The earth’s climate undergoes regular cyclical changes. Those related to changes in the orbit of the earth around the sun have a periodicity of tens of thousands of years. Those related to the seasons have an annual pattern. Superimposed on these are number of oscillation. These are less regular than the cycles mentioned above and have periods from a few months up to several decades. Their cause is not clearly understood but changes in one part of the world can have well defined effects on global weather patterns. The cycles are generally defined by changes in air pressure, sea temperature and wind direction over oceans.
The principal oscillations are:
- El Niño/Southern Oscillation (ENSO) which is observed in the southern Pacific and has a periodicity of 3 to 8 years
- Pacific Decadal Oscillation (PDO) which is observed over the whole Pacific hand has a periodicity of two or three decades
- North Atlantic Oscillation (NAO) which is observed over the northern Atlantic Ocean and has a period of around one decade
Other oscillations affect the Arctic, Antarctic and Indian Oceans.
El NIÑO/Southern Oscillation (ENSO)
This is the most important of the oscillations. The phrase El Niño means the boy child in Spanish and its use came about as it was first noticed by Peruvian fisherman when warmer than usual conditions were experienced at around Christmas . It has a counterpart called La Niña which means the girl child whose effects are the opposite of those of El Niño. Another oscillation was known as the “Southern Oscillation” as this affected the southern Pacific. It is now generally considered that these two are aspects of the same phenomena and are referred to by the single acronym ENSO.
The indicators of an El Niño are:
- A rise in air pressure over the Indian Ocean (Darwin in Australia is used a reference meteorological station) coupled with a fall in pressure over the central and eastern Pacific (Tahiti is used a reference station)
- The westerly tropical trade winds weaken or reverse direction
- Warm, nutrient-poor, water reaches the west coast of South America and replaces the cold, nutrient-rich , waters of the Humboldt Current
- Sea level in the Western Pacific rises by around 0.5 m
The impacts of El Niño and La Niña are widespread: