Currently, the Pacific is in a [converse] La Nina mode.
The plains of North India, along with a few parts of northeast India, are at risk of being rain deficient through the season.
The Indian Ocean Dipole, characterised by a temperature gradient in the western and eastern Indian Ocean, is expected to be slightly on the negative. A positive dipole usually aids the monsoon.
The monsoon in 2019 and 2020 was only the third time in a century of back-to-back years of above normal rainfall (rainfall that is 5% above normal, or 105%) during the season in India.

What is Indian Ocean Dipole?

It is an atmosphere-ocean coupled phenomenon in the tropical Indian Ocean (like the El Nino is in the tropical Pacific), characterised by a difference in sea-surface temperatures.

Key features:

IOD is the difference between the temperature of eastern (Bay of Bengal) and the western Indian Ocean (Arabian Sea).
This temperature difference results into pressure difference which results in flowing of winds between eastern and western parts of Indian Ocean.
A ‘positive IOD’ — or simply ‘IOD’ — is associated with cooler than normal sea-surface temperatures in the eastern equatorial Indian Ocean and warmer than normal sea-surface temperatures in the western tropical Indian Ocean.
The opposite phenomenon is called a ‘negative IOD’, and is characterised by warmer than normal SSTs in the eastern equatorial Indian Ocean and cooler than normal SSTs in the western tropical Indian Ocean.

Impacts:

Studies have shown that a positive IOD year sees more than normal rainfall over central India.
A negative IOD complements El NiNo leading to severe drought.
At the same time, Positive IOD results in more cyclones than usual in Arabian Sea.
Negative IOD results in stronger than usual cyclogenesis (Formation of Tropical Cyclones) in Bay of Bengal. Cyclogenesis in Arabian Sea is suppressed during this time.