Cyclone frequency may rise over Indian coast from the warming of Pacific: study

Subject: Geography

Section: Physical Geography

Context:

Tropical cyclones that originate near the Equator, while being devastating, have been unusually subdued in recent decades.

The last major cyclone of this kind in the Indian neighborhood was the 2017 Cyclone Okchi which devastated Kerala, Tamil Nadu and Sri Lanka.

Rising cyclone frequency near equator:

A combination of global warming and a cyclical event called the Pacific Decadal Oscillation (PDO) that repeats every 20-30 years, could make such cyclones more frequent in the coming years, according to a study.
The number of such equatorial-origin cyclones was 43% fewer in 1981-2010 compared with 1951-1980.
This was because the PDO was in a ‘warmer’ or positive phase.
In 2019, the PDO entered a cooler, negative phase and if it remains so, could mean moretropical cyclones in the post-monsoon months that originate near the equator.
It’s usually rare for cyclones to form near the Equator but when the waters are warm, they can gain more moisture and rise in intensity.
An El Nino is currently developing in the Pacific, the effects of which are already manifested in central and southern India, which have recorded rainfall deficits of 7% and 17% respectively.
ENSO with a positive PDO is generally not good, but when linked with a negative PDO, brings more rain to India.

What is Pacific Decadal Oscillation (PDO):

The PDO is often described as a long-lived El Niño-like pattern of Pacific climate variability.
- When Sea Surface Temperatures (SSTs) are anomalously cool in the interior North Pacific and warm along the Pacific Coast, and when sea level pressures are below average over the North Pacific, the PDO has a positive value.
- When the climate anomaly patterns are reversed, with warm SST anomalies in the interior and cool SST anomalies along the North American coast, or above average sea level pressures over the North Pacific, the PDO has a negative value.

Difference between ENSO cycle and Pacific Decadal Oscillation (PDO)

ENSO	PDO
A warming of the Central Equatorial Pacific, called an El Nino, frequently corresponds to reduced rainfall over India whereas cooler-than-normal temperatures, or a La Nina, is linked to excessive rainfall. This pattern collectively called the El Nino Southern Oscillation (ENSO) phenomenon, repeats in the Pacific over two-seven years.	PDO is a long-term ocean fluctuation of the Pacific Ocean, which waxes and wanes approximately every 20 to 30 years. The PDO isn’t an annual occurrence and, on average, corresponds to a warmer than average Western Pacific Ocean and relatively cooler Eastern Pacific, though this plays out over much longer time scales.
The stage of ENSO cycle can be determined any year	a ‘positive’ or ‘warmer phase’ of a PDO can be known only after several years of measuring ocean temperatures and their interaction with the atmosphere.
ENSO cycles typically only last 6 to 18 months.	The PDO can remain in the same phase for 20 to 30 years

Similarities between ENSO and PDO:

Just like El Nino/La Nina in the tropical Pacific, PDO has a significant impact on the sea surface temperatures and its interaction with the atmosphere, which in turn affects the northeast Indian summer monsoon.
The PDO, like ENSO, consists of a warm and cool phase which alters upper-level atmospheric winds.
PDO can intensify or diminish the impacts of ENSO according to its phase.
- If both ENSO and the PDO are in the same phase, it is believed that El Niño/La Nina impacts may be magnified.
- Conversely, if ENSO and the PDO are out of phase, it has been proposed that they may offset one another, preventing “true” ENSO impacts from occurring.