These two phenomenons are an occurrence in the tropical eastern Pacific ocean where El Niño is a warming and La Niña is a cooling trend of the surface waters. Together with El Niño high surface pressures occurs in the western Pacific and with La Niña a lowering of surface pressures. Both effects are known as the El Niño Southern Oscillation (ENSO). Click the following link for the Oceanic Nino Index, last update Jun. 2015.
Because the ENSO has such a great influence in countries around the Pacific and definately worldwide too, we spend some time with this very interesting phenomenon.
Pilots flying in the pacific theatre will need to have intimate knowledge of this phenomenon as weather is directly influenced by this and tradewinds can change daily.
This page presents a short introduction into ENSO, further study will be required for a complete understanding of the topic.
El Niño (EN) is characterized by a large scale weakening of the trade winds and warming of the surface layers in the eastern and central equatorial Pacific Ocean. El Niño events occur irregularly at intervals of 2-7 years, although the average is about once every 3-4 years. They typically last 12-18 months, and are accompanied by swings in the Southern Oscillation (SO), an inter annual see-saw in tropical sea level pressure between the eastern and western hemispheres.
During El Niño, unusually high atmospheric sea level pressures develop in the western tropical Pacific and Indian Ocean regions, and unusually low sea level pressures develop in the southeastern tropical Pacific. SO tendencies for unusually low pressures west of the date line and high pressures east of the date line have also been linked to periods of anomalously cold equatorial Pacific sea surface temperatures (SSTs) sometimes referred to as La Niña.
The Southern Oscillation Index (SOI), defined as the normalized difference in surface pressure between Tahiti, French Polynesia and Darwin, Australia, is a measure of the strength of the trade winds, which have a component of flow from regions of high to low pressure.
High SOI (large pressure difference) is associated with stronger than normal trade winds and La Niña conditions, and low SOI (smaller pressure difference) is associated with weaker than normal trade winds and El Niño conditions. The terms ENSO and ENSO cycle are used to describe the full range of variability observed in the Southern Oscillation Index, including both El Niño and La Niña events.
Each warming and cooling cycle of the tropical Pacific surface waters will have specific effects on weather in the Pacific area and probably on a larger scale too and it is discussed on major points below.
Creates an increase of warm moist air resulting in rainfall in the east Pacific with its effect stronger in South than in North America. Especially in the winter months. Warm and drier winter months can be expected in North America. Following summer months will be warmer and wetter too. Hurricanes are less active too when El Niño is around.
Africa can also expect wetter than normal rain seasons between March and May. Northern Europe will experience warmer and wetter winters in contrary to middle and Southern Europe.
Causes effects mostly opposite to El Niño in Northern America and cooler and snowier winters in Canada and the US. In South America coastal regions of Peru and Chile will see drought periods and in winter Brazil will be wetter than normal.
Australasia will see the formation of tropical cyclones shift westward across the western Pacific ocean. With increase threat to China and surrounding countries. Wetter weather will occur in Indonesia, Philippines and Malaysia. Expect heavier than normal cyclones throughout the western Pacific.
This multivariate ENSO index (MEI) shows all the El Niño and La Niña occurrences since 1950. What is perfectly clear from this MEI graph above, is that since 1976, we have spent far more time in the positive (warmer) phase of ENSO than we have in the blue (colder) phase. As a result the climate has been a bit warmer than in the period before 1976 (when every climate scientist was proclaiming that an ice was coming, and now they do the opposite ;-).
The effects of El Niño on the Atlantic are a warm tropical North Atlantic in the following spring and summer. Sometimes, El Niño's effect on the Atlantic Walker circulation over South America will increase the easterly trade winds in the western tropical Atlantic area. Resulting in a very unusual cooling in the eastern tropical Atlantic during spring and summer after El Niño peaks in winter
As you can see, the El Niño & La Niña phenomenon in the Pacific Ocean have their effects globally influencing weather for everyone on a greater scale than CO2 will ever be able too.
More detailed technical information on the ENSO page of Wattsupwiththat by Anthony Watts.