What is La Nina?
La Niña (‘little girl’ in Spanish) and El Niño (‘little boy’) refer to the see-sawing of surface temperatures of the Pacific Ocean, which represents more than 50 per cent of the world’s oceanic water.
When sea-surface temperatures (SSTs) go up beyond 27.5 degrees Celsius, it supports evaporation and cloud-building, triggering storms (cyclones or typhoons) and heavy rain. During a La Nina, SSTs are elevated over the West Pacific.
When the SSTs cool, the exact reverse happens; it suppresses evaporation, cloud-building, storm formation and rain. This phase is called El Niño (SSTs are higher in the East Pacific), which has been associated with drought years in India, with exceptions.
What is the impact of La Niña on the North-East monsoon?
While a La Niña is known to enhance rainfall associated with the South-West monsoon, again with exceptions, it has correlated negatively with the North-East monsoon.
Meteorologists ascribe two reasons for this. Low-pressure areas, depressions or cyclones form relatively north to their normal position during a La Nina year.
Two, instead of moving West-North-West towards the East Coast of India, they tend to recurve and move away, robbing the South Peninsula of its share of rainfall.
A classic example is in how cyclone ‘Jawad’, just last week, sped its way initially towards the East Coast only to shift track later.
How is it that La Niña has fared differently this year?
Normally, a La Niña is a dampener on a concurrent North-East monsoon. But this logic may have been blown away by the rain-driving low-pressure areas/depressions in the Bay of Bengal/Arabian Sea so far.
La Niña may trigger extreme cold, prolonged pollution spells
Varying Pacific or Indian Ocean modes are but just two pieces in the grand puzzle of the ‘predictably unpredictable’ weather/climate patterns across the globe. There are many other causative factors and inter-connections that go to make any conjectures based on them preposterous.
So, is La Nina contributing to the excessive NE monsoon?
Yes. The Indian Ocean Dipole (IOD) or Indian Nino (it involves a periodic oscillation of SST between positive, negative and neutral) had gone from a slightly negative mode into neutral even as the North-East monsoon was establishing.
A negative IOD could have wrecked it irrespective of the Pacific mode, since warming of the East Indian Ocean could divert rain systems away from the Bay. Weather systems seek out the nearest warm water pool to thrive and prosper.
Given the neutral IOD phase, it is likely that the flows from upstream South China Sea/West Pacific were directed into the Bay, beefing up the North-East monsoon.
How long will its effect last?
Meteorologists track specific segments (called ‘Nino’ regions) of the Equatorial Pacific to monitor the SSTs. The sheer size of the Pacific and the enormity of scale of La Nina and its alter-ego El Niño are such that they can affect weather and climate patterns with varyingly grave consequences for entire economies across the globe.
These phenomena recur every three to five years and each cycle lasts nine months to a year or perhaps even more, manifesting in floods/drought across geographies.