The ocean and atmosphere are closely interconnected. ENSO (El Niño–Southern Oscillation) and the large circulation involving the deep ocean regulate Earth's climate.
Basic Knowledge
Under normal conditions, trade winds blow from east to west across the equatorial Pacific, gathering warm seawater in the western Pacific. Off Peru in the eastern Pacific, upwelling (the rise of cold deep water) is active, making the sea-surface temperature low.
El Niño is a phenomenon in which trade winds weaken, warm water from the western Pacific spreads eastward, and the sea-surface temperature in the eastern Pacific becomes higher than average. Conversely, La Niña is the phenomenon in which trade winds strengthen and the eastern Pacific becomes even colder. Together they are called ENSO (El Niño-Southern Oscillation), and they alternate on a cycle of 4–7 years.
Global ocean circulation (deep circulation and thermohaline circulation) is a circulation on a scale of 1,000 to several thousand years in which cold, salty, high-density water sinks into the deep ocean and travels throughout the world's oceans. Deep water forms off Greenland in the North Atlantic and around Antarctica.
El Niño (state in which sea-surface temperature in the equatorial eastern Pacific is higher than average; weakened trade winds)
La Niña (state in which sea-surface temperature in the equatorial eastern Pacific is lower than average; strengthened trade winds)
ENSO (El Niño–Southern Oscillation; coupled ocean–atmosphere oscillation with a 4–7-year cycle)
Upwelling (phenomenon in which cold deep water rises to the surface; prominent off Peru; forms fishing grounds)
Thermohaline circulation (deep circulation driven by density differences determined by temperature and salinity; a several-thousand-year scale)
Atlantic Meridional Overturning Circulation (AMOC) (warm North Atlantic current → deep ocean → Southern Ocean; warms the European climate)
Further Study (Background and Significance)
Through teleconnections (remote connections), ENSO affects weather across the entire planet. El Niño years tend to bring cool summers and warm winters to Japan, while La Niña years tend to bring extremely hot summers and cold winters. Typical patterns also include droughts in Indonesia and Australia and heavy rain in Peru and Ecuador.
Thermohaline circulation is also related to the ocean's ability to absorb carbon dioxide. When deep water forms, large amounts of CO₂ are carried into the deep ocean. If climate change melts the Greenland ice sheet, increased freshwater input may weaken deep-water formation, potentially cooling the European climate.
Near Japan, the Kuroshio (North Pacific western boundary current) and the Liman Current (a southward current along the western Sea of Japan) intersect, forming rich fishing grounds. The path of the Kuroshio varies on cycles of several to more than ten years.
- Normal conditions: trade winds → warm water gathers in the western Pacific and upwelling occurs in the eastern Pacific
- El Niño: weaker trade winds → rising sea temperature in the eastern Pacific
- La Niña: stronger trade winds → falling sea temperature in the eastern Pacific
- ENSO cycle: 4–7 years
- Japan: El Niño year = cool summer and warm winter / La Niña year = extremely hot summer and cold winter
- Thermohaline circulation operates on a several-thousand-year scale and transports CO₂ to the deep ocean
- Weakened AMOC → risk of a cooler European climate
Points of Caution (Often Confused)
① Do not reverse El Niño (warming) and La Niña (cooling). ② ENSO is a coupled ocean–atmosphere phenomenon and does not occur in the atmosphere or ocean alone. ③ Thermohaline circulation is a deep current different from wind-driven circulation (surface currents moved by wind). ④ Japan's El Niño influence is a tendency toward cool summers and warm winters, not a certainty.
Practice
- How does the strength of the trade winds change when El Niño occurs?
- What are the two factors that drive thermohaline circulation?
- What is the tendency for Japan's summers during La Niña years?