El Nino & La Nina
Notes
ENSO (El Nino-Southern Oscillation) is a periodic variation of sea-surface temperatures and pressure across the tropical Pacific, with global effects. It recurs every 2-7 years.
Normal conditions (Walker circulation)
- Trade winds blow east to west along the equator, piling up warm water in the western Pacific (near Indonesia). Cold water upwells off Peru (the Humboldt current), which supports a rich fishery.
- Low pressure over Indonesia and Australia; high pressure over the eastern Pacific.
El Nino (Spanish for "the Christ child", named by Peruvian fishermen as it comes around December)
- Warming of the central and eastern Pacific. The Oceanic Nino Index (ONI), a 3-month mean of SST anomalies in the Nino 3.4 region (5°N-5°S, 120°-170°W), is above +0.5°C for several consecutive periods.
- Trade winds weaken or reverse; the Walker cell weakens; the Southern Oscillation Index (the pressure difference between Tahiti and Darwin) becomes negative.
- Effects: weak or deficient Indian monsoon (e.g., 2002, 2009, 2014-15, 2023), droughts in Australia and Indonesia, floods in Peru and Ecuador, collapse of the Peruvian anchovy fishery, fewer Atlantic hurricanes, and higher global temperatures.
La Nina
- Cooling of the central and eastern Pacific; stronger trade winds and a stronger Walker cell.
- Effects: usually a good monsoon in India, floods in Australia and south-east Asia, more Atlantic hurricanes, and drought in parts of the south-western USA.
El Nino Modoki: a warming in the central Pacific ("dateline El Nino"), with different teleconnections.
Caution: not every El Nino year brings a drought in India. In 1997 a strong El Nino coincided with a positive Indian Ocean Dipole and a normal monsoon.
Basis | El Nino | La Nina |
|---|---|---|
Eastern Pacific SST | Warmer | Cooler |
Trade winds | Weaker | Stronger |
Indian monsoon | Usually weaker | Usually stronger |
SOI | Negative | Positive |
Key Points
- ENSO: coupled ocean-atmosphere cycle; every 2-7 years.
- El Nino: warm eastern Pacific, weaker trades, negative SOI; weak Indian monsoon.
- La Nina: cool eastern Pacific, strong trades; good monsoon.
- ONI in the Nino 3.4 region defines the phases.
- Positive IOD can offset El Nino's impact.
MCQs
Q1. El Nino is characterised by:
(a) Cooling of the eastern Pacific
(b) Warming of the central and eastern Pacific
(c) Strengthening trade winds
(d) Heavy Indian monsoon
Ans: (b).
Q2. The Southern Oscillation Index measures the pressure difference between:
(a) Tahiti and Darwin
(b) Hawaii and Fiji
(c) Lima and Sydney
(d) Jakarta and Manila
Ans:
(a).
Q3. La Nina generally brings what to the Indian monsoon?
(a) Deficit rainfall
(b) Good rainfall
(c) No effect
(d) Delayed onset every time
Ans: (b).