The distant ocean and atmospheric patterns that decide whether India's monsoon fails or thrives.
El Niño is the periodic warming of surface waters in the central and eastern equatorial Pacific Ocean, generally associated with weaker, deficient Indian monsoons (though the correlation isn't perfect). La Niña is the opposite phase — cooler-than-normal Pacific waters — usually associated with a stronger, above-normal monsoon in India. Together with the neutral phase, these make up the El Niño-Southern Oscillation (ENSO) cycle.
Jet streams are narrow bands of fast-flowing air in the upper troposphere. The subtropical westerly jet stream retreating northward (beyond the Himalayas) in summer is linked to the onset of the Indian monsoon, while the establishment of the tropical easterly jet stream over peninsular India in summer helps steer monsoon depressions and strengthens monsoon rainfall.
The Indian Ocean Dipole (IOD) — the temperature difference between the western and eastern Indian Ocean — can independently influence the monsoon; a "positive IOD" phase can sometimes offset the drought-inducing effect of an El Niño.
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